Brooke B. answered • 03/27/20
Brooke B. - Medical Student - Biology & Physiology Tutor
A genotype refers to the set of alleles that a person has inherited from both parents, whereas a haplotype refers to the set of alleles that are inherited from just one parent.
HLA antigens show co-dominant expression, so all of the inherited alleles will show up as antigens in the phenotype.
For the mother:
She is showing expression of HLA-A11 and HLA-A36 antigens, so her HLA-A genotype is A11, A36. She is only showing expression of HLA-B8 antigen, but we know that she must have two copies of the HLA-B gene. Therefore, she must be homozygous (have two copies of the same allele) for the B8 allele and her HLA-B genotype is B8, B8. Her full genotype is A11, A36, B8, B8.
Her haplotype is determined by figuring out which set of A and B alleles she inherited from each parent. Since she inherited a B8 allele from each of her parents, A11 from one of her parents, and A36 from the other parent, we know that her haplotypes are A11, B8 and A36, B8.
For the child:
The child is showing expression of two HLA-A antigens and two HLA-B antigens, so the child is heterozygous (has two different alleles) for the HLA-A & B genes. Based on the phenotype, the child's genotype is A3, A11, B5, B8.
To determine the haplotype, we look at the mother. The child could have inherited either the A11, B8 haplotype or the A36, B8 haplotype. Since the child does not carry the A36 allele we know that the haplotype inherited from the mother is A11, B8. From the remaining alleles, we now know that the haplotype inherited from the child's father must be A3, B5.
For the father:
Similar to the mother, the father is expressing two different HLA-A antigens, A1 and A3, and only one HLA-B antigen, B5. This means that the father is heterozygous at the HLA-A gene with a genotype of A1, A3, and homozygous at the HLA-B gene with a genotype of B5, B5. His full genotype is A1, A3, B5, B5.
Again, similar to the mother, the father has inherited a copy of the B5 allele from each parent, A1 from one parent, and A3 from the other parent. This means that his haplotypes are A1, B5 and A3, B5.
Since the paternal haplotype of the child (A3, B5) matches one of the father's haplotypes, we know that the father can be the biological father of the child.
