Life sciences, genetics, gene expression / principles and exploration
Independent assortment of two traits — principles and investigation
When observing two traits together, the separation of each gene and the independence between the two genes must be distinguished. Compare the probability of combining two traits by changing the parent genotype and sample number. 9:3:3:1 or 1:1:1:1 in black crosses are the ratios expected under certain assumptions and breeding conditions.
How does it work?
Two genes on different chromosomes separate independently. In a test cross, the four phenotypes appear with a 1:1:1:1 probability.
(3:1) × (3:1) = 9:3:3:1
Try this investigation
- R/r is round/wrinkled, Y/y is yellow/green. Fill in 16 spaces by combining the four gametes of RrYy, RY·Ry·rY·ry. Homozygous parents show the same gametes repeatedly.
- Record 1000 samples of RrYy×RrYy and compare them to the 9:3:3:1 theoretical distribution.
- Change parent 20000 to rryy and record the test cross. We also compare records that only reduce the number of samples.
Interpreting your results
What is the expected ratio of the four phenotypes in RrYy × rryy?
RrYy makes 1/4 of RY, Ry, rY, and ry, and rryy only makes ry. Assume the two genes are on different chromosomes.
Assumptions and limitations
The assumption is that the two genes are on different chromosomes. Each gene is completely dominant and has the same probability of surviving gametes and offspring. Genetic linkage, epistasis, environment, selection, and cleavage errors are omitted. Sampling distributions may deviate from theoretical values. Dominance does not mean common or excellent.