Chromosome Theory

March 1, 2013

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

SEX!Now that I got your attention…

We’ll be looking at sex determination

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

Sex Determination

Sex Determination

• Sometimes sex is not determined by presence of X and Y…

AlligatorClown fish

Sex-linked traits

Thomas Hunt Morgan

• Morgan used fruit flies, Drosophila melanogaster to study inheritance

Thomas Hunt Morgan

• Noticed that there were white-eyed male among many red-eyed offspring

Thomas Hunt Morgan

– So he mated white-eyed male with a red-eyed female (P-generation)• All members of the F1

generation had red eyes

• Noticed that there were white-eyed male among many red-eyed offspring

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

– So he mated white-eyed male with a red-eyed female (P-generation)• All members of the F1 generation

had red eyes

• Noticed that there were white-eyed male among many red-eyed offspring

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

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

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

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