Final answer:
Clines are the gradual geographic variation in the frequency of a gene variant, such as the ABO blood group alleles - IA, IB, and i - which show codominance and vary across global populations due to evolutionary forces.
Step-by-step explanation:
Clines refer to the gradual changes in the frequency of a particular trait or gene variant - often known as alleles - across a geographic space. This type of variation results from genetic differences in populations that are spread out over different environments. One common example of a cline is seen in the body sizes of warm-blooded animals, which tend to be larger in cooler climates (a latitudinal cline) as a way to conserve body heat more efficiently.
In the case of the ABO blood group system, this too can demonstrate clinal variation. The alleles responsible for blood types A, B, and O - designated as IA, IB, and i respectively - are distributed differently among different populations around the world. For instance, blood type A might be more prevalent in one region, while blood type B is more common in another. This is because the alleles have been subjected to various evolutionary pressures, such as natural selection, genetic drift, and gene flow, which affect their distribution and frequency.
It is important to note that the IA and IB alleles show a pattern of codominance where both alleles are expressed in the phenotype, while both of these alleles are dominant over the i allele, which results in the O blood type when in homozygous form (ii). These variations in blood group alleles across different populations are an intriguing aspect of human genetic diversity and can have implications for medical practices, such as blood transfusions and disease susceptibility in different regions.