IGCSE Biology · 0610

IGCSE Biology: Inheritance and Genetics Practice Questions

Genetics marks are won with clean definitions and a correctly drawn Punnett square. These questions cover dominant and recessive alleles, genotype versus phenotype and the 3 to 1 monohybrid ratio.

What you need to know

A gene is a length of DNA that codes for a protein; alleles are different versions of the same gene.
A dominant allele is expressed even when only one copy is present; a recessive allele is expressed only when two copies are present.
Genotype is the alleles an organism has; phenotype is the observable characteristics.
A cross between two heterozygous parents gives a 3 to 1 ratio of dominant to recessive phenotypes.

Monohybrid cross Tt x Tt gives a 3 to 1 ratio in the offspring.

Practice questions with answers

Question 12 marks

Define the term dominant allele.

A dominant allele is an allele that is expressed in the phenotype whenever it is present, even if only one copy is present in a heterozygous organism.

Mark schemeExpressed or shown in the phenotype (1). When only one copy is present, or in the heterozygous condition (1).

Common mistakeConfusing dominant with the more common allele. Dominant refers to how the allele is expressed, not how frequent it is.

Exam tipMention the heterozygous case to show you understand one copy is enough.

Question 23 marks

In pea plants, tall (T) is dominant to short (t). Two heterozygous tall plants are crossed. State the ratio of phenotypes in the offspring and explain, using a genetic diagram.

Both parents are Tt. The offspring genotypes are TT, Tt, Tt and tt. TT, Tt and Tt are tall, and tt is short. The ratio of phenotypes is 3 tall to 1 short.

Mark schemeParents both Tt, gametes T and t shown (1). Correct offspring TT, Tt, Tt, tt (1). Ratio 3 tall to 1 short (1).

Common mistakeGiving the genotype ratio 1:2:1 when the question asks for phenotypes.

Exam tipDraw a Punnett square even if not asked. It earns the working marks and prevents errors.

Question 32 marks

Explain the difference between genotype and phenotype.

The genotype is the combination of alleles that an organism has for a characteristic. The phenotype is the observable characteristic that results, such as being tall or short.

Mark schemeGenotype is the alleles present (1). Phenotype is the observable or physical characteristic (1).

Common mistakeSwapping the two definitions.

Exam tipGive a quick example, such as Tt for genotype and tall for phenotype, to make the distinction clear.

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Frequently asked questions

What is the difference between an allele and a gene?

A gene is a length of DNA that codes for a characteristic. An allele is one of the different versions of that gene, for example the allele for tall or the allele for short.

What does heterozygous mean?

Heterozygous means having two different alleles for a characteristic, for example Tt. Homozygous means two identical alleles, such as TT or tt.

How do you draw a Punnett square?

Write one parent's gametes across the top and the other parent's gametes down the side, then fill each box by combining the row and column alleles to give the offspring genotypes.

What is a monohybrid cross?

A monohybrid cross follows the inheritance of a single characteristic controlled by one gene, such as height in pea plants.

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