Genetics: Genetic Linkage Mapping

Q: What is genetic linkage?

A: The physical association of genes located on the same chromosome that tend to be inherited together.

Q: Why don’t linked genes follow Mendel’s Law of Independent Assortment?

A: Because they are physically connected on the same chromosome and do not segregate independently.

Q: What is a linkage group?

A: A set of genes located on the same chromosome.

Q: What is synteny?

A: The physical linkage of genes on the same chromosome.

Q: What is crossing over?

A: The exchange of genetic material between homologous chromosomes during meiosis, producing recombinant chromosomes.

Q: During what stage of meiosis does crossing over occur?

A: During pachytene of prophase I in meiosis I.

Q: What are recombinant offspring?

A: Offspring whose combinations of alleles differ from those in either parent due to crossing over.

Q: What are nonrecombinant (parental) offspring?

A: Offspring with the same combinations of alleles as the parental chromosomes.

Q: Who discovered genetic linkage in plants?

A: Bateson and Punnett, in their experiments with sweet pea flower color and pollen shape.

Q: Who provided the first direct evidence of linkage in animals?

A: Thomas Hunt Morgan, through his fruit fly (Drosophila) experiments.

Q: What did Morgan’s experiments show about linked genes?

A: Linked genes are located on the same chromosome, but crossing over can separate them and create recombinants.

Q: What is the relationship between distance and crossing over frequency?

A: The farther apart two genes are on a chromosome, the more likely crossing over occurs between them.

Q: What is genetic mapping (gene mapping)?

A: Determining the linear order and relative distances of genes on a chromosome.

Q: What is a map unit (mu) or centiMorgan (cM)?

A: A unit representing 1% recombination between two genes.

Q: Why do multiple crossovers underestimate map distance?

A: Because even-numbered crossovers restore the parental configuration and go undetected.

Q: What is the maximum recombination frequency possible in a testcross?

A: 50%, the same as if the genes were on different chromosomes.

Q: What is a three-factor cross used for?

A: To determine gene order and calculate map distances among three linked genes.

Q: What is mitotic recombination?

A: A rare event where crossing over occurs during mitosis instead of meiosis.

Q: What is a twin spot in Drosophila?

A: Adjacent patches of tissue with different phenotypes caused by mitotic recombination during early development.