Alleles, genotypes and phenotypes, Punnett squares with probabilities, ratios and percentages, family trees and how sex is inherited.
What you will learn
- Use the words allele, dominant, recessive, homozygous, heterozygous, genotype and phenotype
- Complete a Punnett square and give a probability, ratio or percentage
- Work out genotypes from a family tree
- Explain why each child has a half chance of being a boy
Alleles, genotype and phenotype
A gene can come in different versions called alleles.
Body cells have two alleles of each gene, one from each parent.
- A dominant allele (capital letter, B) shows even if only one copy is present.
- A recessive allele (small letter, b) shows only if there are two copies.
- Homozygous: two identical alleles (BB or bb). Heterozygous: two different alleles (Bb).
- The genotype is the alleles (Bb). The phenotype is the characteristic you see (black fur).
Most characteristics are controlled by several genes working together; a few, like the ones in this lesson, are controlled by a single gene.
The rest of this lesson and its 10 practice questions are locked.
In this lesson
- Alleles, genotype and phenotype
- The Punnett square
- Family trees and sex
- Try it: Your first Punnett square
10 practice questions, marked as you go; about 25 minutes in all.
Key terms
Alleles, Dominant, Recessive, Genotype, Phenotype, Carrier, Sex chromosomes
In this unit: Inheritance, variation and evolution
Meiosis and DNA, Punnett squares and family trees, inherited disorders and embryo screening, natural selection, selective breeding, genetic engineering and cloning, and classification.