People most often select mates who are similar to themselves in terms of traits that can be readily observed and discriminated. For instance, tall people usually mate with tall people. Genotypically similar people more often mate with each other.

The most prevalent type of consanguineous marriage, show that the excess risk for their offspring of having a significant birth defect ranges from 1.7-2.8%.

Consanguineous mating or inbreeding is an extreme form of positive assortative mating. It can result in a variety of homozygous recessive conditions showing up in phenotypes and a subsequent reduction in reproductive potential.

This occurs when there is a small ancestral or founding population. The potentially numerous descendants of the founders often have similar genetic makeups. This explains why rare diseases are more frequent among descendants of people who had them.

Usually closely related individuals are genetically similiar. When they mate, there is a less chance of occurrence of significantly new genetic combinations. Compared to crossbreeding, there will be reduced genetic diversity and increased homozygosity.

It is likely that most of these Native Americans are descended from a small band of closely related early immigrant founders who also shared this blood type.

Genetic drift operates independently of mutation, recombination and other evolutionary mechanisms, i.e. genetic drift can occur in a population whether or not other evolutionary processes are operating.

The significant changes in gene frequencies in small, reproductively isolated populations can occur by chance. They are totally independent of mutation, recombination and natural selection. The smaller the population, the more susceptible it is to such evolution.

The result of such catastrophes is a dramatic reduction in genetic diversity of future generations. The variability is lost at the time of the bottleneck as a result of most individuals dying without reproducing.

Japan has a population of over 125 million people who form one large potentially interbreeding society. Society was divided into two classes in Feudal Japan, the nobility and the peasants. The noble class made up roughly 12% of the population with peasants making up the rest. In such a population, genetic drift would have virtually no effect.

The liquid component of blood, i.e. plasma is the most abundant substance in blood. 

East Asians and Native Americans apparently are the only people in the world who have the Diego positive blood type. This supports the hypothesis of an East Asian origin for Native Americans.

The Rh blood factor is a dominant trait. Therefore, both DD and Dd people have the Rh antigen on the surface of their red cells which makes them Rh positive.

All the above statements characterise mother-fetus Rh incompatibility problems.

A homozygous genotype refers to the inheritance of the same allele for a trait from both parents.

This progressive disease characterized by degeneration of brain cells resulting in severe memory loss usually occurs in old age for normal people, if it occurs at all. It is far more common among Down syndrome sufferers and it occurs at an earlier age.

When genes have a different effect depending on the gender of the parent from whom they were inherited, it is referred to as genome imprinting. Diabetes, psoriasis, and Prader-Willi syndrome are other examples of this phenomenon.

Since type O blood normally lacks both A and B antigens, it will not be recognized as an alien type by the blood of anyone. As a result, anyone usually can be transfused with O blood without concern about rejection for this blood group.

The Rh system apparently is much more complex genetically since it involves 45 different antigens on the surface of red cells that are controlled by 2 closely linked genes on chromosome 1. In most cases, the ABO system is controlled by only two antigens.

The feminizing effects of Klinefelter syndrome can be diminished if boys are treated regularly with testosterone injections from the age of puberty on. As a result of this treatment, most become sufficiently ordinary in appearance to live in society without notice.

This method is used to draw out a sample of chorion tissue for cell culturing. As with amniocentesis, ultrasound monitoring helps to prevent damage to the unborn child during the sampling procedure.

DNA sequence comparison is considered to be far more reliable than one of the other forms of evidence in predicting who could or could not be a parent of a particular child.

People with Turner syndrome are females who inherit only one X chromosome--their genotype is usually written as X0.

When there are complete multiples of sets (e.g. 23 + 23 + 23), the error is known as polyploidy. When there is an addition or loss of chromosomes within a set (e.g. 23 + 22 or 23 + 24), it is known as aneuploidy. Both result in medical aberrations.

Women with Turner syndrome are essentially postmenopausal from early childhood and are sterile because their ovaries do not develop normally and they do not ovulate.