The diagram shows the inheritance of a condition in a family.

(i) Identify the three individuals who must be heterozygous for this condition.
(ii) What are the chances that a second child of individuals 3 and 4 will be a female who
has this condition?

Answer:

14) i) 3, 4, 5    ii) E

i) 7 has inherited the disease in an autosomal recessive fashion since 4 and 5 do not have the disease. Therefore, 4 and 5 must be heterozygous.
Since 7 is affected, 3 must also be heterozygous.

ii) We now know that 3 and 4 are heterozygous.

                                     T                       t

                    T              TT                    Tt

                     t              Tt                      tt

The only way for a person to be affected by a person is if they are homozygous and only have the recessive alleles i.e. ‘tt’

Therefore, the probability of the child being affected by the condition is 1/4.

Probability of a having a girl child = 1/2

Probability of having a girl with disease = 1/2 x 1/4 = 1/8