Title: Sex and the Chromosome (or how gender traits are inherited)
1Sex and the Chromosome(or how gender traits are
inherited)
2Some insects
- X-O system
- XXfemale
- Xmale (no second chromosome)
3Birds, Fish and some insects
- Z-W system
- ZWfemale
- ZZmales
- Determination is which chromosome is in the ovum
(not the sperm)
4Bees and ants
- Haplo-diploid system
- No sex chromosome
- Female are diploid
- Males are haploid (developing from an
unfertilized ova)
5Humans and other mammals
- Two varieties of sex chromosomes
- X Y
- XXfemale
- XYmale
6XY system
- Only relatively short segments at the either end
of the Y are homologous to the X - X Y rarely crossover
7Making babies
- In both testes (XY) and ovaries (XX), the two sex
chromosomes segregate independently during
meiosis - Each ovum (from female) receives an X
- Sperm ½ X ½ Y
- During conception
- 50-50 chance of either sperm fertilizing the ovum
- Therefore 50-50 chance of producing a male or
female offspring
8Genetic control of gender
- In humans anatomical signs of gender first appear
would the 2nd month after conception - In 1990 a gene on the Y chromosome was identified
which is required for development of the testes
(SRY gene) - In individuals possessing SRY gene the embryonic
region develops into testes - Activity of SRY gene triggers a cascade of
biochemical, physiological, and anatomical events
because it regulates may other genes - Other genes on the Y regulate production of
functional sperm - In the absence of these genes, an XY individual
is male but does not produce normal sperm - In individual lacking SRY gene, the embryonic
region develops into ovaries
9Sex linked genes
- Sex chromosomes (especially the X) have a number
of other genes - Sex-linked genegene on the X chromosome
- Fathers pass sex linked alleles
- to all their daughters
- None of their sons
- Mothers pass sex linked alleles to both sons and
daughters
10Inheritance of Sex-Linked Traits
- Normal mother father with trait
- XAXA x XaY
- Carrier mother normal father
- XAXa x XAY
- Carrier mother father with trait
- XAXa x XaY
11Recessive sex linked traits
- Heterozygous femalescarriers
- Chance of female inheriting 2 recessive mutants
on the X are unlikely - Hemizygous males (only one X)has trait
- There fore males are more likely to exhibit
sex-linked recessive disorders than females
12Think about it
- Colorblindness is a recessive trait. Two people
with normal vision have two sons, one colorblind
and one with normal vision. If the couple also
has daughters, what proportion of them will have
normal vision? Use a Punnett square to explain
your answer.
13Think about it
- Redgreen color blindness is caused by a
sexlinked recessive allele. A colorblind man
marries a woman with normal vision whose father
was colorblind. What is the probability that
they will have a colorblind daughter? What is
the probability that their first son will be
colorblind? (Note The two questions are worded
a bit differently.)
14Think about it
- A man with hemophilia (a recessive, sexlinked
condition) has a daughter of normal phenotype.
She marries a man who is normal for the trait.
What is the probability that a daughter of this
mating will be a hemophiliac? That a son will be
a hemophiliac? If the couple has four sons, what
is the probability that all four will be born
with hemophilia?
15Color blindness
- A color blind daughter may be born to a color
blind father if the mother is a carrier - Odds are low
16Duchenne muscular dystrophy
- Affects 13500 males in US
- Individuals rarely live past early 20s
- Due to the absence of an X linked gene for
dystrophin (key muscle protein) - Progressive weakening of muscles and loss of
coordination
17Hemophilia
- Absence of one or more proteins required for
blood clotting - Prolonged bleeding because blood clots form very
slowly - Bleeding in muscles and joints can be painful and
lead to serious damage - Treatment involves IV injections of missing
protein
18Think about it
- Pseudohypertrophic muscular dystrophy is an
inherited disorder that causes gradual
deterioration of the muscles. It is seen almost
exclusively in boys born to apparently normal
parents and usually results in death in the early
teens. Is this disorder caused by a dominant or a
recessive allele? Is its inheritance sexlinked
or autosomal? How do you know? Explain why this
disorder is almost never seen in girls.
19Being Female
- Only one of the X chromosomes is actually active
- Males and Females essentially have same dose of
female genes - During female development one of the X chromsomes
per cell condenses into a compact Barr Body - Barr body lies along the inside of the nuclear
envelope - Barr Body may reactivate in ovarian cells which
produce ova
20Barr Body
- Selection of which X chromosome will form the
Barr Body occurs randomly in embryonic cells at
time of X inactivation - Females are a mosaic of two types of cells
- Some inactive maternal X, some inactive paternal
X
21Mosaic
- After an X is inactivated in a cell, all the
mitotic descendants of the cell have the same
inactive X - If a female is heterozygous for a sex-linked
trait - ½ of her cells will express the trait
- ½ of her cells will not
22Examples of mosaic pattern of inheritance
- In cats
- Orange and black tortiseshell coat color is due
to patches of cells expressing orange allele and
other patches expressing nonorange allele - In humans
- Women possessing X-linked mutation preventing
development of sweat glands - If Heterozygous
- Patches having sweat glands
- Patches lacking sweat glands
23How does Barr body form
- X-inactivation involves modification of DNA by
attachment of methyl (--CH3) groups to cytosine
nucleotides on one of the X chromosomes - XIST (x-inactive specific transcript gene)
- Active only on Barr body chromosome
- Produces multiple copies of an RNA that attaches
to the X chromosome on which they were made - This inactivates X
- The mechanism connecting XIST RNA and DNA
methylation is unknown - Determination of X chromosome to be inactivated
is also unknown - Go to the film
24Using pedigrees to determine inheritance
- Is the trait shown in this pedigree autosomal
dominant, autosomal recessive, sex-linked
dominant, or sex-linked recessive? If
sex-linked, is the trait on the X or the Y
chromosome?
Hemophilia Sex-linked on the X chromosome
25Using pedigrees to determine inheritance
- Is the trait shown in this pedigree autosomal
dominant, autosomal recessive, sex-linked
dominant, or sex-linked recessive? If
sex-linked, is the trait on the X or the Y
chromosome?
Marfans Syndrome Autosomal Dominant
26Using pedigrees to determine inheritance
- Is the trait shown in this pedigree autosomal
dominant, autosomal recessive, sex-linked
dominant, or sex-linked recessive? If
sex-linked, is the trait on the X or the Y
chromosome?
Hairy Ears Sex-linked on the Y chromosome
27Using pedigrees to determine inheritance
- Is the trait shown in this pedigree autosomal
dominant, autosomal recessive, sex-linked
dominant, or sex-linked recessive? If
sex-linked, is the trait on the X or the Y
chromosome?
Albinism Homozygous Recessive