AQA · GCSE · Combined Science
Course version: 8464 · Version from Sept 2018
Genetic inheritance: homozygous
Homozygous describes an individual carrying two identical alleles for a particular gene, for example BB or bb.
Checked against the AQA 8464 specification (opens in a new tab) on .
- Point 8 of 20
- 3 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Homozygous describes an individual carrying two identical alleles for a particular gene, for example BB or bb. Because both alleles are the same, every gamete the organism produces for that gene carries the same allele. A homozygous dominant individual (BB) shows the dominant phenotype; a homozygous recessive individual (bb) shows the recessive phenotype. In a monohybrid cross between two heterozygotes (Bb × Bb), the possible genotypes are BB, Bb and bb in a 1:2:1 ratio. The homozygous genotypes BB and bb therefore each make up one quarter of the expected offspring, while the heterozygous genotype Bb makes up one half. This gives the familiar 3:1 phenotypic ratio of dominant to recessive.
What earns the marks
- Homozygous means two identical alleles of a gene are present in the genotype, such as BB or bb.
- A homozygous dominant organism has two dominant alleles and expresses the dominant phenotype.
- A homozygous recessive organism has two recessive alleles and expresses the recessive phenotype.
- All gametes produced by a homozygote for that gene carry the same allele, which affects predicted offspring ratios.
Common mistakes and how to put them right
Confusing homozygous with heterozygous.
Put it right: Homozygous means identical alleles, heterozygous means different alleles.
Believing homozygous always means dominant.
Put it right: An organism can be homozygous dominant or homozygous recessive.
Assuming a homozygous dominant parent crossed with a homozygous recessive parent gives a 3:1 ratio.
Put it right: That cross (BB × bb) gives all heterozygous (Bb) offspring with the dominant phenotype.
Saying the homozygous genotypes occur in a 1:2:1 ratio.
Put it right: The 1:2:1 ratio describes all three genotypes (BB:Bb:bb), while the homozygous genotypes alone occur in a 1:1 ratio (each one quarter).
Exam tips
- State the genotype using two letters of the same case, for example BB or bb, and name it as homozygous.
- When predicting gametes, remember a homozygote contributes only one type of allele to its gametes.
- Use a Punnett square to show the cross and then quote the ratio of genotypes and phenotypes separately, checking which genotypes are homozygous.
Try the lesson
The start of the Genetic inheritance: homozygous lesson, made from checked teaching notes for this specification point.
Have a go first
What does the term homozygous mean when describing an organism's genotype?
Listen
Homozygous describes an individual carrying two identical alleles for a particular gene, for example BB or bb. Because both alleles are the same, every gamete the organism produces for that gene carries the same allele. A homozygous dominant individual (BB) shows the dominant phenotype; a homozygous recessive individual (bb) shows the recessive phenotype. In a monohybrid cross between two heterozygotes (Bb × Bb), the possible genotypes are BB, Bb and bb in a 1:2:1 ratio. The homozygous genotypes BB and bb therefore each make up one quarter of the expected offspring, while the heterozygous genotype Bb makes up one half. This gives the familiar 3:1 phenotypic ratio of dominant to recessive. A strong answer earns marks for these points. Homozygous means two identical alleles of a gene are present in the genotype, such as BB or bb. A homozygous dominant organism has two dominant alleles and expresses the dominant phenotype. A homozygous recessive organism has two recessive alleles and expresses the recessive phenotype. All gametes produced by a homozygote for that gene carry the same allele, which affects predicted offspring ratios. A common slip: Confusing homozygous with heterozygous. Instead, homozygous means identical alleles, heterozygous means different alleles. Exam tip: State the genotype using two letters of the same case, for example BB or bb, and name it as homozygous.
Key words
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Homozygous
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Homozygous dominant
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Homozygous recessive
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See every definition
- Homozygous
- Describes an individual carrying two identical alleles for a particular gene, for example BB or bb.
- Homozygous dominant
- An individual with two dominant alleles, such as BB, that shows the dominant phenotype.
- Homozygous recessive
- An individual with two recessive alleles, such as bb, that shows the recessive phenotype.
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