Chromosomal Basis of Inheritance

1. Chromosomal Basis of Inheritance Chapter 15

2. Mendel & Chromosomes • Today we know that Mendel’s “hereditary factors” are located on chromosomes • So we can link Mendelian genetics to modern genetics through the genes that lie on the chromosome • Chromosome Theory of Inheritance • Mendelian genes have specific loci (positions) on chromosomes • Chromosomes undergo Segregation & Independent Assortment

4. Converting to Morgan speak • Phenotype • Dominant = wild type • Recessive = mutant • Wild-type – normal or typical • W+ • Mutant type – not normal • W • For example, vg+ = wild type for body size (full body) vg = mutant (vestigial body, smaller size)

5. T.H. Morgan studied flies • He studied Drosophila Melanogaster – Fruit fly • They _____ like flies • Take 2 weeks to breed • Hundreds of offspring per brood • Only 4 pairs of chromosomes • 3 autosomes and 1pair of sex chromosomes

7. Morgan’s Results • He crossed wild type (red eyes) with a mutant (white eyes), but did not get Mendelian results, or did he? • Gender or sex differences • Called Sex-linked gene or trait • ONLY F2 MALES = white eyes !!

9. Sex-linked Inheritance

10. Now… • Cross the other 3 combinations • What are the other combinations? • Are there any patterns?

11. What happens to produce? • All males are mutant, but all females are wild-type? • All females are wild type, but only 50% of males are? • 50% are wild-type, 50% are mutant?

12. Think about it… • IF the mother is homozygous dominant, then sons are • IF the mother is heterozygous, then sons are • IF the mother is homozygous recessive, then sons are

13. Think about it… (Page 2) • IF the father is wild-type, then daughters are • IF the father is mutant, then daughters are • So mothers determine ______ & wild type fathers produce ______ daughters

14. Known Sex-Linked Disorders • Duchenne muscular dystrophy • Progressively weakening of muscles and loss of coordination • Hemophilia • Blood that is unable to clot normally • Due to absence of proteins required for proper clotting

15. Sex-linked Disorders? • Which gender do you think is afflicted at a higher rate? • Why?

16. Hmm? • If XX is female & XY is male, but the Y chromosome contains virtually no genetic material, do females have more genetic information than males?

18. Females = X Inactivation • Although females receive 2 copies of alleles, one chromosome becomes inactivated during embryonic development • Due to XX • Chromosome inactivation is Random • Inactivation is due to methylation • So BOTH females and males are operating on only 1 sex chromosome • Barr Body – inactivated chromosome condenses • Lies on the inside of the nuclear envelope

19. More fly stuff 2 Characters: body color & wing size Body Color b+ = Grey (wild type) b = black (mutant) Wing Size vg+ = normal wings vg = vestigial wings (Reduced wing size)

20. Vocabulary • Linked Genes – located on same chromosome • Tend to be inherited together • Genetic Recombination – Offspring with new combination of genes inherited from parents • Parental Phenotype - at least one of the parental phenotypes • Recombinants – NOT either of the parental phenotypes

22. What should have happened? • What should have been the ratio if the characters were inherited via a Mendelian pattern? • How do the recombinants form?

23. Results • 2,300 offspring • Far higher proportion of parental phenotypes than expected from independent assortment • Genes are inherited together • There were also recombinants or non-parental phenotypes as well • Conclusion = Partial linkage & Genetic recombination (recombinants or recombinant types)

25. Recombination frequency • Calculate by (Total Recombinants / Offspring * 100) • If the genes are located on different chromosomes, then the recombination frequency should be 50% • In the flies, the recombinant frequency was less than 50%; it was about 17% • Evidence of that the 2 genes lied on the same chromosome • So some linkage but incomplete • More recombinants = less linkage

26. Linkage Maps • Crossing Over explains why some linked genes get separated during meiosis • Crossing Over occurs in Meiosis I • Farther apart 2 genes = Higher P(Crossing Over) • Linkage Map – genetic map based on the percentage of cross-over events • Map unit – 1% recombination frequency • Used ONLY for relative distances on the chromosome

28. Explain • We know that Mendel’s seed color and flower color were on the SAME chromosome, but they did not behave as linked genes. Explain.

29. Chromosomal Abnormalities • Nondisjunction – mishap where pairs of homologs do not move apart properly during meiosis • Could happen in Meiosis I or when Sister chromatids fail to separate correctly in Meiosis II • One gamete receives 2 of the same type of chromosome, while another receives no copy

31. Abnormal NUMBERS of chromosomes • Aneuploidy – abnormal number of chromosomes • Nondisjunction could result in a cell with 2n+1 • Here this cell would be considered aneuploid, and considered trisomic for that individual chromosome • -somy = different number of an INDIVIDUAL chromosomes • Trisomy – 3 copies of a chromosome • 2n + 1 • Monosomy – only 1 copy of a chromosome • 2n – 1

32. Abnormal Number of Chromosome SETS • Alteration of an ENTIRE CHROMOSOMAL SET • Called polyploidy • Triploid = 3n • Tetraploid = 4n • Polyploidy plants are fairly common • animals are less common • Polyploids are more normal than aneuploids • Hence, 1 chromosome extra or fewer is more disruptive, than an entire set of chromosomes extra or fewer

34. Down Syndrome • 1 of 700 • Trisomy 21 (each cell has 47 chromosomes, not 46) • Risk increases with maternal age

35. Klinefelter’s Syndrome • Male have extra X chromosome • Possess male sex organs, but are sterile • IF Female, 3 chromosomes (XXX) = healthy & normal

36. Turner Syndrome • Female with only 1 X • Only viable monosomy in humans