12 Biology

INHERITANCE AND VARIATION

Incomplete Dominance

Sometimes, a dominant trait may not completely dominate over a recessive trait. This condition is called incomplete dominance. We know that in a diploid organism, there are two copies of each gene, i.e. as a pair of alleles. One of the genes may be different due to some changes it has undergone.

To understand this, let us take an example of a gene that contains the information for producing an enzyme. There are two copies of this gene. Let us assume that the normal allele produces the normal enzyme. The modified allele could be responsible for production of following:

  • The normal or less efficient enzyme, or
  • A non-functional enzyme, or
  • No enzyme at all.




In the first case, the modified allele is same as the unmodified allele. It will produce the same phenotype, i.e. it will result in the transformation of substrate S. But, if the allele produces a non-functional enzyme or no enzyme, the phenotype may be affected. The unmodified allele is the dominant allele, while the modified allele is generally the recessive allele. The recessive trait is seen due to non-functional enzyme or because no enzyme is produced.

The inheritance of flower colour in the dog flower (Snapdragon or Antirrhinum sp.) is a good example to understand incomplete dominance. When true breeding red flowered (RR) and true breeding white flowered (rr) plants were crossed, all plants in F2 generation produced pink flowers (Rr). When plants of F2 generation were self pollinated, the plants of F2 generation produced Red (RR), pink (Rr) and white (rr) flowers in the ratio 1:2:1. This happened because the red colour could not dominate completely over white colour. Hence, pink flowers were produced.

Co-dominance

To understand the concept of co-dominance, let us take example of ABO blood groupings in human beings. The plasma membrane of RBCs has sugar polymers that protrude from its surface and the kind of sugar is controlled by the gene. The gene (I) has three alleles; IA, IB and i. The alleles IA and IB produce a slightly different form of the sugar while the allele i does not produce a sugar. IA and IB are completely dominant over i. This means that when IA and i are present, only IA expresses. Similarly, when IB and i are present, only IB expresses. But when both IA and IB are present, both of them express their own types of sugars; because of co-dominance. These three alleles give six different types of combinations; resulting in different blood groups. This can be shown by following table.

Genetic Basis of Blood Groups in Humans
Allele from parent 1Allele from parent 2Genotype of offspringBlood Group of offspring
IAIAIAIAA
IAIBIAIBAB
IAiIAiA
IBIAIAIBAB
IBIBIBIBB
IBiIBiB
iiiiO



INHERITANCE OF TWO GENES

Dihybrid Cross: When two pairs of characters are studied in the cross, it is called dihybrid cross.

Mendel selected yellow colour (YY) and green colour (yy) as seed colour. He further selected round seeds (RR) and wrinkled seeds (rr) for seed texture. In this case, yellow colour is dominant over green colour, while round texture is dominant over wrinkled texture.

F1 Generation: When gametes RY and ry were crossed, all plants in F1 generation produced yellow and wrinkled seeds (RrYy). The genotype was heterozygous in these plants. Yellow colour and round texture showed dominance.

When plants of F1 generation were allowed to self pollinate, the result could be shown by following Punette Square.

GAMETESRRrYRyry
RYRRYYRrYyRRYyRrYy
rYRrYYrrYYRrYyrrYy
RyRRYyRrYyRRyyRryy
ryRrYyrrYyRryyrryy

The plants of F2 generation produced 3 types of seeds, i.e. round yellow, wrinkled yellow, round green and wrinkled green in ratio 9:3:3:1. Based on this observation, Mendel proposed the Law of Independent Assortment.

Law of Independent Assortment: When two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters.



Chromosomal Theory of Inheritance: Chromosomes as well as genes occur in pairs. The two alleles of a gene pair are located on homologous sites on homologous chromosomes. Sutton and Boveri argued that the pairing and separation of a pair of chromosomes would lead to the segregation of a pair of factors they carried. Sutton united the knowledge of chromosomal segregation and Mendelian principles and termed it the Chromosomal Theory of Inheritance.

Linkage: The physical association of genes on a chromosome is called linkage.

Recombination: Combination of non-parental genes is called recombination.

Morgan carried out several dihybrid crosses in Drosophila to study genes that were sex-linked. Morgan hybridized yellow-bodied, white-eyed females to brown-bodied, red-eyed males. He intercrossed the F1 progeny. He observed that the two genes did not segregate independently of each other, and the F2 ratio deviated very significantly from the 9:3:3:1 ratio. Morgan was aware that the genes were located on the X chromosome. He could see that when the two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combinations were much higher than the non-parental gene combinations. This was attributed to the physical association or linkage of the two genes. Morgan also found that even when genes were grouped on the same chromosome, some genes were tightly linked, while others were loosely linked. The tightly linked genes showed very low recombination, while the loosely linked genes showed higher recombination. For example; genes for white and yellow colours were tightly linked and showed only 1.3% recombination. On the other hand, genes for white and miniature wing showed 37.2% recombination because they were loosely linked.