1. Chromosomal abnormalities

2. Abnormal Chromosome number Non-disjunction= error in chromosome separation during meiosis –Homologous chromosomes –Sister chromatids 1 gamete receives 2 copies of chromosome, 1 receives none. Results in aneuploidy (incorrect # of chromosomes)

3. Non-disjunction

4. Aneuploidy Trisomy= 3 copies (2n+1) –Down’s syndrome Monoploidy = 1 copy (2n-1) –Turner’s syndrome Mitosis spreads condition to all embryonic cells Non-disjunction can also occur during mitosis  spreads condition to all cells in that cell line

6. Polyploidy  3n, 4n Primarily in plants, some fish and amphibians Fertilization of diploid egg Failure of zygote to go through cytokinesis

7. Abnormal chromosome structure Chromosomes are breakable and flexible –Crossing over in meiosis

8. Chromosomal mutations Non-sister chromatids allign incorrectly during meiosis I –Nonreciprocal crossing over  results in a deletion on one chromosome and a duplication on the other

9. Down’s Syndrome Affects 1/700 children born in America Risk increases with maternal age

10. Aneuploidy of sex chromosomes Cause fewer problems than aneuploidy of autosomes –Y chromosome carries so few genes –Only one X chromosome stays active

11. Kleinfelter’s Syndrome- XXY Turner’s Syndrome- Xo Trisomy X- XXX XYY

12. Exceptions to normal chromosome behavior Genomic imprinting Occurs during gamete formation Certain genes are inactivated Offspring express only one allele Imprints are erased in gamete producing cells, new imprints are applied

13. Imprinting Methylation (-CH 3 ) of cytosine inactivates genes Most imprinted genes are associated with embryonic development

14. Organelle Genes Mitochondria and chloroplasts have their own DNA Extra-nuclear genes Passed from mother  offspring