1. Chapter 15~ The Chromosomal Basis of Inheritance

2. Mendel’s Inheritance w/ Independent Assortment2

3. The Chromosomal Theory of Inheritance
Genes have specific loci on chromosomes and chromosomes undergo segregation and independent assortment 3

4. Chromosomal Linkage Morgan- used Drosophilia melanogaster
XX (female) vs. XY (male)
Discovered:
Sex-linkage: genes located on a sex chromosome
Linked genes: genes located on the same chromosome that tend to be inherited together 4

5. Chromosomal Linkage Sex - Linked
Genes on the X chromosome will only have 1 copy in males 5

6. 6

7. 7

8. Genetic recombination
Crossing over
Genes that DO NOT assort independently of each other
Genetic maps
The further apart 2 genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency
Linkage maps
Genetic map based on recombination frequencies 8

9. Recombination Calculations9

10. Human sex-linkage
SRY gene: gene on Y chromosome that triggers the development of testes
Fathers= pass X-linked alleles to all daughters only (but not to sons)
Mothers= pass X-linked alleles to both sons & daughters
Sex-Linked Disorders: Color-blindness; Duchenne muscular dystropy (MD); hemophilia 10

11. Sex-Linked Disorders 11

12. X- inactivation
X-inactivation: 2nd X chromosome in females condenses into a Barr body (e.g., tortoiseshell gene gene in cats)
A different X will be randomly be inactive in different cells. (The Barr body can be viewed in the nucleus.) 12

13. Chromosomal errors, I
Nondisjunction: members of a pair of homologous chromosomes do not separate properly during meiosis I or sister chromatids fail to separate during meiosis II
Aneuploidy: chromosome number is abnormal
• Monosomy~ missing chromosome
• Trisomy ~ extra chromosome (Down syndrome)
• Polyploidy~ extra sets of chromosomes 13

14. Chromosomal Errors - Turner’s (XO)14

15. Chromosomal Errors - Metafemale15

16. Chromosomal Errors - Klinefelter (XXY) or (XXXY)16

17. Chromosomal Errors - Down’s syndrome (Trisomy 21)17

18. Chromosomal errors, II Alterations of chromosomal structure:
Deletion: removal of a chromosomal segment
Duplication: repeats a chromosomal segment
Inversion: segment reversal in a chromosome
Translocation: movement of a chromosomal segment to another 18

19. Genomic imprinting Def: a parental effect on gene expression
Identical alleles may have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm.
Fragile X syndrome: higher prevalence of disorder and retardation in males (partially caused by maternal imprint) 19

20. Mitochondrial DNA Small circular DNA located in mitochondria
Inherited from the mother (maternally) 20