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.
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
8.
9.
10. Eye colour in humans
• X-linked polygenes
• 9 classes of eye colour
• increasing amount of melanin pigmentation
