Complementarity is the most common type of gene interactions. It is the situation when the final product arises only if there is at least one dominant allele in each of two (complementary) genes. Single dominant allele in one or second gene is not expressed in phenotype by any specific trait. The genotypic ratio in F2 generation is 9:7.
Inherited deafness in humans is an example of complementarity. Two genes “A” and “B” are involved in the development of normal hearing. Dominant allele in each of both genes is needed. If in the genotype of the person dominant allele is missing (in one or second or both genes), this person is deaf (fig. X.10).
X.3.3 INHIBITION
In inhibition, the expression of a dominant allele of one gene is suppressed
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With regard to genetic linkage, three different situations may be observed:
1. Genes are not in linkage. Studied gene pairs are located on different chromosomes and follow the principle of independent assortment.
2. Genes are in complete linkage. Studied gene pairs are located on one chromosomal pair. They are positioned very closely to each other and there is no probability of crossing-over between them. Genes in complete linkage are always inherited together from parents to offspring and they form a haplotype. Children produce same gametes like parents.
3. Genes are in non-complete linkage. Gene pairs are located on one chromosomal pair, but the distance between genes allows for crossing-over. After crossing-over, linkage is annihilated. Children produce, except parental types of gametes, also new, recombinant types. Recombinant gametes are produced with the lower frequency in comparison to parental ones. The degree of crossing-over between any two loci on a single chromosome is proportional to the distance between them (fig. X.13). Considering the positioning of dominant and recessive alleles of linked genes, two possibilities may occur. If two dominant alleles are linked together on one chromosome of the homologous pair and two recessive alleles are linked together on the second chromosome of the homologous pair , this arrangement is referred as cis or coupling. If, in homologous chromosomes, dominant allele is linked with recessive one, this arrangement is referred as trans or repulsion (fig.