Understanding Dominant and Recessive Traits
* Dominant Trait: A trait that is expressed even if only one copy of the dominant allele is present. We represent dominant alleles with uppercase letters (e.g., 'B').
* Recessive Trait: A trait that is only expressed if two copies of the recessive allele are present. We represent recessive alleles with lowercase letters (e.g., 'b').
The Cross
Let's assume the trait is eye color, with brown eyes being dominant (B) and blue eyes being recessive (b).
* Parent 1: Has brown eyes, but could have the genotype (BB or Bb). They carry at least one dominant 'B' allele.
* Parent 2: Also has brown eyes, and could also have the genotype (BB or Bb).
Possible Scenarios
There are four possible combinations of alleles that the parents could pass on to their offspring:
1. BB x BB: Both parents have two dominant alleles. All offspring will have brown eyes (BB).
2. BB x Bb: One parent has two dominant alleles, the other has one dominant and one recessive. All offspring will have brown eyes (BB or Bb).
3. Bb x Bb: Both parents have one dominant and one recessive allele. The offspring have a 25% chance of having blue eyes (bb), a 50% chance of having brown eyes with one dominant and one recessive allele (Bb), and a 25% chance of having brown eyes with two dominant alleles (BB).
4. Bb x BB: Similar to scenario 2, all offspring will have brown eyes (BB or Bb).
Conclusion
The recessive trait (blue eyes in this example) only shows up in the offspring if both parents carry at least one recessive allele (Bb). In the scenario where both parents are heterozygous (Bb), there is a 25% chance of their offspring inheriting two recessive alleles (bb) and therefore expressing the recessive trait.
