The Chromosome Theory of Inheritance

Thomas Morgan & Fruit Flies

• Thomas Morgan was one of the early geneticist, who after Mendel’s work was rediscovered in 1900, wanted to further extend our understanding of inheritance

Fruit Flies

• Small Organisms• Easy to Keep• Short Generation

Time (28 days)• Few Highly Visible

Chromosomes (8)

Morgan’s Experiment

• In an effort to replicate Mendel’s two-trait crosses he complete a cross between flies that were heterozygous for Body Color and Wing Size (GgNn) and flies that were homozygous recessive for Body Color and Wing Size (ggnn)

Independent Assortment or Mendelian Meiosis

• Mendilian Assortment produces all the possible gametes, having the alleles GN, Gn, gN, and gn

Mendelian Punnett Square

GN Gn gN gn

gn GgNn Ggnn ggNn ggnn

gn GgNn Ggnn ggNn ggnn

gn GgNn Ggnn ggNn ggnn

gn GgNn Ggnn ggNn ggnn

Mendelian Expectations

Gray Body and Normal Wings25%

Gray Body and Short Wings 25%Black Body and Normal Wings 25%Black Body and Short Wings 25%

Morgan’s Actual Results

Gray Body and Normal Wings42%

Gray Body and Short Wings 8%Black Body and Normal Wings 8%Black Body and Short Wings 42%

Walter Sutton

• In the study of the production of sperm in grasshoppers, Walter Sutton had noticed that the factor (or alleles) that Mendel had discovered acted just like the chromosomes as they separated during the process of meiosis

• Thus, he linked Mendelian Inheritance with Chromosomes

Gene Linkage

• Knowing the work of Sutton, Morgan wondered if the genes for Body Color and Wing Size were on the same chromosome

• Would this explain his actual results?

Gene Linkage Meiosis

• Gene linkage produces the gametes having the alleles GN and gn only, and can’t produce Gn and gN

Predicted Results

Gray Body and Normal Wings50%

Gray Body and Short Wings 0%Black Body and Normal Wings 0%Black Body and Short Wings 50%

Gene Linkage & Crossing Over

• Next, he wondered if the events of Crossing Over may help to explain his results

Crossing Over Meiosis

Predicted Results

Gray Body and Normal Wings25%

Gray Body and Short Wings 25%Black Body and Normal Wings 25%Black Body and Short Wings 25%

The Final Solution

• The previous results assume that Crossing Over occurs 100% of the time

• In this case, Crossing Over occurs only 32% of the time (the Cross Over Percentage is determined by adding the two smaller phenotypic percentages and multiplying by two)

Gene Mapping

• Morgan’s results lead to the mapping of genes on chromosomes.

• If two genes are on separate chromosomes, then you expect Mendelian Results

• If two genes are on the same chromosome, then the smaller the cross over percentage the closer they are together

Non-DisjunctionMeiosis Errors