9.8.4 Linking Genes to Chromosomes: The Work of Morgan

Question 1

work of Morgan

Answer 1

• T.H. Morgan used the common fruit fly, Drosophila melanogaster, in his pioneering research of genetics.
• In performing their experiments, Morgan and Walter Sutton were able to link Mendel’s findings to chromosomal movement:
· Sutton correlated chromosomal movements during meiosis to Mendel’s laws
of segregation and independent assortment.
· Morgan discovered sex-linked traits meaning that genes are on sex chromosomes.

Question 2

note

Answer 2

• T.H. Morgan determined that there were many advantages to using the common fruit fly, Drosophila melanogaster, for his experiments. The flies are easy to cultivate and have a short lifespan. Many mutant strains exist, providing many genotypes and phenotypes to perform experiments on.
• Mendel’s experiments showed that pairs of discrete factors caused traits. When Mendel’s findings were rediscovered at the turn of the century, it was still unclear where genes were physically located. Morgan’s experiments shed light on this question.
• In one experiment, Morgan crossed red-eyed flies with
  white-eyed flies. All of the F1 individuals had red eyes.
  Crossing F1 flies yielded a 3:1 ratio of red to white-eyed flies. All of the white-eyed flies were male. One explanation is that the white eye trait is lethal in females.
• In the second experiment, Morgan crossed white-eyed males with red-eyed females. In the F1, white-eyed and red-eyed flies appear in a 1:1 ratio. Males and females occurred in equal numbers in both phenotype classes. The explanation that the white eye trait is lethal in females can be ruled out.
• Both Morgan’s and Sutton’s experimental work provided evidence that genes were located on chromosomes.
• Walter Sutton proposed that chromosomal movement during meiosis was correlated to independent assortment and segregation. Pairs of alleles could be located on pairs of chromosomes, which separate during meiosis.
• Morgan observed that one of the four pairs of chromosomes located in fruit flies was responsible for determining sex. Females are XX and males are XY. Morgan hypothesized that the gene for eye color is located on the sex chromosomes, which causes a different pattern of inheritance in male versus female flies.

Question 3

How did Walter Sutton’s work performed in 1902 advance the field of genetics?

Answer 3

• He observed a correlation between Mendel’s law of segregation and the separation of homologous chromosomes during meiosis.

Question 4

With regards to his first experiment, why did Morgan suspect that the white-eye allele might be lethal?

Answer 4

• Because there were no white-eyed females produced in the F2 generation.

Question 5

In Morgan’s second experiment he crossed white-eyed males with red-eyed females. What result did he observe in the offspring of this pairing?

Answer 5

• Half of the females had white eyes.

Question 6

While studying the genetics of various traits in fruit flies, T.H. Morgan observed that certain characteristics have different inheritance patterns in males versus females. Such characteristics are said to be

Answer 6

• sex-linked.

Question 7

Concerning Morgan’s experiments on sex-linkage, which of the following statements is incorrect?

Answer 7

• Morgan observed that when red-eyed F1 offspring were crossed, a 3:1 ratio of red-eyed to white-eyed offspring occurred in both males and females.

Question 8

Which of the following is not one of the reasons that Morgan chose Drosophila sp. fruit flies for his genetics experiments?

Answer 8

• Their genome is comparable to the human genome.