Lesson 5: Non-Mendelian Genetics: Multiple Allele and Polygenic Inheritance

Question 1

Multiple Alleles

Answer 1

Many genes are controlled by more than two alleles.

An example of this is human blood type. This is
controlled by three alleles;

A, B, O

These three alleles create 4 blood types

A, B, AB, O

Each person has two of the three possible alleles that
determine their blood type.

The alleles A and B are co – dominant to each other but
both are dominant to the O allele

Question 2

Notation

Answer 2

The notation used is similar to that of co – dominance:

In the case of human blood types we use the uppercase I with
the superscript A or B for the alleles for A and B

We use the lowercase letter i for the allele for O in order to
indicate that it is recessive to the other 2 alleles.

Question 3

Polygenic
Inheritance

Answer 3

Many traits are controlled
by more than one gene, this
is known as polygenic
inheritance.

The result of many genes
working together is
continuous variation in a
trait.

Ex. Human skin colour
and eye colour

Question 4

Example: Ear
length in Corn

Answer 4

Ear length in corn is controlled
by two genes ( A and B)

In order to get the longest ear
of corn both genes controlling
the trait must be homozygous
dominant ( AABB)

In order to get the shortest
ear of corn both genes
controlling the trait must be
homozygous recessive (aabb)

The combination of alleles
other than those two produces
the whole range of ear length
in between.

Question 5

Epistasis

Answer 5

Epistatic genes are genes that mask
the effect of other genes.

The proteins produced by these
genes bind to the promoter sites of
other genes and prevent RNA
polymerase from binding. Thereby
preventing the protein of that gene
being made.

An example is red hair in humans.
This is due to the gene MCR1. It acts
as a switch controlling whether
eumelanin or phaeomelanin are
produced. If MCR1 is active, the
brown eumelanin is produced.
When MCR1 is not active, the red
phaeomelanin is produced.

Question 6

Example: Coat Colour in
Mice

Answer 6

In mice gene C controls the production of
melanin (the pigment that gives colour) while
gene B determine whether the colour is black
or brown.

In its homozygous recessive form, gene C,
blocks the expression of gene B.

In order for the mouse to be white its
genotype must contain the homozygous
recessive form of gene C ( cc) the alleles for
gene B do not affect the phenotype.
In order to get a brown
mouse, it must be
homozygous recessive for
gene B (bb) and must have
at least one dominant allele
for gene C.

In order to get a black
mouse, it must have at least
one dominant allele for
gene B and gene C.