Science · Cells, genes & microorganisms
Mendelian Genetics and Inheritance
Estimated time includes reading and one quiz. Take the time you need.
Start here: key ideas
- Alleles are different versions of a gene.
- Genotype is an allele combination; phenotype is an observable trait.
- A dominant allele can appear with one copy.
- Punnett squares show probabilities, not guaranteed results.
- Pedigrees show how traits appear across generations.
What you’ll be able to do
- Tell genotype from phenotype.
- Use dominant and recessive alleles.
- Find monohybrid probabilities with a Punnett square.
- Read a simple pedigree.
Learn basic inheritance words, use Punnett squares, and read simple pedigrees.
Optional review
If you’d like to review the basics, visit DNA, RNA, and Protein Synthesis. You can start this lesson without completing those first.
Alleles connect genotype with phenotype
Different versions of one gene are called alleles. A genotype is the allele combination, such as Aa. A phenotype is the observable trait.
A dominant allele can affect the phenotype with one copy. A recessive allele usually needs two copies. AA and aa are homozygous. Aa is heterozygous.
Use uppercase and lowercase letters only as labels. Uppercase does not mean common, healthy, or stronger. It simply marks the allele defined as dominant in that problem. A recessive phenotype requires two recessive alleles in a simple complete-dominance model.
Meiosis supplies the probability model. The two alleles in a diploid individual separate when homologous chromosomes separate, so each gamete receives one allele for the gene. For genotype Aa, a simple model predicts that about half the gametes carry A and half carry a. Fertilization then combines one allele from each parent.
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