Pooling sons and daughters hides X linkage
- The idea
- Predict
- See it
- Why
- Check
The idea
“I can count all the offspring together. The total ratio shows how the gene is inherited.”
Mendel never sorted his peas by sex, and his ratios worked fine. Peas have no X chromosome. Flies do.
The short version
The mistake: I can count all the offspring together. The total ratio shows how the gene is inherited.
What's actually true: X linkage shows up as a difference between sons and daughters, so you have to score each sex separately.
- Females have two X chromosomes. Males have one X and a Y, so a son's only X comes from his mother.
- One copy of an X-linked recessive allele is enough to show in a male. A female needs two.
- Every daughter got a normal X from her father. Each son got his X from a mother carrying one yellow allele, so half were yellow.
- Reciprocal crosses swap which parent has the trait. An autosomal gene gives the same offspring both ways. An X-linked gene doesn't.
On the exam: Count sons and daughters separately before you trust any ratio.