How many gamete types does RrYy produce?

Four: RY, Ry, rY and ry, in equal numbers.

Why do genes on different chromosomes assort independently?

Homologous pairs line up randomly at meiosis, so alleles of different genes enter gametes in any combination.

What are multiple alleles?

Three or more alleles of one gene in a population; an individual carries only two.

Which alleles control ABO blood groups?

I^A and I^B (codominant) and I^O (recessive).

What does sex-linked mean?

The gene is on the X chromosome (or the Y).

Why are X-linked recessive conditions more common in males?

Males have one X chromosome, so one recessive allele is expressed.

Can a father pass an X-linked allele to his son?

No: a son receives his Y from his father and his X from his mother.

What must a fully labelled genetic diagram include?

Parental phenotypes and genotypes, gametes, offspring genotypes and phenotypes with the ratio.

How do you find expected numbers from a 9:3:3:1 ratio?

Divide the total by 16 and multiply by 9, 3, 3 and 1.

Dihybrid inheritance, multiple alleles and sex linkageAQA A-Level Biology: Flashcards

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What is a dihybrid cross?

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What is a dihybrid cross?: A cross that follows the inheritance of two genes at once.
How many gamete types does RrYy produce?: Four: RY, Ry, rY and ry, in equal numbers.
What is the phenotypic ratio from RrYy × RrYy?: 9 : 3 : 3 : 1.
What is the ratio from RrYy × rryy?: 1 : 1 : 1 : 1.
Why do genes on different chromosomes assort independently?: Homologous pairs line up randomly at meiosis, so alleles of different genes enter gametes in any combination.
What are multiple alleles?: Three or more alleles of one gene in a population; an individual carries only two.
Which alleles control ABO blood groups?: $I^A$ and $I^B$ (codominant) and $I^O$ (recessive).
Which genotype gives blood group O?: $I^OI^O$ .
What does sex-linked mean?: The gene is on the X chromosome (or the Y).
Why are X-linked recessive conditions more common in males?: Males have one X chromosome, so one recessive allele is expressed.
Can a father pass an X-linked allele to his son?: No: a son receives his Y from his father and his X from his mother.
A carrier mother and normal father: what proportion of sons are affected?: 1/2.
What must a fully labelled genetic diagram include?: Parental phenotypes and genotypes, gametes, offspring genotypes and phenotypes with the ratio.
How do you find expected numbers from a 9:3:3:1 ratio?: Divide the total by 16 and multiply by 9, 3, 3 and 1.

Exam questions on Dihybrid inheritance, multiple alleles and sex linkage

In a species of garden pea, round seeds are produced by the dominant allele $R$ and wrinkled seeds by the recessive allele $r$ . Yellow seeds are produced by the dominant allele $Y$ and green seeds by the recessive allele $y$ . The two genes are on different chromosomes. A plant breeder crosses plants that are heterozygous for both genes ( $RrYy$ ).
An $RrYy$ plant is crossed with a plant that has wrinkled green seeds. State the expected phenotypic ratio in the offspring and explain how it arises.2 marks
The ABO blood group in humans is controlled by one gene, $I$ , with three alleles. The alleles $I^A$ and $I^B$ are codominant, and the allele $I^O$ is recessive to both. A woman with blood group A (genotype $I^AI^O$ ) and a man with blood group B (genotype $I^BI^O$ ) have children. In a separate paternity enquiry, a woman of blood group A has a child of blood group O.
A man with blood group AB is named as the father of the child of blood group O. Explain why he cannot be the father.2 marks
Haemophilia A is a blood-clotting disorder caused by a recessive allele, $h$ , on the X chromosome. The dominant allele, $H$ , codes for a working clotting factor. There is no corresponding gene on the Y chromosome. A woman who is a carrier and a man with normal clotting plan to have children.
Explain why haemophilia is much more common in males than in females.3 marks

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).