This document discusses polygenic or quantitative inheritance, where multiple genes each have a small effect on a trait, resulting in continuous variation in phenotypes. Key points: - Traits like height, weight, eye color are polygenic, with no clear boundaries between types. - The multiple factor hypothesis proposes that multiple genes, each with small effects, combine to produce quantitative variation in traits. - Characteristics of polygenic traits include each gene having an additive, cumulative effect and no dominance or epistasis between genes. Environmental factors also influence phenotypic expression. Examples of polygenic traits in humans are skin and eye color.

1. 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