Before you read this you may wish to refer to the short explanation of genes, genomes and genetic databases in Evolution-an Introduction.
The human ‘genome’, that set of genetic ‘instructions’ that make us ‘human’, dictates how our bodies should develop and grow. Among many other things, the human ‘genome’ specifies that we should each be born with two eyes.
You can clearly see eyes developing in this sequence of images of a human foetus as it grows inside its mother’s womb.
A foetus inherits half its genes from its mother and the other half of its genes from its father. At 54 days’ old you cannot tell if the foetus has inherited its mother’s eye color or its father’s eye color; all you know is that the foetus will have inherited half its ‘eye color genes’ from its father and half its ‘eye color genes’ from its mother.
You will have to wait until after the baby is born before you can determine what color its eyes actually are.
Each of us has different variations of ‘eye color genes’. It is the ‘variations’ in our ‘eye color genes’ which give genetic instructions about what color our irises will be.
The word we use to describe variations in our genes is ‘alleles’. (pronounced ‘al-e-els’)
It is the alleles, in this example those variations in our ‘eye color’ genes, that determine what color our irises will be. Will our irises be colored brown, blue, gray or green?
One baby might have inherited ‘a blue eyed’ variation of eye genes, or blue eye alleles. Another baby might have inherited ‘a brown eyed’ variation of eye genes, or brown eye alleles.
Let us assume that your mother has blue eyes and two blue ‘eye color alleles’. (two blue variations of eye color genes)
Let us also assume that she inherited one blue eye variation of the ‘eye color’ gene from her mother and one blue eye variation of the eye color gene from her father. In the diagram below both blue eye color variations are written with a small ‘b’. The reason why the blue eye color allele is written with a small ‘b’ will be explained later.
Let us assume that your father has brown eyes and two brown eye color alleles.
Let us also assume that he inherited one brown eye allele from his mother and one brown eye allele from his father. Both brown alleles (one copy inherited from each parent) are written with a capital ‘B’.
Let us now assume that you are one of four children.(ie you have three brothers and sisters) You and your siblings have each inherited one blue eye allele from your mother and one brown eye allele from your father.
Each of you now has one blued eyed allele and one brown eyed allele. What color eyes will you all have? Brown eyes!
We call the brown eye variation of the ‘eye color gene’, shown above with a capital ‘B’, the ‘dominant’ allele. We call the blue eye variation of the ‘eye color gene’, shown above with a small ‘b’, the ‘recessive’ allele.
The dominant variation of the gene (or the dominant allele ‘B’) always shows, even if an individual only has one copy of that allele.
The recessive variation of the gene (or recessive allele ‘b’) never shows when an individual has one recessive allele and one dominant allele.
A recessive allele only shows if the individual has two copies of it. You need two copies of a recessive blue eye allele to have blue eyes.
To summarize, you only need one of your two alleles to be brown for you to inherit brown eyes. In this example the recessive blue eye color alleles that you and your siblings inherited from your mother, as represented by a small blue b, have no influence on the color of your irises.
We call each combination of alleles a ‘genotype’. In our example three distinct ‘genotypes’ are possible – ‘BB’, ‘Bb’ and ‘bb’.
The genetic trait that can be seen, or is visible on the outside, is called the ‘phenotype’ The mixed race girl has a brown eye phenotype; her brown eyes are visible. Her ‘genotype’ (in this case Bb) remains hidden in her alleles.
So she he has a brown eye phenotype and a ‘Bb’ genotype.
We call the table below a ‘Punnett Square’. The Punnett Square is named after Professor Reginald Punnett (b 1875 – d 1967) who devised this way of summarising predicted patterns of inheritance in a simple to understand format:
The Punnett square predicts the probability of different outcomes occurring. The probability of inheriting blue colored eyes is zero while the probability of inheriting brown eyes is 100%.
You have a one in four chance of being born with a ‘BB’ genotype; a two in four chance of being born with a ‘Bb’ genotype and a one in four chance of being born with a ‘bb’ genotype.
Following on from this there is a three in four chance of you inheriting a brown eyes and having a ‘brown eye phenotype’. You have a one in four chance of inheriting blue eyes and having a ‘blue eye phenotype’.