3.3 Meiosis cont.

Number of cells Number of chromosomes Chromatids per chromosome Before the start of meiosis 1 2n 2 At the end of the first division n At the end of the second division 4

Genetic variation A result of random orientation and crossing over Metaphase I Prophase I

Non-disjunction Can lead to chromosomal abnormalities Chromosomes fail to separate during anaphase = non-disjunction. Most common trisomy is trisomy 21  Down’s syndrome. A syndrome is a collection of physical signs/symptoms not a disease. Trisomy can occur on any of the chromosomes but normally leads to the offspring not surviving.

Examples Trisomy 21 Down’s syndrome. Trisomy 18 Edwards syndrome. Varied developmental complications. Low life expectancy. Very rare. Trisomy 13 Patau syndrome. Varied developmental complications. Low life expectancy. Very rare. Klinefleter’s syndrome, sex chromosomes XXY Turner’s syndrome sex chromosome X Klinefelters only affects males, males with an extra X chromosome, do not produce enough testosterone before birth or during puberty so normal male characteristics do not fully develop (e.g. reduced facial hair, infertility). Turners only affects females, short with under developed ovaries. As a result the syndrome would not present itself until puberty.