Polygenic Traits

Definition

Some traits are determined by the combined effect of two or more pairs of alleles. These traits are called polygenic traits.

Each pair of alleles adds something to the resulting phenotype.

Other names for polygenic traits are multi-factorial traits, or quantitative traits.

Polygenic traits are continuous

Because so many alleles contribute to the final phenotype, a variety of phenotypes can occur!

For example, height is a polygenic trait.

Polygenic Traits are Continuos

When dealing with polygenic traits that are only controlled by two pairs of alleles, we can complete Punnett squares to determine the genotypes and phenotypes of the F1 generation.

Examples of Polygenic Traits

Pepper Color

Gene 1: R=red r=yellow

Gene 2: Y=absence of chlorophyll (no green) y=presence of chlorophyll (green)

Pepper Color

Possible genotypes:

R-/Y- : red (red/no chlorophyll)R-/yy : brown/orange

(red/chlorophyll)rr/Y- : yellow (yellow/no chlorophyll)rr/yy : green (yellow/chlorophyll)

Pepper Color

Try crossing a brown pepper (RRyy) with a yellow pepper (rrYY).

Which trait will your offspring (F1 generation) produce?

What traits are produced when you cross two of the peppers found in the F1 generation?

More Polygenic Traits

Chicken Combs:

There are 4 different comb shapes that can appear in a chicken.

Comb shape is controlled by two genes found on two different pairs of chromosomes.Gene 1: RGene 2: P

Chicken Comb Example:

4 different phenotypes result:

Rose Combs (R-pp)Walnut Combs (R-P-)Pea Combs (rrP-)Single Combs (rrpp)

Try these questions:

Indicate the phenotypes of the parents and give the genotypic and phenotypic ratios of their offspring for the following crosses: (Refer to your phenotype chart for chickens)

rrPP x RRpp

RrPp x RrPp

Another example of a polygenic trait:

Hair Color Hair color is controlled by alleles on

chromosomes 3, 6, 10, and 18. The more dominant alleles that appear

in the genotype, the darker the hair!

Polygenics Problem

The height of plants is controlled by 4 pairs of alleles. Alleles A, B, and C contribute 3 cm to the plant's height. Alleles that are recessive do not contribute to the height. Gene L is always found in a homozygous dominant condition and always contributes 40 cm to the height.

a) What would be the height of a plant with the genotype AABBCCLL?

b) What would be the height of a plant with a genotype aabbccLL?

c) What would be the height of the offspring produced from a cross between the plants in a) and b)?

d) What would be the heights of the offspring produced from a cross between AaBbCcLL and AaBbCcLL? (now there is a challenge!)

Let’s Review:

Sex linked traits- X linked recessive- X linked dominant- y linkedSex influenced traitsPolygenicsEpistasisPick up a piece of flip chart paper and a

marker for your table. Summarize the new genetics topics.