Principles of Inheritance
Law of Independent Assortment
When you follow two genes together, each pair separates on its own. One pair does not depend on the other.
Follow seed colour and seed shape together. The colour gene goes into a gamete without depending on which shape allele joins it. So a parent with yellow-round and green-wrinkled can also make new mixes, like yellow-wrinkled.
This free mixing is why a dihybrid cross gives the 9 : 3 : 3 : 1 ratio, and why offspring can look different from both parents.
Where students slip up
This works only when the two genes are on different chromosomes, or far apart on the same one. Genes that sit close together usually travel as a set, and the 9 : 3 : 3 : 1 ratio breaks. So it is the rule for unlinked genes, not for every pair.
Hold onto this
Each gene pair separates on its own. One does not decide the other.
Where this connects
Free mixing is where new combinations, and much of your own uniqueness, come from. And when it fails, because two genes sit close together, that very failure is what let scientists map genes along a chromosome.
Watch it make 9 : 3 : 3 : 1 →Quick check
The law of independent assortment works most cleanly when two genes are:
- AOn the same chromosome, close together
- BOn different chromosomes
- CBoth recessive
- DBoth dominant