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COURSE OUTLINE FOR BIO 201
Introduction-Definition, evolution and history of genetics.
Chromosomes-number, types, sex determination, shapes, length, chemical composition, structure, classification,
physical organization and operation and significance
Cell Division(Mitosis and Meiosis) and Cytokinesis
Gametogenesis
Consequences of Chromosomal aberrations resulting from Mitosis and Meiosis.
Genes(Alleles,Nucleotide bases,location,properties,classification etc)
Mendellian Genetics
Deviation from Mendellian Genetics.
Sources of Genetic Variation and Types.
Mutation.
Hereditable and Non-hereditable Characters
Sexual Dimorphism
Pedigree (Use and symbols)
Genes-definition,properties,location,number,classification, functions, gene effect, chemical basis(400 level)
Chromosomal Abberations(400 level)
Inheritance
INTRODUCTION
Genetics is the aspect of science that deals with transmission of traits or characters from parents to
offsprings.The term genetics was coined from the greek word ‘gene’ which means “to become”
It is divided into two parts:
1) Heredity: the study of the factors responsible for the resemblance between parents and their offsprings.
2) Variation: It is concerned with the forces or influences due to which no two organisms are exactly alike.
IMPORTANCE OF GENETICS
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Knowledge of genetics helps in the understanding of the causes of diseases.
Its helps to understand the principle underlying variations in human beings
It is tremendously useful in agriculture to increase yield and produce disease resistant varieties in plants
and animals.
It is fundamental in the successes of biomedical remedies for diseases of genetic origin such as bone
marrow transplant,stem cell research ,cloning, development of new organs etc
Knowlegde of genetics has led to possible means of prevention of genetic disorders through genetic
counselling and pre and post natal diagnosis.
There are various branches of genetics: Assignment
List various branches of genetics.
HISTORY OF GENETICS
Bateson in 1906 coined the term genetics which he defined as the science that deals with heredity and
variation.Johannsen in 1909 used the term “genes” for factors which are transmitted from parents to offspring in
an attempt to explain heredity(similarity) and variation (difference).
Hippocrates 400 BC theorised that small representative elements of all parts of the parental body are
concentrated in the semen which provide the building blocks for corresponding parts of the embryo.
Aristotle 300BC disproved Hippocrates theory because crippled parent do not give birth to abnormal children .He
postulated the form and substance theory stating that the father’s semen produces the form or plan while the
mother’s blood produces the shape or the substance.
We can go on and on,however,an Austrian monk by the name Gregor Mendel is referred to as the father of
modern genetics.He worked with garden peas and in the year 1866,he published a work on cross fertilization.In
1865,he reported it at a scientific meeting though he was not regarded.
However,Correns,Devries,Tschermak in 1900 rediscovered Mendel’s work which was accepted and also brought
about the basic laws of inheritance.
CHROMOSOMES
Chromosomes are genetic materials that codes for hereditary information.
Number
The number of xsome in each cell is fixed for a particular species.In human beings,it is 46 and this is called the
diploid no(2n).However, it is haploid in spermatozoa and ova(i.e n=23) which is the half the diploid no).These 46
xsomes are arranged in the form of 23 pairs.
Types
Each parent “donates” a member of each pair of xsomes.22 out of the 23 pairs are identical in both sexes and are
therefore called Autosomes. The xsomes in the remaining pair are called sex xsomes.Both sexes are identical in
the female and rae called X-somes while the two sex xsomes in male are not identical and are called X and Y
xsomes. Therefore, the female germ cell always has X-xsomes while a male germ cell may have either X or Y
xsome.
Determination of Sex
Female Gamete
Male Gamete
Fertilized Ovum
Child
Sex Xsome
X
+
Y
=
XY
Male
;;
X
+
X
=
XX
Female
Sex of a child is determined by the mother.True/False?
Why?
Chromosomes usually have different shapes which range from spiral,twisted,curved to rod shaped.
Xsomes are shortest during metaphase of cell division and it varies from 4-6 microns in length in humans.
Chemical composition
Xsomes are composed of
A )Deoxyribonucleic acid(DNA)
b) Ribonucleic acid
c)Histones
d)
Proteins
Xsome Structure.
