D’YOUVILLE COLLEGE
BIOLOGY 102 - INTRODUCTORY BIOLOGY II
LECTURE # 25
LINKAGE and GENETIC MAPPING
5.
Autosomal Linkage:
• linkage: since # of genes far exceeds # of chromosomes, many genes must be
carried by each chromosome
- genes that occur on same chromosome are linked, i.e. their inheritance
does not follow law of independent assortment
• departures from independent assortment: Morgan studied Drosophila (fruit
fly) for inheritance of linked genes: black (b) and wild type (b+) body color alleles &
vestigial (vg) and wild type (vg+) wing shape alleles (ppt. 1)
- test crosses gave unusual results – (not the expected 1:1:1:1)
- classes with very high frequency showed parental combinations (wild type
body + wild type wings or black body + vestigial wings)
- classes with low frequency showed recombination of traits (black body +
wild type wings or wild type body + vestigial wings) (fig. 15 - 9 & ppt. 2)
- concluded genes were on same chromosome (linked) and therefore not
independent
- recombination of linked genes occurs via crossing over (fig. 15 - 10 &
ppt. 3)
• importance of crossing over: large increase in recombinant possibilities
Bio 102, spr. 13
lec. 25 - p. 2
• mapping genes: percentage of crossovers (recombination frequency)
between linked genes roughly relates to distance between them; closely linked
genes show less recombination than distantly linked genes
- order of genes determined by additional test crosses; e.g., if b & vg loci
are 17 units apart and b & cn are 9 units apart, one needs recombination frequency
between cn & vg to determine order (8 or 26?)
• linkage in Drosophila: Morgan studied mutants for body color, wing shape
& eye color that were linked – black body, vestigial wings and cinnabar eyes (figs.
15 – 11 & ppt. 4)
- many linked genes have been mapped & comprise four 'linkage groups
(corresponds with the four chromosomes – see fig. 15 – 12 & ppt. 5)