Biology 9700 · AS & A Level · Passage of information from parents to offspring

Passage of information from parents to offspring — practice question

Huntington’s Disease (HD) is a serious neurological disorder whose symptoms usually start only after a person has reached sexual maturity. These symptoms can include memory loss and changes in personality and mood. HD is caused by a gene mutation on chromosome 4 in which the triplet code CAG is repeated many times. The allele formed is dominant.
(a)[4]

Explain the meaning of the terms gene mutation and triplet code.

(b)[3]

A couple want to begin a family. The man does not have HD, while the woman has the disease. The woman’s father is unaffected. Complete the genetic diagram to show the probability that the couple’s first child will have HD. Key: Huntington allele $= T$, normal allele $= t$. Complete the parts for parental genotypes, gametes, offspring genotypes, offspring phenotypes, and state the probability of the first child having HD.

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