Question

A gene is composed of two alleles. An allele can be either dominant or recessive. suppose that a husband and a wife who are both carriers of the sickle-cell anemia allele but do not have the disease, decide to have a child. because both parents are carriers of the disease, each one has a dominant normal cell allele (S), and one recessive sickle-cell allele (s). Therefore the genotype of each parent is Ss. Each parent contributes one allele to his or her offspring with each allele being equally likely.

a.List the possible genotypes of their offspring.
b. what is the probability that the offspring will have sickle cell- genotype ss?
c. what is the probability that the offspring will not have sickle cell but still be a carrier (one normal cell allele and one sickle cell allele)

Answer 1

a) SS, Ss, ss

b) 1/4

c) 1/2

Explanation:

According to this question, a gene coding for possession or not of sickle cell has two alleles. Allele "S" codes for normal cell while allele "s" codes for sickle cell. A heterozygous individual or carrier will have a "Ss" genotype.

If two carriers of the sickle cell condition decide to have a child and mate i.e. Ss × Ss, the following gametes will be produced by each parent: S and s.

a) Using these gametes (S, s) in a punnet square (see attached image), the four possible offsprings with the following genotypes will be produced: SS, Ss and ss in a GENOTYPIC RATIO 1:2:1.

b) Since offsprings with the genotype "ss" will possess the sickle cell condition/phenotype, the probability of offsprings that will have sickle cell is 1/4 i.e. 1 out of 4 possible offsprings

c) Also,carrier genotype will be "Ss", therefore, the probability that the offspring will not have sickle cell but still be a carrier (one normal cell allele and one sickle cell allele) will be 2/4 or 1/2 i.e. 1 out of 2 possible offsprings.