Mendelian genetics explains how traits can be passed from parents to offspring through units of heredity called genes. Gregor Mendel discovered predictable inheritance patterns by studying pea plants and tracking traits over generations. His ideas matter because they help us predict the chance that an offspring will inherit certain traits.
Punnett squares give students a clear visual method for organizing these genetic possibilities.
Key Facts
- Alleles are different versions of a gene, such as A and a.
- Dominant alleles are expressed with one copy, while recessive alleles are expressed only when both copies are recessive.
- A monohybrid cross tracks one gene at a time, such as Aa x Aa.
- For Aa x Aa, the genotype ratio is 1 AA : 2 Aa : 1 aa.
- For complete dominance in Aa x Aa, the phenotype ratio is 3 dominant : 1 recessive.
- Probability of an outcome = number of favorable boxes / total number of boxes.
Vocabulary
- Gene
- A gene is a segment of DNA that carries instructions for a specific trait.
- Allele
- An allele is one version of a gene, such as a dominant or recessive form.
- Genotype
- A genotype is the allele combination an organism has, such as AA, Aa, or aa.
- Phenotype
- A phenotype is the observable trait produced by the genotype, such as purple flowers or white flowers.
- Punnett Square
- A Punnett square is a grid used to predict possible offspring genotypes from parental alleles.
Common Mistakes to Avoid
- Confusing genotype with phenotype, which is wrong because genotype means allele combination while phenotype means the visible or measurable trait.
- Writing only one allele for an offspring, which is wrong because most organisms inherit one allele from each parent for a gene.
- Assuming a dominant allele is always more common, which is wrong because dominance describes expression in a heterozygote, not how frequent an allele is in a population.
- Reading Punnett square results as guaranteed offspring, which is wrong because the boxes show probabilities for each birth or fertilization event.
Practice Questions
- 1 In pea plants, T is tall and t is short. Cross Tt x Tt. What are the genotype ratio and phenotype ratio of the offspring?
- 2 A black fur allele B is dominant over a brown fur allele b. Cross Bb x bb. What percent of the offspring are expected to have black fur?
- 3 Two parents both show the dominant phenotype, but they have one offspring with the recessive phenotype. Explain what this reveals about the parents' genotypes.