Qunatative inheritance by iqra aslam
- 2. . • 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
- 3. 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.
- 4. Examples • Height • Weight • Eye colour • Yield (bu/A) in crops • Growth rate in farm animals • IQ
- 5. 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.
- 6. 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.
- 7. 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
- 10. Eye colour in humans • X-linked polygenes • 9 classes of eye colour • increasing amount of melanin pigmentation