Q. Choose the correct option that a single gene product may produce more than one effect in
Solution:
The Mendelian concept of a gene controlling a single character has also expanded to take into account genes which affect several characters simultaneously (pleiotropy). It means in pleiotropy, a single gene product may produce more than one effect or control several phenotypes depending on its position. The basis of pleiotropy is the interrelationship between the metabolic pathways that may contribute towards different phenotypes.
Followings are some examples of pleiotropy:
- In phenylketonuria, mutation of the gene that codes for the enzyme phenylalanine hydroxylase results in a phenotypic expression characterized by mental retardation and a reduction in hair and skin pigmentation.
- In Drosophila, white eye mutation leads to depigmentation in many other parts of the body, giving a pleiotropic effect.
- In transgenic organisms, the introduced gene can produce different effects depending on where the gene has introgressed.
- The gene controlling starch synthesis in garden pea. It has two alleles, B and b. Starch synthesis in BB homozygotes is efficient and therefore large starch grains are produced. In bb homozygotes, starch synthesis is less efficient, so that it produces small-sized starch grains. After maturation of seeds, BB seeds are round and bb seeds are wrinkled. Heterozygotes Bb form round seeds but the starch grains are of intermediate size. Now it is clear that if starch synthesis is considered, Bb seeds show incomplete dominance but if seed shape is considered, B allele is dominant and b is recessive.
The criss-cross inheritance is the transmission of a gene from mother to son or father to daughter, for example, X-chromosome linkage.
When one characteristic is controlled by two or more genes, it is referred to as the polygenic inheritance. Often these genes are large in quantity but small in their effect. Examples of human polygenic inheritance are height, skin color, etc.
Quantitative inheritance is the combined result when many factors combine to result in a distinctive trait. An example of a result of quantitative inheritance is the height of humans.
Followings are some examples of pleiotropy:
- In phenylketonuria, mutation of the gene that codes for the enzyme phenylalanine hydroxylase results in a phenotypic expression characterized by mental retardation and a reduction in hair and skin pigmentation.
- In Drosophila, white eye mutation leads to depigmentation in many other parts of the body, giving a pleiotropic effect.
- In transgenic organisms, the introduced gene can produce different effects depending on where the gene has introgressed.
- The gene controlling starch synthesis in garden pea. It has two alleles, B and b. Starch synthesis in BB homozygotes is efficient and therefore large starch grains are produced. In bb homozygotes, starch synthesis is less efficient, so that it produces small-sized starch grains. After maturation of seeds, BB seeds are round and bb seeds are wrinkled. Heterozygotes Bb form round seeds but the starch grains are of intermediate size. Now it is clear that if starch synthesis is considered, Bb seeds show incomplete dominance but if seed shape is considered, B allele is dominant and b is recessive.
When one characteristic is controlled by two or more genes, it is referred to as the polygenic inheritance. Often these genes are large in quantity but small in their effect. Examples of human polygenic inheritance are height, skin color, etc.
Quantitative inheritance is the combined result when many factors combine to result in a distinctive trait. An example of a result of quantitative inheritance is the height of humans.