Inheritance

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Question 35
Medium

Usher syndrome is a rare genetic disorder. People with Usher syndrome have damage to the hair cell receptors in the cochlea (inner ear) and damage to the retina (the light-sensitive layer at the back of the eye).

The diagram below shows the inheritance of Usher syndrome in a family:

Pedigree chart showing two generations of a family. In the first generation, mother P (represented by a circle) and father Q (represented by a square) are connected by a horizontal line. Neither parent is shaded, indicating they do not have Usher syndrome. They have four offspring in the second generation, connected to them by vertical and horizontal branching lines: female R (unshaded circle), male S (shaded square), female T (shaded circle), and male U (unshaded square). The key shows: unshaded square = Male without Usher syndrome; unshaded circle = Female without Usher syndrome; shaded square = Male with Usher syndrome; shaded circle = Female with Usher syndrome.

a.

A doctor makes this statement: "The allele causing Usher syndrome in this family is recessive."

Explain why this statement is correct. Use evidence from the diagram.

[2]
b.

Mother P and father Q are expecting another baby. Complete a genetic cross to find the probability that the baby will have Usher syndrome. Use UUU for the dominant allele and uuu for the recessive allele.

[3]
c.

Person S has difficulty hearing quiet sounds and has trouble seeing in dim light. Explain why this is.

[4]

Inheritance Questions

  1. GCSE
  2. /Biology
  3. /Inheritance