The dominant alleles don't have to share a chromosome
- The idea
- Predict
- See it
- Why
- Check
The idea
“In a double heterozygote, both dominant alleles sit on one chromosome and both recessives on the other.”
We write Tt Pp with the capitals side by side, so it's easy to picture them on one chromosome. The genotype doesn't say which alleles share one.
The short version
The mistake: In a double heterozygote, both dominant alleles sit on one chromosome and both recessives on the other.
What's actually true: Either pairing is possible. In a test cross, the two biggest offspring classes show which alleles shared a chromosome.
- In a test cross, the other parent gives only recessive alleles. So each offspring's traits show exactly which alleles it got from the Tt Pp parent.
- Crossing over between linked genes happens in only some meioses. So the parental chromosomes make the two biggest classes, and recombinants the two smallest.
- Recombination frequency is recombinants over all offspring, and 1% is one map unit. It can't top 50%, so a bigger answer means you swapped the classes.
On the exam: Don't assume the capitals travel together. The two biggest test-cross classes are the parental chromosomes.