Inheritance — Edexcel GCSE Combined Science
Alleles and key genetic terms, monohybrid crosses and family pedigrees, sex determination, and calculating and analysing the outcomes of crosses.
Revision notes
5 short notes, in the order of the specification. Each one in short:
Alleles are different forms of the same gene, such as a purple allele and a white allele of a flower colour gene. Because chromosomes are in pairs, an organism usually has two alleles of each gene, one inherited from each parent. Alleles have slightly different base sequences, so they can code for slightly different versions of a protein.
Key genetic terms describe how alleles are inherited. A dominant allele is expressed with one copy, but a recessive allele only with two copies. Homozygous means two identical alleles, such as TT or tt; heterozygous means two different alleles, such as Tt. The genotype is the alleles an organism has, and the phenotype is the characteristics expressed.
Monohybrid inheritance is the inheritance of a characteristic controlled by a single gene. It can be shown using a genetic diagram, a Punnett square or a family pedigree. Crossing two heterozygous parents, such as Tt × Tt, gives the genotypes 1 TT : 2 Tt : 1 tt, so the phenotype ratio is 3 dominant : 1 recessive.
Sex is determined by one pair of sex chromosomes: females are XX and males are XY. All eggs carry an X chromosome, while half of sperm carry an X and half carry a Y. The sperm that fertilises the egg decides the sex, so there is a 50% probability of a boy or a girl.
The outcomes of a monohybrid cross can be given as a ratio, a probability or a percentage. From a Punnett square, count the boxes with that outcome out of four. For two carrier parents (Ff × Ff), the probability of an affected child is 1/4, 0.25 or 25%. Each child is an independent event, so the probability stays the same.
Exam questions
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Quick check questions
What is an allele?
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A different version of the same gene.
What is the difference between genotype and phenotype?
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Genotype is the alleles an organism has. Phenotype is the observable characteristic that is expressed.
Which sex chromosomes do human females and males have?
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Females XX, males XY.
Two parents without a disorder have an affected child. Is the allele dominant or recessive?
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Recessive, and both parents are carriers.
What is codominance?
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Both alleles are expressed in the phenotype when both are present, as with IA and IB giving blood group AB.
Frequently asked questions
What is a genotype and phenotype?
A genotype is the alleles an organism has for a gene, such as Tt. A phenotype is the characteristics that are expressed and can be observed, such as being tall. A heterozygous plant, Tt, has the phenotype of the dominant allele, so it is tall even though it also carries the allele for short.
What is a Punnett square used for in genetics?
A Punnett square is used to show the possible offspring of a genetic cross. The gametes from each parent are written along two sides, then combined in the boxes to give the possible genotypes. Counting the boxes gives the ratio, probability or percentage of each genotype and phenotype in the offspring.
What is the difference between homozygous and heterozygous?
Homozygous means having two identical alleles of a gene, such as TT or tt. Heterozygous means having two different alleles of a gene, such as Tt. A heterozygous organism shows the dominant characteristic, because a dominant allele is expressed in the phenotype even when only one copy is present.
What decides whether a baby is a boy or a girl?
The father's sperm decides whether a baby is a boy or a girl. Every egg carries an X chromosome, but half of sperm carry an X and half carry a Y. If an X sperm fertilises the egg the baby is XX, a girl; if a Y sperm does, it is XY, a boy. Each has a 50% probability.
Written and checked against the Edexcel GCSE Combined Science (1SC0) specification · Updated October 2026