What you need to know...
- How do our genes affect our features?
You already know that we inherit our DNA and our genes from our parents. Half our DNA came from our mother, half our DNA came from our father. If we know what types of genes two parents have, we can predict the chance that children will inherit certain characteristics. This is useful where parents know that they may be a carrier for a genetic condition.
Let's look at our DNA. We already know that it is organised into chromosomes. You may also know that these chromosomes come in pairs. One chromosome in each pair came from your mother, the other from your father.
Dominant and recessive genes
Here is the same pair of chromosomes but in three different people. We are looking at one particular gene on these pair of chromosomes and that is for eye colour.
Person A has inherited a copy of the brown eyed gene from both parents (BB) - so they will have brown eyes.
Person B has inherited a copy of the blue eyed gene from both parents (bb) - so they will have blue eyes.
But what about person C - they have a copy of both the brown and blue eyed gene (Bb) - what colour of eyes will they have?
Interestingly, the genes do not mix. Person C will have brown eyes. This is because brown eyes are dominant to blue eyes. Blue eyes are recessive in this example.
Dominant genes will always show their effect over recessive genes.
If we know what type of genes two parents have, we can predict what characteristics their children will have.
In this example we have a blue eyed mother and a brown eyed father. The mother must have two copies of the blue eyed gene (bb) because she has the recessive characteristic - blue eyes. The father could have Bb or BB. Lets assume in this case the father is BB.
Genetic cross between a blue eyed mother (bb) and a brown eyed father (BB)
We know that one chromosome will be inherited from the mother and one from the father but it is not possible to predict which one. We need to look at all the possibilities. We can do this by doing genetic crosses. On one side of the cross we put the type of genes held by the mother. On the old side of the cross we put the type of genes held by the father. By completing the cross we can see the different gene combinations that can arise:
In this case the possible gene combinations in the children are all Bb (Brown eyes). This means the couple will only have brown eyed children.
Genetic cross between a blue eyed mother (bb) and a brown eyed father (Bb)
What if the father had a copy of both the brown and blue eyed gene (Bb)? What would our cross look like then? Lets repeat the exercise but with the new genes for the father.
In this case the possible gene combinations in the children are Bb, Bb (Brown eyes) and bb, bb (Blue eyes). This means the couple will have a 50:50 chance of having brown or blue eyed children.