Inheritance — Edexcel GCSE Biology
Mendel's work, alleles and key terms, monohybrid crosses, pedigrees, sex determination, ABO blood groups and sex-linked disorders.
Revision notes
8 short notes, in the order of the specification. Each one in short:
Gregor Mendel
Triple onlyIn the mid-1800s, a monk breeding pea plants worked out the basis of genetics. He showed that characteristics are passed on as separate inherited factors, one from each parent, now called genes, and that some factors are dominant and others recessive. His work was not understood at the time because genes, DNA and chromosomes were unknown.
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.
ABO blood groups
Triple onlyThe ABO blood group is controlled by one gene with multiple alleles: IA, IB and IO. A person inherits two, one from each parent. IA and IB are codominant, so a person with both is blood group AB. IO is recessive to both, so a person is only blood group O with the genotype IO IO.
Sex-linked genetic disorders
Triple onlyHigher tierA sex-linked genetic disorder is caused by an allele on a sex chromosome, usually a recessive allele on the X chromosome, as in haemophilia. Males have only one X chromosome, so one copy of the recessive allele causes the disorder. Females need two copies, and with one copy they are carriers. So sex-linked disorders are more common in males.
Exam questions
8 exam-style questions (28 marks), each with its mark scheme.
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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.
Why are sex-linked disorders more common in males?
Triple onlyHigher tier Sex-linked disorders such as haemophilia are more common in males because they are usually caused by a recessive allele on the X chromosome. Males (XY) have only one X chromosome, so one copy of the allele causes the disorder. Females (XX) need two copies, and with one copy they are carriers.
Written and checked against the Edexcel GCSE Biology (1BI0) specification · Updated October 2026