Each xsome is composed of two rod shaped strands of chromatids.These chromatids are identical and lie parallel
to each other but united to form axsome at a pale staining area called the centromere(Pry
constriction).However,secondary constriction exists in certain xsomes and are therefore called SAT xsomes
Drawing of Xsome structure.
IMPORTANCE
The entire information needed for proper organisation,function and heredity instruction of various organs and
tissues are embedded in xsomes.Each xsome bear on itself large no of structures called GENES which guides the
performance of cellular functions.Thus,each complete diploid set of xsome contains the cell’s hereditary
instruction or genome.
Also, cellular activity in living organisms are controlled by xsome which determines the types of proteins
synthesized within the cells.
CLASSIFICATION OF XSOME-this is based on the location of centromere
1) TELOCENTRIC: axsome with terminal centromere meaning that each chromatid has one arm only.
2) ACROCENTRIC: The centromere is located in a subterminal location resulting in a xsome having one long
arm and the other short(the short arm is called the p-arm and long the q-arm)
3) SUBMETACENTRIC: centromere located slightly away from the mid points of the xsome resulting in
unequal arms.
4) METACENTRIC: centromere located in the middle of the xsome,with the two arms almost equal.
FURTHER READING ON METHODS OF XSOMAL STUDY.
The karyotype of a person with down’s syndrome
karyotyping
IMPORTANCE OF XMOSOMAL STUDIES
1) It helps in new fields involving separation of X or Y bearing sperms.
2) It is helpful in the diagnosis of xmosomal anomalies like Down’s(21),Turner’s and Klinefelter’s
syndrome(xmosomal complement abnormality) etc
3) In determination of an unborn baby’s sex.
4) It helps to detect the effect of occupational/environmental hazards on xsomes in relation to
chemicals,radiations etc.
5) For clinical diagnosis of patients with infertility or abnormalities in sexual development.
CELL DIVISION
Cells are regenerated through the division of pre-existing cells. This is necessary to ensure proper growth and
replacement of dead cells even after birth.
There are two basic types of cell division:
Mitosis
Meiosis
MITOSIS: It is a process in which chromosomes in the cell nucleus of the mother cell are separated into two
identical sets of chromosomes, with each in its own nucleus. These two daughter cells will be similar to the parent
cell in all aspect. Therefore, it can be referred to as a “duplicate process”.It is replicative(in type to that of the
parent cell) and quantitative(in no of xsome it contains).
This is a type of cell division that occurs in the somatic cells (i.e cells in all parts of the body of a plant or animal
except those of the ovaries, testes, or pollen mother cells).Cytokinesis(i.e division of the cytoplasm
,organelles,cell membrane and nucleus(karyokinesis) occurs after mitosis is accomplished.These processes results
in the production of two new genetically identical cells to each other and to their parent cell.
Types
Open mitosis: this is characterised by the breaking down of nuclear envelope before xsomes separate (e.g in
animals.)
Closed mitosis: this characterised by the division of xsomes within an intact cell nucleus(e.g in fungi and yeast
which reproduces through binary fission).
Stages in Mitosis
Prophase, Metaphase, Anaphase and Telophase(PMAT)
(Please note that interphase phase precedes the Mitotic phase(M-phase) but is not included here as it is for next
semester)
These phases are characterised by certain” events” which occurs at each phase to give a result of two new
genetically identical cells.
PROPHASE
1)Onset of xsome coiling,supercoiling and condensation.
2) Disintegration of nuclear membrane occurs which gradually merges with cytoplasmic contents to form
fragments.
3) Centrosome nucleates microtubules(cellular ropes or poles) to form spindles which separates to the opposite
sides of the nuclear membrane.
METAPHASE
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Microtubules invade the nuclearspace.
Each chromosome forms two kinetochores at its centromere, one attached at each chromatid. This serves
as point where microtubules attach themselves to the xsome
Xsomes migrate towards the equatorial plane “middle” of the cell.(This is thought to ensure that xsomes
are predisposed to “equal” separations to the poles.(Note: morphological study of xsomes is done at this
stage as the xsomes are thought to have reached their highest degree of coiling,folding and
condensation).
ANAPHASE
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The proteins binding the sisterchromatids together are cleaved (cut) and
becomes“separate”daughter chromosomes.
Shortening of the kinetochore microtubules causes their attached xsomes to be pulled apart towards
the poles(the cleaved centromeres leading and their chromatids trailing behind in a v/u shape
manner).
