The reason for deviation from Mendel's dihybrid cross in T.H. Morgan's experiment is ________.

Concept:

• T. H. Morgan experimented with Drosophila melanogaster to establish that the basis of variation was caused by sexual reproduction.
• He further showed that when 2 genes in a dihybrid cross were located on the same chromosome, the proportion of parental gene combinations were much higher than the non-parental type.
• This was attributed to the linkage of genes.
• Morgan found some genes to be tightly linked and some to be loosely linked.

Important Points

• Morgan perfortmed dihybrid crosses between yellow-bodied, white-eyed females to brown-bodied, red-eyed males of Drosophila.
• He found that the F2 ratio differed significantly from the Mendelian ratio 9:3:3:1
• This was because many genes were not assorting independently.
• He attributed this phenomenon to the physical association between two genes and named it 'Linkage'.
• Physical association means that they lie very close to each other and the chance of recombination between them is very less.
• Here, the gene for body colour (yellow or brown) and the gene for eye colour (red or white) are close together.

Additional Information

• Pleiotropy - is the phenomenon of a single gene influencing two or more distinct phenotypic traits. Example - Phenylketonuria.
• Polygenic Inheritance - refers to the inheritance type where one character is controlled by more than one gene. Example - Human skin colour.