Download relates Mendel`s discoveries to actual behavior of chromosomes

Chapter 15 Study Guide
Courtesy of Julia Beamsderfer (2005)
Chromosome Theory of Inheritance- relates Mendel’s discoveries to actual behavior of chromosomes
I.
Thomas Hunt Morgan- first person to associate a specific gene with a specific chromosome by using a
species of fruit flies (to take advantage of its ability to produce hundreds of offspring from one
mating, and its having only four pairs of chromosomes)
A. Wild Type- the normal phenotype for a character (such as red eyes in a fruit fly)
B. Mutant Phenotype- a trait that is alternative tot he wild type (such as white eyes)
C. Sex-Linked Genes- genes (such as eye color for fruit flies) that are located exclusively on one’s X
chromosome, with no corresponding locus on the Y chromosome.
1. When a mutant allele is recessive, it is most common in males, because there is no dominant
allele to affect the phenotype like there is in females, who can only receive the mutant if it
comes from both X chromosomes.
D. Linked Genes- Genes located on the same chromosome that tend to be inherited together in
genetic crosses due to chromosomes being passed along as units
1. Morgan’s Testcross- between dihybrid flies with gray bodies with normal wing size (b+ b
vg+ vg) and mutant flies with black wings and vestigial wings (b b vg vg) he expected to get
equal ratios of each of the four phenotypic possibilities.
2. Results- instead, Morgan noticed that the two phenotypes of the parents took up a higher
percentage than ¼, hypothesizing that the two traits were inherited together due to the fact
that they were on the same chromosome.
3. The other two phenotypes (b+ b vg vg) and (b b vg+ vg) were represented, however, as a
result of crossing over of chromosomes.
a. Genetic Recombination- the production of offspring with new combinations of traits
inherited from two parents
- Unlinked Genes (Independent Assortment of Chromosomes)- When breeding pea
plant (YyRr) and (yyrr), half offspring have parental types (phenotypes that match
one of the parental phenotypes), and the other are recombinants (meaning they have
different combinations of traits)
- When half of all offspring are recombinants, there is a 50% frequency of
recombination, which is observed for any two genes that are located on
different chromosomes)
- Basis of recombination of unlinked genes is random orientation of homologous
chromosomes at metaphase I of meiosis (independent assortment)
- Linked Genes (Crossing Over)- Linked genes do not assort independently because
they are located on the same chromosomes and usually move together. In Morgan’s
testcross however, there were recombinants, caused by homologous chromosomes
exchanging segments (crossing over)
- With the ability to cross over during prophase of Meiosis I, parental
chromosomes can alter parental combinations, distributing recombinant
varieties to gametes.
- With this discovery, one of Morgan’s students, Alfred Surtevant, constructed a
genetic map to order genetic loci along a chromosome (hypothesizing that the
further apart two genes were, the higher the probability a crossover will occur
between them)
- EX- in fruit flies, the recombination frequency between the body color (b)
gene and wing size gene (vg) is 17%, and the recombination frequency
between the body color and the eye color gene (cn) is 9%, meaning that b
and cn were closer together on the chromosome
- The distances used to measure units between genes are called map units or
centimorgans, representing 1% recombination frequency
- The maximum recombination frequency between two genes on a
chromosome is 50%, the same as if the two genes were on different
chromosomes
- Linkage Maps- maps based on recombination frequencies that show order
of genes but not actual measures
- Cytological Maps- maps of chromosomes that locate genes with respect to
chromosomal features
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Chapter 15 Study Guide
Courtesy of Julia Beamsderfer (2005)
Sex Chromosomes- in humans and other mammals, there are two varieties of sex chromosomes, designated X and Y
which usually behave as homologous chromosomes. Females are XX, and males are usually XY.
I. Sex Determination- occurs in meiosis. All ova have one X chromosome, where as 50% sperm have X and
other 50% have Y, so when eggs are fertilized, sex depends on the type of sperm
A. The sex of an embryo is triggered by hormonal conditions in embryo. EX- testes are triggered by
SYR gene, which occurs Y chromosomes
1. Sex chromosomes (usually X-chromosomes) also have genes for many characters unrelated
to sex. Fathers can pass X-linked alleles to all daughters but to no sons, where mothers can
pass sex-linked alleles to both.
a. Recessive Sex-Linked Traits- these traits will only be expressed by females if she is a
homozygote, where any male receiving the recessive allele from his mother will express
the trait (explaining why far more males than females have disorders that are inherited as
sex-linked recessives) EX- color blindness
- Distribution: (NOT CARRIER), (CARRIER), (HAVE DISORDER)
- XAXA x XaY = 50% XAXa, 50% XAY
- XAXa x XAY = 25% XAXA, 25% XAY, 25% XaY, 25% XaXA
- XAXa x XaY = 25% XAXa, 25% XAY, 25% XaY, 25% XaXa
II.
