Unformatted text preview: LINKED GENES
Numerous genes are found on the same chromosome. Genes on same chromosome
are described as being in same Linkage Group.
Can be inherited together. Colorblindness and hemophilia linked genes in humans.
Both traits reside on the X chromosome.
Unable to assort independently- will not show 9:3:3:1 ratio. Can only separate
from each other if separated during crossing over.
RECOMBINATION OF UNLINKED GENES
If 2 genes on different chromosomes all 4 phenotypes will be represented
50% Parental 50% Recombinants (pheno. diff. than parent)
If linked-Recomb freq. < 50%
If very close on chromo –Recomb ≥ 0% RECOMBINATION OF LINKED GENES: CROSSING- OVER
Crossing over between homologous chromosomes produces progeny that do not follow
expected ratios of unlinked genes (1:1:1:1) or of linked genes (1:1)
The more room between two sites on the chromosome for breakage to occur,
the more likely a break will occur between those two sites.
The greater the distance between genes, the greater the prob. that crossing
over will form new combinations. Crossover frequencies reflect their
relative distance from each other.
Linkage maps are created using recombination frequencies.
If far apart → ↑ prob. that crossing over will separate them
→ ↓ prob. that crossing over will separate them
Genes arranged in linear sequence (think pearl necklace)
Closer genes: ↓ % Recomb
Far genes: ↑ % Recomb.
1% Recomb Freq. = 1 map unit (centimorgans [cM])
1. Determine Ends- genes farthest apart
2. Det. Recomb. Freq. between 3rd & 1st gene
3. Det. Recomb. Freq. between 3rd & 2nd gene If linked genes have > 50% Recombination frequency == may be unlinked?
½ of all recombinants will have diff. genotypes if assort. independently. ...
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- Fall '08