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Biology · Genetic Inheritance

Genes, Alleles and Genetic Terms

Why do you look like your family but not exactly like them? This lesson is for both Combined Science and separate Biology. It sets out the vocabulary of inheritance: gamete, chromosome, gene, allele, dominant, recessive, homozygous, heterozygous, genotype and phenotype, and shows that most characteristics depend on many genes.

  • 10 key terms
  • All boards

Learning Objectives

  1. 1Explain the terms gamete, chromosome, gene, allele, dominant, recessive, homozygous, heterozygous, genotype and phenotype.
  2. 2Describe how a dominant allele and a recessive allele are expressed.
  3. 3Work out a phenotype from a genotype, and say whether a genotype is homozygous or heterozygous.
  4. 4Explain why most characteristics are controlled by many genes rather than a single gene.

Retrieval practice

  1. 1

    What is a gene?

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    A small section of DNA on a chromosome that codes for a particular sequence of amino acids, to make a specific protein.

  2. 2

    How many chromosomes are in a human body cell?

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    46, in 23 pairs.

  3. 3

    What is a gamete?

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    A sex cell: a sperm or an egg in animals.

  4. 4

    What process makes gametes?

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    Meiosis.

Why offspring are not identical copies

Every child gets half of its chromosomes from each parent, so it carries two copies of every gene, one from the mother and one from the father. The two copies are not always the same. This lesson gives you the words geneticists use to describe those copies and the characteristics they produce, because every genetics question in the exam is written in this language.

The vocabulary of inheritance

These four words describe the genetic material, from the smallest unit to the sex cell.

  • Gamete

    A sex cell: a sperm cell or an egg cell in animals. A gamete carries one copy of each chromosome, so one copy of each gene.

  • Chromosome

    A long molecule of DNA in the nucleus that carries many genes. Human body cells have 23 pairs, so each gene is present twice.

  • Gene

    A small section of DNA on a chromosome. It codes for a particular protein and so helps to control a characteristic, such as fur colour in mice.

  • Allele

    A different form of the same gene. A gene for fur colour might have an allele for black fur and an allele for brown fur.

Dominant and recessive alleles

A body cell has two alleles of each gene, one on each chromosome of a pair. Which one shows depends on whether it is dominant or recessive.

  • Dominant allele

    A dominant allele is always expressed, even if only one copy is present. It is written as a capital letter, such as B.

  • Recessive allele

    A recessive allele is only expressed if two copies are present, so there is no dominant allele. It is written as the lower case of the same letter, such as b.

  • Same letter

    The two forms of one gene use the same letter, in capital and in lower case, so you can see at once that they are alleles of the same gene.

  • Not about strength

    Dominant does not mean stronger or more common. It only means that the allele shows when it is present.

Homozygous and heterozygous

The two alleles of a gene can be the same or different.

  • Homozygous

    The two alleles are the same. BB is homozygous dominant and bb is homozygous recessive. Zygous refers to the zygote, the first cell of a new organism.

  • Heterozygous

    The two alleles are different, for example Bb. The dominant allele is expressed, and the recessive allele is hidden.

  • Remember the prefix

    Homo means the same and hetero means different.

Homozygous or heterozygous?

Homozygous

  • The two alleles are the same
  • BB (dominant) or bb (recessive)
  • Only one kind of gamete can be made
  • Organism is true breeding for that characteristic

Heterozygous

  • The two alleles are different
  • Bb
  • Two kinds of gamete can be made, B and b
  • The dominant allele is expressed and the recessive is hidden

Genotype and phenotype

Genotype is what is in the cells. Phenotype is what you can see or measure.

  • Genotype

    The combination of alleles that an organism has, written as letters such as Bb. The alleles operate at a molecular level, by coding for proteins.

  • Phenotype

    The characteristic that is expressed, such as black fur. It is the result of the genotype working with the environment.

