Lesson 10 - Sex Linkage

Question 1

The Chromosomal Theory of Inheritance

Answer 1

1. A chromosome carry many different genes
2. Each chromosome segregates during meiosis. Each gamete has half the number of
   chromosomes found in somatic cells
3. Chromosomes assort independently during
   meiosis
4. Females have two X-chromosomes, while males have one X and a Y chromosome. (where Y chromosome does not carry any alleles due to it’s small size)
5. The genes that are located on the same chromosome tend to be inherited together.
6. Gene linkage refers to genes that occur on the same chromosome and do not assort independently like Mendel’s Law
7. Sex linked traits are controlled by genes located on the sex chromosomes.

A recessive trait located on the X chromosome is more likely to express itself in males than in females since males need only one copy of the recessive allele while females need two for sex-linked recessive traits.

Question 2

Sex - Linkage

Answer 2

American Geneticist Thomas Hunt Morgan (1866 - 1945) provided a deeper understanding of gender and inheritance

He was the first to use fruit flies, Drosophila melanogaster, to study inheritance…WHY?
- Reproduce rapidly (many offspring)
- Short life cycle (many generations in a short period of time)
- Very small in size for “housing”
- Males are easily distinguished from females
- In fruit flies, red eyes are dominant over white eyes.
- Morgan noticed a white eye male amongst many red-eyed offspring in F2 generation –the female never had white eyes in F2!
- Morgan hypothesized that the Y – Y-chromosome found in males only did not have the eye colour gene, SO the gene for eye colour must be on the X chromosome