The monogenic herediting. The polygenic heredity

As it was said above, monogenic heredity is heredity of traits controlled by one gene. The main principles of monogenic heredities were discovered by G. Mendel; perhaps by his hybridologic method. The essence of such a method is in following:

1. We need to conduct analysis of alternative, contrast trait pairs in several generations of parents having these contrast traits. In each generation, we need to count only definite trait pair ignoring other differences between crossed organisms.

2. We need to count hybrids in line of following generation.
3. We need to use personal analysis of offspring for each hybrid organism.

The cross in which parents are analyzed by one alternative traits pair is called monohybrid cross, by two pairs - dihybrid cross and by many pairs - polyhybrid cross.

To write a scheme of cross it is necessary to know some useful signs. The female organism is placed on a first place; male is placed on a second. Crossing is pointed by letter “x”. Parents are putted on a first line and are pointed as “P” generation (from Latin parentis - parent). The gametes, which are produced by parents, are putted in second line. The offspring are putted in a third line. They are labeled as the F1 generation (from Latin filia - daughter). Index is used for representing generation number.

The hybrids of F1 have only one trait expressed. Second one is suppressed. This is an essence of First Law of Heredity. It can be formulated by this way: In a cross between homozygous-dominant am homozygous- recessive individuals, all of the F1 progeny will be heterozygous; they will all resemble the homozygous dominant parent in their phenotype. The First Law was also named as Law of dominating (pic 8.1a).

Ріс. 8.1. The scheme of crossing and cytological basements of First (A) and Second Mendel’s Laws

Having analyzed F2 generation hybrids Mendel formulated Second Law of Heredity or Law of Segregation: in crossing two heterozygous individuals analyzed by one alternative traits pair, we can predict phenotypic ratio 3:1 and genotypic ratio 1:2:1 (pic8.1b). The outcome of such cross can be illustrated by a Punnett square, suggested by English geneticist Reginald Crundall Punnett.

To explain results of 2nd Mendel’s Law W. Batson suggested a thesis of “gametes purity”. It can be formulated by this way: genes in gametes of hybrids are discrete (pure) and not blended. Such thesis and Mendel’s Laws are the best illustration of philosophic categories “cause and effect”. The cause why traits are not blended is that genes for these traits are in different homologues chromosomes. These chromosomes in meiosis come to different gametes.

To analyze a genotype of individual with dominant phenotype we can use a testcross. It is because of individual with dominant phenotype may be either homozygous or heterozygous. In a testcross, analyzing individuals are crossed with homozygous recessive one. If all offspring are the same, it is homozygous dominant individual. If it will be 1:1 ratio among offsprings, it is heterozygous dominant individual.

It was founded in monohybrid crosses that numerous traits have a phenotypic ratio in F2 generation 3:1. To perform dihybrid cross Mendel took homozygous organisms having two pairs of alternative traits. The hybrids of first generation look similar to their dominant parents. Analyzing hybrids of F2 generation, it was founded that independent assortment of different traits occurs. Such event was named Third Law of Heredity. It states that genes located on different chromosomes assort independently of one another. To make cross scheme easy to write we may use so called phenotypic radical - it is dominant genes of organism, which determines it phenotype. For Third Law it will be the following: 9A-B-: 3A-bb: 3aaB-: 1aabb.

Each trait pair gives phenotypic ratio 3:1 in F2, which is provided by independent assortment of homologues chromosomes in meiosis. In polyhybrid cross, the acquired ratio of hybrids in F2 can be described by formula (3+1 )*n, where “n” - is number of alternative traits pairs.

As every natural law, Mendel’s laws may work only in definite conditions, which are:
1. The same probability of all kinds of gamete formation by all hybrids while monohybrids cross.
2. The same probability of all possible gametes combinations while fertilization.

3. The same survival rate of zygotes of any genotype.
4. Full trait expression independently from development conditions.
5. Gene location on different chromosomes in dihybrid and polyhybrid crosses.

6. The same probability of all kinds of gamete formation on a basis of independent assortment of non homologous chromosomes in meiosis while dihybrid and polyhybrid cross.

As it was said above the main mechanism providing traits splitting in hybrid’s generation is meiosis. It provides independent assortment of chromosomes during gametes formation. That means splitting occurs in haploid gametes, on a level of genes and chromosomes, but it is analyzed in diploid organisms on a level of traits. These two moments are divided by long period of time. During such period many environmental factors may act on gametes and developing organisms.

That’s why some deviations may occur in real traits ratio. Pointed above conditions bring an element of probability to such ratio. Therefore, to analyze it, we need to use several statistic methods which allow to prove inherited theoretical ratio principle or to deny it. One of them is X2-method. Using it, we can determine is it deviation occasional or regular.

Analyzing patterns of heredity in garden pea Mendel worked with several traits pairs. But human has thousands of traits which follow Mendel’s Laws of Heredity. They are hair color, ear color, nose shape, teeth shape, finger shape, and so on. The definite knowledge of traits and their description are the aims of medical genetics. Many hereditary diseases follow Mendel’s Laws of Heredity. Among them are achondroplasia, diabetes insipidus, albinism, pancreas fibrosis, syndactilia, glaucoma, hemophilia, and so on. In 1970 American geneticists V. Mac Quisic was first to publish catalog of hereditary human traits. Since, it has being updated every year. Thus, if in 1958 it was known only 412 hereditary human traits, in 1978-2511, in 1981-3217.

The polygenic heredity. All previously discussed types of gene relation concerned alternative traits. However, such traits as weight, pigmentation level and etc., are hard to divide on phenotypic classes. They are often called quantitative traits. Each of them is formatted under influence of several genes or polygenes. This event was named polygenic heredity or polymeria. And such genes are called polymeric genes. All polymeric genes act similarly in trait development. For example in com and oats the seeds color is determined by several genes. The level of trait expression depends on number of dominant polymeric genes that means on gene dose.

The human height is determined by interaction of three allelic gene pairs, using principle of cumulative polymeria: A and a, В and b, C and c. Individuals with genotype aabbcc have a smallest height (around 150 cm), but individuals with genotype AABBCC have highest height (around 180 cm). Heterozygous height will depend on dominant genes number.

There are four dominant genes PI, P2, P3 and P4, which are presented in double dose. They are responsible for the integuments pigmentation intensity. If all genes in genotype are dominant, skin pigmentation is maximal like in native Africans (P1P1P2P2P3P3P4P4). If all genes are recessive, skin pigmentation is minimal like in European Caucasians (p1p1p2p2p3p3p4p4). Mulatto’s pigmentation depends on dominant genes number.

Polygenic heredities are directed by following rules:
- Variations of quantitative traits depend on dominant genes number of polymeric genes.
- The measurement of traits diversity is amplitude of traits variation.
- The limits of variation of quantitative traits are under genetic control.
- The amplitude of traits variation corresponds with polygenes number in species genotype. The more polygenes are in genotype, the larger amplitude of traits variation the species has.