GCSE Biology revision

Genetic inheritance

Reproduction

AQA 4.1.1.1

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AQA student objectives

Learning pathway

All · Most · Some

ALL 🎯

Identify and describe What is a Gene?, Alleles: Different Versions of the Same Gene and Homozygous and Heterozygous in the context of genetic inheritance.

MOST 🎯🎯

Explain the relationships between What is a Gene?, Alleles: Different Versions of the Same Gene and Homozygous and Heterozygous and how they contribute to genetic inheritance.

SOME 🎯🎯🎯

Apply and analyse knowledge of What is a Gene?, Alleles: Different Versions of the Same Gene and Homozygous and Heterozygous to interpret unfamiliar information about genetic inheritance.

Revision summary

Key knowledge

Read on screen, then print for Cornell-style active revision.

What is a Gene?

A gene is a segment of DNA that codes for a particular type of protein, and together inherited genes determine the characteristics an organism develops
Some traits are controlled by a single gene, such as fur colour in mice or red-green colour blindness in humans
Many characteristics, like height, are determined by several genes interacting with each other, which is why there is so much variation between individuals

1. What is a Gene?AQA

Notes

A gene is a segment of DNA that codes for a particular type of protein, and together inherited genes determine the characteristics an organism develops
Some traits are controlled by a single gene, such as fur colour in mice or red-green colour blindness in humans
Many characteristics, like height, are determined by several genes interacting with each other, which is why there is so much variation between individuals

Alleles: Different Versions of the Same Gene

Alleles are different versions of the same gene, each providing a slightly different genetic code for a particular protein
Because we inherit one copy of each gene from each parent, every individual carries two alleles for each gene
The two alleles an individual carries can either be identical or different from one another

2. Alleles: Different Versions of the Same GeneAQA

Notes

Alleles are different versions of the same gene, each providing a slightly different genetic code for a particular protein
Because we inherit one copy of each gene from each parent, every individual carries two alleles for each gene
The two alleles an individual carries can either be identical or different from one another

Homozygous and Heterozygous

An organism is described as homozygous for a gene when both of its alleles for that gene are the same
An organism is described as heterozygous for a gene when it carries two different alleles for that gene

Dominant and Recessive Alleles

When an organism is heterozygous, the dominant allele is always expressed and determines the observable characteristic
The recessive allele is only expressed when both alleles present are recessive, ie the organism is homozygous recessive
For example, if the purple fur allele is dominant over the green fur allele, a heterozygous mouse will always appear purple

4. Dominant and Recessive AllelesAQA

Notes

When an organism is heterozygous, the dominant allele is always expressed and determines the observable characteristic
The recessive allele is only expressed when both alleles present are recessive, ie the organism is homozygous recessive
For example, if the purple fur allele is dominant over the green fur allele, a heterozygous mouse will always appear purple

Genotype vs Phenotype

The genotype refers to the complete collection of alleles an organism possesses, representing its genetic makeup
The phenotype refers to the observable characteristics that result from the genotype, such as fur colour or height
Two organisms can share the same phenotype but have different genotypes — for example, a homozygous dominant and a heterozygous mouse may both appear purple
A homozygous recessive organism will have both a different genotype and a different phenotype compared to dominant or heterozygous organisms

5. Genotype vs PhenotypeAQA

Notes

The genotype refers to the complete collection of alleles an organism possesses, representing its genetic makeup
The phenotype refers to the observable characteristics that result from the genotype, such as fur colour or height
Two organisms can share the same phenotype but have different genotypes — for example, a homozygous dominant and a heterozygous mouse may both appear purple
A homozygous recessive organism will have both a different genotype and a different phenotype compared to dominant or heterozygous organisms

Representing Alleles

Dominant alleles are conventionally represented by a capital letter (eg P for purple fur), whilst recessive alleles are shown with a lowercase letter (eg p for green fur)
A homozygous dominant individual is written as PP, a heterozygous individual as Pp, and a homozygous recessive individual as pp
Only the pp genotype results in the recessive phenotype, whilst both PP and Pp result in the dominant phenotype

6. Representing AllelesAQA

Notes

Dominant alleles are conventionally represented by a capital letter (eg P for purple fur), whilst recessive alleles are shown with a lowercase letter (eg p for green fur)
A homozygous dominant individual is written as PP, a heterozygous individual as Pp, and a homozygous recessive individual as pp
Only the pp genotype results in the recessive phenotype, whilst both PP and Pp result in the dominant phenotype