Genetics Exam 2 Franciscan University Dr. Perozich Spring 2012

sex-linked characteristics

traits determined by genes located on sex chromosomes
mostly x-linked, although a few y-linked

First discovered x-linkage in Drosophila?

Thomas Hunt Morgan

Findings of Thomas Hunt Morgan

different phenotypic ratio in sexes
males are hemizygous


one copy of gene, directly determines phenotype


recessive x-linked gene appears dominant in males as only one copy

Drosophila white eye male and red eye female cross?
Reciprocal cross?

cross: 1/2 female red eyes, 1/4 male red eyes, 1/4 male white eyes
reciprocal cross: 1/4 female red eyes, 1/4 female white eyes, 1/4 male red eyes, 1/4 male white eyes

When phenotypic ratios are different in initial and reciprocal cross, what does that mean?

sign of sex-linkage

criss-cross pattern of inheritance

fathers give x-linked recessive trait to their daughters who pass it in to sons; skips a generation

non-disjuction with x chromosomes

Calvin Bridges crossed white-eyed female with red-eyed male and expected all white-eyed males and red-eyed females. 5% of males had red eyes and 5% of females had white eyes. Likely due to non-disjunction.

Red-Green Colorblindness

X-linked recessive
eyes have pigments in cone to detect color
absorb red:green:blue
genes for red and green are close together on X, brain can mix signals together and get a wide spectrum of colors
Xc allele for red-green colorblindness
blue is on chromosome 7
X-linked traits more common in males because they only have one X -> pseudodominance

dosage compensation

different number of x's in males and females present problems to development (females would make twice as much protein as males from x-linked genes, would be highly detrimental to development)
animals correct this problem via dosage compensation
different in mammals and Drosophila

Sex-lethal gene (Sxl)

dosage compensation in Drosophila
on in females - each X produces one equivalent each of protein
off in males - protein complex binds to X, doubles amount of protein produced to equal that of females

Lyon hypothesis

in female cells, one X is inactivated
- inactivation by DNA methylation
- most, but not all, genes are turned off
- both males and females only have one functional X
- inactivation occurs within first few weeks of development (all somatic cells descending from that cell will have the same X turned off)
- X is random (50 dad/50 mom)
inactivated cell known as Barr Body (darkly staining structure)

Barr Body

darkly staining structure resulting from X inactivation in mammals
normal female has one Barr Body/nucleus
normal male has none

active genes on inactive x

not all genes on the inactive x are turned off
15% escape inactivation
active genes tend to occur in blocks in the distal portion of arms (especially short arm)
further away from Xise gene, more likely inactive
transcription levels range from 15-50% on active genes on inactive X as compared to those on active X


traits very from cell-to-cell depending on which X is active
only in females
ex. tortoiseshell cats
ex. anhidrotic ectodermal dysplasia
ex. RGCB

Anhidrotic Ectodermal Dysplasia

example of mosaicism
recessive allele blocks formation of sweat glands
heterozygous women have patches of skin that lack sweat glands

Z-linked characteristics

similar to XX-XY
males = ZZ; females = ZW
ex. Indian Blue Peafowl (wildtype and cameo)

Y-linked traits

traits only present in males
all offspring of affected male will have trait
little genetic info on Y
ex. hairy ears
- shows variable expressivity
- age of appearance is variable
- may have incomplete penetrance (not totally clear if Y-linked)
- maybe sex-limited
Y chromosome collects mutations over time (unique markers for family or tribe)

Pseudoautosomal genes

ex. bobbed locus (bb) on Drosophila X & Y, recessive
- 3 WT: 1 bobbed
- not reciprocal
- top = males 1/2 WT, 1/2 bobbed
- bottom = females 1/2 WT, 1/2 bobbed

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