1. 11/05/08
Lecture 25 - Genetics 4
 Garden pea (di-hybrid cross)
 Parent (P) generation
Species Purple flowers Red flowers
Phenotype Long pollen X Short pollen
Genotype PPLL ppll
 F generation
1
 Purple flowers
 Long pollen
 PpLl
 F generation
2
- strange ratio
 Thomas T Morgan
 Geneticist of the 1900's
 Discovered PpLl x ppll = 7:1:1:7 ratio (due to crossing over)
 Linkage map: The distance between genes (after arrangement of the genes in the chromosomes).
 Sex-linked traits: traits determined by alleles on the sex chromosomes (X and Y).
 For example, color blindness (red/green)
 C, dominant genotype (normal vision; c, recessive genotype (blind)
c c C
 Male: X X (blind), X Y (normal vision)
c c C C C c
 Female: X X (blind), X X or X X (normal vision)
 Non-disjunction: an error in cell division where the chromosomes fail to disjoin.
 Aneuploidy: either an extra copy or the absence of a chromosome.
 Down syndrome (Trisomy 21): an extra copy of the chromosome pair #21 in humans.
 Polypolidy: three or more copies of a chromosome.
 Trisomy X
 A person with three X chromosomes.
 This occurrence happens one in one thousandths of births.
 The offspring will be a normal female.
 Turner syndrome (monosomy X)
 A person with only a single X chromosome.
 The offspring will be a sterile female
 This occurrence happens one in five thousandths of births.