Download Chromosomal Basis of Inheritance Final

2. CHROMOSOMAL BASIS OF INHERITANCE
DROSOPHILA AS A MATERIAL FOR EXPERIMENTAL GENETICS
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Thomas hunt morgan (the father of experimental genetics) selected fruitfly (drosophila
melanogaster) (the jackpot of genetics) as experimental material though it is small in size (2mm).
Following advantages of drosophilaIts easily available hovering over ripe mango/banana fruits where it feeds over yeast cells present
over the fruit surface.
The flies can be reared inside bottles having yeast culture over medium containing cream of wheat,
molasses and agar.
A new generation can be raised within two weeks from single mating producing hundreds of
individuals.
The animals can be temporarily inactivated with ether and examined by hand lens / dissection
microscope.
Female is distinguishable from male by its larger size and ovipositor.
The animals possess 4 pairs of chromosomes of different sizes, Y chromosome is hooked and easily
distinguished.
Polytene chromosomes occur in salivary glands of larva which can indicate any type of
abnormality.
MORGAN
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Thomas hunt Morgan, an American geneticist and Nobel prize winner 1933 s considered as father
of experimental genetics for his work and discovery of linkage, crossing over, sex linkage, criss
cross inheritance, linkage maps, mutability of genes.
He is considered fly man of genetics.
He wrote the book the theory of gene.
He discovered the basis for variations due to sexual reproduction.
In 1910 he discovered linkage and differentiated between linked and unlinked gene.
Morgan and castle in 1911 proposed chromosome theory of linkage showing that genes are located
in chromosomes and show linear order.
The strength of linkage between genes increases with the decrease in distance between them.
He proposed chaisma type hypothesis showing that chaismata causes crossing over.
Morgan and Sturtevant (1911) found that frequency of crossing between two linked genes is
directly proportional to the distance between the two.
1% recombination is considered equal to 1 centi morgan (cM) or 1 map unit.
LINKAGE
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Linkage is the phenomenon of certain genes staying together during inheritance through
generations without any change or separation due to their being present on the same chromosome.
Linkage was first suggested by Sutton and boveri. (1902- 1903).also gave chromosomal theory of
inheritance.
In 1910 Morgan clearly proved and defined linkage on the basis of his breeding experiments in
fruitfly drosophila melanogaster. In 1911, Morgan proposed chromosome theory of linkage. It
states thatLinked genes occur on the same chromosome.
They lie in a linear sequence in the chromosome.
There is tendency to maintain the parental combinations of genes except for occasional cross
over's.
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Strength of the linkage between two genes is inversely proportional to the distance between the two
i.e. two linked genes show higher frequency of crossing over if the distance between them is higher
and lower frequency if the distance is small.
LINKED GENES
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These genes are placed very closely on the chromosome an do not show independent assortment
at the time of gamete formation.
They show dihybrid ratio of 3: 1.
In dihybrid cross, they show test cross ratio of 1:1.
UNLINKED GENES
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These genes are located distantly and undergo assortment (segregation).
Show dihybrid ratio of 9:3:3:1.
Show test cross ratio of 1:1:1:1.
LINKED GENES
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Are those genes which occur on the same chromosome while unlinked genes are the ones found on
different chromosomes.
Linked and unlinked genes can be easily known from breeding experiments.
TYPES OF LINKAGE
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Complete linkage.
Incomplete linkage.
COMPLETE LINKAGE
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Morgan 1919.
The genes located in the same chromosome do not separate and are inherited together over the
generations due to absence of crossing over.
Complete linkage allows the combination of parental traits to be inherited as such.
It is rare but has been reported in male drosophila and some other heterogametic individuals.
INCOMPLETE LINKAGE
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Genes present in the same chromosomes have a tendency to separate due to crossing over and
hence produce recombinant progeny besides the parental type.
The number of recombinant individuals is usually less than the number expected in independent
assortment. In independent assortment all the four types (two parental types and two recombinant
types) are each 25%.
In linkage each of two parental types is more than 25% while each of the recombinant types is less
than 25%.
KEY CONCEPTS
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A gene is a region of DNA within the chromosome.
Each gene has a specific location on the chromosome.
In humans, males have one X and one Y chromosome, and females have two X chromosomes.
A specific gene on the Y chromosome is required for human embryos to develop as males.
Unless they are located far from each other, genes on the same chromosome tend to be inherited
together, or linked. Genes on different chromosomes are not linked.
Homologous chromosomes that pair during meiosis can exchange genes in a process called
crossing-over.
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The genotypes of offspring can be different from that of either parent as a result of crossing-over,
the random distribution of maternal and paternal chromosomes into gametes, and fertilization.
Many inherited genetic disorders in humans are caused by mutations of single genes.
A far smaller number of human genetic disorders are caused by abnormalities in chromosome
number or structure.
THE ROLE OF CHROMOSOMES IN INHERITANCE
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Mendel did not know what the physical properties of his “particles” were when he proposed his
laws of inheritance.
August Weismann suggested that chromosomes (discovered in 1882) were the location of
hereditary material.
Genes are located on chromosomes
The idea that genes are located on chromosomes is known as the chromosome theory of
inheritance.
Chromosomes are composed of a single DNA molecule and many proteins.
The physical location of a gene on a chromosome is called a locus.
Chromosomes that pair during meiosis and contain the same gene loci and structure are called
homologous chromosomes
AUTOSOMES AND SEX CHROMOSOMES
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Chromosomes that determine gender are called sex chromosomes; all other chromosomes are
called autosomes.
Autosomes are homologous pairs.
Sex chromosomes can be homologous or non-homologous pairs
Sex determination in humans
Human females have two X chromosomes, and all their gametes contain one X chromosome.
Human males have one X and one Y chromosome; half their gametes contain an X chromosome,
and the other half contain a Y chromosome.
The chromosome carried by the sperm determines sex in humans.
LINKAGE AND CROSSING-OVER
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Exceptions to the law of independent assortment
Thomas Hunt Morgan discovered some genes that were inherited together in his research on fruit
flies
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Genes that are located on the same chromosome and do not assort independently are said to be
genetically linked
WHAT IS LINKAGE?
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Linkage is defined genetically: the failure of two genes to assort independently.
Linkage occurs when two genes are close to each other on the same chromosome.
However, two genes on the same chromosome are called syntenic
Linked genes are syntenic, but syntenic genes are not always linked. Genes far apart on the same
chromosome assort independently: they are not linked.
