Question

Many patterns of inheritance deviate considerably from the pattern of inheritance as explained by Mendel. List any four such inheritance patterns and explain any three with the help of an example each.

Answer 1

1. There were a great number of inheritance patterns that were significantly different from the pattern of heredity that Mendel had described. These deviations were brought about by a variety of factors and natural occurrences.
2. Incomplete Dominance: When experiments on peas were repeated using other traits in other plants, it was found that sometimes the F1 had a phenotype that did not resemble either of the two parents and was in between the two. The inheritance of flower colour in the dog flower (Snapdragon or Antirrhinum sp.) is a good example to understand incomplete dominance.
3. Co-dominance: In the case of co-dominance, the F1 generation resembles both parents. A good example is different types of red blood cells that determine ABO blood grouping in human beings. ABO blood groups are controlled by the gene I. The plasma membrane of the red blood cells has sugar polymers that protrude from its surface, and the kind of sugar is controlled by the gene.
4. Pleiotropy: We have so far seen the effect of a gene on a single phenotype or trait. There are, however, instances where a single gene can exhibit multiple phenotypic expressions. Such a gene is called a pleiotropic gene. The underlying mechanism of pleiotropy in most cases is the effect of a gene on metabolic pathways, which contribute towards different phenotypes. An example of this is the disease phenylketonuria, which occurs in humans. The disease is caused by a mutation in the gene that codes for the enzyme phenyl alanine hydroxylase (single gene mutation). This manifests itself through phenotypic expression characterised by mental retardation and a reduction in hair and skin pigmentation.
5. Linkage and Recombination: Morgan and his group knew that the genes were located on the X chromosome and saw quickly that when the two genes in a dihybrid cross were situated on the same chromosome, the proportion of parental gene combinations was much higher than the non-parental type. Morgan attributed this due to the physical association or linkage of the two genes and coined the term linkage to describe this physical association of genes on a chromosome and the term recombination to describe the generation of non-parental gene combinations.