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.
Punnett squares organize probability
A Punnett square lists possible allele combinations in offspring. Put one parent’s possible alleles across the top and the other parent’s down the side. Fill each box by combining one allele from each parent.
The boxes show probability, not a promise. For Aa × Aa, the genotype chances are 1/4 AA, 1/2 Aa, and 1/4 aa for each child.
Worked example. Cross Bb × bb. The first parent can give B or b; the second gives only b. The boxes are Bb, bb, Bb, bb. Each child therefore has a 1/2 chance of the dominant phenotype and a 1/2 chance of the recessive phenotype.
From words to a Punnett model: (1) define the allele symbols and dominance pattern; (2) translate each parent’s phenotype or description into every genotype still possible; (3) list each parent’s gametes; (4) combine gametes; and (5) count genotypes before translating them into phenotypes. A 3:1 phenotype ratio does not mean a 3:1 genotype ratio.
Work a testcross systematically
A testcross can help reveal an unknown dominant-looking genotype. Cross the unknown individual with a homozygous recessive individual. If any recessive offspring appear, the unknown parent must carry the recessive allele.
Example: T_ × tt produces a short tt offspring. The unknown parent gave a t allele, so its genotype is Tt.
A testcross result gives evidence, not perfect certainty when the sample is small. A heterozygous parent can by chance produce several dominant-looking offspring. More offspring make the inference stronger.
Pedigrees show family patterns
A pedigree shows a trait through a family. Squares usually represent males, circles represent females, and shaded symbols show people with the trait.
A recessive trait can skip a generation because unaffected carriers can pass the allele. Use several family relationships before deciding the inheritance pattern.
For a rare recessive trait, two unaffected parents with an affected child are likely carriers. Each parent supplied a recessive allele. The chance for another child is found from the parental genotypes, not from the outcomes of earlier children.
Autosomal recessive inheritance
For this recessive trait, a child needs two recessive copies to show it. A carrier has one copy without showing the trait. The picture uses C and c; the same reasoning works with A and a. Two carriers have a 1-in-4 chance of an affected child in each pregnancy, not a guarantee of exactly one among four children.
Autosomal recessive inheritance — OpenStax College, Anatomy & Physiology (2014 CC BY archive); source figure credit: U.S. National Library of Medicine. CC BY 3.0. Original source image reproduced unchanged; explanatory caption added.
Not every trait follows simple dominance
Simple dominance is only one pattern. In incomplete dominance, a heterozygote has an intermediate phenotype. In codominance, both alleles are clearly expressed, as in AB blood.
Many traits are affected by several genes or by the environment. A Punnett square is useful only when its inheritance assumptions fit the trait.
Worked example. Two pink flowers with incomplete dominance are Rr × Rr. The offspring are 1/4 red RR, 1/2 pink Rr, and 1/4 white rr. For AB blood, both A and B features appear together; that is codominance, not blending.
Optional: combining inheritance models
For two independently assorting genes with complete dominance, find each trait probability separately, then multiply: in AaBb × AaBb, the chance of both recessive traits is 1/4 × 1/4 = 1/16. The full phenotype distribution can be derived the same way; memorizing 9:3:3:1 is not required here. Linked genes may not assort independently; sex-linked inheritance follows chromosome location; one gene may affect another gene’s expression (epistasis). These models need their own assumptions, not just new labels.
Terms to remember
- allele
- A version of a gene.
- genotype
- An organism’s allele combination.
- phenotype
- An observable trait.
- dominant
- An allele expressed when at least one copy is present.
- recessive
- An allele usually expressed only when two copies are present.
- homozygous
- Having two matching alleles.
- heterozygous
- Having two different alleles.
- pedigree
- A family diagram used to track a trait.
Your quick summary
- Alleles are versions of a gene.
- Genotype describes alleles; phenotype describes the trait.
- Punnett squares show possible offspring and probabilities.
- Pedigrees track traits in families.
- Many traits do not follow simple dominant-recessive rules.
Lesson quiz
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