The Chromosomal Basis of Inheritance

1. The Chromosomal Basis of Inheritance

2. In 1902 Sutton noted the parallels between the behavior of chromosomes and the behavior of Mendel’s factors. Chromosomal theory of inheritance: genes have specific loci on chromosomes, and the chromosomes undergo segregation and independent assortment

4. Morgan traced a gene to a specific chromosome - Morgan used Drosophila “fruit flies” for his experiments - only 4 pairs of chromosomes - wild type flies have red eyes - white eyes is a mutant phenotype What do you call a Drosophila who likes to drink? A bar fly.

5. - Morgan mated the white-eyed male to a red-eyed female - all F1 offspring were red-eyed - The F2 offspring were not 3:1; instead all females were red-eyed while half of the males had red and half had white eyes - eye color was linked to the fly’s sex

6. Genes located on sex chromosomes are called sex-linked genes - Morgan’s evidence that a specific gene is carried on the X chromosome helped confirm the chromosomal theory of inheritance.

7. Concept 15.1

8. linked genes: genes that are located on the same chromosomes and tend to be inherited together - linked genes deviate from expected Mendelian ratios -ex. in flies body color and wing shape are inherited together

10. Genetic recombination: the production of new combinations of traits inherited from two parents yellow-round x green-wrinkled YyRr x yyrr Parental types: when the offspring phenotypes are identical to the parents - ex ¼ YyRr, ¼ yyrr

11. Recombinants: when the offspring phenotypes are new combinations ex ¼ Yyrr, ¼ yyRr When 50% of all offspring are recombinants, we say there is a 50% frequency of recombination - 50% frequency is observed for genes located on different chromosomes

12. Geneticists can use recombination data to map a chromosomes genetic loci Genetic map: an ordered list of the genetic loci along a particular chromosome Linkage map: a genetic map based on recombination frequencies

13. Map of body-color (b), wing-size (vg) and cinnabar (cn) - cn and b is 9% - cn and vg is 9.5% - b and vg is 17%

14. Concept 15.2

15. Sex-linked genes have unique patterns of inheritance - fathers pass sex-linked alleles to daughters, but not sons - mothers pass sex-linked alleles to both sons and daughters

16. Because males only have one locus, they cannot be heterozygous Sex-linked disorders in humans - muscular dystrophy: 1/3500 males in the US - hemophilia: absence of proteins for blood clotting

18. Nondisjunction: the members of a pair of homologous chromosomes do not move apart properly during meiosis I or II. - gametes are (n+1) or (n-1) if they have too many or too few chromosomes

19. Concept 15.4 Aneuploidy: having an abnormal chromosome number Trisomic: a chromosome in triplicate (2n+1); ex. trisomy 21 (Down’s syndrome) Monosomic: if a chromosome is missing (2n-1)

21. Polyploidy: organisms that have more than two complete chromosome sets - triploidy (3n) - tetraploidy (4n) Polyploids are more normal in appearance than aneuploids

22. Breakage of a chromosome can lead to four types of changes in chromosome structure Deletion: occurs when a chromosomal fragment lacking a centromere is lost during cell division

23. Duplication: a fragment becomes attached as an extra segment to a sister chromatid Inversion: a chromosomal fragment can reattach to the original chromosome in reverse order Translocation: the fragment joins a nonhomologous chromosome

25. Concept 15.4

26. Alterations in chromosome number and structure are associated with several human disorders - Down syndrome: 1/700 children born; extra 21st chromosome - Klinefelter syndrome: XXY male - Turner syndrome: XO female