Also,this shortening causes the polar microtubules to push against each other and stretch the cell into
an oval/peanut shape implying that the cell is on the verge of dividing.
TELOPHASE
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A new nuclear membrane is formed using the membrane vesicles of the parent cell and the nucleolus
reappears.
Both sets of chromosomes surrounded by new nuclei uncondensed back into chromatin to end the
process of mitosis.
CYTOKINESIS
This is separated from cell division which is nuclear division while cytokinesis is cytoplasmic division.
In animal cells, a cleavage furrow (pinch) containing a contractile ring develops at the equatorial plane i.e the
cell’s metaphase plate pinching off the separated nuclei. In both animaland plant cells, cell division is also driven
by vesicles derivedfrom the Golgi apparatus, which move along microtubulesto the middle of the cell.
Cytokinesis in plant cells is different from that of animal cell because of its rigid cell wall.Small membrane
bounded vesicles formed by golgi apparatus filled with cell wall precursors-(“cell wall producing”)contract and
merges into the centre(cell plate) of the microtubule structure in plants called phragmoplast and develops into a
cell wall, separating thetwo nuclei. The phragmoplast is a microtubule structuretypical for higher plants, whereas
some green algae use aphycoplast microtubule array during cytokinesis.Eachdaughter cell has a complete copy of
the genome of
itsparent cell.
Insert a diagram showing the formation of cell wall and process of cytokinesis in plants
SIGNIFICANCE
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For the maintenance of the cell’s chromosomal set. Each cell formed receives chromosomes thatare alike
in composition and equal in number to the chromosomesof the parent cell.
For the development and growthtissues and organs for proper functioning in living organisms.
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For cell maintenance and replacement in some parts of body where cells are constantly dying or worn off
e.g. skin cells, digestive tract, red blood cellsetc
Regeneration of body parts e.gstarfish regeneratelost arms through mitosis.
Asexual reproduction:some organisms produce geneticallysimilar offspring through asexual
reproduction.For example, the hydra reproduces asexuallyby budding.
MEIOSIS
This is a mechanism where one parent gives rise to four daughter cells .It is a process whereby the genome
number of xsomes is halved i.e from diploid no to haploid no
This process takes place in germ (reproductive) cells of living organisms. When two gametes fuse during
fertilisation, the number of sets of chromosomes in theresulting zygote is restored to the original number which is
diploid in number.
Meiotic division is essential as it provides an avenue for the production of haploid gametes from diploid cells in
the germinal tissues of plants and animals.
Meiosis occur in two successive cell division namely : Meiosis I and Meiosis II
MEIOSIS I
Prophase I
This is a very critical stage and its sub-stages are:
1. Leptotene(leptonema):
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Chromosomes condense into visible strands within the nucleus though it may appear as a chromatin
thread under the microscope.
lateral elementsof the synaptonemal complex assemble.
progressive condensation andcoiling of chromosome fibres takes place
2. Zygotene (zygonema):
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This is the point where xsomes pair point for point with one another.They line up with each
other into homologous chromosome pairs.
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Synapsis(pairing/coming together) of homologous chromosomestakes place, facilitated by assembly of
central element ofthe synaptonemalcomplex. The point for point pairing is enhanced by the lateral
elements and pairing can start at any point.
The paired chromosomesare called bivalent or tetrad chromosomes.
How many tetrad complexes are in a cell if 2n=4?
In a cell of 2n=20,how many centromeres do you see?
3.PACHYTENE(pachynema)
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Chromosomal cross over occurs which leads to exchange of genetic material
DIPLOTENE(diplonema)
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The synaptonemal complex degrades andhomologous chromosomes separate slightly from one another.
The chromosomes uncoil a bit to allowingsome transcription of DNA. However, the
homologouschromosomes of each bivalent remain tightly bound atchiasmatawhich remain on the
chromosomes until they are separated in anaphase I.(The more the chiasma formed the more exchange
and variation).
(Note-This happens in female foetus where oocytes developto this stage and stop before birth. This
suspended stateis referred to as the dictyotenestageand remains so untilpuberty when one egg is released
(one/2 oocyte per menstrual cycle)to complete meiosis in the fallopian tubes).
3.Diakinesis
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Chromosome condensation reaches its peak
Chiasma moves towards the tip of tetrads
chiasmatabecomes clearly visible.
