Reproduction » Genes, Alleles and Inheritance

The language of genetics

Why do you look a bit like your parents, but not exactly like them? The answer is in the genes you inherited. Genetics has its own vocabulary, so we start by getting the words straight.

You already know that a chromosome is a long thread of DNA found in the nucleus, and that each chromosome carries many genes. A gene is a short section of DNA that codes for a protein. Remember too that gametes are the sex cells (sperm and egg cells) that join at fertilisation, so a baby gets genes from both parents.

Here is the new part. Each gene may have different forms called alleles. Take a gene for fur colour in mice. One allele makes the fur black. Another allele of the same gene makes the fur brown. Same gene, same place on the chromosome, different version.

Key terms:

• Allele: a different form of the same gene.
• Genotype: the alleles an organism has for a characteristic.
• Phenotype: the characteristic that you can see or measure, such as black fur.

Genotype and phenotype

How does one become the other? The alleles present operate at a molecular level. The code in the alleles is used to make proteins, and those proteins help to build the characteristic. So the genotype leads to the phenotype.

Think of a recipe and a cake. The recipe is the genotype. The cake you can eat is the phenotype.

Single gene characteristics

Some characteristics are controlled by a single gene. Two examples are:

• Fur colour in mice.
• Red-green colour blindness in humans. People with it find it hard to tell red and green apart.

For these characteristics, which alleles you have decides which version you show.

Dominant and recessive alleles

For most genes you have two copies, one from each parent. They may be the same allele or different alleles. What happens then?

We show alleles with letters. A capital letter is a dominant allele and the same letter in lower case is the recessive allele. Let us say that B is the dominant allele for black fur and b is the recessive allele for brown fur.

• BB: black fur.
• Bb: black fur, because the dominant allele is expressed.
• bb: brown fur, because there is no dominant allele to hide the recessive one.

Key terms:

• Dominant: an allele that is always expressed, even if only one copy is present.
• Recessive: an allele that is only expressed if two copies are present.

Homozygous and heterozygous

Look at the letters again. Some pairs have two matching alleles. Others have two different alleles.

A handy trick: homo means same, and hetero means different.

Key terms:

• Homozygous: having two alleles the same for a characteristic.
• Heterozygous: having two different alleles for a characteristic.

Most characteristics have many genes

Fur colour in mice and colour blindness are useful examples, but they are the simple cases. Most characteristics are the result of multiple genes interacting, rather than a single gene.

Your height, for example, is not decided by one gene with a tall allele and a short allele. Many genes work together, so the characteristic cannot be explained by just one pair of alleles. This is why not every feature can be predicted with the simple dominant and recessive idea.

Exam-style question

In rabbits, the allele for black fur (B) is dominant to the allele for white fur (b).

(a) What is meant by the term allele? [1 mark]

(b) A rabbit has the genotype Bb. State its phenotype and explain your answer. [2 marks]

(c) A white rabbit has two b alleles. Is it homozygous or heterozygous? Give a reason. [2 marks]