IGCSE · Practice questions · Pedigrees, codominance and sex linkage

Follow the alleles through the family.

Six original Cambridge-style questions on deducing genotypes from a pedigree, ABO blood group crosses with codominant alleles and sex-linked crosses for colour blindness.

Original questions All questions on this page are original work, written in the Cambridge IGCSE style. They are not from past papers. They test the same concepts and skills the syllabus rewards.

Keep these straight

Pedigrees, codominance, sex linkage.

Pedigree: affected people first (aa for a recessive feature), then their parents and children.
Extended codominance: both alleles contribute to the phenotype; I^AI^B = group AB, I^o is recessive.
Extended sex linkage: write alleles on the X: X^B, X^b; the Y carries none.
Extended: sons get their X from their mother; colour blindness is more common in males.

Analysis

[4 marks]

A pedigree shows a feature caused by a recessive allele, d. Two unaffected parents, Ana and Ben, have three children: two unaffected and one affected. (a) State the genotype of the affected child. (b) Deduce the genotypes of Ana and Ben, explaining your answer. (c) State the possible genotypes of the unaffected children.

(a) dd. ✓
(b) Both Dd: the affected child received d from each parent. ✓
Both parents are unaffected, so each must also carry the dominant allele D. ✓
(c) DD or Dd. ✓

Recall

[2 marks]

Extended. Describe what is meant by codominance, using the ABO blood groups as an example.

Codominance is when both alleles in a heterozygous organism contribute to the phenotype. ✓
For example I^AI^B gives blood group AB, with both antigen A and antigen B on the red blood cells. ✓

Analysis

[5 marks]

Extended. A man with blood group A and a woman with blood group B have a child with blood group O. (a) State the genotype of the child. (b) Draw a genetic diagram to work out the genotypes of the parents and all the possible blood groups of their children, with the ratio.

(a) I^oI^o. ✓
(b) The child received I^o from each parent, so the parents are I^AI^o (group A) and I^BI^o (group B). ✓
Gametes: I^A or I^o from the man; I^B or I^o from the woman. ✓
Offspring: I^AI^B, I^AI^o, I^BI^o, I^oI^o. ✓
Blood groups AB, A, B and O in the ratio 1 : 1 : 1 : 1. ✓

Recall

[2 marks]

Extended. Describe what is meant by a sex-linked characteristic and name one example.

A feature in which the gene responsible is located on a sex chromosome (the X), which makes it more common in one sex than in the other. ✓
Example: red-green colour blindness. ✓

Analysis

[5 marks]

Extended. A woman with normal colour vision whose father was colour blind has children with a man with normal colour vision. Use a genetic diagram to find the probability that a son of theirs is colour blind.

The woman received X^b from her colour-blind father, and she has normal vision, so she is X^BX^b. The man is X^BY. ✓
Gametes: X^B or X^b from the mother; X^B or Y from the father. ✓
Offspring: X^BX^B, X^BX^b, X^BY, X^bY. ✓
Sons are X^BY (normal) or X^bY (colour blind). ✓
Probability that a son is colour blind = 1 in 2 (0.5). ✓

Explain

[3 marks]

Extended. Explain why a colour-blind man cannot pass colour blindness to his sons, but all his daughters will carry the allele.

The allele is on the X chromosome; the Y carries no copy of the gene. ✓
A father passes his Y chromosome to his sons, so his X^b never reaches them; a son's X comes from his mother. ✓
A father passes his only X (X^b) to every daughter, so every daughter carries the allele. ✓

Mark this once you have attempted all six and checked your working. It records a Practiced badge on the topic and adds a one-time bonus. Revealing the solutions alone does not count.