Disintegration of nuclear membrane.
METAPHASE I
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Bivalents align themselves along the equatorial plane.
ANAPHASE I
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Kinetochore (bipolar spindles) microtubules shorten which pulls homologous chromosomes apart (i.e
whole chromosomes are pulled toward opposing poles, forming two haploid set-DYADS(the centromere
doesn’t break).
The separated xsomes are genetically different from that of their parent cell.
TELOPHASE I
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new nuclearmembrane surrounds each haploid set.
Chromosomesuncoil back into chromatin.
Cytokinesis occurs.(NOTE-Sister chromatids remain attached during telophase I)
Meiosis II
Similar to mitosis mechanically but has different genetic result.. The end result is production of fourhaploid cells
(23 chromosomes, N in humans) from thetwo haploid cells (23 chromosomes, N * each of thechromosomes
consisting of two sister chromatids) producedin meiosis I. The four main steps of Meiosis II are:Prophase II,
Metaphase II, Anaphase II, and TelophaseII.
Prophase II we see the
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Disappearance of the nucleoliand the nuclear envelope.
Shorteningand thickening of the chromatids.
Centrioles move to thepolar regions and arrange spindle fibres for the secondmeiotic division.
Metaphase II
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the centromeres contain two kinetochores move to spindle equator
Anaphase II,
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xsomes are cleaved to allow microtubules attached to the kinetochorespull the sister chromatids apart.
The sisterchromatids(sister chromosomes)as they move toward opposite poles.
Telophase II
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Uncoiling and lengthening of the chromosomes.
disappearance of the spindle
Nuclear envelopes reform and cleavage or cell wall formationeventually produces a total of four daughter
cells,each with a haploid set of chromosomes.
SIGNIFICANCE
It brings about variation in populationse.g the observable difference between progeny and their parent.
It helps to keep the normal diploid xsome no in sexually reproductive organisms from generation to generation
thus ensuring stability of xsome complement of every organism.
Helps to reshuffle genetic combination thus providing material for evolution.
Differences between Mitosis and Meiosis
Mitosis takes place in the somatic cells while meiosis takes place in the germinal tissues and result in the
formation of gametes.
Mitosis is completed in a single cell division after xsome replication while meiosis is completed after two
successive cell divisions.
Synapsis of homologous xsome occur in meiosis but does not occur in mitosis.
Mitosis produces two genetically identical cells while meiosis results in four genetically different daughter cells.
During mitotic metaphase,sister xsomes orientate themselves independent of each other along the equator of
the spindle while bivalents/tetrads align themselves along the equator of the spindle in meiosis I.
During mitotic metaphase, centromere of each xsome divides while the division of centromere is delayed until
the second metaphase of meiosis.
At mitotic anaphase, daughter xsomes segregate to different poles while dyads separates to the poles during
anaphase I of meiosis.
Note: a dyad is a pair of sister chromatids.
A tetrad is a group of 2 sets of sister chromatids (metaphase stage)
A bivalent has 2 chromosomes.
Monads : any of the four chromatid of a tetrad(single chromosome-like chromatid)
Quiz: An organism has a diploid number of 16 in a primary oocyte. (a) How many tetrads are present in the
second meiotic prophase? (c) How many monads migrate to eachpole during the second meiotic anaphase?
GAMETOGENESIS
Cell division,fertilization and embryonic development or growth are the major features of any organism’s life
cycle.Though some plants complete their life cycles without fusion of gametese.g cassava,yam etc sexually
reproducing plants and animals can only complete their life cycles through the two processes of cell division.
Gametogenesis is the gamete formation process in sexually reproducing organisms.Oogenesis is the female form
of gametogenesis (the production of female gamete) while the male equivalent is spermatogenesis(production of
a male gamete).
Spermatogenesis takes place in the testes, the malereproductive organs. This can undergo repeated roundsof
mitosis to produce more spermatogonia (2n).The process begins with the enlargementof an undifferentiated
diploid germ cell called a spermatogonium. This cell grows(through mitosis) to become a primary spermatocyte,
which undergoes the first meiotic division to produce secondary spermatocytes. The secondaryspermatocytes
then undergo meiosis II, and each of thesecells produces two haploid spermatids eachwhich goes through a
series of developmental changes(spermiogenesis)to become highly specialized, motile spermatozoa, orsperm. All
sperm cells produced during spermatogenesis contain the haploid number of chromosomes and equalamounts of
cytoplasm.
