Inheritance is a key topic in Biology. It explains why we share traits with our parents, such as our appearance, and how characteristics are passed down between generations. Conversely, it also explains how siblings can look different.
This guide explains the role of genes, chromosomes and alleles in inheritance. We'll look at dominant and recessive alleles, genotype and phenotype, and the use of Punnett squares to predict characteristics in children. Exam questions are included to test your understanding. This article is suitable for GCSE Biology revision across all major exam boards, including AQA.
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What is Inheritance?
Inheritance means passing genetic information between parent and child. It's stored in DNA, which is passed from parents to their offspring. This information is stored in DNA, which is found in a cell's nucleus.
DNA code is stored in chromosomes. The normal number of chromosomes is 23 pairs in a human's cells. Each pair has a chromosome from the mother and the other from the father.
Genes are short sections of DNA inside these chromosomes. They contain instructions for making proteins, which affect an organism's traits. For example, there may be DNA code for brown eyes or for whether a person can roll their tongue.
There are two copies for every gene. These are called alleles. One comes from each parent, and the combination of alleles decides what characteristics you inherit.
The environment of an organism also affects its characteristics. For example, diet and health can also affect a person's height as they age.
Key terms - Biology revision notes
Make a note of the following key inheritance terms. They will be relevant to the rest of this article, particularly when we explore genetic crosses (selective breeding with parents containing different traits to see if their traits are passed to the offspring):
- Chromosome - A long, coiled structure of DNA. It carries genes and is found in the nucleus of a cell.
- Gene - A small section of DNA with instructions to create a protein. Genes influence characteristics.
- Allele - A different version of a gene. For instance, a gene for flower colour could have a purple allele and a white allele.
- Gamete - A sex cell, e.g. a sperm cell or egg cell. They have half the usual number of chromosomes.
- Genotype - This is the combination of alleles in an organism, e.g. BB, Bb or bb.
- Phenotype - The characteristics of an organism, such as the colour of their eyes or their height.
Dominant and recessive alleles
Some alleles are dominant, and others are recessive. If an allele is dominant, this means it affects the phenotype when an organism has one or two copies of it. If the allele is recessive, it only affects the phenotype when both copies in the organism are recessive.
A capital letter shows a dominant allele, and a lowercase letter signifies a recessive allele. For example, tall pea plants are controlled by a dominant allele (T), while short pea plants are controlled by a recessive allele (t):
- TT = tall plant.
- Tt = tall plant.
- tt = short plant.
If a plant has Tt, this means it has a dominant and a recessive allele. In this situation, the plant will be tall because the dominant allele is present. The recessive t allele is also present, but its effect is hidden.
Remember that a dominant allele is not stronger, better, or more common. It will simply be expressed if it's present in the phenotype.
Homozygous and heterozygous alleles
Homozygous and heterozygous are the two alleles an organism has for a gene.
If an organism is homozygous, this means the two alleles are the same. It can be homozygous dominant (TT) or homozygous recessive (tt). When an organism is heterozygous, the two alleles are different. In the pea plant example above, this is heterozygous, as the plant is tall despite the recessive allele also being present (Tt).
A homozygous organism is called pure-breeding. It will always pass the same allele for a characteristic to its offspring. For instance, a TT plant can only pass on a T allele, and a tt plant can only pass on a t allele.
Punnett squares
A Punnett square is a grid that shows the possible allele combinations in a parent's child. It doesn't predict the exact traits of a child, but suggests the probabilities for certain characteristics.
Let's say we cross two heterozygous tall pea plants: Tt x Tt. Each parent can pass a T allele or a t allele. The diagram shows the possible genotypes: TT (tall), Tt (tall), Tt (tall), tt (short). The genotype ratio is 1 TT : 2 Tt : 1 tt.
| T | t | |
| T | TT | Tt |
| t | Tt | tt |
If we continue with this diagram, it shows that T is dominant, and 3 out of 4 possible combinations create tall plants. The phenotype ratio is 3 tall : 1 short. As a percentage, there is a 75% chance of a tall offspring and a 25% chance of a short offspring.
When you need to draw a Punnet square, start by writing out the parent genotypes. Put their possible gametes around the grid. Finally, combine one allele from each parent in every box until the diagram is complete.
Complexity in gene inheritance
Punnett squares help when characteristics are controlled by one gene. This is called monohybrid inheritance.
Many characteristics are controlled by multiple genes working together. This is called polygenic inheritance. For example, the height or skin colour of an organism. There's a wide range of possible phenotypes, so it can't be accurately shown using a Punnett square.
An organism's environment can impact its characteristics. For example, a person may have strong athletic ability due to their genes, which can be further enhanced by training, nutrition and sustained coaching.
Memorise the summary of inheritance characteristics. Remember that most characteristics are derived from a mixture of the following:
- Inherited variation - The alleles you receive from your parents.
- Environmental variation - Changes in your traits due to your surroundings, lifestyle or experiences.
Quiz questions
1
What is an allele?
2
Which genotype has a recessive characteristic?
3
What is a phenotype?
4
What description fits a heterozygous genotype?
5
Two heterozygous tall plants are crossed: Tt x Tt. How likely is it that the offspring will be short?
Exam tips
- Check that you understand the difference between gene (a small section of DNA with protein instructions), allele (a different version of a gene), genotype (an organism's allele combination), and phenotype (an organism's characteristics). They all have different meanings.
- Be careful with uppercase and lowercase when describing dominant and recessive alleles. For example,
- Before tackling a Punnett square, simplify your workings by listing the parent genotypes.
- List a parent's possible gametes and combine one allele from each parent in every box.
- Always answer in the form the question requires. For example, a question may ask for a genotype ratio or a fraction.
- The dominant allele is expressed when one copy is present. This doesn't mean it's more common.
Final thoughts on Genetic Inheritance - GCSE Biology Revision
Genetic information is passed from parents to children of living organisms through genes and inheritance. A gene is a small section of DNA on chromosomes that is present in all living things. Different versions of a gene are called alleles.
Make sure you understand the main terms, can identify dominant and recessive alleles, and understand the difference between a genotype and a phenotype. You should expect to complete a Punnett square diagram to find the probability of possible characteristics in children.
For further reading, learn about Gregor Mendel's pea-plant experiments. He was crucial in creating the principles that underpin modern genetic science. You can also learn about genetic engineering, which is when we change an organism's DNA in research, agriculture, and medicine. These articles are by Your Genome.
If you need support with variation and evolution revision, TeachTutti has experienced GCSE Biology tutors who can teach you in-person or online using TeachTutti's learning environment. They will provide bespoke tuition to your needs, such as understanding how genes are inherited or explaining polygenic inheritance and single-gene inheritance.
