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Huntington’s disease is an autosomal dominant condition caused by a mutation of a gene on chromosome 4 - HSC - SSCE Biology - Question 28 - 2019 - Paper 1
Question 28
Huntington’s disease is an autosomal dominant condition caused by a mutation of a gene on chromosome 4. It causes nerve cells to break down. Stargardt disease is an... show full transcript
Worked Solution & Example Answer:Huntington’s disease is an autosomal dominant condition caused by a mutation of a gene on chromosome 4 - HSC - SSCE Biology - Question 28 - 2019 - Paper 1
Step 1
Complete the tables, showing the TWO alleles the patient inherited from each parent.
Answer
The patient's father has one allele for Huntington’s disease (H) and one that is healthy (h), giving him the genotype Hh. The father also has one allele for Stargardt disease (r) and one healthy allele (R) which means he is Rr. Therefore, the patient's inherited alleles from the father are:
Alleles from father
H, r
The patient's mother is homozygous for both genes, meaning she has two healthy alleles. Thus, her genotype is hh for Huntington’s, and RR for Stargardt disease. The inherited alleles from the mother are:
Alleles from mother
h, R
Step 2
Add the relevant alleles to the diagram to model the production of possible gamete combinations.
Answer
Before crossing over, the patient’s homologous pair of chromosome 4 can be represented as:
Homologous pair of chromosome 4 before crossing over
H H
r r
After crossing over and separation, the chromosomes can be modeled as:
Homologous chromosomes after crossing over and separation
H r
H r
The gametes produced will be:
Gametes
H r
H r
A key for the alleles is provided:
KEY
H = Huntington’s
h = Non-Huntington’s
R = Healthy retina
r = Stargardt disease retina