Oogenesis involves the development of the various stages of the immature ovum. This takes place in the ovary
(the female reproductive organ) and occurs mainly at the foetal stage in mammals. The daughter cells resulting
from the two meioticdivisions of this process receive equal amounts of genetic material but unequal amounts of
cytoplasm.
Instead, during each division, almost all the cytoplasm ofthe primary oocyte is concentrated in one of the two
daughter cells. The concentration of cytoplasm is necessary because a major function of the mature ovum is to
nourish the developing embryo after fertilization.
Oogonia in the ovaries may either undergo repeated rounds of mitosis or enter prophase 1 becoming primary
oocyte(primary oocyte) 2n.Each primary oocyte completes meiosis I producing a large secondary oocyte and a
smaller polar body which distintegratesafterwards.This secondary oocyte completes meiosis II producing an
ovum(n) and a second polar body which also disintegrates.
Oogonium (oocytogenesis)pry oocyte(Meiosis I)First Polar Body (degenerates afterwards) and
Secondary oocyte(Meiosis II)Second Polar Body (degenerates) and Ovum
DIFFERENCES BETWEEN SPERMATOGENESIS AND OOGENESIS
Spermatogenesis
Preceded by 5 mitotic divisions
All daughter cells are viable
Starts after puberty
just after birth
Oogenesis
preceded by 2
only one is viable as a result of oogenesis.
Prophase of all oocytes are completed before or
Consequences of Chromosomal Separation Errors
NONDISJUNCTION: this occurs when a chromosome fail to separate during anaphase resulting in one daughter
cell having both sisterchromosomes and the other receiving none. Thisresults in the former cell having three
chromosomes containingthe same genes (two sisters and a homologue) which results in a condition condition
known as trisomy (2n+1)and the latter cell having onlyone chromosome (the homologous chromosome), a
conditionknown as monosomy(2n-1).
XXX(trisomy) XO(monosomy)
Illustration with 2 sister chromatids
Damaged xsome: due to a broken arm/fragment of a chromosome causing deletion.This results in a loss of
segment from a functional length of a chromosome e.g cats cry syndrome
Broken fragment may become incorrectly attached to another non-homologous chromosome, resulting in
translocation.
A-1--2---3---4------ B---11---12---13--14----------a--1--2--3----4----
b---11----12---13--14----------
A-1--2--//-3---4------ (a deletion occurs)= A—1---2
A-1--2--
B---11---12---13--14---3----4----
a---1--2--3----4----
b---11----12---13--14----------
It may also reattach to the original chromosome in a reverse orientation causing inversion or may reoccur as a
separate chromosome causing chromosomal duplication. The effect of these genetic abnormalities depends on
the specific nature of the error. I .e
A-1--2---4---3(inversion)
A-1--2-- ,A---3—4 (xsomal duplication)
a-1--2--3----4----
GENES
Genes are units of heredity located in xsomes. They are responsible for the coding of particular proteins /transfer
of trait or xter from parents to offspring.
DNA is a nucleic acid that contains the genetic instructions used in the development and functioning of all known
living organisms and some viruses.with backbone made of nucleic acid ,bases sugar and phosphate group.
It is also a set of blue prints needed to construct other components of cells such as proteins and RNA
molecules.The DNA has a double helical structure with two strands running in opposite direction to each
other.Chemically,The DNA consist of 2 long polymers of a single unit called nucleotides
.2 Two types of nitrogenous bases are present in a nucleic acid: pyrimidines, which contain a single ring, and
purines, which contain two rings
DNA contains two different pyrimidines, thymine (T) and cytosine (C), and two different purines, guanine (G) and
adenine (A).Uracil takes the place of thymine in RNA and siffers thymine by lacking a methyl group on its ring.
Assignment.
Differences between DNA and RNA in a tabular form.
Proteins: these are also known as polypeptides and are made up of amino acids arranged in a linear chain and
fold into a globular form.The sequence of amino acids in a protein is defined by the sequence of a gene which is
encoded in the genetic code.The genetic code specifies 20 standard amino acids.
Diagram below shows the 20 amino acids and their Genetic codons
MENDELLIAN GENETICS
Mendel is known as the father of genetics because he firdt gave evidence of inheritance by studyinging
monohybrid and dihybrid characters in Pisum sativum (garden peas).
