Question

A man and a woman are preparing to have a child. The man finds out that his father has

Answer 1

Answer:

• Man´s father → Huntington's disease → Dd
• Man´s mother → Normal → dd
• Man´s sister → Normal → dd        
• The mother of the child is unaffected → dd
• Man → Dd → If affected → 50% or 1/2 of probability

• Man → dd → If not affected
• Child → Dd → If affected → 50% or 1/2 of probability

• Child → dd → If not affected

Explanation:  

Huntington´s disease is Autosomal and Dominant, which means that by getting only one copy of the altered gene coming from any of the parents, the receiving person will express the disease.

Let us name the dominant allele D and the recessive allele d.

• DD or Dd genotypes express the disease
• dd genotype expresses the normal phenotype.

Man´s father → Huntington's disease → Dd

Man´s mother → Normal → dd

Man´s sister → Normal → dd        

The mother of the child is unaffected → dd

If the sister is normal, dd, means that the father is heterozygous for the trait. The sister inherited a recessive allele from her mother and the other recessive allele must have been inherited from the father, who expresses the disease.  

The man has 1/2 of the chances of having the dominant allele because his mother could only pass a recessive allele (100%), and his father could pass either a recessive (50%) or a dominant allele (50%).

If the man inherited the dominant allele from his father, he is heterozygous for the trait, and he has 1/2 of the probability of providing a D allele to the child. The man´s wife is normal, dd, so she can only provide a recessive allele to the child.

Pedigree:

When drawing a pedigree, remember the symbols you need to use

• Circles represent females
• Squares represent males
• Rhombus represents not specified sex (unborn)
• Full-shaded figures represent affected individuals
• Empty figures represent unaffected individuals.  
• Generations are represented with roman numbers
• Individuals can be represented with numbers, genotypes, or names

You will find the pedigree in the attached files