Linkage, Recombination, and Eukaryotic Gene Mapping

Flashcards covering the core concepts of linkage, recombination, crossing over, gene mapping (two-point and three-point), coupling vs. repulsion, interference, and related principles from the lecture notes.

What observation in the sweet pea cross indicates that flower color and pollen shape do not assort independently?

F2 progeny do not follow the 9:3:3:1 ratio, indicating linkage between the two genes.

Define genetic linkage.

Genes located on the same chromosome that tend to be inherited together unless crossing over occurs.

Define genetic recombination.

The production of new combinations of alleles in offspring due to crossing over during meiosis.

What is a recombinant gamete?

A gamete carrying new combinations of alleles (e.g., Ab or aB) that differ from parental combinations.

What is a nonrecombinant (parental) gamete?

A gamete carrying the same allele combination as one of the parental chromosomes (e.g., AB or ab).

What is crossing over?

The exchange of corresponding segments between homologous chromosomes during meiosis I, creating recombinant chromosomes.

How does complete linkage affect progeny in a dihybrid cross?

No crossing over occurs; only nonrecombinant progeny are produced, typically in parental combinations.

What are recombinant vs nonrecombinant progeny?

Recombinant progeny have new allele combinations; nonrecombinant progeny retain parental allele combinations.

What does a dihybrid testcross Aa Bb × aa bb reveal?

Gene order and recombination frequencies between two genes by analyzing progeny classes.

What four phenotypic classes are expected in a dihybrid testcross under independent assortment, and in what proportions?

AB, Ab, aB, ab — each class expected at about 25%.

What is expected under complete linkage for a dihybrid testcross?

Only the two parental classes (e.g., AB and ab) appear, in about 50% each; no recombinants.

What does some crossing over indicate about recombinant proportions?

Nonrecombinant progeny are more frequent than recombinant progeny, but recombinant classes do appear.

What is a map unit?

1 map unit (centiMorgan) equals 1% recombination between two genes.

What is recombination frequency?

The percentage of offspring that are recombinant; used as a proxy for distance between two genes on a chromosome.

What is coupling configuration?

AB on one chromosome and ab on the homolog; alleles are in coupling on the two homologs.

What is repulsion configuration?

Ab on one chromosome and aB on the other; alleles are in repulsion on the two homologs.

Why is a double crossover informative in three-gene mapping?

A double crossover can swap the middle gene, helping determine the order of the three genes.

What did McClintock and Creighton demonstrate?

Crossing over is a physical exchange between homologous chromosomes.

What is a three-point testcross used for?

To determine the order of three genes and their relative distances on a chromosome.

What are single crossovers in a three-point cross?

Crossovers between adjacent genes that create two recombinant and two nonrecombinant classes.

What is a coefficient of coincidence?

Observed double crossovers divided by expected double crossovers; used to quantify interference.

What is chromosomal interference?

The phenomenon that one crossover reduces the probability of a second nearby crossover.

How do you calculate the expected number of double crossovers?

Estimate using the product of the frequencies of the two single crossovers.

Why are large genetic distances often underestimated in mapping?

Because multiple crossovers between distant genes can occur, making recombination frequency underestimate true distance.

In Drosophila mapping, which sex exhibits crossing over?

Crossing over occurs only in females; males do not show recombination in most mapping scenarios.

What is a double heterozygote in mapping crosses?

Aa Bb, the starting genotype for two-point or three-point mapping crosses.

What is the significance of a three-point cross showing a double crossover?

Helps determine the correct gene order by revealing which gene lies between the other two.