Linkage is based on the frequency of crossing over between the two genes. Crossing over occurs in
prophase of meiosis 1, where homologous chromosomes break at identical locations and rejoin
with each other.
LINKAGE AND CROSSING-OVER
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Crossing-over disrupts genetic linkage
If the linkage between two genes on a chromosome were complete, all offspring would be of a
parental type.
Morgan’s experiments showed that complete linkage was not occurring as a result of the presence
of nonparental genotypes
To explain the appearance of nonparental genotypes in linked genes, Morgan proposed that genes
are physically exchanged between homologous chromosomes during meiosis.
The exchange of genes between homologous chromosomes is called “crossing-over”
Genes that are far from each other on a chromosome are more likely to be separated by crossingover than are genes that are close to each other.
Genes that are very distant from one another on a chromosome will assort independently.
CROSSING OVER
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Chromosomal crossover (or crossing over) is an exchange of genetic material between homologous
chromosomes. It is one of the final phases of genetic recombination, which occurs during prophase
I of meiosis (pachytene) in a process called synapsis.
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HISTORY
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Crossing over was described, in theory, by Thomas Hunt Morgan. He relied on the discovery of the
Belgian Professor Frans Alfons Janssens of the University of Leuven who described the
phenomenon in 1909 and had called it 'chiasmatypie'.
The term chiasma is linked if not identical to chromosomal crossover. Morgan immediately saw the
great importance of Janssens' cytological interpretation of chiasmata to the experimental results of
his research on the heredity of Drosophila.
Janssens 1909 was the first person to discover chaisma formation and related process of crossing
over.
Morgan 1910 found phenomena of linkage and recombination.
The recombination or new recombination of genes is possible only due to exchange of genetic
material between homologous chromosomes. Linkage is incomplete in such cases.
PROCESS
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In a non-dividing cell, chromosomes are not visible by light microscopy, because chromatin
spreads throughout the nucleus.
During the metaphase of cell division, the chromatin condenses and becomes visible as
chromosomes.
At this time, each chromosome has been duplicated. A chromosome becomes two sister
chromatids attached at the centromere.
Chromosomes condense, and the spindle apparatus begins to form. However, after the
chromosomes condense, the homologous chromosomes (each one itself a pair of sister chromatids)
pair up with their homologues
The resulting pair is called a tetrad, due to the fact that it has a total of four sister chromatids (two
pairs of two)
The chromosomes in a tetrad are bound side by side along their length by a protein apparatus
called the synaptonemal complex. The chromosomes are perfectly lined up with each other, such
that analogous genes are side by side.
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DYAD AND TETRAD
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The familiar pattern of a two-chromatid chromosome (seen during mitosis) is called a dyad. During
mitosis dyads line up at the metaphase plate and the division of the dyad (chromosome) during
anaphase "creates" the new chromosomes.
When the two homologous pairs are aligned (side by side) we call the pair a tetrad. Therefore, a
tetrad is composed of two chromosomes - one maternal (M) and one paternal (P). A tetrad will have
two centromeres and four chromatids (because it is made from two chromosomes).
A dyad was a single (X-shaped) chromosome so a tetrad is composed of two dyads.
MECHANISM OF CROSSING OVER
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The process of crossing over occurs by a mechanism called breakage and reunion theory of
Darlington.
It comprises four steps: synapsis, tetrad formation, exchange of chromatids and disjunction.
SYNAPSIS
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The homologous chromosomes come to lie close together in pairs in the zygotene substage of the
prophase of meiosis -1.
Their pairing is called synapsis.
The paired homologus chromosomes are called bivalents.
Their chromatids are not visible at this stage.
TETRAD FORMATION
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The chromatids of each synapsed chromosome slightly separate and become visible in the
pachytene substage of the prophase of meiosis.
A group of four homologous chromosomes chromatids is called is called a tetrad.
Crossing over occurs in the four stranded or tetrad stage.
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CROSSING OVER
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Crossing over occurs in the pachytene substage.
The adjacent non sister chromatids break at homologous sites, mutually exchange small
corresponding segments and rejoin.
The process leads to physical exchange of segments and is so exact that neither chromatid gains or
looses any genes.
It occurs with the help of enzymes endonuclease, exonuclease, recombinase.
DISJUNCTION
x After the completion of crossing over, the synaptic forces end and the homologous chromosomes
move apart.
x The sites where crossing over occurs are called chaismata.
x Therefore the above explained mode of crossing over is called chaisma-type hypothesis.
SEX DETERMINATION
x In diploid organisms with separate sexes , a specific pair of chromosomes determine the sex of the
individual.
x They are called sex chromosomes or allosomes or heterosomes.
x All other chromosomes are termed autosomal chromosomes.
DISSIMILAR SEX CHROMOSOMES
x Dissimilar sex chromosomes exhibit four conditions in animals
x xx-xy- in mammals including man, and most insects , including fruitfly ,one sex chromosome is
smaller than the other in males.
a. The larger one is known as x chromosome and the smaller one is known as y chromosome.
b. The similar and dissimilar sex chromosomes of females and males are described as
homomorphic and heteromorphic respectively.
X AND Y CHROMOSOMES
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The sex chromosomes (X and Y) have 2 regions each: homologous and non homologous or
differential.
The differential regions carry completely sex linked genes as they do not undergo crossing over.
The homologous regions carry incompletely sex linked genes because they undergo crossing over.
The genes present in the differential regions of the y chromosome are called holandric genes.
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PORTIONS OF CHROMOSOMES
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Homologous portion is that in which there are genes having alleles in both Y and X sex
chromosomes.
The homologous portions are situated more in the central part of the sex chromosomes, near the
centromere.
Heterologous portion is that whose genes do not have correspondent alleles in the other sex
chromosome. These genes are located more in the peripheral regions of the arms of the Y and X
chromosomes.
XX-XO
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In certain insects, such as cockroach and some roundworms , the y chromosomes is missing so
that the male has only one sex chromosome.
The condition in the male is XO (O means absence of one sex chromosome) and in the female it is
XX.
ZW-ZZ
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In many vertebrates (fishes, reptiles, birds) and insects butterflies, moths. The female has
heteromorphic sex chromosomes and the male has homomorphic sex chromosomes.
These conditions are briefly indicated as ZW and ZZ respectively.
These letters are used to avoid confusion with XX- XY condition.
ZW- ZZ TYPE
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The male has two homomorphic sex chromosomes (zz) and is homogametic, and the female has two
heteromorphic sex chromosomes(zw) and is heterogametic.
