Exercises: Linkage and Independent Assortment

Define the term linkage and state where on a chromosome two linked genes are located relative to each other.

Mendel's law of independent assortment states that alleles of different genes separate independently during gamete formation.

1. a) A plant has the genotype $AaBb$ and the genes $A$ and $B$ are on different chromosomes. List the four types of gamete it produces and state the expected ratio of these gametes.

1. b) State the phase of meiosis during which independent assortment takes place.

Two genes are linked on the same chromosome. An individual has the genotype $AB/ab$ (alleles $A$ and $B$ are on one chromosome, and $a$ and $b$ on its homologue). If no crossing over occurs, list the gametes produced.

A heterozygous individual has the genotype $AB/ab$. It is test-crossed with an individual of genotype $ab/ab$.

1. a) State the phenotypes and the expected ratio of the offspring if the genes assort independently.

1. b) State the phenotypes and the expected ratio of the offspring if the genes are completely linked.

1. c) Explain why test-crossing is used to investigate linkage.

In a test cross of a pea plant heterozygous for two linked genes (tall/dwarf and purple/white flowers) with a double-recessive plant, 1,000 offspring were produced:

• tall, purple: 460

• dwarf, white: 440

• tall, white: 50

• dwarf, purple: 50

1. a) Identify the parental types and the recombinant types.

1. b) Explain how the recombinant offspring arise.

1. c) Calculate the percentage of recombinant offspring.

A student crossed fruit flies and obtained offspring in the ratio 1 : 1 : 1 : 1 in one cross and in a ratio of approximately 9 : 1 : 1 : 9 in another cross, both test crosses involving different pairs of genes.

1. a) State which cross suggests that the genes are linked and explain your answer.

1. b) Explain why the recombinant classes are the smallest in the linked cross.

1. c) Suggest how the frequency of recombinants would change if the two genes were located further apart on the chromosome.

Two genes are on the same chromosome. A dihybrid individual is test-crossed and 800 offspring are obtained, of which 96 are recombinants.

1. a) Calculate the recombination frequency as a percentage.

1. b) Explain why the recombination frequency between linked genes can never exceed 50%.

1. c) Explain how recombination frequencies can be used to deduce the relative positions of genes on a chromosome.

A dihybrid cross of $AaBb \times AaBb$ is expected to give a 9 : 3 : 3 : 1 phenotypic ratio when genes assort independently. A farmer in Co. Meath obtains instead mostly the two parental phenotypes in an approximately 3 : 1 ratio, with very few of the other two classes.

1. a) Explain this result in terms of the location of the genes.

1. b) Explain the role of crossing over in producing the few unexpected phenotypes.

1. c) Discuss the effect of linkage on genetic variation in a population, and the significance of this for a plant breeder.

Genes $P$, $Q$ and $R$ are on the same chromosome. The recombination frequencies are: $P$–$Q$ = 8%, $Q$–$R$ = 12%, $P$–$R$ = 20%.

1. a) Deduce the order of the three genes on the chromosome and draw a linear map in words or text with the distances.

1. b) Predict, with a reason, whether the genes would be expected to show independent assortment in a cross.

1. c) Explain why the observed $P$–$R$ frequency might be slightly lower than the sum of the two shorter distances in some real experiments.