I can't figure out how to do the punnet square for any of this or what it all means? Can anyone please explain how to
understand the solution? Looks like a bunch of letters and numbers to me.
Red-green color-blindness is an X-linked recessive trait in humans. Polydactyly (extra fingers and toes) is an autosomal dominant trait. Martha has normal fingers and toes and normal color vision. Her mother is normal in all respects, but her father is color-blind and polydactylous. Bill is color-blind and polydactylous. His mother has normal color vision and normal fingers and toes. If Bill and Martha marry, what types and proportions of children can they produce?
The first step is to deduce the genotypes of Martha and Bill. Because the two traits are independent, we can deal with just one trait at a time.
Starting with the X-linked color-blind trait, Bill must be XcY because he is color-blind. Bill’s mother must be a carrier (X+Xc). Martha must be X+Xc, a carrier for color blindness because her father is color blind (XcY).
For polydactyly, Bill must be Dd (D denotes the dominant polydactyly allele). Since his mother has normal fingers (dd), he can’t be homozygous DD. Martha, with normal fingers, must be dd.
If Martha (dd,X+Xc) marries Bill (Dd,XcY), then we can predict the types and probability ratios of children they could produce.
For polydactyly, ½ of children will be polydactylous, and ½ will have normal fingers.
For color-blindness, ¼ of children will be color-blind girls, ¼ will be girls with normal vision but carrying the color-blindness allele, ¼ will be color-blind boys, and ¼ will be boys with normal vision.
Combining both traits, then:
1/8 color-blind girls with normal fingers
1/8 color-blind girls with polydactyly
1/8 girls with normal vision and normal fingers
1/8 girls with normal vision and polydactyly
1/8 color-blind boys with normal fingers
1/8 color-blind boys with polydactyly
1/8 boys with normal vision and normal fingers
1/8 boys with normal vision and polydactyly
This analysis can also be carried out with a Punnett square.
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