0.3 Genetics in clasical understanding  (Page 3/6)

However, early in the twentieth century, William Bateson and Reginald Crundall Punnett, two British geneticists, observed that sometimes the expected Mendelian ratio of phenotypes did not occur. Their best explanation was that in some manner the phenotypic classes, the alleles, were coupled, and so did not sort independently into gametes. Proof of their explanation was provided by Thomas Hunt Morgan, using Drosophila eye color as the examined trait.

Morgan observed that test crosses between mutants in eye color and wing development deviated from the expected Mendelian 1:1:1:1 ratio for independent assortment. The observed ratio was, rather, consistent with the non-independent segregation of two genes that were close to each other on the same chromosome.

Linked genes do not observe the genotypic or phenotypic relationships predicted by Mendelian crosses that assume independent assortment of chromosomes and genes. In a cross the parental generation is designated P1 and the first generation of offspring are designated F1(first filial generation), and the offspring resulting from the fertilization between individuals of the F1 generation are called the F2 (second filial generation). When the F1 and F2 ratios deviate from the predicted Mendelian ratios, this is evidence of gene linkage.

The linkage of genes is used to generate so-called linkage maps which give a measure of the distance between genes on a chromosome. The linkage map technique, which is based on the use of the percentage of recombinants, in which crossing over of DNA and expression of traits due to gene linkage has occurred, was devised in 1911 by Alfred Henry Sturtevant, an undergraduate student of Morgan's. The technique remains in use today as a means of producing an index of the distance between two genes.

To see concrete experiments for analyzing as well as for applying gene linkage, click over to MITOPENCOURSEWARE ( PDF ) for the complementaton test and gene function; click to ( PDF ) for tests of gene position, starting with the position of genes on chromosomes in general; click to ( PDF ) for experiments to map genes relative to one another on sex chromosomes, and, finally, click to ( PDF ) for mapping genes on autosomes by test-cross and other measures.

Lecture 31. genetic complex traits

Genetic complex traits refer to those traits which are determined by either many genes or vice versa a single gene influences multiple phenotypic traits .

This lecture note will present the main types of the traits of the kind-- Pleiotropy, Polygenic Inheritance, Genetic Heterogeneity, Twinnings and Siblings.

Pleiotropy