R. A. Emerson crossed two different pure-breeding lines of...

R. A. Emerson crossed two different pure-breeding lines of corn and obtained a phenotypically wild-type $\mathrm{F}_{1}$ that was heterozygous for three alleles that determine recessive phenotypes: an determines anther; br, brachytic; and $f,$ fine. He testcrossed the $\mathrm{F}_{1}$ with a tester that was homozygous recessive for the three genes and obtained these progeny phenotypes: 355 anther; 339 brachytic, fine; 88 completely wild type; 55 anther, brachytic, fine; 21 fine; 17 anther, brachytic; 2 brachytic; 2 anther, fine. a. What were the genotypes of the parental lines? b. Draw a linkage map for the three genes (include map distances). c. Calculate the interference value.

R. A. Emerson crossed two different pure-breeding lines of corn and obtained a phenotypically wild-type $\mathrm{F}_{1}$ that was heterozygous for three alleles that determine recessive phenotypes: an determines anther; br, brachytic; and $f,$ fine. He testcrossed the $\mathrm{F}_{1}$ with a tester that was homozygous recessive for the three genes and obtained these progeny phenotypes: 355 anther; 339 brachytic, fine; 88 completely wild type; 55 anther, brachytic, fine;
21 fine; 17 anther, brachytic; 2 brachytic; 2 anther, fine.
a. What were the genotypes of the parental lines?
b. Draw a linkage map for the three genes (include map distances).
c. Calculate the interference value.

Key Concepts

Genetic Linkage

Genetic linkage refers to the phenomenon where genes that are physically close on the same chromosome tend to be inherited together. This is because the likelihood of a crossover event occurring between linked genes is lower compared to genes that are far apart, which results in non-independent segregation of alleles during meiosis.

Recombination Frequency and Map Distance

Recombination frequency is the proportion of offspring in a cross that exhibit recombinant phenotypes due to crossing-over events between linked genes. This frequency is used to calculate map distances between genes, expressed in centimorgans, providing a quantitative measure of the physical distance on a chromosome.

Interference in Crossing Over

Interference is the phenomenon where the occurrence of one crossover event in a chromosomal region reduces the probability of another nearby crossover. It is quantified by the interference value, which helps determine how crossover events are distributed along the chromosome and is crucial in accurately constructing linkage maps.

Testcross

A testcross is a genetic cross between an individual with an unknown genotype (typically heterozygous for certain traits) and an individual that is homozygous recessive for the traits being studied. This type of cross is used to reveal the genotype of the test individual by analyzing the phenotypic ratios of the progeny.

Parental and Recombinant Phenotypes

Parental (or non-recombinant) phenotypes are the trait combinations observed in the original pure-breeding parental lines, while recombinant phenotypes result from crossover events between linked genes. The ratio of parental to recombinant types is a key indicator in mapping gene order and estimating recombination frequencies.

Pure-breeding Lines and Heterozygosity

Pure-breeding lines are populations that are homozygous at specific loci, consistently expressing particular phenotypes. When two pure-breeding lines with different trait expressions are crossed, the F1 generation is heterozygous, setting the stage for further analysis such as testcrosses to determine the linkage relationships and map distances between genes.

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