Alpha thalassemia is a blood condition resulting from abnormalhemoglobin production, due to loss of function of copies of eitherHBA1 or HBA2 genes. These genes are very similar, and forsimplicity’s sake we talk about having four copies of the HBA gene(two copies of each).
HBA1 and HBA2 genes are located next to each other, andtherefore are described within the same allele. For example, ααindicates that both HBA1 and HBA2 copies on an allele arefunctioning, α- indicates an allele with one of the two genesfunctioning, and -- indicates an allele with neither genefunctioning.
There are multiple versions of alpha thalassemia, which dependon how many of the four HBA copies are functioning. This isoutlined in the table below, in order of severity (most severe-> least severe):
Phenotype | Genotype |
α-thalassemia major | Loss of all 4 α-globin genes |
Hemoglobin H (HbH) disease | Loss of 3 α-globin genes |
α-thalassemia trait | Loss of 2 α-globin genes in cis (--/αα) ortrans (-α/-α) |
α-thalassemia silent carrier | Loss of 1 α-globin gene (-α/αα) |
A man who is a silent carrier has a child with a woman who hasα-thalassemia trait.
i)Â Â Â Â Â Â Â Â Â Â Â Â Â Â Assumingthe woman has mutations in cis, what are thepossible offspring phenotypes for this child? Give the probabilityfor each possible phenotype.
ii)Â Â Â Â Â Â Â Â Â Â Â Â Â Assumingthe woman has mutations in trans, what are thepossible offspring phenotypes for this child? Give the probabilityfor each possible phenotype.
