Genetics Exam 2
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.
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
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
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)
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
similar to XX-XY males = ZZ; females = ZW ex. Indian Blue Peafowl (wildtype and cameo)
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)
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