S4.2.5 For the adult
Dominant and recessive
A pea plant with round seeds is crossed with one with wrinkled seeds. All the offspring have round seeds. Where did "wrinkled" go?
What this chapter teaches
Genes come in pairs of alleles. A dominant allele shows with one copy, and a recessive allele shows only with two. The genotype is the pair of alleles and the phenotype is what you see. A Punnett square gives ratios such as 3 : 1, and a ratio is a chance, not a promise.
Three questions to ask
There is no one right wording. Listen for the idea.
- What is the difference between a genotype and a phenotype, with one example of each?
- Why can two round seeded pea plants have a wrinkled offspring?
- Does dominant mean most common? Why not?
After the chapter, the learner can
- Use the words allele, dominant, recessive, genotype and phenotype correctly
- Predict offspring ratios with a Punnett square
What they copy into their notebook
Alleles are versions of a gene. A dominant allele shows with one copy. A recessive allele needs two. Genotype is the alleles, phenotype is what you see. Rr × Rr gives 3 round : 1 wrinkled.