19- 18 - 17 16 15 14 13 12 11 10 9- 7- 6- 5- 4- 2- 0- 10 20 30 40 50 60 70 80 Generations FIGURE 28.11 Results of selective breeding for a high and low oil content, a quantitative trait in corn. Percentage of oil content
Genetic Variation
Genetic variation refers to the variation in the genome sequences between individual organisms of a species. Individual differences or population differences can both be referred to as genetic variations. It is primarily caused by mutation, but other factors such as genetic drift and sexual reproduction also play a major role.
Quantitative Genetics
Quantitative genetics is the part of genetics that deals with the continuous trait, where the expression of various genes influences the phenotypes. Thus genes are expressed together to produce a trait with continuous variability. This is unlike the classical traits or qualitative traits, where each trait is controlled by the expression of a single or very few genes to produce a discontinuous variation.
A large, genetically heterogeneous group of tomato plants was used
as the original breeding stock by two different breeders, named Mary
and Hector. Each breeder was given 50 seeds and began an artificial
selection strategy, much like the one described in the figure. The
seeds were planted, and the breeders selected the 10 plants with the
highest mean tomato weights as the breeding stock for the next generation. This process was repeated over the course of 12 growing seasons, and the following data were obtained:
Mean Weight of Tomatoes (pounds)
Year Mary’s Tomatoes Hector’s Tomatoes
1 0.7 0.8
2 0.9 0.9
3 1.1 1.2
4 1.2 1.3
5 1.3 1.3
6 1.4 1.4
7 1.4 1.5
8 1.5 1.5
9 1.5 1.5
10 1.5 1.5
11 1.5 1.5
12 1.5 1.5
A. Explain these results.
B. Another tomato breeder, named Martin, got some seeds from
Mary’s and Hector’s tomato strains (after 12 generations), grew
the plants, and then crossed them to each other. The mean
weight of the tomatoes in these hybrids was about 1.7 pounds.
For a period of 5 years, Martin subjected these hybrids to the
same experimental strategy that Mary and Hector had followed,
and he obtained the following results:
Mean Weight of Tomatoes (pounds)
Year Martin’s Tomatoes
1 1.7
2 1.8
3 1.9
4 2.0
5 2.0
Explain Martin’s data. Why was Martin able to obtain tomatoes
heavier than 1.5 pounds, whereas Mary’s and Hector’s strains
appeared
to plateau at this weight?
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