IB Biology Genetics

1. 10.3 Polygenic Inheritance
Topic 10 Genetics

2. Polygenic Inheritance
 10.3.1 Define polygenic inheritance.
 10.3.2 Explain that polygenic inheritance can contribute to
continuous variation using two examples, one of which
must be human skin colour.

3. Polygenic Inheritance
 So far, the characteristics we have studied have been
controlled by one gene.
 Some characteristics are controlled by more than one
gene.
 This is called Polygenic Inheritance.
 Examples of traits controlled by polygenic inheritance
include:
 skin colour, eye colour, seed colour in wheat,

4. Polygenic Inheritance
 It is important not to confuse multiple alleles with
polygenic inheritance.
 Blood type is an example of multiple alleles, whereby
blood type is controlled by three possible alleles (IA
, IB
, i)
all occurring at one locus.
 Polygenic inheritance involves many genes at several
different loci.

5. Continuous vs Non-continuous Variation
 Variation is expressed by the different phenotypes for a
characteristic.
 Variation can be:
 Continuous.
 Exhibits a wide range of phenotypes ranging from one extreme to
the other.
 Discontinuous.
 Usually only has 2 forms.
 Polygenic Inheritance can contribute to continuous
variation.

6. Human Skin Colour
 Human skin colour is a result of polygenic inheritance.
 Skin colour is controlled by 3 or 4 genes.
 Skin colour depends on the amount of a black pigment
called melanin present.
 Each gene has alleles which promote melanin production
and alleles which do not.
 Thus there is a wide range of phenotypes possible ranging
from all alleles promoting melanin production (black
Skin) to non alleles promoting melanin production (white
skin).

7. Continuous Variation in Human Skin
Colour as a Result of Polygenic
Inheritance Ref: Yr12 Biology, Biozone

8. Seed Colour in Wheat
 Seed colour in wheat is also the result of polygenic
inheritance.
 Seed colour is controlled by 3 genes, each on a different
chromosome.
 Each gene has two alleles, coding for the production of a
red pigment or not.
 Thus a seed can have up to 6 red alleles.
 Seed colour exhibits continuous variation, with seed
colours ranging from dark red to white.

9. Ref: Advanced Biology, Roberts etal

10. Ref: Biology for the IB Diploma, Allott

11. IBO guide:
 10.3.1 Define polygenic inheritance.
 10.3.2 Explain that polygenic inheritance can contribute to
continuous variation using two examples, one of which
must be human skin colour.

12. IBO guide:
 Aim 8: This is one of the most obvious opportunities to
develop the theme of parity of esteem for all humans. The
selective advantage of dark skin to protect against
ultraviolet light and light skin to allow vitamin D
production could be mentioned. The correlation between
skin colour and intensity of sunlight is clear, though the
selective advantages of particular skin colours can now be
overcome by the use of sun-block creams and vitamin D
supplements.