Transcript Inheritance Patterns Related to Gender Determination

Inheritance Patterns
Related to Gender Determination
A. Mechanisms of gender determination
B. X-linkage
C. Sex-limited and sex-influenced traits
A. Mechanisms of Gender
Determination
Chromosomal mechanisms
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Gender determined by the presence or absence of
certain chromosomes called sex chromosomes
Other “non-sex” chromosomes are called
autosomes
Not all genes on a sex chromosome are
necessarily related to gender or reproduction
Genic mechanisms
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Gender determined by the activity of genes located
on several chromosomes
A. Mechanisms of Gender
Determination
X-O mechanisms
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In grasshoppers and related insects
A single sex chromosome (X)
Females are have two copies of the X
chromosome (XX); each ovum produced will carry
an X chromosome (females are homogametous)
Males have only one copy of X, and no other sex
chromosome (XO); half the sperm carry an X, and
the other half have no sex chromosome (males are
heterogametous)
A. Mechanisms of Gender
Determination
X-Y mechanisms
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Females are XX (homogametous gender)
males are XY (heterogametous gender)
In mammals:
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The embryonic gonads develop into testes in response
to the expression of a “testes determining factor” gene
on the Y chromosome; Otherwise, the gonads develop
into ovaries
The Y chromosome also has a region that has
sequence homology with the X chromosome, allowing
pairing and segregation during meiosis
A. Mechanisms of Gender
Determination
X-Y mechanisms
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In Drosophila and similar insects, gender is
determined by the ratio of X chromosomes and
autosomes
In both mammals and Drosophila, the X
chromosome carries many genes not related to
gender or reproduction
Genes located on the X chromosome are called
X-linked genes
A. Mechanisms of Gender
Determination
Z-W mechanisms
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In birds and some reptiles
Z chromosome is similar to the X; W similar to Y
Except . . .
Males are homogametous (ZZ)
and females are heterogametous (ZW)
A. Mechanisms of Gender
Determination
Diploid-haploid mechanisms
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In bees and wasps
Females (queen and worker bees) develop from
fertilized eggs and have diploid cells
Males (drones) develop from unfertilized eggs and
have haploid cells
B. X-linkage
Discovered by T.H. Morgan around 1910
First X-linked gene to be discovered:
white eye gene in Drosophila
Morgan’s students found a single rare, mutant
white eyed male fly and, of course, wanted to
characterize it by a monohybrid cross
B. X-linkage
P:
F1:
F2:
White eyed male X wild type female
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All wild type, males & females
F 1 X F1

½ wild type females
¼ wild type males
¼ white eyed males
B. X-linkage
Obviously, the gene for white eye is recessive to its
allele for normal eye color
What was unusual was that all of the F2 recessive
offspring were males – no females
Morgan would have expected to find equal numbers of
males and females in the F2 phenotypes
With autosomal genes, one expects an F2 ratio of
3/8 dominant females: 1/8 recessive females:
3/8 dominant males: 1/8 recessive males
B. X-linkage
Morgan deduced that the white eye results could
be explained if the gene for white eye color is
located on the X chromosome
This was the first direct evidence that genes are
located on chromosomes
B. X-linkage
Genotypic explanation of Drosophila white eye:
– One gene, X-linked, with two alleles, w+ & w
– Different genotypes in males and females:
w+ w+ : Homozygous wild type females
w+ w : Heterozygous wild type females
w w : Homozygous white-eyed females
w+ Y : Hemizygous wild type males
w Y : Hemizygous white-eyed type males
B. X-linkage
Note that, with respect to X-linked genes, males
are neither homozygous nor heterozygous, but
hemizygous
Also note that the “Y” designates the Y
chromosome in males
During spermatogenesis, ½ the sperm get the
X chromosome (that has the X-linked genes),
and ½ the sperm get the Y chromosome
B. X-linkage
So, for the white-eye monohybrid cross:
P:
w Y male
X
w+ w+ female
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F 1:
w+ Y males & w+ w females
F1 X F 1
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F 2:
¼ w+ w+ females
¼ w+ w females
¼ w+ Y males
¼ w Y males
B. X-linkage
Morgan found that a testcross would produce
white-eyed (w w) females:
w Y male & w+ w females
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¼ w+ w females
¼ w w females
¼ w+ Y males
¼ w Y males
B. X-linkage
There are many X-linked traits in humans
– Example: Red-green color blindness
Normal female (N N) X Colorblind male (n Y)
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½ N n (Carrier females) & ½ N Y (Normal males)
Carrier female (N n) X Normal male (N Y)
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¼ N N, ¼ N n, ¼ N Y, ¼ n Y
B. X-linkage
Z-linked traits in birds exhibit similar inheritance
patterns, except that it is the male bird that is
homogametous
– Example: barred feathers in chickens
C. Sex-limited &
Sex-Influenced Traits
Sex-limited trait:
– A trait, produced by an autosomal gene, in which the
expression of a specific genotype is limited to only
one gender
– Example: Hen-feather vs. rooster-feather patterns in
chickens
C. Sex-limited &
Sex-Influenced Traits
Sex-influenced trait:
– A trait, produced by an autosomal gene, in which an
allele is dominant in one gender but is recessive in
the other gender
– Example: Pattern baldness in humans