8. Chromosomal theory Sex determination and fruit flies

Question 1

What is the chromosomal theory?

Answer 1

A theory that proposes that genes are present in chromosomes

Question 2

Explain the chromosomal theory from T. Boveri, W. Sutton and TH. Morgan’s perspective.

Answer 2

Boveri and Sutton’s theory states that genes are found at specific locations on chromosomes, and that the behaviour of chromosomes during meiosis can explain Mendel’s laws of inheritance.
- Sutton : separation of chromosomes into gametes during meiosis (making egg and sperm) under a microscope.
- Boveri : embryonic development of sea urchins does not occur unless chromosomes are present.

Morgan discovered a mutation that affected fruit fly eye colour. Observed mutation was inherited differently by male and female flies
- Based on the inheritance pattern, Morgan concluded that the eye colour gene must be located on the X chromosome.

Question 3

Explain sex determination in humans and other organisms.

Answer 3

Sex determination in Drosophila is based on the number of ‘X’ chromosomes
XX = female
XY = male

Sex determination in humans is based on the presence of a ‘Y’ chromosome
XX (XXX, X0) = female
having a Y chromosome (XY, XXY, XYY) = male

Question 4

Discuss the difference between wild-type and mutant phenotypes and aware of how they can be used to understand inheritance patterns.

Answer 4

Wild type: individual having the normal phenotype (over half of pop.)

Mutant type: individual having a phenotype that differs from the normal phenotype (not present in majority)

Example: fruit flies with white eyes are mutants while those with red eyes are wild type

Question 5

Outline why fruit flies are good experimental subjects in genetics.

Answer 5

1. easily bred in a lab
2. prolific breeders
3. short life cycle (≈ 2w) and length can be manipulated by altering incubation temp.
4. prominent charac. used in sex determination
5. fairly simple chromosome organization of 3 pairs of autosomal chromosomes and 1 pair of sex chromosomes.

Question 6

What are Barr bodies?

Answer 6

In X-inactivation, an X chromosome is compacted to make a small, dense structure called a Barr body
- found in the nucleus and can be seen if stained
Examples
- XX (one active X one barr body)
- XY (one active X)
- XXY (one active X one barr body)
- XXX (one active X two barr bodies)

Question 7

When and how do you apply the Chi Square test?

Answer 7

• applies when the cage doesn’t align with punnett square
• (O-E)^2/E
• Find DF (n-1)
• go to 0.05 column
• observed chi square has to be wayyy smaller than expected