The Most Important Patterns of Inheritance in Human Genetics are:
1. Dominant and Recessive Character:
In the Mendel’s experiment it was seen that the hybrid plants showed only one of the pair of contrasted characters. Intermediate forms did not appear. In the generation all the plants were tall. For this reason, Mendel termed tallness the dominant and darkness the recessive character.
The recessive characters are present in the F, generation but remained suppressed. Thus the character that expresses itself in the F, generation is known as dominant and the character that remains suppressed (or apparently suppressed) is known as recessive.
2. Segregation of Unit Characters:
In the Mendel’s experiment, the cross-bred plants of the generation were allowed to fertilize themselves; and it was found that their offspring’s exhibited both tall and dwarf plants, showing on the average three dominants to one recessive.
Thus though the dwarf characters disappeared for the time being, it was not lost, or even permanently altered. The dwarf character, the recessive, had turned up again, and when such plants were allowed to self-fertilize they produced pure recessive only.
The hereditary unit (such units are called genes) tending to produce dwarfness had not been altered by its association in the hybrid with the dominant gene for tallness. This proves that characters are units by themselves and are independent of one another.
3. Contrasting Characters and Alleles:
A gamete may contain one of the two contrasting characters or allelomorphs. The genes occupying] the same place or locus in homologous chromosomes are called allelomorphs or, more frequently, alleles.
Thus in the previous experiment a given gamete carried gene of tallness or dwarfness, not both. The two were manually exclusive so far as the gametes are concerned. It must be pure for one or the other of such a pair. This is known as the purity of gametes, which forms an essential part of Mendel’s law.
4. Genotypes and Phenotypes:
When two gametes fuse together the resulting zygote had both the genes. Thus in the F, generation in the zygotes, genes for both tallness and dwarfness were present, although tallness only expressed itself. But ultimately the characters were segregated when two gametes were formed, as is evident by the offspring of the F2generation, where segregation of characters took place in the proportion of 1: 2: 1, i.e., one-fourth pure tails, half impure tails and one fourth pure dwarfs.
The above 1:2:1 ratio is based on the genotypes of the plants. By the term Genotype, the total gene component in an organism is meant, while phenotype is the external appearance of an organism resulting from the interaction of its genotype and its environment.
Thus the ratio 3: 1 is based on the phenotypes of the plants. Though the genotype of the impure tall plants, are quite different from that of the pure tall plants, they are similar to one another in respect of their external characters or in other words, phenotypic ally they are same.
5. Multiple Alleles:
A T (D) plant is said to be in heterozygous condition since it possesses two different alleles—D for dwarfness and T for tallness; while D and T plants are homozygous plants, as in both the cases two alleles are alike. D plants have two genes, both being responsible for dwarfness. Similarly, a T plant has two genes of tallness.
(i) Multiple Allelism.
(ii) Histocompatibility alleles