Chapter 8 - Genes and Chromosomes

Question 1

What happens when 2 genes are close together on a chromosome? How can the distance between genes be calculated?

Answer 1

When two genes are close together on a chromosome, they are less likely to be separated when the chromosomes cross over.

The distance between the two genes is calculated by calculating the percentage of offspring that are recombinants. These offspring have had these two genes separated, as they do not express the same phenotypes as either of the parents.

This does not yield the actual position of the gene on the chromosome, but instead yields the distance separating the two genes analyzed.

Question 2

What are the 4 methods of sex determination? Give one example for each.

Answer 2

1. XY : humans
2. XO: grasshopers. Males simply do not have a second X chromosome.
   3: ZW system: birds.
3. Haploid vs diploid: bees.

Question 3

What are non-gene related methods of sex determination?

Answer 3

Temperature in alligators

First settler is female, rest are male in marine worms.

Dominant fish is female, rest are male.

Question 4

What are barr bodies? How do they affect fur color in calico cats?

Answer 4

Barr bodies occur in cells containing two X chromosomes, as the cell only needs one. Barr bodies are deactivated X chromosomes that migrate towards the outside of the cell.

In calico cats, when females are heterozygote for black and orange fur, the cell whose orange X is deactivate will cause black fur to grow, and the cell whose black X chromosome has been deactivated will grow orange fur.

Question 5

Where in the female egg cell development are there pauses before fertilization?

Answer 5

First, the egg cells pause in Prophase I, waiting to be ovulated. Once ovulated, the egg cell pauses once again in Metaphase II, waiting to be fertilized. As soon as it is fertilized, mitosis begins, and the cell duplicates again and again.

Question 6

What are the two main methods of genetic screening a foetus?

Answer 6

Amniocentesis, where amniotic fluid is withdrawn and analized, as it contains fetal cells.

CVS, where chorionic villi are sampled and analyzed.

Both methods allow for the karyotyping of the baby.