Lesson 3.3.1

3.3.1 Loci, gene linkage and sex linkage Quiz: Pearson Edexcel Biology A (Salters-Nuffield), Unit 3

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

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Lesson 3.3.1, Loci, gene linkage and sex linkage: 20 multiple choice questions for the Pearson Edexcel Biology A (Salters-Nuffield) (9BI0), Unit 3: Voice of the Genome, written with Revision Ninja.

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The 20 questions

  1. What is a locus?

    • The total DNA content of a cell
    • The pattern of inheritance in a family
    • The form of an allele that is expressed
    • The position of a gene on a chromosome

  2. Genes located on the same chromosome are said to be what?

    • Linked, so they tend to be inherited together
    • Dominant over one another
    • Codominant in every individual
    • Epistatic to each other

  3. What event breaks the linkage between two genes on the same chromosome?

    • Mitosis of somatic cells
    • Crossing over during meiosis
    • Dominance of one allele
    • Sex determination at fertilisation

  4. Sex-linked genes in humans are usually located on which chromosome?

    • Mitochondrial DNA
    • Chromosome 21
    • The X chromosome
    • The Y chromosome only

  5. Haemophilia is an example of which type of condition?

    • Mitochondrial
    • X-linked recessive
    • Y-linked
    • Autosomal dominant

  6. A carrier woman (X^c X) for colour blindness has a colour-blind husband (X^c Y). What is the probability that a son is colour blind?

    • 1/2
    • 0
    • 1
    • 1/4

  7. Why are males more often affected by X-linked recessive conditions than females?

    • Males have only one X chromosome
    • Males have two X chromosomes, and one is always inactivated
    • Males produce more recessive alleles in their gametes
    • Females have a stronger immune system that prevents expression

  8. Recombinant gametes are produced most often when linked genes are:

    • Very close together on the chromosome
    • Far apart on the chromosome
    • On different chromosomes
    • On the Y chromosome

  9. In a test cross, 4% of offspring are recombinant. What does this suggest?

    • The genes are on different chromosomes and assort independently
    • The genes are close together on the chromosome
    • The genes are on the Y chromosome
    • Crossing over occurred in 96% of gametes

  10. A father carries an X-linked allele on his X chromosome. What is the consequence for his daughters?

    • All of his daughters are affected because they receive his Y chromosome
    • All of his daughters receive his X chromosome
    • None of his daughters receive any sex-linked allele from him
    • His daughters will be affected only if the mother is also affected

  11. In a test cross for two linked genes, which result is most consistent with linkage?

    • All four phenotype classes appear in equal numbers
    • Parental phenotype combinations are much more common than recombinant combinations
    • Recombinant classes are more common than parental types
    • Only one phenotype appears in the offspring

  12. A test cross gives offspring in the ratio 46:4:4:46 for four phenotypes. Which conclusion is correct?

    • The genes are on different chromosomes
    • The genes are sex-linked on the Y chromosome
    • The two genes are linked, with about 8% recombinants
    • The genes assort independently, giving a 9:3:3:1 ratio

  13. The alleles of a gene at a locus on homologous chromosomes may be what?

    • Always one dominant and one recessive
    • Always found on the sex chromosomes
    • Always identical in every individual
    • Identical or different alleles of the same gene

  14. A female carrier of haemophilia (X^H X^h) has a normal husband (X^H Y). What is the probability that a daughter is affected?

    • 1
    • 0
    • 1/4
    • 1/2

  15. Why does a Y-linked allele occur only in males?

    • Females carry Y genes but they are always silenced
    • Only males carry a Y chromosome
    • Y genes are expressed only in females
    • Y chromosomes are passed only through the egg

  16. A man has an X-linked trait. To which of his children is the allele passed?

    • Half of his sons and half of his daughters
    • All his daughters, but none of his sons
    • All his sons, but none of his daughters
    • None of his children

  17. How many alleles of a given gene does a diploid cell carry at one locus?

    • One
    • Three
    • Four
    • Two

  18. Two loci on the same chromosome are tested in a cross and show 50% recombinant offspring. Which conclusion is correct?

    • The genes are sex-linked on the X chromosome
    • The genes are unlinked, either on different chromosomes or very far apart on one chromosome
    • The genes are tightly linked and never recombine
    • The genes are on the mitochondrial genome

  19. Why is an X-linked recessive allele in a female with two X chromosomes not always expressed?

    • The Y chromosome cancels out X-linked alleles
    • The allele works only in males
    • The allele is destroyed during mitosis
    • X-chromosome inactivation silences one X in each cell, so the allele may be masked

  20. A recessive X-linked allele is written X^r and the normal allele X^N. Which genotype describes a female who expresses the condition?

    • X^r X^r
    • X^N X^r
    • X^r Y
    • X^N Y

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