Principles of Inheritance and Variation — BIOLOGY STD 12 Science — Question

1. Pleiotropy: A single gene that has an effect on the expression of two or more phonotypic traits is said to have a pleiotropic effect on the traits. For example, testosterone controls the development of what are referred to as secondary sexual characteristics (e.g., a hon’s mane), but testosterone also relates  behavioral traits like aggraession/Thus, a gene that controls the level of testosterone would have a pleiotropic iffect on the expression of many secondary sexual traitjs which are morphological, as well as behavioral. The concept of pleiotropy is intimately related to the concept of trade-off (Steams- 1976). Pleiotropy describes the proximate genetic gene controls the expression of two or more traits and those traits are related to a fitness trade-off, then we have identified the proximate source of the trade-off.

Polygenic: If two or more genes are responsible for a single trait, the phenotypic. 

2. If the gene for eye color is on the X chromosome, the red eyed male in the second cross will pass his red eyed X to only his daughters, who in turn received only a recessive white- carrying X from their mother. Thus all females had trend eyes like their father. Since the male fruit fly passes only the Y to his sons, their eye color is determined entirely by the single X chromosome they receive from their mother (in this case white). Thus all the males in the second cross were white eyed.

These experiments introduced the concept of sex-linkage, the occurrence of genes on that part of the X that lack a corresponding location on the Y. Sex- linked recessives (such as white eyes in fruit flies, hemophilia, baldness, and colorblindness in humans) occur more commonly in males, since there is no chance of them being heterozygous. Such condition is termedhemizygous.