Thus there are two types of eggs: each with Z and with W, and only one type of sperms i.e. each
with Z.
Fertilization of an egg with Z chromosome by a sperm with Z chromosome gives a zygote with ZZ
chromosomes. This zygote develops into a male.
Fertilization of an egg with W chromosome by a sperm with Z chromosome yields a zygote with ZW
chromosomes. This zygote produces a female.
The offspring have a sex ratio of 1:1.
ZO-ZZ
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In some butterflies and moths the W chromosomes is lacking, so that the condition in the female is
ZO and that in the male is ZZ
The female is heterogametic and produces two types of eggs: half with Z and half without Z
chromosome.
Male has homomorphic sex chromosomes and is homogametic.
It forms only one kind of sperms, each with Z chromosome. On fertilization by a sperm with Z
chromosome, the Z containing egg gives rise to a male offspring (zz) and the Z lacking egg produces
a female offspring (zo).
MECHANISM OF SEX DETERMINATION
x The mechanism that fixes the sex of an individual as it begins life Is called sex determination.
1. genetic or chromosomal.
2. environmental or nongenetic.
3. nonchromosomal.
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CHROMOSOMAL SEX DETERMINATION
x XX-XY-though dissimilar the x and y chromosomes synapse in meiosis ,segregate and pass into
different gametes.
x The eggs are all alike each with an x chromosome.
x The female is homogametic.
x The male is heterogametic as it produces two types of sperms. This condition is called male
digamety. The Y containing sperms and the X containing sperms are respectively called
androsperms and gynosperms.
x The two are produced in equal proportion.
INFLUENCE OF SEX CHROMOSOMES
x Early embryo develops rudimentary undifferentiated gonads and reproductive tracts. The gonad
rudiments can give rise to testes or ovaries. The sex determining gene of the Y chromosome makes
the embryo develop into a male. This gene codes for a gene regulatory protein called testis
determining factor (TDF), which makes the gonads differentiate into testes.
x The testes then produce the hormone testosterone which induces the development of male
reproductive tract.
x The testes also produce an inhibitory substance that causes regression of the female tract.
x If testes do not differentiate till the sixth week of embryonic life, the gonads differentiate into
ovaries.
x Thus female is a default sex.
SRY (SEX-DETERMINING REGION Y) GENE
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Since its discovery, the importance of the SRY gene in sex determination has been extensively
documented:
Humans with one Y chromosome and multiple X chromosomes (XXY, XXXY etc.) are usually males.
Individuals with a male phenotype and an XX (female) karyotype—XX male syndrome—have been
observed; these males have the SRY gene in one or both X chromosomes, moved there by
chromosomal translocation. (However, these males are infertile.)
Similarly, there are females with an XXY or XY karyotype. These females have no SRY gene in their
Y chromosome, or the SRY gene exists but is defective (mutated).
PLANTS
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The flowering plants are mostly bisexual and lack sex chromosomes.
The unisexual flowering plants tend to have ZZ- ZO type of sex chromosomal mechanism for sex
determination.
The female plants are ZZ and male plants ZO.
1. SEX DETERMINATION IN DROSOPHILA
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Calvin bridges demonstrated that in drosophila sex determining factor is the ratio of number of X
chromosomes to the set of autosomes.
1. Individuals having X/A ratio of 1.0 are females and fertile.
Eg. 2A+ XX or 3A + XXX.
2. If X/A ratio exceeds 1.0 meta females are produced which are weak and infertile. (2A + XXX).
3. X/A ratio of 0.5 is necessary for male sex differentiation (2A + XY).
4. When X/A decreases below 0.5 (3A +XY ; X/A ratio = 0.33) infertile metamales are produced.
These findings suggested that in drosophila male development factors are localized on autosomes.
The X chromosomes perhaps carry some female determining genes.
This mode of sex differentiation is known as genic balance theory of sex determination.
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2. TIME OF SEX DETERMINATION
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In the animals with two types of sperms , the sex of the offspring is determined at the time of
fertilization by a chance event.
It is determined by the kind of sperm that fuses with the egg.
HAPLOID DIPLOID MECHANISM OF SEX DETERMINATION
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Hymenopterous insects, such as bees, wasps saw flies and ants: an unfertilized egg develops into a
male and a fertilized egg develops into a female.
Female is diploid (2n), and the male is haploid (n).
Eggs are formed by meiosis and sperms by mitosis.
Fertilization restores the diploid number of chromosomes in the zygote which gives rise to the
female.
If the egg is not fertilized, it will develop but into a male.
Thus sex is determined by the number of chromosomes.
HONEY BEE
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Male honey bee has 16 chromosomes, undergoes mitosis and forms sperms.
Female honey bee has 32 chromosomes has meiosis and makes eggs with 16 chromosomes each.
If the sperm and egg undergo fusion i.e. fertilization they form female honey bee with 32
chromosome.
If the egg with 16 chromosomes remains as such. It undergoes parthenogenesis and becomes male
honey bee.
In honey bee, the quality of food determines whether a diploid larva becomes a fertile queen or a
sterile worker female.
A larva fed on royal jelly, grows into queen.
Thus the environment determines fertility or sterility of the bee but it does not alter the genetically
determined sex.
Queen is the ultimate controller????????
ROYAL JELLY
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TYPES OF BEES
ENVIRONMENT
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Chemical plays a unique role in the worm's sexual differentiation.
Free-swimming Bonellia larvae are initially sexually undifferentiated.
Larvae which land on unoccupied sea-floor mature, over the period of years, into adult females.
Most larvae, however, come in contact with the bonellin in the skin of an adult female and are
masculinised by this exposure.
The chemical causes these larvae to develop into the tiny males, which cling to the female's body or
are sucked inside it by the feeding tube, to spend the remainder of their lives inside her genital sac,
producing sperm to fertilize her eggs, reliant on her for all other needs.
The sex of a Green Spoonworm is thus determined by external, environmental factors (the presence
or absence of bonellin), not by internal, genetic factors (chromosomes), as is the case with most
other sexually-differentiated organisms.
This environmental sex determination helps Green Spoonworm populations respond to the
availability of burrows.
In American alligator, snapping turtle and other reptiles sex is determined by environmental
temperature.
The temperature at which alligator eggs develop determines their sex.
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Those eggs which are hatched in temperatures ranging from 32 to 34 °C become males, while those
in temperatures from 23 to 30 °C become female.
Intermediate temperature ranges have proven to yield a mix of both male and females.
The female will remain near the nest throughout the 65-day incubation period, protecting the nest
from intruders. When the young begin to hatch they emit a high-pitched croaking noise, and the
mother quickly digs them out.
