KEY CONCEPT Genes can be mapped to specific locations on chromosomes.

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KEY CONCEPT Genes can be mapped to specific
locations on chromosomes.
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Gene linkage was discovered by deviations from
Mendels rules

• Mendel crossed pea plants with different discrete
  traits
• Alleles assort independently with 9331 ratio

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R.C. Punnett and William Bateson

• Developed Punnett squares. Crazy chicken
  breeder.
• Found that rates often did not follow 9331
  proportions
• Therefore, some genes are linked

Cream Legbar chicken. Originally bred by Punnett
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Gene linkage was explained using fruit flies.

• 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.

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• 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.

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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)

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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.

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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.

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• 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

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• 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

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