The Chromosomal Basis of Inheritance

1. The Chromosomal Basis of Inheritance Chapter 15

2. The importance of chromosomes • In 1902, the chromosomal theory of inheritance began to take form, stating: genes have specific locations (loci) on chromosomes, and you randomly get one chromosome from each parent.

3. Fruit flies • Fruit flies are Drosophila melanogaster • They have 4 pairs of chromosomes • They use the same XX, XY sex determination as people • They breed quickly (every 2 weeks, hundreds of offspring) • Wild type is the phenotype most common in natural populations

4. Linked genes • Genes on the same chromosome tend to be inherited together “linked genes” So why do offspring look different from parents?

5. Genetic Recombination • Genetic recombination is the production of offspring with combinations of traits different from those found in either parent. • 1. Independent Assortment • 2. Crossing Over

6. Independent Assortment • The phenotypes of the parents are called parental types. • The offspring, with new and different phenotypes, are called recombinant types or recombinants. • This happens because offspring receive one chromosome from each parent, and end up looking different.

7. Crossing Over • In prophase I of meiosis, genes of homologous chromosomes switch around, called crossing over. • This creates recombinant chromosomes, which makes offspring look different than parents.

8. Linkage Mapping • A genetic map is an ordered list of the locations of genes on a chromosome.

9. Linkage Mapping • Based on a linkage map, one can assume: the farther apart 2 genes are, the more likely a crossover will occur between them, therefore the recombination frequency is higher.

10. Linkage Mapping • A linkage map is a genetic map based on recombination frequencies. • Units are called map units and show the distance between genes. • 1 map unit = a 1% chance of recombination. • If two genes are 50 map units apart, how likely is recombination?

11. Cytogenic Map • A cytogenic map locates genes on chromosomes with respect to visible features like stained bands.

12. Sex linked traits • Men are XY, Women are XX • Eggs only carry X, sperm carry X or Y • Significant genes are only carried on the X chromosomes, which is larger and longer.

13. Sex-linked genes • A gene located on the X chromosome is called a sex-linked gene. • Females ONLY express the trait if they inherit the trait on both chromosomes: XaXa • If they are XAXA(normal) or XAXa (carrier) they do not have it • Men ONLY need ONE copy of the dominant gene to have the trait: XaY • If they are XAY, they don’t have it.

14. Sex-linked genes • Therefore, men are much more likely to have a sex-linked trait or disease, they only need one gene to get it, women need two! • Color-blindness • Baldness • Muscular dystrophy • Hemophilia

15. Practice: Hemophilia is sex linked. • Cross a female carrier with a normal male. • XAXa x XAY • If they have a girl, what are the chances she is a carrier? Normal? Has hemophilia? • If they have a boy, what are the chances he has hemophilia? Is normal? Is a carrier?

16. Practice: Hemophilia is sex linked. • Cross a man with hemophilia with a carrier woman. • XaY x XAXa • If they have a girl, what are the chances she is a carrier? Normal? Has hemophilia? • If they have a boy, what are the chances he has hemophilia? Is normal? Is a carrier?

17. X inactivation in females • Sometimes, the second X is inactivated in females, and is called a Barr body. • She is still normal and fertile, but sometimes has patchy skin. • This is what causes calico cats, every calico cat you see MUST be female.

18. Abnormal Chromosome Number • Nondisjunction is when chromosomes do not separate correctly during meiosis. • This causes an abnormal chromosome number, called aneuploidy • Trisomy is when you have 3 chromosomes instead of 2 (2n + 1) • Monosomy is when you have 1 chromosome instead of 2 (2n – 1) • Polyploidy is having more than one complete set of chromosomes • If any of the above organisms survive to birth, it will have major developmental abnormalities

19. Down Syndrome • Trisomy 21 is Down Syndrome, where the child has 3 copies of chromosome 21 (instead of 2) • Different facial features • Short stature • Mental retardation • Prone to leukemia and Alzheimer’s • Sexually underdeveloped or sterile • Reduced Life expectancy

20. Aneuploidy of Sex chromosomes • XXY is Klinefelter Syndrome: male sex organs, small testes, sterile • XYY: taller than average • XXX: trisomy X, healthy and normal • XO: Turner Syndrome, Monosomy X, sterile, appear female, no secondary sex characteristics develop without hormone therapy • All of the above have normal intelligence

21. Alterations of chromosome structure • Deletion: chromosomal fragment is deleted • Duplication: a chromosomal fragment is doubled • Inversion: chromosomal fragment gets reversed • Translocation: chromosomal fragments get switched around

22. Other Diseases • Cri du chat: deletion from chromosome 5, mentally retarded, meows, die early • Chronic myelogenous leukemia: 22 and 9 trade pieces, cancer of white blood cells results

23. 2 exceptions to this chapter • 1. Genomic imprinting: a zygote expresses only one allele (either mom’s or dad’s) instead of both. • 2. Inheritance of organelle genes: we have DNA in our mitochondria, and it comes only from mom