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7.3 Gene Linkage and Mapping
KEY CONCEPT
Genes can be mapped to specific locations on
chromosomes.
7.3 Gene Linkage and Mapping
Gene linkage was discovered by deviations from Mendel’s
rules
• Mendel crossed pea plants with different discrete traits
• Alleles assort independently with 9:3:3:1 ratio
7.3 Gene Linkage and Mapping
R.C. Punnett and William Bateson
• Developed Punnett squares. Crazy chicken breeder.
• Found that rates often did not follow 9:3:3:1 proportions
• Therefore, some genes are linked
Cream Legbar chicken.
Originally bred by Punnett
7.3 Gene Linkage and Mapping
Gene linkage was explained using fruit flies.
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Thomas Hunt Morgan studied fruit flies
Found some traits were inherited together
Linked traits fell into four groups
Determined fruit flies have four pairs of chromosomes
Chromosomes, not genes, assort independently during
meiosis.
Wild type
Mutant
7.3 Gene Linkage and Mapping
• Linked genes are not inherited together every time.
• Chromosomes exchange homologous genes during
meiosis.
• Genes that are closer together will more often cross over
together.
7.3 Gene Linkage and Mapping
Alfred Sturtevant (student of Morgan)
Hypothesis:
– Frequency of genes crossing-over together depends
on distance between the genes on the chromosomes
Experiments:
– Found 3 linked traits in fruit flies: body color, eye color,
wing size.
– Recorded how many times the genes crossed over
together. Number out of 100 (i.e. percent)
7.3 Gene Linkage and Mapping
Alfred Sturtevant (student of Morgan)
Conclusions:
– Determined rates of crossover between each of the
three genes.
– Linkage Maps show relative distance between genes
on a chromosome
– If you know crossover rates of three or more genes,
you can determine their relative locations.
7.3 Gene Linkage and Mapping
Linkage maps estimate distances between genes.
• The closer together two genes are, the more likely
they will be inherited together.
• Linkage maps show the relative locations of genes.
7.3 Gene Linkage and Mapping
• Cross-over frequencies can be converted into map units.
– gene A and gene B cross over 6.0
percent of the time
– gene B and gene C
cross over 12.5 percent
of the time
– gene A and gene C cross over 18.5 percent of the
time
7.3 Gene Linkage and Mapping
7.3 Gene Linkage and Mapping
7.3 Gene Linkage and Mapping
• Quiz Question 5:
– gene A and gene B cross over 9.5 % of the time
– gene B and gene C cross over 14.5 % of the time
– gene A and gene C cross over 5 % of the time
C
A
B
B
A
C
7.3 Gene Linkage and Mapping