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

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

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

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

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?

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

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

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

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

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