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Key points

  • Characteristics like eye colour and genetic diseases are .
  • A Punnett square can be used to work out the probability of offspring inheriting some characteristics.
  • Heredity is the study of inheritance.

Video - gene inheritance

Can you answer these questions based on the video?

1. Which characteristic of cats is inherited by one pair of genes?

2. What are alleles?

  1. Hair (or fur) length

  2. Pairs of genes for one characteristic, one inherited from each parent.

Monohybrid inheritance

Monohybrid inheritance is the inheritance of characteristics controlled by one pair of only such as fur in animals and red-green colour blindness in humans. Very few characteristics are controlled by one pair of genes.

Scientists now think that the classic examples used in monohybrid inheritance may actually controlled by more than one gene. These include tongue rolling and ear lobe structure. You might learn about these examples as models to understand the process.

two ears. One has an ear lobe which is attached to the neck, and the other is unattached
Scientist are not sure how ear lobe structure is inherited


The sperm and the ovum (egg cell) each carry half of the from the parent. These join together during to form a new organism, with approximately half of the DNA from each parent. So there are almost always two copies of each gene. Pairs of genes for a characteristic are called alleles. Therefore there are alleles for almost every one of your characteristics.

Predicting alleles in offspring

Monohybrid inheritance can be shown using Punnett squares. These show the probability of alleles in offspring.

A Punnett square

The alleles for one parent go into the orange boxes along the top and the alleles for the other go into the purple boxes on the left. It doesn’t matter which way around the parents go. The four possible alleles for the offspring are shown in the white boxes.

For example, here are the three possible and their for hair length in cats. Remember short hair is

HH (homozygous dominant)Short hair
Hh (heterozygous)Short hair - because H is dominant over h
hh (homozygous recessive)Long hair
A cat with the homozygous recessive genotype

Here is how to work out the probabilty percentage of genotype and phenotype of the offspring of two long-haired cats (hh).

Step 1

Add the parent genotypes to the left and top boxes.

Step 2

Copy the letters from the orange and purple boxes as shown. This reveals the genotype of the offspring.

Step 3

Continue for the other white boxes until all four are filled.

Step 4

Analyse the results. It is important to do this for all three genotypes and both phenotypes.

  • HH – zero out of four so 0%
  • Hh – zero out of four so 0%
  • hh – four out of four so 100%
  • Short hair – zero out of four so 0%
  • Long hair – four out of four so 100%

It is important that the genotypes (HH, Hh, hh) always add up to 100% and so do the phenotypes (short and long hair).

Repeat the process for two heterozygous short-haired cats.

Complete the table then analyse the results to see the percentages of genotypes and phenotypes.

  • HH – one from four so 25%
  • Hh – two from four so 50%
  • hh – one from four so 25%
  • Short hair – three from four so 75%
  • Long hair – one from four so 25%

Inheritance of genetic disorders

A genetic disorder is an inherited medical condition. It can be passed from parents to their children. Examples include cystic fibrosis, sickle cell disease and haemophilia.

Parents who are heterozygous for these conditions are called carriers. They do not usually have the disorder themselves. Two heterozygous parents (carriers) have a one in four chance of passing it to their children.

This is shown in this Punnett square for cystic fibrosis.

An individual who is homozygous (cc) with the allele will develop cystic fibrosis.

  • CC – one from four so 25%
  • Cc – two from four so 50%
  • cc – one from four so 25%
  • Without cystic fibrosis – three from four so 75%
  • With cystic fibrosis – one from four so 25%

Quiz - Inheritance