Chromosomal abnormalities

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)

Non-disjunction

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

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

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

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

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

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

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

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

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

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