Sex-Linked Disorders
A. Duchenne Muscular Dystrophy- X-linked disorder that affects one out of every 3500 males in US
1. Rarely live past early twenties
2. Characterized by progressive weakening of the muscles and loss of coordination
3. Caused by absence of a key muscle protein called dystrophin that has been tracked to a gene
of a specific locus on the X chromosome
a. Hemophilia- sex-linked recessive trait
- Defined by absence of a certain protein required for blood clotting, causing
hemophiliacs to bleed excessively when injured
- Hemophilia has plagued Europe’s royal families, starting with Leopold, son of
Queen Victoria (a carrier of trait)
III.
X-Inactivation in Female Mammals- In embryonic development, one X-chromosome in each female
embryo cell becomes almost completely inactivated
A. Purpose- to make sure females and males have the same effective dose of genes with loci on the
X chromosome
B. The inactive X in each cell of a female condenses into a Barr body, which remains mostly
inactive until they are reactivated in ovary cells that produce ova
C. Because selection of inactive X is done randomly in each embryonic cell, a female heterozygous
for a sex-linked trait will have approximately half the number of her cells expressing one allele,
and half the alternate.
1. EX- the calico cat / a female that is heterozygous for a recessive X-linked mutation that
prevents the development of sweat glands has half skin with sweat glands and half without
D. Inactivation involves attachment of methyl groups (CH3) to cytosine, one of the nitrogenous
bases of DNA nucleotides.
E. A gene that is active only on the Barr body chromosome does determining which X is
inactivated. The gene, XIST has a product (specific transcript) that is an RNA molecule, that
when multiple copies are present, attach to the X chromosome from which they are made, causing
it to initiate inactivation. It is still not known how this occurs, or what determines which Xchromosome will have an active XIST gene.
Errors and Exceptions in Chromosomal Inheritance
I.
Nondisjunction- when members of a pair of homologous chromosomes do not move apart properly
during meiosis I, or in which sister chromatids fail to separate during meiosis II
A. Causes one gamete to receive two of the same type of chromosome and another gamete to receive
no copy
B. Aneuploidy- When an abnormal gamete unites with a normal one at fertilization, causing the
offspring to have an abnormal chromosome number which is transported to all cells through
mitosis
1. Trisomic- when a chromosome is present in triplicate in the fertilized egg (2n + 1
chromosomes)
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Chapter 15 Study Guide
Courtesy of Julia Beamsderfer (2005)
Monosomic- when a chromosome is missing (2n – 1 chromosomes)
Aneuploidy can also occur in mitosis, and when this happens at an early age it my pass
condition to large number of cells
C. Polyploidy- when an organism has more than two complete chromosome sets
1. Triploidy- when an organism has three sets of chromosomes (3n)
a. May occur when an abnormal diploid egg, produced by nondisjunction of all its
chromosomes is fertilized
2. Tetraploidy- organism has four sets of chromosomes (4n)
a. May occur when a normal zygote fails to divide after replicating its chromosomes
3. Relatively common in the plant kingdom and plays an important part in plant evolution
4. Rare in animal kingdom, yet polyploidy does not affect individual as severely as aneuploidy
Alterations of Chromosome Structure Due to Breakage of Chromosome
A. Four Major Types
1. Deletion- occurs when a chromosomal fragment lacking a centromere is lost during cell
division, causing the chromosome it came from to lack genes
2. Duplication- when a deleted fragment joins to the homologous chromosome, producing a
duplication
3. Inversion- when a deleted fragment rejoins its chromosome but in the reverse orientation
4. Translocation- when a deleted fragment joins a nonhomologous chromosome
B. Sometimes during meiosis homologous chromatids sometimes break and rejoin at incorrect
places (nonreciprocal crossover), resulting in a deletion and a duplication
C. Duplications and Translocations are harmful, but in reciprocal translocations and inversions, the
gene balance is normal, but phenotype may be altered
D. An organism inheriting a large homozygous deletion (or single X chromosome with deletion in
male) is usually missing essential genes
Human Disorders Due to Chromosomal Alterations
A. While aneuploidy occurs frequently in humans, most are so disastrous that the embryo is
spontaneously aborted long before birth
B. Down Syndrome- an aneuploid condition affecting approximately one out of every 700 children
born in the US, usually caused by nondisjunction of gamete production
1. Usually a result of cells that are trisomic for chromosome 21, resulting in cells of 47
chromosomes; however it often occurs in individuals with normal chromosome count which
have the presence of part or all of the third chromosome 21 attached to another chromosome
by translocation
2. Includes characteristic facial features, short stature, heart defects, susceptibility to respirator
infection, mental retardation, prone to development of leukemia and Alzheimer’s disease
3. Usually results in a shortened lifespan, as well as sexually underdeveloped and sterile
organisms.
