ByIQRA ASLAM

Quantative inheritance

.

• pattern of inheritance ……..single phenotypic trait is governed by more than one pair of genes.

• formation of many possible genotypes • wide range of phenotypes. • trait is called polygenic or multifactor trait and

metric traits • the pattern of inheritance is called polygenic or

quantitative inheritance and cumulative gene action

Quantitative inheritance

Cont….

• do not show clear cut differences between individuals

• forms a spectrum of phenotypes which blend• imperceptivity from one type to another to

cause continuous variations.

Examples

• Height• Weight• Eye colour• Yield (bu/A) in crops• Growth rate in farm animals • IQ

Multiple Factor Hypothesis

• Each gene with a small effect along with a few or many other genes exerting a slight effect on the phenotype are segregating to produce quantitative variation

• contributing, effective or active alleles• non-contributing, non-effectiveor null alleles.

CHARACTERISTICS OF MULTIPLE GENES

• Each contributing allele in the series of multiple genes produces an equal effect.

• Effects of each contributing allele are cumulative or additive.• There is no dominance, rather, there exist pairs of contributing

and non-contributing alleles.• There is no epistasis (masking of the phenotypes) among genes

at different loci.• There is no linkage involved.• The environmental conditions have considerable effect on the

phenotypic expression of poly- genes for the quantitative traits.

Skin colour • Davenport (1913) in Jamaica.• two pairs of genes, A-a and B-b cause the difference in

skin pigmentation between negro and caucasian people. • genes were found to affect the character in additive

fashion.• a true negro has four dominant genes, AABB, • and a white has four recessive genes aabb. • The F1 offspring will have intermediate skin colour

(mulatto). • mulattoes produces a wide variety of skin colour in the

offspring

Eye colour in humans

• X-linked polygenes• 9 classes of eye colour• increasing amount of melanin pigmentation

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