Introduction to Quantitative Genetics

Example: Quantitative genetics of ear length in corn

Generation Mean (cm) Standard Deviation (cm)
Tom Thumb (P1)

16.80 

0.817

BMS (P2)

  6.63

1.887

F1

12.12

1.519

F2

12.89

2.252

Several observations can be made from the above data.

  1. Even though the mean ear length of the BMS is smaller, the standard deviation is larger. This suggests that it is more variable than the long ear line.
  2. Because the F1 population is derived from two pure lines, it should be genetically homogeneous (i.e. all plants are heterozygous). Thus all the variance associated with that population is environmental variance.
  3. The mean of a quantitative trait in a F1 population is intermediate to the two parents, and the mean of the F2 is approximately equal to that of the F1.
  4. The F2 population is more variable than the F1.
  5. The extreme values of the distribution should be equivalent to the two parents used in the cross because this small portion of the population will have the same genotypes as the parents. If two genes control the trait 1/16 of the F2 populations will be equal to either of the two parents. If five genes control the trait then 1/243 of the F2 populations will be equal to either parent.