Download Sex and the Chromosome

Sex and the Chromosome
(or how gender traits are inherited)
Some insects
• X-O system
• XX=female
• X=male (no second
chromosome)
Birds, Fish and some insects
•
•
•
•
Z-W system
ZW=female
ZZ=males
Determination is which chromosome is
in the ovum (not the sperm)
Bees and ants
•
•
•
•
Haplo-diploid system
No sex chromosome
Female are diploid
Males are haploid (developing from an
unfertilized ova)
Humans and other mammals
• Two varieties of sex
chromosomes
– X&Y
– XX=female
– XY=male
XY system
• Only relatively short
segments at the
either end of the Y are
homologous to the X
– X & Y rarely crossover
Making 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
Genetic 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
Sex linked genes
• Sex chromosomes (especially the X) have a
number of other genes
• Sex-linked gene=gene 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
Inheritance 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
Recessive sex linked traits
• Heterozygous females=carriers
– 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 sexlinked recessive disorders than females
Think 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.
Think about it:
Red–green color blindness is caused by
a sex–linked recessive allele. A color–
blind man marries a woman with
normal vision whose father was
color–blind. What is the probability
that they will have a color–blind
daughter? What is the probability that
their first son will be color–blind?
(Note: The two questions are worded
a bit differently.)
Think about it:
A man with hemophilia (a recessive, sex–
linked 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?
Color blindness
• A color blind daughter may be
born to a color blind father if
the mother is a carrier
– Odds are low
Duchenne muscular dystrophy
• Affects 1:3500 males in
US
• Individuals rarely live past
early 20’s
• Due to the absence of an
X linked gene for
dystrophin (key muscle
protein)
• Progressive weakening of
muscles and loss of
coordination
Hemophilia
• 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
Think 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 sex–
linked or autosomal? How do you know?
Explain why this disorder is almost never
seen in girls.
Being 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
Barr 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
Mosaic
• 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 sexlinked trait
– ½ of her cells will express the trait
– ½ of her cells will not
Examples 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
How 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
Using 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
Using 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?
Marfan’s Syndrome: Autosomal
Dominant
Using 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
Using 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