Edexcel IGCSE Biology · Spec 3.19-3.20
Genetics: Key Words
Key genetics terms you must know — gene, allele, dominant, recessive, genotype and phenotype.
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Genetics: Key Words
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Explained
The words genetics questions are built from
Almost every genetics question uses this vocabulary, so getting the definitions exact pays for itself many times over. They are also recall marks, which makes them among the cheapest in the paper.
Gene and allele
A gene is a section of DNA that codes for a specific protein.
An allele is a different version of a gene. There is one gene for eye colour, and the alleles are the versions of it that produce different colours.
You have two alleles of each gene, one inherited from each parent, because you have two copies of each chromosome.
A genome is all the DNA of an organism, meaning the complete set of genetic material.
Dominant and recessive
A dominant allele shows in the phenotype even when only one copy is present. It is written as a capital letter.
A recessive allele only shows when two copies are present, because a single dominant allele would mask it. It is written as the same letter in lower case.
Using the same letter in both cases is not a convention you can vary. B and b are versions of one gene; B and c would be two different genes.
Genotype and phenotype
The genotype is which alleles an organism has, such as Bb.
The phenotype is the characteristic that shows, such as brown eyes.
Different genotypes can give the same phenotype. BB and Bb both give brown eyes, which is why you cannot always tell an organism's genotype by looking at it.
Homozygous and heterozygous
Homozygous means the two alleles are the same, either BB or bb. Homozygous dominant is BB and homozygous recessive is bb.
Heterozygous means the two alleles are different, Bb.
The prefixes are worth knowing: homo means same, hetero means different. That alone prevents the commonest mix up in the whole topic.
What the mark scheme accepts and rejects
An Edexcel International GCSE Biology mark scheme asks for the difference between a gene and an allele, for two marks. It credits that a gene is a section of DNA that codes for a specific protein or polypeptide, and that an allele is an alternative form or version of a gene.
The examiner report on that paper notes that candidates tended to be better at explaining an allele than a gene. The allele definition is short and memorable; the gene definition has three parts, and the words codes for a protein are the ones people drop.
A neighbouring part asks what a genome is, and credits all the DNA of an organism, the complete set of genes, or all of the genetic material of an organism. The report records that most candidates managed this one.
Notice that the gene definition names what the DNA does, not just what it is. A section of DNA is only half the answer, because the whole chromosome is a section of DNA.
Chromosome, DNA and gene
These three are a nested set, and questions test whether you have them in the right order.
DNA is the molecule. A gene is a section of that molecule coding for one protein. A chromosome is a long DNA molecule containing many genes, found in the nucleus.
Human body cells contain 23 pairs of chromosomes, which is 46 in total. Gametes contain 23 single chromosomes, which is why fertilisation restores the pairs.
Spec 3.19-3.20
What you need to know
- Understand what alleles are
- Use the words dominant and recessive
- Use genotype, phenotype, homozygous and heterozygous
Active recall
Quick check
Answer each question before opening the answer.
Define genotype and phenotype.
Genotype — the alleles an organism has; phenotype — the observable characteristics.
What is the difference between a dominant and a recessive allele?
A dominant allele is expressed with only one copy; a recessive allele is only expressed when two copies are present.
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