IGCSE Biology: Inheritance Practice Questions
Inheritance is the transmission of genetic information from generation to generation. A gene is a length of DNA coding for a protein, and an allele is one version of a gene. The genotype is the alleles present and the phenotype is the observable features.
Topic 17 of Cambridge IGCSE Biology 0610 is where careful layout earns marks. Genetic diagrams are marked on the parental genotypes, the gametes, the offspring genotypes and the ratio, so a Punnett square drawn properly scores even when the final ratio is misread. The questions below use that layout throughout.
What you need to know for Inheritance
- Key termsGene: a length of DNA that codes for a protein. Allele: an alternative form of a gene. Genotype: the alleles present. Phenotype: the observable features. Homozygous: two identical alleles. Heterozygous: two different alleles.
- Dominant and recessiveA dominant allele is expressed if it is present. A recessive allele is only expressed when there is no dominant allele present, so only in the homozygous recessive genotype.
- MitosisNuclear division giving rise to genetically identical cells with the same number of chromosomes as the parent cell. Used in growth, repair and asexual reproduction.
- MeiosisReduction division in which the chromosome number is halved, producing genetically different haploid gametes.
- Sex determinationFemales are XX and males are XY. Each egg carries an X. Half of sperm carry X and half carry Y, so the sperm determines the sex and the expected ratio is 1 to 1.
- CodominanceBoth alleles in a heterozygote contribute to the phenotype. Human blood group is the standard example, with alleles I superscript A and I superscript B codominant and I superscript O recessive.
IGCSE Biology Inheritance questions and answers
4 exam-style questions written to the 0610 syllabus. Try each one on paper first, then open the worked answer to check your method against the marks.
In pea plants, tall (T) is dominant to short (t). Two heterozygous tall plants are crossed. Draw a genetic diagram to show the expected genotypes and phenotypes of the offspring, and state the expected ratio.
Show the worked answer
- Parental genotypes: Tt and Tt, both tall.
- Gametes from each parent: T and t.
- Punnett square combinations: T with T gives TT, T with t gives Tt, t with T gives Tt, t with t gives tt.
- Offspring genotypes: 1 TT, 2 Tt, 1 tt.
- Phenotypes: TT and Tt are both tall because T is dominant, and tt is short. The ratio is 3 tall to 1 short.
State three differences between mitosis and meiosis.
Show the worked answer
- Difference 1, number of cells produced: mitosis produces two daughter cells, meiosis produces four.
- Difference 2, chromosome number: mitosis keeps the chromosome number the same as the parent cell, meiosis halves it from diploid to haploid.
- Difference 3, genetic variation: mitosis produces genetically identical cells, meiosis produces genetically different cells.
- A fourth difference: mitosis is used for growth, repair and asexual reproduction, meiosis is used to produce gametes.
Explain why approximately half of all babies born are male, using a genetic diagram.
Show the worked answer
- The mother's sex chromosomes are XX, so meiosis produces eggs that all carry an X chromosome.
- The father's sex chromosomes are XY, so meiosis produces sperm of which half carry X and half carry Y.
- Combinations at fertilisation: X from the egg with X from the sperm gives XX, a female. X from the egg with Y from the sperm gives XY, a male.
- Since the two types of sperm are produced in equal numbers and are equally likely to fertilise the egg, the expected ratio is 1 female to 1 male, or 50 per cent male.
A plant with red flowers may be homozygous or heterozygous. Red (R) is dominant to white (r). Describe a cross the breeder could carry out to find out which, and explain how to interpret the results.
Show the worked answer
- Cross the unknown red plant with a homozygous recessive white plant, genotype rr. This is a test cross.
- If the red plant is homozygous RR, all gametes carry R, so every offspring is Rr and all are red.
- If the red plant is heterozygous Rr, half its gametes carry R and half carry r, so about half the offspring are Rr and red and half are rr and white.
- Therefore any white offspring at all proves the parent was heterozygous. All red offspring, from a reasonably large sample, indicates it was homozygous.
Common mistakes in this topic
- Using the same letter shape for dominant and recessive alleles, such as S and s written identically. Make the capital clearly larger.
- Writing gametes as pairs. A gamete carries one allele, not two.
- Giving a ratio without naming the phenotypes.
- Saying meiosis produces two cells.
- Confusing genotype with phenotype. Tt is a genotype, tall is a phenotype.
Exam tips
- Set out every genetic cross the same way: parental phenotypes, parental genotypes, gametes in circles, Punnett square, offspring genotypes, offspring phenotypes, ratio.
- Choose letters where the capital and small versions look clearly different, such as T and t rather than C and c.
- Write ratios as expected ratios. Real offspring numbers vary because fertilisation is random.
- For pedigree diagrams, look for two unaffected parents with an affected child. That proves the condition is recessive.
- In codominance questions, use superscript notation for the alleles and state that both are expressed in the heterozygote.
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Inheritance FAQs
What is the difference between genotype and phenotype?
The genotype is the combination of alleles an organism carries, written as letters such as Tt. The phenotype is the observable characteristic that results, such as tall. Two different genotypes, TT and Tt, can produce the same phenotype when one allele is dominant.
How do I draw a Punnett square?
Write the gametes of one parent along the top and the gametes of the other down the side, then fill each box with the combination of the two alleles that meet there. Show parental genotypes and gametes above the square, then list offspring genotypes, phenotypes and the ratio underneath. Each of those stages earns marks.
What is the difference between mitosis and meiosis?
Mitosis produces two genetically identical daughter cells with the same chromosome number as the parent, and is used for growth, repair and asexual reproduction. Meiosis produces four genetically different haploid cells with half the chromosome number, and is used to make gametes.
Does the mother or the father determine the sex of a baby?
The father. The mother is XX so every egg carries an X chromosome. The father is XY, so half his sperm carry X and half carry Y. Whether the fertilising sperm carries X or Y determines whether the zygote is XX, female, or XY, male.
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Written to the published Cambridge IGCSE Biology (0610) syllabus. Check your school entry code and syllabus year, because Core and Extended candidates are assessed on different content. Last reviewed 2026-08-12.