Polygenic Inheritance and Genetic Variation Explained

Polygenic Inheritance vs. Monogenic Inheritance

Polygenic inheritance refers to the kind of inheritance in which a trait is produced from the cumulative effects of many genes. This is in contrast to monogenic inheritance, wherein a trait results from the expression of one gene (or one gene pair). In humans, height, weight, and skin color are examples of polygenic inheritance, which does not follow a Mendelian pattern of inheritance.

Linkage Groups, Sex Chromosomes, and Autosomal Chromosomes

• Linkage group: A pair or set of genes on a chromosome that tend to be inherited together.
• Sex chromosomes are the ones that determine your gender. These are X and Y (XX in females, XY in males).
• Autosomal chromosomes are the remaining chromosomes that are not sex chromosomes. There are 22 pairs of these in humans.

This means that there is a total of 23 pairs of chromosomes in humans (22 autosomal pairs + 1 sex chromosome pair = 23 pairs of chromosomes).

Independent Assortment and Dihybrid Crosses

During metaphase I of meiosis, the homologous pairs of chromosomes align along the equator. The orientation of the chromosomes is random. This means that when the pairs of homologous chromosomes move to opposite poles during anaphase I, either chromosome can end up at either pole. This depends on which way the pair is facing (occurs randomly). Also, whichever way the pair is facing does not affect which way the other homologous chromosome pairs are facing. This is known as independent orientation and forms the basis of Mendel's law of independent assortment. Unlinked genes are found on different chromosomes, so when the homologous chromosome pairs separate, it allows the formation of daughter cells with random assortments of chromosomes and alleles.

A dihybrid cross is a cross between first-generation offspring of two individuals who have two different characteristics. These two characteristics are controlled by two genes. Allele pairs separate independently during gamete formation, which means that the transmission of traits to offspring is independent of one another.

We can use dihybrid crosses to calculate and predict the genotypic and phenotypic ratio of offspring involving unlinked autosomal genes. For example, let's say we cross two cats with two different characteristics, such as fur color and fur length.

Processes Resulting in Infinite Genetic Variety

Two processes result in the infinite genetic variety in gametes. These are crossing over in prophase I and the random orientation of chromosomes in metaphase I.

Crossing Over

Crossing over is important for genetic variety as it allows the exchange of genetic material between the maternal and paternal chromosomes. This forms chromatids with new combinations of alleles (recombination of linked genes). The chromatids that have a combination of alleles different from that of either parent are called recombinants. It is also important to note that crossing over occurs at a random point, and more than one chiasma can form per homologous pair. This means that meiosis can result in almost an infinite amount of genetic variety.

Random Orientation

The random orientation of homologous chromosomes at the equator in metaphase I also plays a vital role in genetic variety. Since the homologous pairs of chromosomes are orientated randomly at the equator, either maternal or paternal homologue can orient towards either pole. The number of possible orientations is equal to 2 raised to the power of the number of chromosome pairs. For example, for a haploid number of n, 2ⁿ is the number of possible outcomes. Humans have a haploid number of 23. 2²³ gives a value of over 8 million. This means that there are over 8 million possible combinations just through the random orientation of the homologous chromosomes. If we add the effects of crossing over, the number of combinations increases even further. Therefore, these two processes allow infinite genetic variety in gametes.