·wtures on Hardy-Weinberg Equilibrium Pedigrees 27. The pedigree below shows the

Question

·wtures on Hardy-Weinberg Equilibrium Pedigrees 27. The pedigree below shows the passing on of straight thumbs (recessive) and Hitchhiker's thumb (dominant) in a family. Sha straight thumb. Hitchhiker's thumb is NOT an X-linked trait ded shapes mean the person has a GenerationI Cieneration1l Generation I Generation III Generation IV a. Is individual IV-2 heterozygous or homozygous for this trait? Explain your answer b. Why is there a double line (as opposed to a single line) between individuals II-1 and II-2? (Hint: what is different about this pairing than the other matings shown on the pedigree, represented by a single line) c. In this pedigree, straight thumbs is not a very common phenotype. Propose an explanation for why the only individuals on this pedigree that have the recessive phenotype are the offspring of lll-1 and Ill-2 and not any of the other matings shown on the pedigree

Explanation / Answer

The individual IV-2 will be heterozygous for the trait. Considering T dominant allele for Hitchhiker's thumb and t allele recessive for straight thumb. As the progeny in generation IV shows the majority of individuals with straight thumbs (tt- genotype), we can say that the probability of an individual having straight thumbs in generation IV is 2/3 or 66.66%. Also the parents III-1 and III-2 do not show the recessive phenotype with straight thumbs, they show a dominant phenotype. Therefore, they can either have a genotype TT or Tt. However, if either of the parents have TT phenotype (TT x Tt --> TT, Tt, TT, Tt) OR (Tt x Tt--> TT, Tt, Tt, tt), the progeny would consist of a majority of individuals with the dominant phenotype. This is not observed in the above pedigree. Therefore, we can conclude that the individual IV-2 has a heterozygous genotype (Tt). The double line between III-1 and III-2 indicates a consanguineous marriage between the two individuals. In such a case, marriage between two closely related individuals (first cousins here). Such marriages result in increased probability of recessive phenotype in the progeny. Infact, in case of autosomal recessive disorders, such marriages often lead to affecting a larger proportion of majority. Consanguineous marriage (double lined) are different from normal marriages (single lined) as the former results in inbreeding whereas the latter is a case of outbreeding. Straight thumb is a recessive phenotype. However, the phenotype is observed in majority of progeny from III-1& III-2 due to the consanguineous marriage between the two closely related individuals( a case of inbreeding) Thus, it results in higher occurrence of recessive alleles in the progeny as the genetic diversity of alleles is reduced (inbreeding depression - - > increased homozygosity for recessive traits). Often autosomal recessive traits get expressed in higher frequencies in the progeny of such individuals. In other normal marriages, the second partner in the marriage is from a different genetic background would provide a variable allele (increased genetic diversity in outcross - - > increased heterozygosity).

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