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Biology · Inheritance

Alleles, Genotype and Phenotype

Why do some people have blue eyes and others brown? This lesson introduces the key words of genetics: alleles, dominant and recessive, homozygous and heterozygous, genotype and phenotype.

  • 10 key terms
  • All boards

Learning Objectives

  1. 1Explain why there are differences in inherited characteristics as a result of alleles.
  2. 2Define the terms chromosome, gene, allele, dominant, recessive, homozygous, heterozygous, genotype, phenotype, gamete and zygote.
  3. 3Use letters to write genotypes, and say what the phenotype is.
  4. 4Explain how alleles are passed on from parents to offspring through gametes.

Retrieval practice

  1. 1

    What is a gene?

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    A section of DNA that codes for a protein.

  2. 2

    How many chromosomes are in a human gamete?

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    23.

  3. 3

    What happens at fertilisation?

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    A male gamete and a female gamete fuse, forming a zygote.

  4. 4

    What type of cell division makes gametes?

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    Meiosis.

Different versions of a gene

Every person has genes for the same characteristics, such as eye colour or blood group, in the same places on their chromosomes. But the genes come in different versions, and these versions are the reason why people differ. Each different version of a gene is called an allele. A body cell has two copies of each chromosome, so it has two alleles of each gene, one from the mother and one from the father. The combination of alleles a person has decides which characteristics they show.

Chromosomes, genes and alleles

These three words are often confused, so be clear about each one.

  • Chromosome

    A long molecule of DNA that carries many genes. Body cells have 23 pairs.

  • Gene

    A section of DNA that codes for a particular protein, and so for a characteristic such as flower colour.

  • Allele

    A different version of the same gene. For example, a pea plant has a gene for height with an allele for tall and another for dwarf.

  • Pairs

    In a pair of chromosomes, the two alleles of a gene lie at the same position, one on each chromosome.

Dominant and recessive alleles

When the two alleles in a person are different, one may hide the effect of the other.

  • Dominant allele

    An allele that has an effect even if only one copy is present. It is written with a capital letter, such as T.

  • Recessive allele

    An allele that only has an effect if there are two copies, with no dominant allele present. It is written with the lower-case letter, such as t.

  • Using one letter

    The same letter is used for both alleles, so that it is clear that they are versions of the same gene.

  • Not about being common

    Dominant does not mean most common or stronger. Some dominant alleles, such as the one for Huntington's disease, are rare.

Homozygous and heterozygous

Homozygous

  • Two identical alleles of a gene
  • Written TT (homozygous dominant) or tt (homozygous recessive)
  • Always makes the same type of gamete
  • TT is tall and tt is dwarf

Heterozygous

  • Two different alleles of a gene
  • Written Tt
  • Makes two types of gamete, half with T and half with t
  • The dominant allele shows, so Tt is tall

Genotype and phenotype

These two words describe what a person has and what it looks like.

  • Genotype

    The combination of alleles that an organism has for a gene, written as letters. For example Tt.

  • Phenotype

    The characteristic that the organism shows, such as being tall. It is the result of the genotype and sometimes also the environment.

  • Same phenotype, different genotype

    TT and Tt both have the phenotype tall, but different genotypes.

  • Recessive phenotype

    The recessive phenotype is only seen in the homozygous recessive genotype, tt.

Naming the genotype and the phenotype

In pea plants, the allele for purple flowers (P) is dominant to the allele for white flowers (p). State the phenotype of plants with genotypes PP, Pp and pp, and say whether each is homozygous or heterozygous.

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  1. 1 Look at the letters P is dominant, so it shows if there is at least one P.
  2. 2 PP Two P alleles, so the flowers are purple, and it is homozygous (same alleles).
  3. 3 Pp One of each, so the dominant allele shows and the flowers are purple. It is heterozygous.
  4. 4 pp No dominant allele, so the recessive phenotype is shown. The flowers are white and it is homozygous.

AnswerPP is purple and homozygous dominant. Pp is purple and heterozygous. pp is white and homozygous recessive.

How alleles are passed on

Alleles are passed on in gametes, one from each parent.

  • Gametes carry one allele

    Meiosis separates the pair of chromosomes, so each gamete has only one allele of each gene.

  • A heterozygous parent

    A parent with Tt makes two kinds of gamete, half carrying T and half carrying t.

  • Zygote

    At fertilisation a gamete from the mother and one from the father fuse, so the zygote has two alleles again.

  • Chance

    Which gamete fertilises which is random, so the allele passed on is a matter of chance.

Case study

Cystic fibrosis

Cystic fibrosis is an inherited disorder that causes thick, sticky mucus to build up in the lungs and gut. It is caused by a recessive allele, which we can call f. A person with two recessive alleles, ff, has the disorder. A person with Ff does not have it, but is a carrier who can pass the allele on, and a person with FF is unaffected. About 1 in 25 people in the UK are carriers, and most do not know it.

ff The genotype of a person with cystic fibrosis.
About 1 in 25 People in the UK who carry the cystic fibrosis allele.

Summary and exam focus

  • An allele is a different version of a gene.
  • A dominant allele shows if one copy is present, and a recessive allele shows only if two copies are present.
  • Homozygous means two identical alleles, and heterozygous means two different alleles.
  • The genotype is the alleles that an organism has, and the phenotype is the characteristic that it shows.
  • Each gamete contains one allele of each gene.

Exam focus

In cats, the allele for black fur (B) is dominant to the allele for ginger fur (b). Explain what is meant by the terms genotype and phenotype, and state the phenotype of a cat with the genotype Bb. (4 marks) (4 marks)

Define each term in one sentence, with a use of the example. Remember that a heterozygous organism shows the dominant phenotype, so Bb is black. Always say whether a letter is dominant or recessive when you define the alleles in your answer.

Key terms

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

Allele
A different version of the same gene.
Dominant
An allele that shows its effect even when only one copy is present.
Recessive
An allele that only shows its effect when 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 for a gene.
Phenotype
The characteristics that an organism shows.
Gamete
A sex cell, with one allele of each gene.
Zygote
The cell formed when two gametes fuse.
Carrier
A person with one copy of a recessive allele who does not show the characteristic but can pass the allele on.

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