2. Chromosomes and Inheritance
Gene for eye color located on X chromosome
A trait determined by a gene on the sex chromosome is said to be sex-linked.
These experiments were the first to demonstrate that genes were carried on chromosomes
The segregation of white eyes corresponded to segregation of X chromosome

5. CALVIN KNEW WHAT FLY CHROMOSOMES LOOKED LIKE
He knew that Drosophilla had 4 pairs of chromosomes numbered 1-4
Chromosomes 2, 3, & 4, are the same in both genders
Chromosome pair # 1 looked different in males and females
Males had an X and a Y chromosome
Females had 2 X chromosomes
Calvin reasoned that the X and Y chromosomes determined gender

6. CALVIN WAS RIGHT
Males get their X chromosome only from their mother and their Y chromosome from their father
If the gene for white eyes is on the X chromosome it explains the fact that males carry only one copy of the gene that they got from their mother
Calvin correlated an unusual pattern of inheritance, X linked, with an unusual pattern of chromosome segregation, XY for males and XX for females

8. GENDER DETERMINATION IN FRUIT FLIES
Gender in flies is determined by the number of X chromosomes
XX Female X Male
Calvin realized these white eye daughters carried 2 X chromosomes, ww, from their mother and a Y from their father
They were wwY but still female and had white eyes
He would see these flies about 1/1000 offspring

9. GENDER DETERMINATION IN FRUIT FLIES
The males had only 1 X chromosome from their father that was W
The red eye males were actually W -
They had no Y chromosome but they were still males
How was this strange distribution of chromosomes happening?

10. NONDISJUNCTION
Calvin realized that such flies could arise if 2 X chromosomes had failed to segregate from each other during the first meiotic division in the mother
When the female is making eggs both of her X chromosomes go into 1 egg
So she will produce an egg with 2 X chromosomes
And an egg with no X chromosome

13. GENES ON CHROMOSOMES
Gender in flies is determined by the number of X chromosomes
XX Female X Male
The Y doesn’t make any difference
It was this finding that the chromosomes an individual carried could determine something as important as gender that convinced the last of the skeptics that chromosomes were the units of heredity
Calvin’s paper on non disjunction was the first paper published in the Journal of Genetics
Non disjunction provided the proof that genes reside on chromosomes

14. NONDISJUNCTION
Nondisjunction of X chromosome in humans like flies leads to XXY individual
This individual will be male because in humans Y chromosome determines maleness
In Drosophila would be female because has two X chromosomes
In humans severe non disjunction causes death in utero
That is why the study of non disjunction is so important it proved that genes are carried on chromosomes and it causes such havoc in organisms

15. Mendelian Inheritance
Mendel’s Second Law of Heredity
(Law of Independent Assortment)
Genes that are located on different chromosomes assort independently of one another.
The presence of an allele of one trait does not preclude the presence or absence of any other allele of any other trait

16. HUMAN GENETICS Humans have about 25,000 genes
If 2 genes are located very close together on the same chromosome they may not segregate independently
2 genes that are very close together on a chromosome may only rarely be separated by a crossover event
The father apart 2 genes are the more chance there is they will be separated by a crossover event and behave as independent genes