1. Chromosome Theory
March 1, 2013

2. Chromosome Theory Chromosomes are the carriers of genetic material
Genes are units of heredity
Theory correctly explains mechanisms of Mendelian’s law of inheritance
Paired chromosome segregate during meiosis
Each sex cell (aka gamete) has half the number of chromosomes found in the somatic cells
Explains why each gamete only possess one of each of the paired alleles
Important to note that both egg and sperm contribute equally to the genetic endowment of offspring

3. Now that I got your attention… We’ll be looking at sex determination

4. Sex Determination Humans have 23 pairs of chromosomes
22 pairs are autosomes
1 pair is a pair of sex chromosomes
XX in females
XY in males

5. Sex Determination

6. Sex Determination
Sometimes sex is not determined by presence of X and Y…
Clown fish
Alligator

7. Sex-linked traits

8. Thomas Hunt Morgan
Morgan used fruit flies, Drosophila melanogaster to study inheritance

9. Thomas Hunt Morgan
Noticed that there were white-eyed male among many red-eyed offspring

10. Thomas Hunt Morgan
Noticed that there were white-eyed male among many red-eyed offspring
So he mated white-eyed male with a red-eyed female (P-generation)
All members of the F1 generation had red eyes

11. Thomas Hunt Morgan
Noticed that there were white-eyed male among many red-eyed offspring
So he mated white-eyed male with a red-eyed female (P-generation)
All members of the F1 generation had red eyes
Mated 2 hybrids of the F1 generation
F2 generation produced ¾ red and ¼ white (explained by Mendelian Genetics)
But all the females had red eyes
The only ones with white eyes were male
Half the males had red eyes and half the males had white eyes

12. Thomas Hunt Morgan Mated 2 hybrids of the F1 generation
F2 generation produced ¾ red and ¼ white (explained by Mendelian Genetics)
But all the females had red eyes
The only ones with white eyes were male
Half the males had red eyes and half the males had white eyes

13. Thomas Hunt Morgan Morgan looked at the cell
More specifically the chromosomes
Found females have four homologous pairs
Males have three homologous pairs (the 4th pair was only partially homologous and determined sex)
Males were found to have an X paired with a Y chromosome
X and Y not completely homologous, they contain different genes

14. Thomas Hunt Morgan Morgan explained why males had white eyes
Y chromosome doesn’t contain genes for eye colour
Traits located on sex-chromosomes are called sex-linked traits

15. Colour-blindness Seen in more males than females
Extremely rare to be present in females
Why?
Under what circumstances can a female be colour-blind? Let’s look at the probability