Lesson 3: Non-Mendelian Genetics: Incomplete and CoDominance

Question 1

Inheritance Beyond
Mendel’s Laws

Answer 1

Mendel found that inherited traits were
either dominant or recessive

However, some organisms show different
patterns of inheritance

These do not follow simple Mendelian
genetics

Question 1

Incomplete Dominance

Answer 1

Not all traits are purely dominant or purely
recessive, sometimes neither of the alleles
controlling a trait are dominant

This leads to a blending of the two traits called
incomplete dominance

Examples of incomplete dominance can be seen in
many plant species.
The homozygous dominant genotype for flower colour
expresses as red

The homozygous recessive genotype for flower colour
expresses as white

The heterozygous genotype for flower colour expresses as
pink

The incomplete dominance of the alleles for flower colour
lead to a blending of the traits in the heterozygous
genotype

Inheritance of traits that show incomplete dominance
follows Mendel’s laws of segregation and independent
assortment.

However, as neither of the alleles are dominant the
offspring do not display the expected Mendelian ratios.

Question 2

Co-dominance

Answer 2

In some cases, both
alleles for a trait may
be dominant.

These alleles are said
to be co – dominant
because both alleles
are expressed in the
heterozygous
individual.

Question 3

Example of Co-dominance:
Coat Colour in Cows

Answer 3

In cows coat colour is controlled by
several dominant alleles. This is
written by using the same
uppercase letter to denote the trait
and a superscript is added to note
which allele is present.

CWCW = white cow

CRCR = red cow

CWCR = roan cow

Co – dominance also follows the
laws of segregation and
independent assortment.

However, in the heterozygous
genotype the alleles are expressed.