In turtles, a temperature below 28 °C produces more males, above 33 °C produces more females,
and between 28 degree and 33°C produces male and female in equal proportion.
MUTATION
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The term mutation was introduced by Hugo de Vries in 1901.
Mutation is defined as a rare, random, discontinuous, inheritable variation in the amount or the
structure of the genetic material of a cell or organism.
Mutation becomes part of the genotype of the cell. Mutations are caused by radiation, viruses,
transposons and mutagenic chemicals, as well as errors that occur during meiosis or DNA replication
TYPES OF MUTATIONS
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Mutations taking place in the germ cells are called germinal mutations.
These are inherited by the next generation.
Mutations appearing in the somatic cells are termed somatic mutations.
They are inherited only by the daughter cells formed by mitosis from the mutant cells.
TWO MAIN TYPES
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Chromosomal mutations
Gene mutations
The term mutation is usually restricted to alterations in the genes; the alterations in chromosomes
are also inheritable variations and may be included In the term mutations.
Chromosomal alterations are called chromosomal aberrations
CHROMOSOMAL MUTATIONS OR ABBERATIONS
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These mutations affect large portions of the chromosome and are observable under a microscope.
They involve morphological modifications in chromosomes.
They are of two types1. Intra-chromosomal modifications.
2. Inter-chromosomal modifications.
INTRA-CHROMOSOMAL MODIFICATIONS
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These changes affect a single chromosome. They occur in two ways: deletion (deficiency) and inversion.
In both cases, the process involves breakage and reunion of segments of chromosomes.
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DELETION (DEFECIENCY)
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x A segment of a chromosome lacking a centromere separates and is lost
x Deletion may be terminal or intercalaray (interstitial).
x The affected chromosome loses certain genes, and becomes shorter than normal.
x Deletion is noticeable at the time of synapsis of the homologous chromosomes.
A segment of a longer chromosome remains unpaired in terminal deletion and forms a loop in
intercalary deletion.
DELETION INTRA CHROMOSOMAL
EXAMPLE
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Cri du chat syndrome results by deletion of half of short arm of chromosome 5 in humans
Cry of an infant having this disorder resembles the sound of a cat in pain; caused by abnormal
larynx development
INVERSION
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A segment of a chromosome separates and rejoins it in an inverted position
Inversion may also be terminal or intercalary.
Intercalary inversion may be pericentric if the inverted segment includes the centromere in it, or
paracentric if the inverted segment is on one side of the centromere
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INVERSION INTRA CHROMOSOMAL
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Deletions and inversions produce new locations for genes and may change phenotypic expression
They may make affected chromosomes unable to pair in meiosis and sterility
INTER-CHROMOSOMAL MODIFICATIONS
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These changes affect two chromosomes simulatneously.
They also occur in two waysTranslocation
Duplication
TRANSLOCATION
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A segment of a chromosome breaks off and joins a non homologous chromosome
Both the affected chromosome gets modified. The donor suffers deletion and becomes shorter than
normal
The recipient has an extra set of genes and becomes longer than normal
This is simple or interstitial translocation
RECIPROCAL TRANSLOCATION
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Two non homologous chromosomes mutually exchange segments
The affected chromosomes retain their original size but have rearranged gene blocks
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E.g.- Chronic Myeloid Leukemia (CML)- is caused by chromosomes 22 of malignant cells shortened
through translocation of a segment of its long arm
DUPLICATION
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It’s the occurrence of a chromosome segment more than once in a chromosome.
Duplication may be in tandem sequence or reverse.
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EFFECT OF MORPHOLOGICAL CHROMOSOMAL ABERRATIONS
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The chromosomal mutations result in changes in the number of genes or in new positions on the
genes in the chromosomes.
This alters the base sequence of the DNA and hence genetic message sent to the mRNA.
These changes if not lethal, may cause profound changes in the phenotype.
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Inversion and translocation interfere in synapsis and reduce chances of crossing over.
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CHROMOSOME NUMBERS
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Any heritable change in the make up of an organism comes under the word mutation.
The term also refers to the process by which such a change takes place
Such changes under mutation include changes in chromosome either in numbers or in structure
(chromosomal mutation).
PLOIDY
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Ploidy is the number of sets of chromosomes in a biological cell.
Human sex cells (sperm and egg) have one complete set of chromosomes from the male or female
parent.
Sex cells, also called gametes, combine to produce somatic cells.
Somatic cells therefore have twice as many chromosomes.
The haploid number (n) is the number of chromosomes in a gamete. A somatic cell has twice that
many chromosomes (2n).
HETEROPLOIDY: VARIATION IN CHROMOSOME NUMBER
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Changes in chromosome number can occur by the addition of all or part of a chromosome
(aneuploidy)
The loss of an entire set of chromosomes (monoploidy)
or the gain of one or more complete sets of chromosomes (euploidy).
Each of these conditions is a variation on the normal diploid number of chromosomes and each of
these can have drastic effects on phenotypic expression
HETEROPLOIDY
Aneuploidy - the abnormal condition were one or more chromosomes of a normal set of
chromosomes are missing or present in more than their usual number of copies
x Monoploidy - the loss of an entire set of chromosomes
x Euploidy - an entire set of chromosomes is duplicated once or several times
x Euploidy refers to cells with an exact multiple of the haploid number, N:
x Diploid=twice the haploid number, 2N.
x Triploid is 3x the haploid number, 3N.
x Tetraploid=4x the haploid number, 4N, etc.
x Aneuploidy are cells with an uneven number of chromosomes.
x So, aneuploidy like trisomy 21 would be the same as diploid + 1 chromosome.
Turner syndrome in humans is a case of aneuploidy because it's diploid - 1 or 45 chromosomes...
EU=true, AN=not
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ANEUPLOIDY
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A normal chromosomal number is euploid
Cells with extra or missing chromosomes are aneuploid
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Most autosomal aneuploids are spontaneously aborted
Those that are born are more likely to have an extra chromosome (trisomy) rather than a missing
one (monosomy)
CONDITIONS OF ANEUPLOIDY
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Nullisomy: loss of both pairs of homologous chromosomes; individuals are called nullisomics and
their chromosomal composition is 2N-2.
Humans with this condition will not survive.
