Question

Haemophilia is sex linked recessive disorder of humans. The pedigree chart given below shows the inheritance of haemophilia in one family. Study the pattern of inheritance and answer following questions-
a. Give all the genotypes of members of 4, 5, and 6 in the pedigree chart.
b. A blood test shows that the individual 14 is a carrier of haemophilia. The number 15 has recently married number 14. What is the probability that their first child will be a haemophilic male?

Answer 1

Hint: In a pedigree analysis, genotypes and phenotypes of individuals of a family can be studied collectively. The genotypes and phenotypes of future generations can be predicted by following the pattern of Mendel’s crosses.

Complete Answer:
Let‘s assume the dominant allele to be 'A' and the recessive allele 'a'.
On the basis of principles of Mendel’s crosses the genotypes can be-
a. Member 4 could be either a carrier Aa or normal aa. Member 5 will be aa (affected). Member 6 could be a carrier Aa or aa, normal. The probability of the first child being a haemophilic male is 1/4.

b. Member 14 is a carrier female with genotype Aa and number 15 is an unaffected male with genotype AA.

Additional information:
- Pedigree analysis: Pedigree charts are used to analyse an existing population. This is used for studying human genetics and the inheritance of genes in humans. Pedigree analysis is a useful tool for studying a population limited data related to progeny is available. For studying species with a long generation time, pedigree analysis is preferred. Various symbols are used to represent the aspects of a pedigree.

- Haemophilia: It is a sex linked recessive disorder linked to X chromosome. The affected individuals continue to bleed even after a minor cut as the blood cannot coagulate. Mostly males are affected as they have only one X chromosome which can express itself in absence of dominant gene, which is why it occurs less in females, females are mostly carriers of the disease. A female will be haemophilic only in a recessive homozygous condition. Defective clotting factors are produced by the defective genes. The Royal family of Europe has this disease in their bloodline. Queen Victoria was a carrier and passed on the defective gene along the progeny.

Note: “A” is used as dominant allele and “a” as the recessive to explain cross between haemophilic people. Mendel’s principles will be used to solve such questions. The symbols of pedigree analysis should be remembered.