Lecture 6 – Sex linkage
I. X-linked inheritance
A. Drosophila white gene behaves oddly
so why is this unusual?
B. Is white restricted to males?
C. Is it a new pattern of inheritance?
D. Characteristics of X-linked inheritance
1. Reciprocal crosses may result in
different phenotypic ratios between sexes.
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2. Heterozygous females transmit mutation to 1/2 their sons
3. Males that inherit X-linked recessive display mutant phenotype because Y
doesn’t carry the gene.
E. X-linkage strongly supports chromosomal theory of inheritance.
prediction based
on genetics
1/4 red-eyed female
1/4 red-eyed male
1/4 white-eyed female
1/4 white-eyed male
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observed
karyotype
II. Sex chromosomes
A. Sex chromosomes differ between sexes
- autosomes are
B. Types of sex chromosomes:
1. X chromosome:
2. Y chromosome:
C. patterns of sex chromosomes
1. XY pattern (human, other mammals, Drosophila)
2. XO pattern (many insects, nematodes)
3. WZ pattern (birds, butterflies)
D. homogametic vs. heterogametic
1. Homogametic produces single type of gamete with respect to sex
chromosomes.
2. Heterogametic produces two types of gametes.
E. What happens in WZ animals – 2 examples (chickens)?
1. nonbarredfemale(white feathers) X barred male (black-striped feathers)
2.
barred female X nonbarred male
3. Reciprocal crosses may produce different results, but the exact pattern is
reversed in WZ animals because female is XY (or WZ).
F. X-linked diseases in humans
1. hemohilia A & B
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2. color blindness
3. Lesch-Nyhan
4. Duchenne muscular dystrophy
5. X-linked diseases more common in males
eg. 1:
eg. 2:
eg. 3:
III. Nondisjunction
A. Bridges found rare exceptions to
pattern of white inheritance.
1. How did these individuals
arise?
– nondisjunction
2. non-disjunction in meiosis I
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3. nondisjunction in
meiosis II
B. nondisjunction as proof of chromosome theory on inheritance
red-eyed male (w+/Y) X white-eyed female (w-/w-)
nearly all females have red eyes
nearly all males have white eyes
~1/2000 red-eyed male
~1/2000 white-eyed female
predicted from genetics
observed karyotype
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