Waxy endosperm (w x), shrunken endosperm (s h), and yellow...

Waxy endosperm $(w x),$ shrunken endosperm $(s h),$ and yellow seedlings (v) are encoded by three recessive genes in corn that are linked on chromosome $5 .$ A corn plant homozygous for all three recessive alleles is crossed with a plant homozygous for all the dominant alleles. The resulting $\mathrm{F}_{1}$ are then crossed with a plant homozygous for the recessive alleles in a three-point testcross. The progeny of the testcross are $$\begin{array}{cccc} w x & s h & V & 87 \\ W x & S h & v & 94 \\ W x & S h & V & 3,479 \\ w x & s h & v & 3,478 \\ W x & s h & V & 1,515 \\ w x & S h & v & 1,531 \\ w x & S h & V & 292 \\ W x & s h & v & \frac{280}{10,75 t} \\ \text { Total } & & & 10,75 \end{array}$$ a. Determine the order of these genes on the chromosome. b. Calculate the map distances between the genes. c. Determine the coefficient of coincidence and the interference among these genes.

Waxy endosperm $(w x),$ shrunken endosperm $(s h),$ and yellow seedlings
(v) are encoded by three recessive genes in corn that are linked on chromosome $5 .$ A corn plant homozygous for all three recessive alleles is crossed with a plant homozygous for all the dominant alleles. The resulting $\mathrm{F}_{1}$ are then crossed with a plant homozygous for the recessive alleles in a three-point testcross. The progeny of the testcross are
$$\begin{array}{cccc}
w x & s h & V & 87 \\
W x & S h & v & 94 \\
W x & S h & V & 3,479 \\
w x & s h & v & 3,478 \\
W x & s h & V & 1,515 \\
w x & S h & v & 1,531 \\
w x & S h & V & 292 \\
W x & s h & v & \frac{280}{10,75 t} \\
\text { Total } & & & 10,75
\end{array}$$
a. Determine the order of these genes on the chromosome.
b. Calculate the map distances between the genes.
c. Determine the coefficient of coincidence and the interference among these genes.

Key Concepts

Genetic Interference

Genetic interference describes the phenomenon where the occurrence of one crossover event reduces the likelihood of another crossover occurring nearby. It is quantified as one minus the coefficient of coincidence and is important in understanding how physical constraints on the chromosome can affect the distribution of crossover events during meiosis.

Coefficient of Coincidence

The coefficient of coincidence is a ratio that compares the observed number of double crossover events to the expected number, calculated based on the product of the individual recombination frequencies. This measure provides insight into the extent of crossover interference, where fewer double crossovers than expected indicate interference.

Crossing Over and Double Crossovers

Crossing over is the exchange of genetic material between homologous chromosomes during meiosis, contributing to genetic diversity. Double crossovers refer to events where two crossover events occur between the loci of interest. These events are essential in mapping studies because they help refine the distances between genes and confirm their order on the chromosome.

Recombination Frequency

Recombination frequency is the percentage of recombinant offspring produced in a cross and is used as a measure of the physical distance between two genes. It is calculated by dividing the number of recombinant progeny by the total number of progeny, and it gives an estimate of the map distance in centimorgans where 1 centimorgan roughly corresponds to a 1% recombination rate.

Linkage Mapping

Linkage mapping is a process used to determine the relative positions of genes on a chromosome based on how frequently they are inherited together. The closer two genes are located on a chromosome, the less likely they are to be separated by recombination during meiosis, which can be exploited to determine their order and the distances between them.

Three-Point Cross

A three-point cross is a genetic mapping strategy that involves three genes to analyze their order and the frequency of recombination between them. This method is crucial in deducing gene order because comparing recombination frequencies between various pairs of genes can reveal the relative positions and distances, as well as the occurrence of single and double crossover events.

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