Question

Draw an invented pedigree in which a couple has three children. The eldest two children, a daughter (Wilma) and son (Fred) both have Huntington’s Disease (HD - a genetic disorder where nerve cells in the brain progressively breakdown affecting physical and mental abilities and is fatal and individuals show signs during their 20-40’s) like their father. HD is NOT a sex-linked gene. The couple’s youngest daughter (Betty) does not have HD.

Fred marries a woman with no HD. They have five kids: a son with HD, normal daughter, daughter with HD, normal son and their youngest daughter with HD. Fred’s oldest son marries a woman with no HD. They have three kids, two daughters with HD and their son is not affected. Fred’s middle daughter with HD marries a man without HD and they have four children (a daughter, son and two more daughters) without HD.

Fred’s youngest daughter marries a man that does not have HD. They have two sons, one with and without HD. Construct a pedigree analysis for this family and give the possible genotypes for each member.

Answer 1

Explanation: In the diagram of the above pedigree

Key : colored circle indicates affected female

Colored square indicates affected male

Blank circle indicates unaffected female

Blank square indicates unaffected male

Horizontal line between the two individuals indicates marriage.

Huntington’s disease is an autosomal dominant disease, the gene will be expressed in homozygous as well as heterozygous condition.

Let the allele for the disease be ‘H’ and the normal allele be ‘h’

All the unaffected individuals are having the genotype ‘hh’

And all the affected individuals are having the genotype ‘Hh’

Note: if the affected individuals were HH then all their offspring would have the disease which is not the case, hence their genotype is Hh

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