AQA · GCSE · Combined Science

    Course version: 8464 · Version from Sept 2018

    Variation: the genes they have inherited (genetic causes)

    Genetic variation arises because offspring inherit different combinations of alleles from their parents.

    • 4 key terms

    Study notes

    From the checked teaching notes for this specification point.

    The idea

    Genetic variation arises because offspring inherit different combinations of alleles from their parents. Each gamete carries one allele of each gene, and fertilisation combines two gametes at random, so siblings differ even though they share parents. Some characteristics, such as blood group or natural eye colour, are determined mainly by genes. Others, such as height, are affected by many genes and by the environment. A mutation can create a new allele, adding further genetic variation. For example, a child may inherit an allele for brown eyes from one parent and an allele for blue eyes from the other, giving brown eyes because brown is dominant. The inherited alleles are the genetic cause of that characteristic.

    What earns the marks

    • Genetic variation is caused by the alleles inherited from both parents.
    • Gametes contain one allele of each gene, so fertilisation produces a new allele combination.
    • Different offspring inherit different allele combinations, which is why siblings are not identical.
    • Some characteristics are determined mainly by genes, for example blood group or natural eye colour.
    • A mutation can produce a new allele and so increase genetic variation.
    • The genetic cause of a characteristic is the set of alleles an organism has inherited.

    Common mistakes and how to put them right

    • Writing that genes are learned or acquired during life.

      Put it right: Genes are inherited from parents and are present from fertilisation.

    • Confusing genes with chromosomes.

      Put it right: A gene is a short section of DNA that codes for a characteristic, while a chromosome is a structure that carries many genes.

    • Stating that all genetic characteristics are controlled by a single gene.

      Put it right: Many, such as height, are controlled by several genes and are also affected by the environment.

    Exam tips

    • Use the terms allele, gamete and fertilisation when explaining how genetic variation arises.
    • Link each inherited allele to the characteristic it affects, for example an allele for brown eyes.
    • If a question asks for a genetic cause, name the inherited alleles rather than describing the environment.

    Try the lesson

    The start of the Variation: the genes they have inherited (genetic causes) lesson, made from checked teaching notes for this specification point.

    Have a go first

    Which cause of variation in a person is described as genetic?

    Listen

    Genetic variation arises because offspring inherit different combinations of alleles from their parents. Each gamete carries one allele of each gene, and fertilisation combines two gametes at random, so siblings differ even though they share parents. Some characteristics, such as blood group or natural eye colour, are determined mainly by genes. Others, such as height, are affected by many genes and by the environment. A mutation can create a new allele, adding further genetic variation. For example, a child may inherit an allele for brown eyes from one parent and an allele for blue eyes from the other, giving brown eyes because brown is dominant. The inherited alleles are the genetic cause of that characteristic. A strong answer earns marks for these points. Genetic variation is caused by the alleles inherited from both parents. Gametes contain one allele of each gene, so fertilisation produces a new allele combination. Different offspring inherit different allele combinations, which is why siblings are not identical. Some characteristics are determined mainly by genes, for example blood group or natural eye colour. Exam tip: Use the terms allele, gamete and fertilisation when explaining how genetic variation arises.

    See it

    A flowchart running from top to bottom. The first box says parents have alleles and points to a box saying gametes carry one allele of each gene. That points to a box saying fertilisation combines two gametes at random, which points to a box saying new allele combination in offspring. This points to a box saying different offspring inherit different allele combinations, which points to a box saying siblings are not identical. A separate box saying mutation points to a box saying new allele, which also points into the new allele combination in offspring box. The new allele combination box also points to a final box saying inherited alleles are the genetic cause of a characteristic.
    It shows how inherited alleles, gametes, fertilisation and mutation produce genetic variation in offspring.

    Key words

    Say what each word means before you turn the card.

    Genetic variation

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    Gametes

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    Mutation

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    Genetic cause

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    See every definition
    Genetic variation
    Arises because offspring inherit different combinations of alleles from their parents.
    Gametes
    Each carries one allele of each gene, and fertilisation combines two gametes at random.
    Mutation
    Can create a new allele, adding further genetic variation.
    Genetic cause
    The inherited alleles are the genetic cause of a characteristic.

    Label the diagram

    Some boxes have lost their names. Pick the right name for each number.

    The diagram again, with 3 boxes numbered instead of named. A flowchart running from top to bottom. The first box says parents have alleles and points to a box saying gametes carry one allele of each gene. That points to a box saying fertilisation combines two gametes at random, which points to a box saying new allele combination in offspring. This points to a box saying different offspring inherit different allele combinations, which points to a box saying siblings are not identical. A separate box saying mutation points to a box saying new allele, which also points into the new allele combination in offspring box. The new allele combination box also points to a final box saying inherited alleles are the genetic cause of a characteristic.

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    • 3Box 3

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