4. A rare form of polydactyly (extra fingers and toes) in humans is due to an X-

Question

4. A rare form of polydactyly (extra fingers and toes) in humans is due to an X-linked recessive gene. The chromosomal location of this gene is unknown. Suppose a geneticist studies a family with the following pedigree. She isolates DNA from each member of this family and analyzes these samples by Southern blotting, using a probe that consists of a cloned DNA sequence from the X chromosome. Results from this analysis are shown below. ? A) For each person in the pedigree, give his or her genotype based on RFLPs revealed by the probe. Your genotypes should clearly indicate which individuals share common alleles based on the RFLP patterns. gene for polydactyly? Why or why not? polydactyly? Why? B) Is there evidence for close linkage between the probe sequence and the X-linked C) How many of the daughters in the pedigree are likely to be carriers of X-linked

Explanation / Answer

XA = Dominant WT allele

Xa = Recessive mutant allele

Genotype of I-1 = XAY

Genotype of I-2 = XAXa

Genotype of II-1 = XAY

Genotype of II-2 = XaY

Genotype of II-3 = XAXa

Genotype of II-4 = XaY

Genotype of II-5 = XAXA

Genotype of II-6 = XaY

Genotype of II-7 = XAY

Yes. The marker is closely linked to the mutant phenotype.

One daughter is a carrier for the mutant alelle.

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