  • Several genotypes, one phenotype

    BB and Bb are different genotypes but give the same phenotype (black fur), because B is dominant.

  • Only one way for a recessive

    A recessive phenotype can only come from one genotype, the homozygous recessive bb.

Working out the phenotype from a genotype

In mice, the allele for black fur (B) is dominant to the allele for brown fur (b). State the phenotype of mice with the genotypes BB, Bb and bb, and say whether each genotype is homozygous or heterozygous.

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  1. 1 Identify the dominant allele B is dominant, so one B is enough for black fur
  2. 2 BB Two dominant alleles, so black fur. Both alleles are the same, so it is homozygous
  3. 3 Bb One dominant allele, so black fur. The alleles are different, so it is heterozygous
  4. 4 bb No dominant allele, so the recessive allele is expressed: brown fur. Both alleles are the same, so it is homozygous

AnswerBB is black and homozygous. Bb is black and heterozygous. bb is brown and homozygous.

One gene or many genes?

Some characteristics are controlled by a single gene, but they are the exception.

  • Single gene characteristics

    Fur colour in mice and red-green colour blindness in humans are controlled by a single gene, so each has a simple pattern of inheritance.

  • Most characteristics

    Most characteristics, such as height, skin colour and intelligence, result from many genes interacting rather than a single gene.

  • Genes and environment

    Even for one gene, the phenotype can be affected by the environment. A plant with the genes for tallness will still be short if it has too little light and too few minerals.

  • Why it matters in the exam

    You can only predict simple ratios for single gene characteristics. For most features, a cross cannot predict the offspring so neatly.

Case study

Red-green colour blindness

Red-green colour blindness is a single gene characteristic in humans. The allele that causes it is recessive and is carried on the X chromosome, which is why it is more common in males, who have only one X chromosome. A person with the condition has difficulty telling red from green because of how the cells in the eye respond to colour. It is an example of a characteristic where one gene has a big effect.

About 1 in 12 Roughly how common it is in males
About 1 in 200 Roughly how common it is in females

The big idea

Genotype plus environment gives phenotype, and a dominant allele shows even when only one copy is present.

Genetic terms

Genetic terms

  • Gamete

    • sex cell
    • one copy of each gene
    • sperm and egg

  • Gene and allele

    • gene is a section of DNA
    • alleles are its different forms

  • Dominant and recessive

    • dominant always expressed
    • recessive needs two copies

  • Homozygous and heterozygous

    • same alleles
    • different alleles

  • Genotype and phenotype

    • alleles present
    • characteristic expressed

  • Many genes

    • most characteristics
    • plus the environment

Summary and exam focus

  • A gene has different forms called alleles, and a gamete carries one allele of each gene.
  • A dominant allele is always expressed, and a recessive allele is only expressed if two copies are present.
  • Alleles that are the same are homozygous and alleles that are different are heterozygous.
  • The genotype is the alleles present and the phenotype is the characteristic expressed.
  • Most characteristics are controlled by many genes, not a single gene.

Exam focus

In cats, the allele for short fur (S) is dominant to the allele for long fur (s). Explain why a cat with the genotype Ss has short fur, and state the term that describes this genotype. (3 marks) (3 marks)

Use the key words in the right place. Say that S is dominant, so it is expressed even when only one copy is present, and that the long fur allele is recessive so it is not expressed. Then name the genotype as heterozygous. Do not just say that short is stronger.

Key terms

The words this lesson expects you to use. Each one is linked from the first place it appears above.

Gamete
A sex cell, such as a sperm cell or egg cell, that carries one copy of each gene.
Chromosome
A long molecule of DNA in the nucleus that carries many genes.
Gene
A small section of DNA on a chromosome that codes for a particular protein.
Allele
A different form of the same gene.
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
Having two identical alleles of a gene.
Heterozygous
Having two different alleles of a gene.
Genotype
The combination of alleles that an organism has.
Phenotype
The characteristic that is expressed, which results from the genotype and the environment.

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