Chegg Guided Solutions for Human Heredity 9th Edition: Chapter 12.C1

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    Penetrance is a term that describes the amount of a population that expresses symptoms from an inherited allele. In this problem for example, seventy-five percent of the people who carry an allele for colon cancer show symptoms, while twenty-five percent do not. This is an example of incomplete penetrance, in contrast to complete penetrance, where 100% of the people who inherit an allele will express it.

    Hereditary nonpolyposis colon cancer (HNCC) is a type of colon cancer that is derived from a faulty DNA repair enzyme. If a person inherits one of the alleles for HNCC then they are at risk for developing colon cancer because mutations in their DNA are not corrected properly. DNA repair enzymes that the normal HNCC alleles would code for would normally correct mutations that occur in DNA. But since M has inherited the alleles for HNCC, he has a high risk of developing colon cancer because DNA mutations normally acquired throughout life are not corrected.

    Some people do not develop colon cancer even though they have HNCC in their genome. This is most likely due to chance and a healthy lifestyle, devoid of a diet that contains carcinogens and mutagens. The twenty-five percent of the people who do not develop colon cancer from HNCC probably keep their DNA mutation rate low.

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