Chromosome theory of inheritance, linkage and crossing over
Sutton and Boveri showed that genes sit on chromosomes. Morgan found that genes on one chromosome are linked, but can separate by crossing over, giving recombinants.
From Mendel's factors to chromosomes
Mendel published his work in 1865, but it was ignored for many years. His 'factors' could not be seen, and few biologists then used mathematics. In 1900, de Vries, Correns and von Tschermak rediscovered his results on their own. By then, microscopes showed how chromosomes behave during cell division. Walter Sutton and Theodore Boveri noticed that chromosomes behave exactly like Mendel's factors. Sutton joined chromosome segregation with Mendel's principles and called it the chromosome theory of inheritance. Thomas Hunt Morgan later gave the experimental proof using the fruit fly Drosophila melanogaster.
Genes and chromosomes behave alike
| Mendel's factors (genes) | Chromosomes |
|---|---|
| Occur in pairs | Occur in pairs (homologous chromosomes) |
| The two alleles segregate at gamete formation; only one goes to each gamete | Homologous chromosomes segregate in meiosis; only one of each pair goes to each gamete |
| One gene pair segregates independently of another | One chromosome pair segregates independently of another |
Why Morgan chose Drosophila
- It grows on a simple synthetic medium in the laboratory.
- It completes its life cycle in about two weeks.
- A single mating gives a large number of offspring.
- Males and females are easy to tell apart.
- Many hereditary variations can be seen with low-power microscopes.
Linkage vs crossing over (recombination)
Morgan crossed yellow-bodied, white-eyed female flies with brown-bodied, red-eyed males and intercrossed the F1. The F2 ratio was far from 9 : 3 : 3 : 1, because the two genes lie on the X chromosome and are linked.
Making a gene map from recombination frequencies
Alfred Sturtevant used recombination frequency as a measure of the distance between genes (1% recombination = 1 map unit). For three linked genes, recombination is P and Q 8%, Q and R 3%, P and R 11%. Find the gene order and the distance from P to R.
- Take each recombination frequency as a distance: 1% = 1 map unit.
- The largest value, P to R (11 units), must join the two end genes.
- Check: P to Q (8) + Q to R (3) = 11, which equals P to R.
- So Q lies between P and R.
Gene order P, Q, R; P and R are 11 map units apart.
Rice breeders use gene maps to find DNA markers lying close to useful genes, such as those for disease resistance. A marker close to a gene is rarely separated from it, so young seedlings can be screened early.
2 marks: “Distinguish between linkage and recombination.” Linkage: physical association of genes on one chromosome, so they tend to be inherited together. Recombination: formation of non-parental gene combinations by crossing over in meiosis.
3 marks: “Why did Morgan choose Drosophila? What did he observe about the genes for body colour, eye colour and wing size?” Simple medium, two-week life cycle, many offspring per mating, sexes easy to tell, visible variations. Body colour and eye colour genes: 1.3% recombination, tightly linked. Eye colour and wing size: 37.2%, loosely linked.
5 marks: “State the chromosome theory of inheritance. How did Morgan's experiments on Drosophila lead to the ideas of linkage and recombination?” Sutton and Boveri; parallels between genes and chromosomes; Morgan's dihybrid crosses with X-linked genes; F2 far from 9 : 3 : 3 : 1; tight and loose linkage with values; crossing over makes recombinants; Sturtevant's gene maps.
Close genes cling (linked, few recombinants); far genes swap (more crossing over).