Monosomy: the loss of a single chromosome; individuals are called monosomics and their
chromosomal composition is 2N-1
Trisomy: the gain of an extra copy of a chromosome; individuals are called trisomics and their
chromosomal composition is 2N+1
Tetrasomic: gain of an extra pair of homologous chromosomes; individuals are called tetrasomics
and their chromosomal composition is 2N+2
In addition to these conditions, more than one pair of homologous chromosomes may be involved.
For example, a double monosomic is missing one chromosome from each of two pair of homologous
chromosome (designated 2N-1-1), and a double tetrasomic contains an extra pair of two pairs of
homologous chromosomes (2N+2+2).
NONDISJUNCTION- CAUSE OF ANEUPLOIDY
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The failure of chromosomes to separate normally during meiosis
Produces gamete with an extra chromosome and another with one missing chromosome
Nondisjunction during Meiosis I results in copies of both homologs in one gamete
Nondisjunction during Meiosis II results in both sister chromatids in one gamete
Chromosomal Basis of Inheritance
42
Chromosomal Basis of Inheritance
43
Chromosomal Basis of Inheritance
44
MONOSOMY
x
x
Monosomy is a form of aneuploidy with the presence of only one chromosome (instead of the
typical two in humans) from a pair.
Partial monosomy occurs when only a portion of the chromosome has one copy, while the rest has
two copies.
HUMAN CONDITIONS DUE TO MONOSOMY
x
x
x
Turner syndrome - Women with Turner syndrome typically have one X chromosome instead of the
usual two sex chromosomes. Turner syndrome is the only full monosomy that is seen in humans—
all other cases of full monosomy are lethal and the individual will not survive development.
Cri du chat syndrome -- (French for "cry of the cat" after the distinctive noise by affected persons'
malformed larynx) a partial monosomy caused by a deletion of the end of the short p (from the
word petit, French for small) arm of chromosome 5.
1p36 Deletion Syndrome - a partial monosomy caused by a deletion at the end of the short p arm
of chromosome 1.
MONOSOMY
x
x
x
x
x
x
x
Monosomy refers to lack of one chromosome of the normal complement.
Monosomy of the sex chromosomes (45,X) causes Turner syndrome
Turner's Syndrome: 2n-1.
45,X.
1/2000 females.
99% of affected fetuses die in utero
Features include short stature, webbing at back of neck, incomplete sexual development (infertile),
impaired hearing
TURNERS
TRISOMIES
x
x
Most autosomal aneuploids cease developing as embryos or fetuses
Most frequently seen trisomies in newborns are those of chromosomes 21, 18, and 13
- Carry fewer genes than other autosomes
Chromosomal Basis of Inheritance
45
TRISOMY 21
x
x
x
Down syndrome
Most common trisomy among newborns
Distinctive facial and physical problems
Varying degrees of developmental disabilities
Individuals more likely to develop leukemia
Link with one form of Alzheimer disease
TRISOMY 13
x
x
Patau syndrome
Very rare and generally do not survive 6 months
Serious mental and physical disabilities
A distinctive feature: Eye fusion
Chromosomal Basis of Inheritance
46
TRISOMY 18
x
x
Edwards syndrome
Most due to non-disjunction in meiosis II in oocyte and do not survive
Serious mental and physical disabilities
A distinctive feature: Oddly-clenched fists
KLINEFELTER SYNDROME
x
x
x
Called the XXY syndrome
1 in 500 male births
Phenotypes include:
- Incomplete sexual development
- Rudimentary testes and prostate
- Long limbs, large hands and feet
- Some breast tissue development
Chromosomal Basis of Inheritance
47
x
Most common cause of male infertility
KLINEFELTERS SYNDROME
x
x
x
Klinefelter syndrome, or XXY syndrome is a condition in which human males have an extra X
chromosome.
While females have an XX chromosomal makeup, and males an XY, affected individuals have at
least two X chromosomes and at least one Y chromosome.
Because of the extra chromosome, individuals with the condition are usually referred to as "XXY
Males", or "47, XXY Males"
TRIPLE X SYNDROME
x
Triple X syndrome is a form of chromosomal variation characterized by the presence of an extra X
chromosome in each cell of a human female. Triple-X is a chromosomal condition which occurs
only in females. A female affected by triple-X syndrome has an XX pair of chromosomes, as well as
an additional chromosome, resulting in the formation of XXX
AMNIOCENTESIS
Detects about 1,000 of the more than 5,000 known chromosomal and biochemical problems
Ultrasound is used to follow needle’s movement
CHORIONIC VILLI SAMPLING
Performed during 10-12th week of pregnancy
Provides earlier results than amniocentesis
However, it does not detect metabolic problems
- And has greater risk of spontaneous abortion
Chromosomal Basis of Inheritance
48
TETRASOMY
x
x
A tetrasomy is a form of aneuploidy with the presence of four copies, instead of the normal two, of
a particular chromosome.
Cat eye syndrome where tetrasomy of chromosome 22 is present.
SEX CHROMOSOMAL TETRASOMIES
x
x
48, XXXX syndrome
48, XXYY syndrome
MONOPLOIDY
x
x
x
x
x
An individual that contains one half the normal number of chromosomes is a monoploid and
exhibits monoploidy.
Monoploids are very rare in nature because recessive lethal mutations become unmasked, and
thus they die before they are detected
These alleles normally are not a problem in diploids because their effects are masked by dominant
alleles in the genome
Some species such as bees, ants and male bees are normally monoploid because they develop from
unfertilized eggs
Consequently, these individuals will be sterile.
Chromosomal Basis of Inheritance
..49..
EXERCISE – 1
1.
In Down’s syndrome (Mongolism), the number of
chromosomes in the somatic cells is
(a)
45
(b)
47
(c)
23 – 1
(d)
21 – 1
11.
Puffs in the salivary gland chromosomes represent the
site of
(a)
Protein synthesis (b)
RNA synthesis
(c)
DNA synthesis
(d)
Duplication of DNA
2.
Hopoploidy resulting from the loss of a pair of
chromosomes is called
(a)
Trisomy
(b)
Tetrasomy
(c)
Nullisomy
(d)
Monosomy
12.
3.
A diploid plant for which monosomics have been isolated
(a)
Potato
(b)
Wheat
(c)
Tomato
(d)
Rice
What are the chromosomes with more than two chromatids
called ?
(a)
Lampbrush chromosomes
(b)
Polytene chromosomes
(c)
Aerocentric chromosomes
(d)
Allosomes
13.
Which of the following is not a polytene chromosome ?
(a)
Salivary gland chromosome of fruit fly
(b)
Salivary gland chromosome of Cheironomous
(c)
Lampbrush chromosome of an amphibian oocyte
(d)
All of above
4.
