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Biology
Gregor Mendel: Father of Genetics
Pea plant experiments, dominant and recessive alleles
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Gregor Mendel was an Austrian monk and scientist who studied how traits pass from parents to offspring. Working with pea plants in the 1850s and 1860s, he carefully counted thousands of plants and compared traits such as flower color, seed shape, and plant height. His work matters because it revealed predictable patterns of inheritance long before DNA was discovered. Today, Mendel is called the father of genetics because his ideas form the starting point for modern heredity.
Key Facts
- Mendel studied pea plants because they grow quickly, have clear traits, and can self-pollinate or be cross-pollinated.
- An allele is a version of a gene, such as P for purple flowers or p for white flowers.
- A dominant allele can mask a recessive allele in a heterozygous organism.
- Law of segregation: the two alleles for a gene separate during gamete formation, so each gamete gets one allele.
- Monohybrid cross ratio for two heterozygotes: Pp x Pp gives a 3:1 dominant to recessive phenotype ratio.
- Law of independent assortment: alleles of different genes are inherited independently when the genes are not linked.
Vocabulary
- Genetics
- Genetics is the study of heredity and how traits are passed from parents to offspring.
- Allele
- An allele is one version of a gene that can influence a trait.
- Dominant
- A dominant allele shows its effect when at least one copy is present.
- Recessive
- A recessive allele shows its effect only when two copies are present.
- Punnett Square
- A Punnett square is a diagram used to predict possible genotypes and phenotypes of offspring.
Common Mistakes to Avoid
- Calling every visible trait dominant is wrong because dominance depends on allele interactions, not on how common or noticeable a trait is.
- Confusing genotype with phenotype is wrong because genotype means the allele combination, while phenotype means the observable trait.
- Assuming a heterozygous organism is purebred is wrong because heterozygous means it has two different alleles for a gene.
- Using Mendel's 3:1 ratio for every cross is wrong because that ratio applies to a monohybrid cross between two heterozygous parents with complete dominance.
Practice Questions
- 1 In pea plants, purple flowers P are dominant over white flowers p. If two heterozygous plants are crossed, Pp x Pp, what are the expected genotype and phenotype ratios?
- 2 Round seeds R are dominant over wrinkled seeds r. A heterozygous round plant is crossed with a wrinkled plant, Rr x rr. Out of 80 offspring, how many would be expected to have round seeds?
- 3 Explain why Mendel's use of large sample sizes and careful counting made his conclusions about inheritance stronger.