Chapter 15 Chromosomal Basis of Inheritance

1. Chapter 15 Chromosomal Basis of Inheritance

2. Mendel & Chromosomes • Mendel was ahead of his time. 19th C cytology suggested a mechanism for his earlier findings. What did they find? • Chromosomes and genes are both present in pairs in diploid cells. • Homologous chromosomes separate and alleles segregate during meiosis. • Fertilization restores the paired condition for both chromosomes and genes.

3. Chromosome Theory of Inheritance • Mendelian genes have specific loci on chromosomes • Chromosomes are what physically undergo segregation and independent assortment.

4. Morgan’s Fruit Flies • Morgan first associated a specific gene with a specific chromosome. • Why fruit flies? • Breed quickly (two week generations) • 4 pairs of chromosomes (3 pair of autosomes, 1 pair of sex chromosomes) • Females = XX • Males = XY

5. Morgan’s Fruit Flies • Wild Type flies are the most common natural phenotype. (Red Eyes) • After a series of crosses, Morgan produced mutants with white eyes. • After a few generations, Morgan noted that only males displayed the white eyes. • He concluded that certain genes are located on the sex chromosome and thus linked to sex. • Sex-linked genes (ie: hemophilia)

6. Sex-linked Traits

7. Sex-linked Traits • Morgan concluded the gene with the white-eyed mutation is on the X chromosome. Y chromosome = no info • Males (XY) only need one copy of recessive allele to show trait.

8. Linked Genes • All genes located on the same chromosome tend to be inherited together. • Chromosome passed on as a unit. • Testcross results varied from those predicted by the law of independent assortment. • This showed that certain genes will assort together. (on same chromosome)

9. Linked Genes

10. Linked Genes • Body color and wing shape are usually inherited together (same chromosome)

11. Recombinants • Where did the other phenotypes come from? (grey-vestigial and black normal) • Genetic recombination= offspring with new combinations of traits inherited from two parents • How?? • independent assortment of genes (non-homologous) • crossing over of genes (homologous)

12. Recombinants • Mendel’s dihybrid crosses produced recombinant genotypes. • 50% parental : 50% recombinant genotypes typical for nonhomologues • Metaphase I • YR, Yr, yR, and yr • Seed shape and color tetrads are independent from one another

13. Recombinants • Linked genes tend to move together during meiosis/fertilization • If Independent assortment of genes • Expect a 1:1:1:1 phenotype ratio • If Complete linkage of genes • 1:1:0:0 ratio (all parental) • Observed 17% recombinant flies • Suggested Incomplete linkage of genes

14. Crossing Over • Prophase I: homologous chromosomes can “swap” alleles • More variable gametes than simple mendelian rules would predict

15. Therefore, Crossing Over Explains:

16. Linkage Maps • Ordered list of genetic loci along chromosome • Based on recombination frequencies between two genes • Higher % of recombination = farther apart • More places in between genes for crossing over to occur and separate the genes

17. Linkage Maps • The recombination frequency between cn and b is 9%. • The recombination frequency between cn and vg is 9.5%. • The recombination frequency between b and vg is 17%.

18. Linkage Maps • Map units are the distances between genes on a chromosome. • 1 map unit = 1% recombination • 50% recombination = so far apart that crossing over is all but certain • Remember, 50% recomb. = ind. assortment (non-homologous) • Linkage maps show relative order/distance • More recent studies show exact distances and order

19. Sex Chromosomes

20. X-Y Sex Determination • X and Y behave as homologues • Each egg receives an X from XX mother • One sperm receives X and one Y • Results in 50/50 chance of male or female • SRY Gene • Present (on Y) : gonads develop into testes (male) • Not present (no Y): gonads become ovaries (female) • SRY also regulates other genes

21. Sex-Linked Genes • Sex chromosomes also contain other genes. (ie: drosophila eye color) • Females must be homozygous recessive to display trait (XX – second X can mask recessive) • Females can be carriers • Males only need to inherit a single copy to show trait • Can a male be a carrier?

22. Sex-Linked Disorders • Duchenne Muscular Dystrophy • 1/3500 males • Progressive muscular weakening • Die by mid-20’s • Missing X-linked gene • No production of dystrophin (muscle protein)

23. Sex-Linked Disorders • Hemophilia • Absence of one or more clotting factors • affected individuals cannot stop bleeding normally • treated with protein injections

24. Barr Bodies • Only one of the females X chromosomes is active • The other becomes a Barr body • When assorted into an ovum, the Barr body becomes activated again • Which X becomes Barr body is random in each cell • Approx. 50% express each allele (if hetero)

25. X-Inactivation in Females

26. Nondisjunction • Errors with meiotic spindle • Meiosis I: Homologous tetrad doesn’t separate OR • Meiosis II: Sister chromatids don’t separate • Some gametes receive two of the same type of chromosome and another gamete receives no copy

27. Aneuploidy • Results from fertilization involving nondisjoined gamete(s) • Trisomy three copies of a particular chromosome (2n + 1) • Monosomy only one copy of a particular chromosome (2n – 1)

28. Down Syndrome • Three copies of chromosome 21 • 1/700 children born each year • Definite link with maternal age

29. Aneuploidy in Sex Chromosomes • XXY Male (Klinefelter’s Syndrome) • Male sex organs, sterile w/ femininity • XYY Males • Tend to be taller than normal

30. Aneuploidy in Sex Chromosomes • XXX Females • Will develop as normal females • XO Females (monosomy – Turner syndrome) • Immature females • 1/2500 live female births

31. Changes in Chromosomes