A chromosome with terminal centromere is known as
(a)
Acrocentric
(b)
Telocentric
(c)
Metacentric
(d)
Polycentric
5.
The chromosome with median centromere and almost
equal arms is called
(a)
Acrocentric
(b)
Metacentric
(c)
Submetacentric
(d)
Telocentric
14.
The part of the chromosome which governs its movement
is
(a)
Spindle fibres
(b)
Nucleosome
(c)
H1 protein
(d)
Kinetochore
6.
Which shapes are assumed by acrocentric and telocentric
chromosomes respectively at the anaphase of meiosis
(a)
I & V shape
(b)
V & J shape
(c)
J & I shape
(d)
I & J shape
15.
Lamp brush chromosomes occur through
(a)
Diplotene of meiosis
(b)
Prophase of meiosis
(c)
Interphase
(d)
Metaphase of meiosis
7.
The terminal end of a chromosome is called
(a)
Satellite
(b)
Metamere
(c)
Telomere
(d)
Centromere
16.
Which of the following is a correct match ?
(a)
Down syndrome
21st chromosome
(b)
Sickel cell anaemia X-chromosome
(c)
Haemophilia
Y-chromosome
(d)
Parkinson disease X & Y chromosome
17.
Trisomy of 21st chromosome is called
(a)
Edward’s syndrome
(b)
Down’s syndrome
(c)
Patau’s syndrome
(d)
Cri-du-chat syndrome
18.
Down’s syndrome is characterised by
(a)
21 trisomy
(b)
Two X and one Y chromosome
(c)
19 trisomy
(d)
Only one X chromosome
8.
Which prevent the union of chromosomes
(a)
Centromeres
(b)
Kinetochores
(c)
Telomeres
(d)
Nucleolar organisers
9.
What are the filaments constituting a chromosome called ?
(a)
Chromomere
(b)
Chromatin
(c)
Centromere
(d)
Chromonema
10.
The chromosomes which remain condensed during
interphase are known as
(a)
Polytene chromosomes
(b)
Heterochromosomes
(c)
Euchromosomes
(d)
Megachromosomes
Chromosomal Basis of Inheritance
..50..
19.
The sub-units of chromatids are
(a)
Chromonemata
(b)
Chromosomes
(c)
Telomeres
(d)
Secondary constrictions
20.
One function of the telomeres in a chromosome is to
(a)
‘Seal’ the ends of the chromosomes
(b)
Start RNA synthesis
(c)
Identify the correct member of the homologous
pair of chromosomes
(d)
Help two chromatids to move towards poles
21.
The polytene chromosomes were discovered for the first
time in
(a)
Drosophila
(b)
Musca domestica
(c)
Chironomus
(d)
Musca nebulo
27.
Given below is a pedigree chart shwoing the inheritance of a
certain sex-linked trait in humans
The trait traced in the above pedigree chart is
(a)
dominant X-linked (b)
recessive X-linked
(c)
dominant Y-linked (d)
recessive Y-linked
22.
Mutual exchange of chomosome segments between
nonhomologous chromosomes is called
(a)
Deletion
(b)
Duplication
(c)
Inversion
(d)
Translocation
28.
In a mutational event, when adenine is replaced by
guanine, it is the case of
(a)
frameshift mutation (b)
transcription
(c)
transition
(d)
transversion
23.
Datura is a classical example of
(a)
Trisomy
(b)
(c)
Monosomy
(d)
29.
The “cri-du-chat” syndrome is caused by change in
chromosome structure involving
(a)
deletion
(b)
duplication
(c)
inversion
(d)
translocation
30.
Given below is a pedigree chart of a family with five
children. It shows the inheritance of attached earlobes as
opposed to the free ones. The squares represent the male
invididuals and circles the female individuals. Which one
of the following conclusions drawn is correct ?
24.
Triploidy
Monoploidy
A polyploid with genomes derived from same original
species is
(a)
Amphidiploid
(b)
Allopolyploid
(c)
Autopolyploid
(d)
Autoallopolyploid
25.
Genes located on Y-chromosomes are called
(a)
holandric genes
(b)
autosomal genes
(c)
sex-linked genes
(d)
mutant genes
26.
Barr body represents
(a)
heterochromatin in male and female cells
(b)
all heterochromatin in female cells
(c)
Y-chromosomes in somatic cells
(d)
one of the two X-chromosomes in somatic cells of
female
(a)
(b)
(c)
(d)
Chromosomal Basis of Inheritance
The parents are homozygous recessive
The trait is Y-linked
The parents are homozygous dominant
The parents are heterozygous
..51..
31.
Given below is a representation of a kind of chromosomal
mutation. What is the kind of mutation represented ?
(a)
(b)
(c)
(d)
32.
33.
Deletion
Duplication
Inversion
Reciprocal translocation
Sickle cell anaemia has not been eliminated from the
African population because
(a)
it is controlled by recessive genes
(b)
it is not a foetal disease
(c)
it provides immunity against malaria
(d)
it is controlled by dominant genes
If a colourblind woman marries a normal visioned man,
their sons will be
(a)
all normal visioned
(b)
one-half colourblind and one-half normal
(c)
three-fourths colourblind and none-fourth normal
(d)
all colourblind
34.
The term ‘crossing over’ was introduced by
(a)
Morgan and Cattell (b)
Sutton and Boveri
(c)
Bateson and Punnett (d)
Beadle and Tatum
35.
In human, sex linked trait is
(a)
Down’s syndrome (b)
(c)
Curly hair
(d)
Colour blindness
Sickle cell anaemia
36.
Sex determination by environmental factors is commony
observed in
(a)
Drosophila melanogaster
(b)
Mirabilis jalapa
(c)
Melandrium album
(d)
Bonellia viridis
37.
The absence of Y chromosome in human beings causes
(a)
Turner’s syndrome
(b)
Klinefelter’s syndrome
(c)
Down’s syndrome
(d)
No abnormality
38.
A man with certain disease marries a normal woman. They
have eight children (3 daughters and 5 sons). Al the
daughters suffer from their father’s disease but none of
the sons are affected. Which of the following mode of
inheritance do you suggest for this disease ?
(a)
Sex limited recessive (b)
Autosomal dominant
(c)
Sex linked dominant (d)
Sex linked recessive
39.
The first attempt to show linkage in plants was done in
(a)
Zen mays
(b)
Oenothera lamarckiana
(c)
Pisum sativum
(d)
Lathyrus odoratus
40.