4. Frequency of Down syndrome correlates with age of mother.
a. Down syndrome occurs in .04% of children born to women under age 30
b. Down syndrome occurs in 1.25% for mothers in early thirties and up
c. While unexplained, it is believed to be caused the possible ability for an older women to
carry a Down baby to full term
C. Nondisjunction of Sex Chromosomes produce a variety of aneuploid conditions which seem to
upset genetic balance less than aneuploid conditions in autosomes
1. Klinefelter Syndrome- when an extra X chromosome is present in a male, producing XXY in
approximately 1 out of every 2000 births
a. People with Klinefelter have male sex organisms, but malfunctioning and sterile testes,
and often breast enlargement and other feminine characteristics
2. Males with an extra Y chromosome (XYY) do not have a well-defined syndrome but are
usually somewhat taller than average
3. Females with trisomy X (XXX), which occurs once in every 1000 live births, are healthy and
undistinguishable except for karyotype
4. Turner Syndrome- Monosomy X, which occurs once in every 5000 births
a. X0 individuals are phenotypically female
2.
3.
II.
III.
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Chapter 15 Study Guide
Courtesy of Julia Beamsderfer (2005)
b.
IV.
V.
X0s do not have mature sex organs at adolescence, their secondary sex characteristics
fail to develop, they are sterile, and of short stature, usually with normal intelligence.
D. Human Disorders Caused by Structural Alterations in Chromosomes
1. Cri Du Chat (“Cry of the Cat”)- occurs in children born with a specific deletion in
chromosome 5
a. Results in mental retardation, small heads, unusual facial features, and children whose
cry sounds like the mewing of a distressed cat
b. Individuals with cri du chat usually die in infancy or early childhood
2. Chronic Myelogenous Leukemia (CML)- a cancer affecting the cells that give rise to white
blood cells.
a. In cancerous cells of CML patients, a reciprocal translocation has occurred between a
portion of chromosome 22 and a fragment of chromosome 9
Genomic Imprinting- the paternal effect on gene expression.
A. Identical alleles may have different effects on offspring, depending on whether they were
received by zygote from father (sperm) or mother (ova)
B. In each generation, imprints on alleles are erased and reprinted according to that individual’s sex
C. Parental imprints are believed to consist of methyl (CH3) groups added to nucleotides at specific
loci on chromosomes
D. About 20 mammalian genes are identified to be subject to imprinting
E. Disorders Inherited Through Genomic Imprinting
1. Prader-Willi Syndrome- characterized by mental retardation, obesity, short stature, small
hands and feet
a. Caused by a deletion of a particular segment of chromosome 15 inherited from father
2. Angelman Syndrome- causes patients to exhibit spontaneous uncontrollable laughter, jerky
movements, and other motor and mental symptoms
a. Caused by a deletion of a particular segment of chromosome 15 inherited from mother
3. Fragile X Syndrome- named for the physical appearance of an abnormal X chromosome
whose tip hangs on to the rest by a thin thread of DNA
a. Occurs in one of every 1500 males and one in every 2500 females
b. Causes mental retardation
c. Much more common when abnormal chromosome is inherited from the mother rather
than the father, which shows consistency with its being more common in males—if he
receives a fragile X chromosome, it has to be from mother
d. Abnormal because maternal imprint causes syndrome rather than silence it
Extranuclear genes exhibit a non-Mendelian pattern of inheritance
A. Not all of a eukaryotic cell’s genes are located in the nucleus—extranuclear genes can be found
in mitochondria and plastids in plants
B. These genes are reproduced and transmitted to daughter cells independently from others, by the
mitochondria and plastids themselves
C. In 1909 Karl Correns studied inheritance of yellow or white patches of leaves of green plants,
which he determined were inherited by the maternal parent only. More research shows that such
coloration is due to genes in plastids, which in most plants are all received from the cytoplasm of
the ovum, not the pollen.
D. This is also seen in mitochondrial genes in mammals, which also come from cytoplasm of ovum
E. Rare human disorders can be caused by mutations in mitochondrial DNA, which reduce the
amount of ATP a cell can make, most affecting nervous systems and muscles, areas most
susceptible to energy deprivation
a. Mitochondrial Myopathy- disease that causes patients to suffer weakness, intolerance of
exercise, and muscle deterioration
b. Mitochondrial Mutations inherited from mothers may also contribute to diabetes, heart
disease, and Alzheimer’s disease
c. Mutations gradually accumulate in this DNA, possibly playing a role in aging process
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