The Chromosome Theory of Inheritance

1
The Chromosome Theory of Inheritance
2
Thomas Morgan Fruit Flies

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

3
Fruit Flies

• Small Organisms
• Easy to Keep
• Short Generation Time (28 days)
• Few Highly Visible Chromosomes (8)

4
Morgans Experiment

• In an effort to replicate Mendels 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)

5
Independent Assortment or Mendelian Meiosis

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

6
(No Transcript)
7
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
8
Mendelian Expectations

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

9
Morgans Actual Results

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

10
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

11
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?

12
Gene Linkage Meiosis

• Gene linkage produces the gametes having the
  alleles GN and gn only, and cant produce Gn and
  gN

13
Predicted Results

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

14
Gene Linkage Crossing Over

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

15
Crossing Over Meiosis
16
Predicted Results

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

17
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)

18
Gene Mapping

• Morgans 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

19
(No Transcript)
20
Non-DisjunctionMeiosis Errors