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 normally begin only after sexual maturity has been reached. These symptoms can include memory loss together with changes in personality and mood. HD results from a gene mutation on chromosome 4, where the triplet code $\text{CAG}$ is repeated numerous times. The allele produced is dominant.
(a)[4]

Explain what is meant by the terms gene mutation and triplet code.

(b)[3]

A couple are planning to have a family. The man is free from HD but the woman has the disease. The woman’s father is unaffected. Complete the genetic diagram below to show the probability that the couple’s first child will have HD. Key: Huntington allele $= T$ normal allele $= t$ Parental phenotypes: man without HD, woman with HD Parental genotypes: ......................... , ......................... Gametes: ......................... , ......................... Offspring genotypes: ......................... Offspring phenotypes: ......................... Probability of first child having HD: .........................

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