In mice with genotype $A^1A^1$, position 64 in the $\alpha$-polypeptide chain contains aspartic acid. In mice with genotype $A^0A^0$, this position contains glycine. Suggest how the substitution of aspartic acid by glycine in the $\alpha$-polypeptide chain could have occurred.
The genotypes of deer mice from three separate populations, each inhabiting a different altitude, were examined. Fig. 5.1 displays the relative proportions of deer mice with aspartic acid (white sections) and glycine (black sections) at position 64 in the $\alpha$-polypeptide of haemoglobin.
Describe how altitude affects the frequency of the haemoglobin alleles in these 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 account for the difference in allele frequency in deer mice living at high altitudes and low altitudes.