sexlinkage and sex determination

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sex-linkage and sex determination X and Y
chromosomes are responsible for sex
determination X and Y chromosomes are
unequally distributed females XX males
XY in humans, Y determines male X-linked
inheritance patterns easy to identify 1,100
genes on the X chromosome many associated with a
disease phenotype
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sex-linked genotypes hemophilia A Xh
(wild-type XH) males are hemizygous females can
be homozygous or heterozygous male genotypes
XHY - normal XhY hemophiliac female
genotypes XH/XH homozygous, normal
XH/Xh heterozygous, normal Xh/Xh
homozygous, hemophiliac
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sex-linked genotypes males have one copy or
dose of each X-linked gene females have
two copies or doses of each X-linked gene
quantity of product from single allele in males
and pair of alleles in females is usually
equivalent dosage compensation
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dosage compensation is explained by the Lyon
hypothesis In somatic cells of females, only one
X chromosome is active in transcription. The
second is heterochromatic and inactive. It
appears in interphase as a Barr
body. Inactivation occurs very early in
embryonic life (7-10 days after
fertilization) In any one somatic cell, the
inactive X could be either the paternal or
the maternal X chromosome. heterochromatin
is DNA that remains in the condensed state
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dosage compensation inactivation of one X
chromosome Inactivation is a matter of
chance in any one cell. All cells derived
from that cell have the same X chromosome
inactivated.
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dosage compensation inactivation of one X
chromosome result functional
mosaicism female is a mosaic with respect to
expression of genes on the X chromosome
example??
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dosage compensation inactivation of one X
chromosome result functional
mosaicism female is a mosaic with respect to
expression of genes on the X chromosome
calico cat
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calico cat gene on the X chromosome for
orange O dominant, orange o recessive
gene, non-orange Genes for other colors are
located on autosomes B black b
brown W white, masks B w color
at B locus exhibited orange patch O
expressed black patch o expressed
white patch o expressed W gene expressed
Can you get a male calico cat?
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dosage compensation inactivation of one X
chromosome result functional
mosaicism female is a mosaic with respect to
expression of genes on the X chromosome
in humans anhidrotic ectodermal
dysplasia absence of sweat glands
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X-linked recessive traits expressed in
phenotype of males expressed only in
homozygous females frequency much lower than in
males transmitted from affected male
through daughter to grandson 100
chance male will pass the gene to a daughter
may not be expressed 50
chance daughter will pass the gene to her sons
and 50 she will pass the gene to her daughters
most X-linked genes are rare unusual to find
affected females e.g., classical hemophilia 1
in 10,000 in males 1 in 100,000,000 in
females
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(No Transcript)
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X-linked recessive traits all daughters of
affected male are carriers female is XX one X
from each parent no sons of affected male
are affected male is XY with the Y from the
father both sons and daughters of
heterozygous female have a 50 chance of
inheriting the gene sons will exhibit the
disease Xd X chromosome with recessive allele
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X-linked dominant traits all daughters of
affected male are affected female is XX one X
from each parent no sons of affected male
are affected male is XY with the Y from the
father both sons and daughters of
heterozygous female have a 50 chance of
inheriting the gene XD X chromosome with
dominant allele
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Fragile X syndrome predominance of males in
institutionalized mentally retarded
patients due to one of several Z-linked
conditions fragile X is most common
syndrome mental retardation (less in
females) long face, large ears hypermobile
joints breaks gaps near tip of long arm of
X chromosome trinucleotide repeat CGG
6-50 copies in normal X chromosomes
230-1000 in fragile X cases
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Y chromosome few genes male determining
genes esp. SRY spermatogenesis factors minor
histocompatibility antigen HY
transmission father to son
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Sex-limited and sex-influenced traits
sex-limited occurs only in one sex inherited
traits involving ovaries, uterus or testes
sex-influenced variable expression in
sexes male-pattern baldness acts as
autosomal dominant in males acts as
autosomal recessive in females females can
transmit the trait, but they dont exhibit it
homozygous females significant thinning