10.2 Inheritance

Polygenic inheritance

is where a heritable characteristic is influenced by more than one gene. This usually leads to continuous variation. Examples in humans could be skin colour and height.

Multiple Gene (Polygenic) Inheritance of skin colour

If we assume that skin colour is influenced by only three alleles. In fact it is controlled by more than three. If these alleles have two forms a dominant and a recessive this then gives these alleles.

A and a B and b C and c. The dominant forms control how much melanin is present in the peoples skin. So a person with AABBCC genotype will have very dark skin and a person with aabbcc will have extremely light skin. The more capital letters in the genotype then the darker the phenotype skin colour. This then gives a range of shades of skin colour.

The table below shows all the possible offspring of two people with these genotypes and (AaBbCc x AaBbCc). These people have medium dark skin.

image

In reality, with more alleles influencing skin colour and some being codominant, we have effectively a continuous range of skin colours in the human population.

Another interesting example is the inheritance of the Rhesus (Rh) blood groups.

There are three alleles all found on different loci on chromosome pair 1. There are alleles for the C and c type antigen, D type and E and e type.

We talk about the phenotype being Rhesus positive Rh+ or Rhesus negative Rh-. Actually this only refers to the D type antigen which is very important in blood immunity and transfusions in particular. The lower case d refers to the absence of the D type antigen whereas the lower c and e are the presence of a different form of antigen to the capital C and E.

Genotype

symbol

Rh(D) status

cde/cde

rr

Negative

CDe/cde

R1r

Positive

CDe/CDe

R1R1

Positive

cDE/cde

R2r

Positive

CDe/cDE

R1R2

Positive

cDE/cDE

R2R2

Positive

The different genotypes gives a range of antigen phenotypes. If we just look at Rhesus positive and negative (based on the D type antigen only remember) it looks like discontinuous variation. However taking all of the antigens into account we see a type of continuous variation.

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HL Topic 10 Genetics Copyright © by Alex Black. All Rights Reserved.

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