Question

part 1: analyzing pedigrees
date:
period: 1st

use the pedigree for huntington’s disease above to answer questions 1-6.

1. huntington’s disease is an autosomal dominant disease. given this information:

a. what are the possible genotypes of someone who has huntington’s disease? ____ or ____
b. what must be the genotype of someone who does not have huntington’s disease? ______

1. list the members of the family that have huntington’s. ______
2. how many children did individuals i-1 and i-2 have? ______
3. how is individual i-1 related to individual iii-5? ______
4. how is individual ii-4 related to individual ii-5? ______
5. what is the genotype of individual ii-1? ____ what about individual ii-2? ____ explain how you know:

______
______

Explanation:

Question 1a

Step1: Recall autosomal dominant inheritance

In autosomal dominant inheritance, a dominant allele (H) causes the disease. So a person with the disease can have either one dominant allele (heterozygous) or two dominant alleles (homozygous dominant), because the dominant allele is sufficient to express the disease.

Step2: Determine possible genotypes

For a dominant disease, the possible genotypes for someone with the disease are $HH$ (homozygous dominant) or $Hh$ (heterozygous), since having at least one $H$ allele will result in the disease phenotype.

Step1: Recall normal genotype in dominant inheritance

In autosomal dominant inheritance, the normal (no disease) phenotype occurs when there are two recessive alleles, because the dominant allele (H) is not present to cause the disease. The recessive allele is $h$, so the genotype must be $hh$ (homozygous recessive).

Step2: Confirm the genotype

Since the disease is dominant, only individuals with two recessive alleles (hh) will not have the disease, as they lack the dominant H allele.

Step1: Identify affected individuals by symbol

Red circles (females) and red squares (males) represent Huntington's disease. Looking at the pedigree:

• Generation I: I - 1 (red square)
• Generation II: II - 2 (red circle), II - 3 (red square), II - 7 (red circle)
• Generation III: III - 3 (red circle)

Step2: List all affected members

So the members with Huntington's are I - 1, II - 2, II - 3, II - 7, III - 3.

Answer:

$HH$ or $Hh$

Question 1b