SB 07: Ch 5.2-5.4 & 5.6 - Gene Recombination & Mapping

What did Thomas Hunt Morgan discover about Janssens’ chiasmata?

• Chiasmata = structures occurring during prophase of the Meiosis 1 division

• Regions where non-sister chromatids of homologous chromosomes cross over each other

• Chiasmata observed through the light microscope were sites of chromosome breakage and exchange resulting in genetic combination

Physical vs. Genetic Markers

• Physical: Cytologically visible abnormalities that make it possible to keep track of specific chromosome parts from one generation to the next.

• Genetic: Genetic loci identifiable through phenotypic variants that can serve as points of reference in determining whether particular progeny are the result of recombination.

Genetic recombination between the car and Bar genes on Drosophila X chromosome is accompanied by the exchange of physical markers

• Cross of two heterozygous females

• In the females, one X chromosome has mutations producing carnation eyes that were kidney-shaped, this chromosome also has a physically marked discontinuity

• The other X chromosome has wild-type alleles for both the car and Bar genes with a physical marker consisting of part of the Y chromosome that has become attached to the X chromosome centromere

What are the two roles recombination (crossing over) plays, beyond just reshuffling alleles for diversity

• (1) Contributes to genetic diversity by reshuffling alleles between homologous chromosomes.

• (2) Ensures proper chromosome segregation during meiosis — without it, nondisjunction would be common rather than rare, and species couldn't maintain a stable chromosome number across generations.

What was Morgan's key deduction about why different gene pairs show different linkage frequencies?

• Genes are arranged in a line along a chromosome, and different gene pairs show different linkage frequencies

• Closer genes have a smaller chance of being separated by a crossover event.

If chiasmata form anywhere along a chromosome with equal likelihood, how does distance between genes relate to crossover probability and recombination frequency?

Crossover probability increases with the distance separating two genes, so recombination frequency also increases with distance between genes.

What does an RF (recombination frequency) below 50% indicate about two genes?

Linkage — the proportion of recombinant classes is smaller than the proportion of parental classes.

What are the two reasons genes on nonhomologous chromosomes always show ~50% RF (independent assortment)?

(1) The two chromosome pairs can line up on the meiosis I spindle in either of two equally likely configurations, so summed over many meioses, parental and recombinant gamete numbers end up equal.

(2) If a crossover occurs between a gene and its centromere, that single meiosis produces two parental and two recombinant gametes. Together, a dihybrid on nonhomologous chromosomes produces all 4 gamete types (AB, Ab, aB, ab) at roughly equal frequency.

What is 1 map unit (m.u.) defined as?

The distance between two genes that yields 1% recombinant progeny (an RF of 1%).

Sturtevant's logic with genes w, y, m: if genes lie in a line, what should be true of the genetic distance between the two outermost genes vs. the distances involving the middle gene?

The distance between the two outside genes should be the greatest, and should roughly equal the sum of the two distances from the middle gene to each outside gene.

What did Sturtevant's data show about the order of w, y, and m, and why is the left-right orientation of the resulting map arbitrary?

• Only relative order matters, left/right orientation does not

How do you go about calculating recombination frequency in a three-point cross?

• RF functions from a pair of genes

• With three genes, analyze two at a time (ignore the third gene’s phenotype) and calculate the RF between those two

When do DCOs occur?

• Adding a third gene is what makes a DCO evident

• That produces a genotype distinct from both the true parental class and the single-crossover classes

• This is the smallest, rarest progeny class

Why must DCO progeny be counted twice?

• Each double-crossover individual is the result of two separate exchange events between vg and b (one in region 1, one in region 2)

• Counting them only once would undercount the total recombination that occurred in that interval.

What is the coefficient of coincidence (CoC), and how is it calculated?

• A quantitative measure of interference in a chromosomal interval =

• (observed frequency of double crossovers) ÷ (expected frequency of double crossovers, based on independent probabilities of each single crossover).

How can you identify which of three genes is "in the middle" using only the double-crossover progeny (no recombination frequency math needed)?

Find the gene whose alleles have recombined relative to both other genes in the double-crossover classes (while the other two genes retain their original parental associations) — that gene is the one in the middle.