Inherited characteristics

Let’s learn more about genetics and how inherited characteristics are determined through the combination of genes. As you learned on the previous page, genes have two different versions, called alleles. In a pair of alleles, there is a dominant allele and a recessive allele.

We can use an example to understand how alleles combine to determine physical characteristics – such as eye colour.

Click through the slide panel below to learn more.

The gene for eye colour is made up of two alleles:
1. An allele for brown eyes.
2. An allele for blue eyes.
In a pair of alleles, one will be dominant and one will be recessive. In the case of eye colour:
- The allele for brown eyes is dominant
- The allele for blue eyes is recessive
A dominant allele is usually indicated with a capital letter (for example, B), while a recessive allele uses a lower case letter (for example, b).
A child will inherit one allele from the mother, and one from the father. Depending on the combination of these alleles, the child’s eye colour will be determined.

The two alleles will combine in one of three ways – the child may inherit either:
- Two alleles for brown eyes (BB)
- Two alleles for blue eyes (bb)
- One allele for brown eyes and one for blue eyes (Bb)
As long as one dominant allele is present, then it will be expressed in the physical characteristic – so, if a child inherits at least one brown eye allele, they will have brown eyes (BB or Bb). Blue eyes will only be expressed if the child inherits two (recessive) blue eye alleles (bb).

When identical alleles are inherited (either two dominant alleles or two recessive alleles) for a characteristic, this is referred to as being a homozygous characteristic. However, if a characteristic is determined by two different alleles (one dominant and one recessive), this is referred to as heterozygous.

You can use a punnett square to determine how likely it is that a certain characteristic will be inherited.

Let’s imagine that parents have the following allele combinations for eye colour:

Mother – Bb (brown eye)
Father – bb (blue eyes)

You would begin by plotting these letters onto a grid, like the one shown below (the mother’s alleles across the top, the father’s down the side):

	B	b
b
b

Then, you fill in the grid by combining the alleles to determine the likelihood of their child having brown or blue eyes, as shown below:

	B	b
b	Bb	bb
b	Bb	bb

You can see from this grid that there is a 50% chance the child will have brown eyes (Bb) and a 50% chance it will have blue eyes (bb).