DNA Replication video: http://highered.mcgraw-hill.com/sites/0072437316/student_view0/chapter14/animations.html#

Drosophila are easily bred and produce a large number of offspring have 4 pairs of chromosomes species of choice by Thomas Hunt Morgan hit them with X-rays trying to find mutants after 2 years of searching he finally found a mutant fly – one with white eyes

XX XY X X X Y XX XX XY XY XX XY X X X Y XX XX XY XY interbred the white eyed fly and the red eyed fly F1 generation – all red eyed F2 generation – typical 3:1 ratio of red to white eyed flies but…..no females had white eyes – and half the males had red eyes and half had white eyes so Morgan hypothesized that the gene for eye colour was on the X chromosome X X X Y XX XX XY XY

X-linked diseases Carrier female: XX Affected Male: XY XX XY X X X Y X-inactivation XX XX XY XY e.g. hemophilia Duchenne Muscular Dystrophy

Genes located on the same chromosome – the genes tend to be inherited Linked genes: Genes located on the same chromosome – the genes tend to be inherited together linked genes go through if independent assortment happened then see what really did happen

frequency of recombination is 50% (independent assortment) Note: if two genes are on different chromosomes – the frequency of recombination is 50% (independent assortment) crossing over changes the frequency of recombination frequency depends on the distance between linked genes

Down Syndrome What happens if there are problems during meiosis? trisomy: a gamete is produced with an extra chromosome 21 Robertsonian translocation -

Philadelphia chromosome - CML