Different mutation referrable to the same locus of a
chromosome give rise to
(a)
Multiple alleles
(b)
Pseudoalleles
(c)
Polygenes
(d)
Oncogenes
41.
Which of the following is sex linked ineritance ?
(a)
Thalassemia
(b)
Phenylketonuria
(c)
Goitre
(d)
Haemophilia
42.
Hypertrichosis or hairy ears is sex linked character
associated with the
(a)
Y chromosome
(b)
X chromosome
(c)
XX chromosome
(d)
XY chromosome
43.
People who carry an allele for normal haemoglobin and an
allele for sickle cell are resistant to malaria they are example
of
(a)
diploidy
(b)
outbreeding
(c)
heterozygotic advantage
(d)
recessive superiority
44.
Criss cross inheritance means
(a) X chromosome from female will pass to female of
next-generation
(b) X chromosome from a male will pass to a female of
next generation
(c) X chromosome from male will pass to a male of next
generation
(d) none of the above
Chromosomal Basis of Inheritance
..52..
45.
In rabbit, two recessive genes produce a solid body colour
and long hair in contrast to dominant spotted body colour
and short hair. The result of a cross between heterozygous
spotted short haired rabbit to salid long haired rabbit gives
spotted, short haireed 48, spotted long haired 5, solid short
haired 7, solid long haired 40, total 100. In terms of cross
over units, how far are there two genes on the
chromosome?
(a)
40 map units
(b)
7 units
(c)
12 map units
(d)
48 map units
46.
Drosophila is used in genetic studies because
(a)
its chromosome complement is simple
(b)
a single mating produces over 100 off springs
(c)
life cycle time in small (10-20 days)
(d)
all of these
47.
Baldness is more common in men than in women. It could be
explained on the basis that
(a) it is due to male hromone and genes are not
involved
(b) baldness genes are located on Y chromosome
(c) genes of baldness are autosomal but influenced by
androgens
(d) genes of baldness are located on X chromosome
only
48.
One way of determine sex linked inheritance is
(a)
son resembles mother and daughter resembles
father
(b)
both son and daughter resemble mother
(c)
both son and daughter resemble father
(d)
son resembles father and daughter resembles
mother
49.
A normal woman whose father was albino marries a man
who is albino. What proportion of normal and albino can
be expected among their offspring ?
(a)
All normal
(b)
All albino
(c)
1 normal : 1 albino (d)
2 normal : 1 albino
50.
Child has blood groups ‘O’ and his father is ‘B’ type. Then
genotype of the father should be
0 0
B 0
(a)
II
(b)
I I
A B
B B
(c)
I I
(d)
I I
Chromosomal Basis of Inheritance
51.
Usually the recessive character is expressed only when
present in a double recessive condition. However single
recessive gene can express itself in human beings when
the gene is present on
(a)
the X chromosome of the female
(b)
the X chromosome of the male
(c)
any autosome
(d)
either an autosome or X chromosome
52.
A man with type A blood married a woman who has type
AB blood. we don’t known weather the man is
homozygous or heterzygous for the I allele. Which one of
the following types in the progeny of this couple would
indicate that the man is heterozygous ?
(a)
O
(b)
A
(c)
B
(d)
AB
53.
A colourblind girl is rare and can be born when
(a)
her father is colourblind and mother has normal
vision but her mother’s father (maternal grandfather)
is colourblind
(b)
her mother is colourblind, even if father has normal
vision
(c)
even when both her parents have normal vision
provided the grand parents were colourblind
(d)
her mother is colourblind and her father had normal
vision but her paternal grandfather (father’s father
was colourblind).
54.
The example of sex linked trait in human being is
(a)
curly hair
(b)
colour blindness
(c)
sickle cell anaemia
(d)
all of these
55.
The gene for haemophilia is located on chromosome of humans.
It is normally impossible for a
(a)
haemophilic father to pass the gene to his son
(b)
carrier mother to pass the gene to her son
(c)
haemophilic father to pass the gene to his daughter
(d)
carrier mother to pass the gene to her daughter
56.
One child is haemophilic (sex linked trait) while its fratermal
twins brother is normal. Which one of the following
information is most appropriate ?
(a)
Haemophilic child is male
(b)
The other child is female and the father is
haemophilic
(c)
Child is monozygotic twin
(d)
The mother must have been heterozygous
..53..
57.
58.
59.
60.
The gene for brown eyes is dominant over the gene for
blue eyes. Following are given some statements
(i)
For the child to be blue-eyed, at least one parent
should be blue-eyed.
(ii)
Both parents with blue eyes will have a blue-eyed
child
(iii) Blue-eyed person will always show both the alleles
different from brown-eyed person
(iv) Identical twins of blue-eyed parents will both have
either blue or brown eyes
(v)
For the child to be brown-eye one parent should
be brown
The correct statement are
(a)
(i), (iv) and (v)
(b)
(ii), (v)
(c)
(ii), (iii)
(d)
(i), (iii)
Haemophilia is a X-linked disease. A haemophilic male
marries a normal woman, whose father is also a
haemophilic. It is expected that
(a)
half their children will be haemophilics
(b)
all the children will be haemophilics
(c)
one fourth of the children will be haemophilics
(d)
one of the children will be haemophilics
Carrier of a genetic disorder
(a)
are indicated by solid symbols on a family pedigree
(b)
are involved in consanguineous mating
(c)
will produce children with the disease
(d)
are heterozygotes for the gene that can cause the
disorder
Sickle cell anaemia
(i)
follows the same inheritance pattern as that of
haemophilia
(ii)
follows the same inheritance pattern as that of
albinism
(iii)
is a sex linked trait
(iv)
is due to a single recessive point mutation leading
to the substitution of a single amino acid in the E charn of haemoglobin of the sentences written
above
(a)
all are correct
(b)
all but (i) are correct
(c)
all but (iii) are correct (d)
(ii) and (iv) are correct
61.
Match the columns :
Column I
Down’s syndrome
Cri-du-chat
syndrome
Klinefelter’s
syndrome
Turner’s
syndrome
(a)
(b)
(c)
(d)
(a)
(c)
a-s, b-q, c-p, d-r
a-s, b-q, c-q, d-r
p.
q.
r.
s.
t.
Column II
An additional sex
Loss of a part of
chromosome 5
Absence of sex
Presence of an extra
chromosome
Presence of two extra
chromosome
(b)
(d)
a-t, b-s, c-p, d-q
a-s, b-q, c-r, d-p
62.
