In mice with genotype $A^1A^1$, the amino acid at position 64 in the $\alpha$-polypeptide chain is aspartic acid. In mice with genotype $A^0A^0$, the amino acid at this position is glycine. Suggest how the change from aspartic acid to glycine in the $\alpha$-polypeptide chain could have occurred.
Genotypes from three deer-mouse populations, each inhabiting a different altitude, were examined. Fig. 5.1 presents the relative proportions of deer mice with aspartic acid (white areas) and glycine (black areas) at position 64 in the $\alpha$-polypeptide of their haemoglobin.
Describe how altitude affects the frequencies of the haemoglobin alleles in deer mouse populations.
The partial pressure of oxygen is relatively low at high altitudes. Haemoglobin containing glycine at position 64 in the $\alpha$-polypeptide chain has a higher affinity for oxygen than haemoglobin with aspartic acid at this position. Suggest how natural selection could explain the difference in allele frequency in deer mice living at high altitudes and low altitudes.