Inheritance

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Question 18
Hard

Spinocerebellar Ataxia Type 1 (SCA1) is a genetic condition caused by a dominant allele.

a.

Explain what is meant by the term dominant allele.

[2]
b.

The symptoms of SCA1 usually appear in middle age, with progressive coordination and balance problems. There is currently no cure.

Scientists have established that the severity and age of onset of SCA1 are related to the number of CAG trinucleotide repeats in the ATXN1 gene.

People can undergo a genetic test to find out if they carry the mutated allele and the exact number of CAG repeats they have.

The graph below shows the relationship between the number of CAG repeats and the age at which symptoms first develop.

A line graph with a shaded region showing the age at onset of symptoms for SCA1. The x-axis is labeled 'Number of CAG repeats' and ranges from 40 to 80 in increments of 10. The y-axis is labeled 'Age at onset of symptoms (years)' and ranges from 0 to 80 in increments of 20. There is a key: a light shaded region represents the 'Area showing the age range of patients when they first develop symptoms', and a thick dark line represents the 'Mean age of patient when they first develop symptoms'. The shaded region starts wide at 40 repeats (spanning from 35 to 65 years) and narrows as the number of repeats increases, ending very narrow at 80 repeats (spanning from 5 to 12 years). The mean line starts at approximately 48 years at 40 repeats and drops non-linearly to approximately 8 years at 80 repeats.

Discuss how useful the genetic test and the graph are for a person whose parent has been diagnosed with SCA1.

[6]
c.

A potential therapeutic approach involves using synthetic antisense oligonucleotides (ASOs) that bind specifically to the mRNA transcribed from the mutant ATXN1 gene, preventing its translation.

Explain how this drug could prevent or delay the onset of SCA1 symptoms.

[2]

Inheritance Questions

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