Cat cry syndrome is due to
nd
(a)
monosomy of 22 chromosome
th
(b)
deletion in an arm of the 5 chromosome
th
(c)
trisomy of 13 chromosome
(d)
monosomy of the sex chromosome
63.
Tay Sach’s disease is
(a)
Recessive and autosomal
(b)
Dominant and autosomal
(c)
Sex-linked
(d)
No a hereditary disease
64.
Number of barr bodies present in Turner’s syndrome is
(a)
0
(b)
1
(c)
2
(d)
2 or 3
65.
Sickle cell anaemia is
(a)
autosomal hereditary disease
(b)
allosomic hereditary disease
(c)
epistatic effect
(d)
nutritional disorder
66.
Christmas disease is another name of
(a)
Sleeping sickness (b)
Down’s syndrome
(c)
Haemophilia B
(d)
Hepatitis
67.
Cri-du-chat is due to chromosomal aberration called
(a)
Duplication
(b)
Inversion
(c)
Deletion
(d)
Translocation
68.
Which of the following chromosomal alterations does not
alter genic balance but may alter phenotype because of
differences in gene expression ?
(a)
Deletion
(b)
Inversion
(c)
Duplication
(d)
Nondisjunction
Chromosomal Basis of Inheritance
..54..
69.
70.
71.
72.
73.
Drosophila flies with XXY genotype are females, but
human beings with such genotype are abnormal males. It
shows that
(a)
Y-chromOsome is essential for sex determination
in Drosophila.
(b)
Y-chromosome is female determinant in
Drosophila.
(c)
Y-chromosome is male determination in human
beings.
(d)
Y-chromosome has no role in sex determination
either in Drosophila or in human beings.
The trait traced in the above pedigree chart is
(a)
dominant X-linked
(b)
recessive X-l inked
(c)
dominant Y-l inked
(d)
recessive Y-linked
74.
Bird females have chromosome arrangement as
(a)
XY
(b)
XO
(c)
WZ
(d)
WW
75.
The formation of multivalents at meiosis in diploid
organism is due to
(a)
monosomy
(b)
deletion
(c)
inversion
(d)
reciprocal translocation
76.
Colchicine prevents the mitosis of cells at which of the
following stage?
(a)
Anaphase
(b)
Metaphase
(c)
Prophase
(d)
Interphase
Which disease has XXY chromosome constitution ?
(a)
Down’s syndrome
(b)
Turner’s syndrome
(c)
Klinefelter’s syndrome
(d)
Okazaki syndrome
77.
Genes of which one of the following is present exclusively
on the X-chromosome in humans ?
(a)
Baldness
(b)
Red-green colour blindness
(c)
Facial hair/moustaches in males
(d)
Night blindness
Barr-body in mammals represents
(a) One of the two X chromosomes in somatic cells of
females.
(b) All heterochromatin of male & female cells.
(c) Y chromosomes of male.
(d) All heterochromatin of female cells
78.
XO-chromosomal abnormality in human beings causes
(a)
Turner’s syndrome
(b)
Down’s syndrome
(c)
Klinefelter’s syndrome
(d)
none of the above
79.
A normal woman whose father was colour blind, is married
to a normal man. The sons would be
(a)
75% colour blind
(b)
50% colour blind
(c)
all normal
(d)
all colour blind
80.
Mother and father both have blood group 4A’. They have
two children one with blood group ‘O’ and second one
with blood group ‘A’. They have
(a) mother has homozygotic gene father has
heterozygot I’A 1 A.
(b) both are homozygotic (IAIA).
(c) mother is heterozygotic (l Ai) and father is
homozygotic (IAIA).
(d)
both are heterozygotic (IAi).
XO chromosomal abnormality in humans causes
(a)
Turner’s syndrome
(b)
Down’s syndrome
(c)
Patau’s syndrome
(d)
Klinefelter’s syndrome
Given below is a pedigree chart showing the in-heritance
of a certain sex-linked trait in humans.
Chromosomal Basis of Inheritance
..55..
81.
A person having 45 chromosomes and Y-chromosome
absent, is suffering from :
(a)
Down's syndrome
(b)
Klinefelter's syndrome
(c)
Turner's syndrome
(d)
Gynandromorph
82.
Monosomy and trisomy can be represented as:
(a)
2n + l, 2n + 3
(b)
2n – l, 2n – 2
(c)
2n, 2n + 1
(d)
2n – l, 2n + l
83.
A diseased man marries a normal woman and they get
three daughters and five sons. All the daughters were
diseased and sons were normal. The gene of this disease
is :
(a)
sex-linked dominant (b)
sex- linked recessive
(c)
sex-linked character (d)
autosomal dominant
84.
Haemophilic man marries a normal woman. Their offspring
will be :
(a)
all boys haemophilic (b)
all normal
(c)
all girls haemophilic (d)
all haemophilic
85.
Genetic identity of a human male is determined by
(a)
autosome
(b)
nucleolus
(c)
sex-chromosome
(d)
cell organelles
86.
Father of human genetics is :
(a)
Bateson
(b)
(c)
Mendel
(d)
Watson
Archibald Garrod
Chromosomal Basis of Inheritance
..56..
ANSWER KEY
EXERCISE – 1
1. (b)
2. (c)
3. (c)
4. (b)
5. (b)
6. (c)
7. (c)
8. (c)
9. (d)
10. (b)
11. (b)
12. (b)
13. (c)
14. (d)
15. (a)
16. (a)
17. (b)
18. (a)
19. (a)
20. (a)
21. (c)
22. (d)
23. (a)
24. (c)
25. (a)
26. (d)
27. (a)
28. (c)
29. (a)
30. (d)
31. (c)
32. (c)
33. (d)
34. (d)
35. (b)
36. (d)
37. (a)
38. (c)
39. (d)
40. (a)
41. (d)
42. (a)
43. (c)
44. (b)
45. (c)
46. (d)
47. (c)
48. (a)
49. (c)
50. (b)
51. (b)
52. (c)
53. (a)
54. (b)
55. (a)
56. (d)
57. (b )
58. (a)
59. (d)
60. (d)
61. (a)
62. (b)
63. (a)
64. (a)
65. (a)
66. (c)
67. (c)
68. (b)
69. (c)
70. (a)
71. (c)
72. (b)
73. (b)
74. (c)
75. (d)
76. (c)
77. (a)
78. (a)
79. (b)
80. (d)
81. (c)
82. (d)
83. (a)
84. (b)
85. (c)
86. (d)
Dream on !!
[\]^[\]^
Chromosomal Basis of Inheritance