AQA · GCSE · Biology
Course version: 8461 · Version from Sept 2018
If the two alleles present are the same the organism is homozygous for…
Full specification wording
Genetic inheritance: If the two alleles present are the same the organism is homozygous for that trait, but if the alleles are different they are heterozygous.
Homozygous and heterozygous describe whether the two alleles for a gene are the same or different.
Checked against the AQA 8461 specification (opens in a new tab) on .
- Point 6 of 11
- 4 key terms
Study notes
From the checked teaching notes for this specification point.
The idea
Homozygous and heterozygous describe whether the two alleles for a gene are the same or different. A homozygous organism has two identical alleles, such as TT or tt, and is described as homozygous dominant or homozygous recessive. A heterozygous organism has two different alleles, such as Tt, and shows the dominant characteristic. These terms apply to a particular gene or trait, so an organism can be homozygous for one gene and heterozygous for another. Correct notation uses one letter for the gene, with a capital for the dominant allele and lower case for the recessive allele. Being able to classify genotypes in this way is essential for predicting offspring ratios in genetic crosses.
What earns the marks
- Define homozygous as having two identical alleles for a gene, and heterozygous as having two different alleles.
- Distinguish homozygous dominant, such as TT, from homozygous recessive, such as tt.
- State that a heterozygous organism, such as Tt, shows the dominant characteristic.
- Use one letter for a gene, capital for the dominant allele and lower case for the recessive allele.
- Apply the terms to a named trait and use them to predict offspring from a genetic cross.
Common mistakes and how to put them right
Using two different letters for the alleles of one gene.
Put it right by using one letter, capital for the dominant allele and lower case for the recessive allele.
Thinking homozygous means two dominant alleles only.
Put it right by stating that homozygous can be two dominant alleles or two recessive alleles.
Applying the term to the whole organism rather than to a specific gene.
Put it right by stating that an organism can be homozygous for one gene and heterozygous for another.
Exam tips
- Write the genotype first, then label it homozygous dominant, homozygous recessive or heterozygous before describing the phenotype.
- Keep allele notation consistent throughout a genetic diagram so that gametes and offspring genotypes are easy to follow.
- When predicting ratios, combine gametes systematically and count genotypes before converting them to phenotypes.
Try the lesson
The start of the Genetic inheritance: If the two alleles present are the same the organism is homozygous for that trait, but if the alleles are different they are heterozygous. lesson, made from checked teaching notes for this specification point.
Have a go first
An organism has two identical alleles for a particular gene. Which term describes this organism for that trait?
Listen
Homozygous and heterozygous describe whether the two alleles for a gene are the same or different. A homozygous organism has two identical alleles, such as TT or tt, and is described as homozygous dominant or homozygous recessive. A heterozygous organism has two different alleles, such as Tt, and shows the dominant characteristic. These terms apply to a particular gene or trait, so an organism can be homozygous for one gene and heterozygous for another. Correct notation uses one letter for the gene, with a capital for the dominant allele and lower case for the recessive allele. Being able to classify genotypes in this way is essential for predicting offspring ratios in genetic crosses. A strong answer earns marks for these points. Define homozygous as having two identical alleles for a gene, and heterozygous as having two different alleles. Distinguish homozygous dominant, such as TT, from homozygous recessive, such as tt. State that a heterozygous organism, such as Tt, shows the dominant characteristic. Use one letter for a gene, capital for the dominant allele and lower case for the recessive allele. A common slip: Using two different letters for the alleles of one gene. Instead, using one letter, capital for the dominant allele and lower case for the recessive allele. Exam tip: Write the genotype first, then label it homozygous dominant, homozygous recessive or heterozygous before describing the phenotype.
See it
Key words
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Homozygous
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Heterozygous
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Homozygous dominant and homozygous recessive
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Allele notation
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See every definition
- Homozygous
- Describes an organism when the two alleles present for a gene are the same, such as TT or tt.
- Heterozygous
- Describes an organism when the two alleles present for a gene are different, such as Tt, and it shows the dominant characteristic.
- Homozygous dominant and homozygous recessive
- A homozygous organism has two identical alleles, which can be two dominant alleles, such as TT, or two recessive alleles, such as tt.
- Allele notation
- One letter is used for the gene, with a capital for the dominant allele and lower case for the recessive allele.
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