or of isolation or both. Also called the Sewall Wright
effect and non-Darwinian evolution, it has been the
subject of considerable controversy.
Genetic drift is used to refer to various random processes that affect gene frequencies in small, relatively isolated
populations. It is also known as the Sewall Wright effect. One variety of genetic drift occurs when a small population
recently derived from a larger one expands in relative isolation. Called the founder effect, this process might occur,
for example, if a family with ten children--all of whom exhibit a particular rare, but neutral, trait--moved to a tiny
country hamlet. If all members of the family remained in the hamlet, married, and produced children who also stayed
there and produced more children, and if the hamlet remained isolated from the outside world, the trait might become
relatively common among residents of the area in a few generations.
The near absence of Blood Type B in the American Indian has been attributed to genetic drift
However, B-type blood reaches its highest frequency in some Mongoloid groups thought to be ancestral to the early
inhabitants of the New World.
An allele is an alternate
form of a gene at a given locus (location on a chromosome).
Alternate forms of a gene are known as alleles. For example, given the gene for eye color, blue eyes, hazel eyes,
and brown eyes are some of the alleles for that gene. A chromosome is an X-shaped collection of genes, and a locus
is a specific position on that chromosome. Certain genes are always found on a certain chromosome (Humans have
23 pairs of chromosomes) at a certain locus. Another example of alleles--the gene for ABO blood group has three
alleles, A, B, and O.
Allele Frequency is the commonness
of a particular allele in a given population, stated as a number, from
0 to 1, or as a percentage, from 0 to 100.
Genotypic Variations are caused by differences in number or structure of chromosomes
or by differences
in the genes carried by the chromosomes.
Eye color, body form, and disease resistance are genotypic variations. As you recall, a person's genotype is the set
of genes they carry. Typically differences in genotype are simply because one person has a unique set of alleles for
his various traits given to him by his parents. Other possible differences, which are out of the ordinary, are having a
different number or structure of chromosomes (instead of the ordinary 23 pairs of chromosomes).
Genetic variation causes people to have different