Inheritance - Genetic Diagrams (GCSE Biology)

Genetic Diagrams

Punnett Squares (Gene Crosses)

With some genes, you can predict the phenotype that is expressed, dependent on the alleles of the parents. This can be done with single gene crosses which explore the inheritance of a single characteristic. This is known as monohybrid inheritance. Therefore, these phenotypes must only be influenced by one set of alleles. When more than one set of alleles influences a phenotype, it is much harder to predict the probability of the characteristics shown.

Worked Example with Probabilities and Ratios

The best way to explain genetic inheritance is through a worked example of a monohybrid inheritance – the single characteristic of hair colour.

Let us pretend that the father has dark hair and the mother has blonde hair.

Therefore, the alleles for the father could be DD (homozygous dominant) or Dd (Heterozygous) and the alleles for the mother must be dd (homozygous recessive).

If the father is heterozygous, you can use the following Punnett square:

As you can see, two Dd (heterozygous) and two dd (homozygous recessive) are formed.

This means that the probability of having dark hair is 2/4 – therefore 0.5. The probability of having blonde hair is then also 2/4 – therefore 0.5. The ratio therefore would be 1:1.

This technique can be used to work out the proportion of having certain characteristics. Using direct proportion, 2 out of 4 offspring are Dd and 2 are dd.

Drawing Punnett Squares

You can use Punnett Squares with any combination of alleles as long as they are single gene crosses.

How do you draw one of these?

First, find the allele of the parents. The place the alleles of one parent at the top of the punnet square and one set of alleles on the side. Then write one allele from each parent per box. Do this for four boxes. This will allow you to predict the alleles.

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Codominance and Blood Groups

• There are 4 good groups. They are A, B, AB and O.
• There are 3 different alleles for blood groups. They are IA, IB and IO.
• There are codominant alleles. IA and IB are codominant with each other so you you have both alleles, both are expressed to give the blood group AB.
• I° is recessive. When you get two of these alleles, your blood group is O. If you get one of these alleles and say the other is IB, only IB will be expressed giving you the blood group B.

• You can use a punnett square to predict blood groups. This can be done in the same way as recessive and dominate alleles but instead using the alleles IΑ, IB and I°.

Family Pedigrees

A pedigree is a family tree that shows how genetic disorders are passed down the family.

We can understand pedigrees by looking at cystic fibrosis in a family as an example.

• Cystic fibrosis is a recessive disease. So if a person is homozygous recessive, they will have the disease.
• If they are heterozygous, they will be carriers of the disease. This means that they don’t have the disease but it can still be passed down the family.
• If they are homozygous dominant, they won’t have the disease or be carriers.

You can tell that the cystic fibrosis allele isn’t dominant because Katie has the disease when neither of her parents had the disease – they were carriers. The carriers have the genotype Ff.

You can predict the probability of the newborn baby having cystic fibrosis. As both parents are carriers of the disease, there is a 25% chance the baby will have the disease (genotype ff), 25% they will be unaffected (genotype FF) and a 50% chance that they will be a carrier (genotype Ff). Try drawing a punnett square for this and see if you get the same probabilities.