DR RACHEL DAVIES
hetero disomy M1
anaphase 1 of meiosis
in division 1 homologous chromosomes do not seperate
one of the daughter cells contains all 4 chromatids and the other does not contain these chromatids
these 2 daughter cells are both haploid one has one too many chromosomes and one has one less chromosome
nondisjunction has taken place (no seperation)
the daughter cell that had nothing in it divides and gives rise to two cells that have one less chromosome than they should (both are n-1)
the other daughter cell that had a chromosome too many divides and produces two daughter cells with one too many chromosome (both are n+1) e.g. chromosome 21 from both parents is present in the cell
when fertilisation occurs there is another chromosome added which makes the cell diploid but with one too many chromosome 21 but the other two cells have 1 too few chromosme 21
produces 2 cells with one more chromosome than normal and 2 cells with one less than normal
isodisomy M2
occurs in anaphase 2 of meiosis
normal anaphase 1 occurs two normal haploid cells produces one with either paternal or maternal chromosomes