Huntington's disease, which causes degeneration of neurons, runs in Mark's family. Huntington's is a heritable autosomal dominant genetic disease, in an autosomal dominant disease, only one abnormal allele is required to cause the disease in offspring. However, it is possible not all offspring will receive the abnormal allele from an affected heterozygous parent. Family members that have the disease include Mark's maternal grandfather and Mark's maternal aunts. Mark's maternal grandmother does not have the disease.
Because symptoms of Huntington's disease do not appear until later in life, genetic testing is necessary to determine whether a younger individual possesses the allele for the disease. These tests determined that Mark's mother does not carry the allele for Huntington's disease.
Mark's paternal aunt suggested that Mark create a pedigree to track the disease in his family. Use the symbols provided to complete the pedigree. Each symbol may be used more than once.

Huntington's disease is an autosomal dominant disorder in which the presence of even one dominant allele can cause the disorder.

The autosomal disorder can not skip a generation, have to be present in every generation.

Condition Given:

Affected Member: Mark's Maternal grandfather and Maternal aunts.

Normal: Mark's Maternal grandmother and Mark's Mother.

As MArk's Maternal Grandfather is affected then his genotype must be Aa because Mark's Maternal grandmother is unaffected (aa), this is the only condition where Mark's mother will be normal.

A- Dominant (Huntington)

a- Recessive

Maternal means on the mother's side and paternal means on the father's side.

The diagram is attached below.

I-1: Mark's maternal Grandmother

I-2: Mark's maternal Grandfather

I-3&4: Mark's Paternal Grandparents

II-1&2: Mark's Maternal aunts

II-5: Mark's paternal aunt

II-3&4: Mark's mother and father respectively will not have the disease as both father and mother of Mark are normal.

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As it is given that Mark's paternal grandmother and grandfather does not carry the disease. The symbols on the right side of the Punnett represent Mark's paternal family. If both his paternal grandparents are unaffected then his father and paternal aunt are also not affected by the disease.

His maternal grandfather is affected as mentioned and also his maternal aunts are affected. Mark's mother is unaffected as she does not have the disease-causing alleles determined by genetic testing.

Both of the marks parents are unaffected which indicates that marks also do not carry the allele of the disease.

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