Chromosomes and Genes Chapter 10 1 Chromosomes and

Chromosomes and Genes Chapter 10 1

Chromosomes and Inheritance • Mendel demonstrated the principle of inheritance of genes with a detailed study of pea plants. 2

Chromosomes and Inheritance • These same principles can be demonstrated in a few months using fruit flies (Drosophila) as experiment subjects. 3

Sex Determination • Drosophila have four pairs of chromosomes while humans have twenty three. • Three pairs are always the same, while the fourth pair differs among the sexes. 4

Sex Determination • In the females they are rod shaped like the others, while in the males they appear to be hooked like a candy cane (Y chromosome). 5

Sex Determination • Because these chromosomes are linked to the gender or sex of the individual, they are called the sex chromosomes. • All the other chromosomes are called autosomes. 6

Male or Female? • In Drosophila the sex of the fly is dependent on the number of X chromosomes not the presence of a Y chromosome. • A normal female has two X chromosomes, a normal male has one. 7

Prince or Princess? • Like the other autosomes, the sex chromosomes segregate during meiosis. • When gametes combine during fertilization, the offspring have a 50% chance of being either a Prince or Princess. 8

Boy or Girl? X XX XX Y XY XY 9

Humans • Humans have 23 pairs of chromosomes. • 22 pairs are autosomes and 1 pair are the sex chromosomes. • In humans, it is the presence of a Y chromosome that determines sex. 10

Gender Possibilities • A normal human female has the sex chromosomes XX. • A normal male has XY. • Variations can occur due to mutation and problems in cell replication. • X, XXY, XYY are examples of abnormal genotypes. • Note that there must be an X chromosome for the person to survive. 11

Sex Linkage and Linkage Groups • The fact that Drosophila has only four chromosome pairs and many traits raised the issue of a chromosomes having more than one gene. • Genes on the same chromosome belong to the same linkage group. 12

Where on the chromosomes? • Genes on a chromosome are like beads on a string, and they do not change position from one generation to the next. • The linkage groups are stable. 13

Where on the chromosomes are the genes located? • The presence of unique characteristics can be used to indicate where specific genes are located on chromosomes. 14

Thomas Hunt Morgan • Geneticist working at Columbia University on Drosophila. • noticed male with white eyes - different than the ‘normal’ red eye phenotype • crossed mutant male with normal female to determine where genes are located 15

My White Eyed Boy 16

F 1 cross XR XR Xr Y X RX r X RY 17

F 1 Cross but are carriers 18

F 2 Cross XR XR Xr Y X RX R X RY X RX r Xr Y 19

F 2 Cross All females have red eyes, one is a carrier 20

Sex Linkage • sex-linked genes - genes that are located on the X chromosome • sex-linked trait - traits determined by sexlinked genes 21

Expression of sex-linked genes • Normally, the dominant allele masks the recessive allele. • In sex-linked traits, the Y chromosome does not have a copy of the allele. • Which ever allele the X chromosome has will be expressed. 22

Sex linked traits in Humans • Examples of traits that are sex linked in humans are red-green colour blindness and hemophilia. 23

Red-Green Colour Blindness 24

Red-Green Colour Blindness 25

Red-Green Colour Blindness This is what we see 26

Red-Green Colour Blindness This is what you would see if you were colour blind. 27

Red-Green Colour Blindness 28

Red-Green Colour Blindness 29

Hemophilia in the Royal family 30