110 Gene Mapping

Summary

Thomas Hunt Morgan's groundbreaking research on fruit flies in the early 20th century helped establish the concept of gene linkage and recombination frequencies. Morgan studied two genes related to body color and wing shape in Drosophila melanogaster to determine if they were linked. Through test crosses, he observed that most offspring resembled parental traits, but some exhibited recombinant traits due to crossing over during meiosis. This led to the discovery that the frequency of crossing over could be used to estimate the relative distance between genes on chromosomes, laying the foundation for genetic mapping.

Takeaways

• 😀 Gene linkage refers to the situation where two genes are located on the same chromosome and are inherited together.
• 😀 Thomas Hunt Morgan's research on fruit flies (Drosophila melanogaster) helped establish the concept of gene linkage.
• 😀 Fruit flies were chosen for genetic studies because of their rapid generation time (7-10 days), allowing for quick observations of inheritance patterns.
• 😀 The two genes Morgan studied were for body color (gray vs. black) and wing shape (normal vs. vestigial).
• 😀 Gray body color is dominant to black, and normal wings are dominant to vestigial wings in fruit flies.
• 😀 Morgan used a test cross to determine whether the genes for body color and wing shape were linked, crossing heterozygous F1 females with homozygous recessive males.
• 😀 If the genes were unlinked, the offspring would show a 1:1:1:1 phenotypic ratio, but linked genes produced a 1:1 ratio of gray normal to black vestigial flies.
• 😀 The presence of gray vestigial and black normal flies in smaller numbers suggested the genes were linked but occasionally underwent crossing over.
• 😀 Crossing over during meiosis swaps genetic material between chromosomes, creating new combinations of alleles and leading to recombinant phenotypes.
• 😀 The recombination frequency, a measure of crossing over, can be used to calculate the relative distance between two genes on a chromosome, with a 17% frequency indicating the genes were 17 map units apart.

Q & A

What was the main objective of Thomas Hunt Morgan's experiment?

Morgan aimed to investigate whether genes for different traits were linked, meaning they were located on the same chromosome, and to determine how their relative positions on the chromosome could be mapped.

Why were fruit flies (Drosophila melanogaster) chosen for Morgan's experiment?

Fruit flies were chosen because they reproduce quickly, with a new generation appearing every 7-10 days, allowing for rapid observation of multiple generations in a short period.

What two traits did Morgan study in fruit flies, and how are they inherited?

Morgan studied body color (gray vs. black) and wing shape (normal vs. vestigial). Gray body color (G) is dominant over black (g), and normal wings (N) are dominant over vestigial wings (n).

What was the result of the cross between a gray normal female and a black vestigial male?

The F1 generation consisted of all gray and normal flies, as those traits are dominant, with the offspring being heterozygous for both traits.

What is a test cross, and why did Morgan use it in his experiment?

A test cross involves crossing an individual with an unknown genotype (in this case, heterozygous for both traits) with a homozygous recessive individual. Morgan used it to determine if the two genes were linked by observing the offspring's phenotypes.

What were the two possible outcomes of the test cross if the genes were unlinked?

If the genes were unlinked, the offspring would show a 1:1:1:1 ratio of all four possible phenotypes: gray normal, gray vestigial, black normal, and black vestigial.

What did Morgan observe in the test cross, and what did it suggest about the genes?

Morgan observed that most offspring were either gray normal or black vestigial, suggesting the genes were linked. However, gray vestigial and black normal flies were also present, indicating some recombination had occurred.

What is recombination, and how did it affect Morgan’s results?

Recombination occurs when genetic material is exchanged between homologous chromosomes during meiosis, leading to new allele combinations. This caused some offspring to have different combinations of traits than the parental phenotypes, indicating crossover had occurred.

How can the recombination frequency be used to map the distance between two genes on a chromosome?

The recombination frequency, calculated as the percentage of recombinant offspring, can be used to estimate the relative distance between two genes on a chromosome. A higher recombination frequency indicates greater distance between the genes.

What does a recombination frequency of 17% mean in terms of map units?

A recombination frequency of 17% means the two genes are 17 map units apart, with each map unit corresponding to a 1% frequency of crossover between the genes.