Mendel’s theories were accepted as the basis for the study of what is known as Transmission genetics which
explains how genes are transmitted from parents to offspring. These principles were derived from Mendel’s
experimentation.
Note (a test cross is crossing the offspring with a homozygous parent)
Monohybrid Inheritance.
This is usually referred to as Mendel’s first law of inheritance or law of segregation.
The Basis.
A monohybrid cross is made by mating through breeding individuals from two parent strains,each exhibiting one
of the two contrasting forms of the character.For example,tall and dwarf/short(T and t);smooth & wrinkled(S and
s);long and short pods(L and l). Etc.
The Experiment and result
The first generation of offspring was crossed and then he reconsidered the offspring from the first generation i.e
self fertilization of individuals from the first generation.The original parent constitute a)P1 or parental generation
b)Their offspring which are the F1 or fisrt filial generation.
c)F2 which are individuals resulting from selfed F1 generation(seconf filial generation).
He found the immediate progeny/hybrids of the crosses involving these pairs of contrasting xters took after one
of their parents.When he crossed the seeds by selfing the hybrids,he found that the two parent contrasting xters
were re established in a particular proportion and the idea of recessive and dominat was derived from here.This
means that the expressed xter in the f1 generation of progeny is the dominant xter or trait while the suppressed
xter which express itself in the 2nd generation is the recessive trait or xter.
e.g a tall man marries a woman from a short family
Give the P1
Gametes of P1=Tt
Phenotype(all heterozygous tall)
Quiz .A man with AA genotype decides to marry a woman with SS genotype.What will be the F1 generation of the
progeny of the cross.
ii)What will be the F2 progeny if 2 of the F1 progenies decides to get married?
Mendel’s Law of Segregation
This states that each xter of an organism is controlled by a pair of gene alleles, one of whose pair is being
inherited from each parent through their haploid gamete.
Dihybrid Inheritance
This law assumes that two independent xters( e.g height and colour of flowers) have their genes located on two
non-homologus xsome pairs.For example,if two plants in which one was dwarf with red flowers and the other tall
with white flowers were crossed bred,the genotypes would be a mixture of both traits.
ttRR XTTrr
t R
T r
Tt
Rr
Using Trig ratio to get the gametes =TR,Tr,tR,tr.
Dihybrid
notation
TR
Tr
tR
tr
TR
TTRR
TTRr
TtRR
TtRr
Tr
TTRr
TTrr
TtRr
Ttrr
tR
TtRR
TtRr
ttRR
ttRr
tr
TtRr
Ttrr
ttRr
ttrr
Mendel’s second law-Law of independent assortment
This law states that genes or alleles do not contaminate or blend with one another in their hybrid or
heterozygotic conditions but segregate independent of each other in the following generation and each member
of a pair of allele may combine randomly with either of another pair i.e two independent xters (e.g height and
colour of flowers) have their genes located on two non-homologus xsome pairs.
MENDELLIAN GENETICS
Genes(Alleles,Nucleotide bases,location,properties,classification etc) The characteristics of the plants were
governed by distinct factors (or units) of inheritance, which were later termed
genes. An individual plant possessed two copies of a gene that controlled the development of each trait, one
derived from each parent. The two copies could be either identical to one another or nonidentical. Alternate
forms of a gene are called alleles. For each of the seven traits studied, one of the two alleles was dominant over
the other.When both were present together in the same plant, the existence of the recessive allele was masked
by the dominant one.
2. Each reproductive cell (or gamete) produced by a plant contained only one copy of a gene for each trait. A
particular gamete could have either the recessive or the domi-nant allele for a given trait, but not both. Each
plant arose by the union of a male and female gamete. Consequently, one of the alleles that governed each trait
in a plant was inherited from the female parent, and the other allele was inherited from the male parent.
3. Even though the pair of alleles that governed a trait remained together throughout the life of an individual
plant, they became separated (or segregated ) from one another during the formation of the gametes. This
finding formed the basis of Mendel’s law of segregation.
4. The segregation of the pair of alleles for one trait had no effect on the segregation of alleles for another trait. A
particular gamete, for example, could receive a paternal gene governing seed color and a maternal gene
governing seed shape. This finding formed the basis of Mendel’s law of independent assortment