Mendelian genetics
Also known as: mendelian inheritance, classical genetics
Mendelian genetics is the set of inheritance principles derived from Gregor Mendel's pea plant experiments, describing how discrete alleles segregate and assort during gamete formation. Its core rules are the laws of segregation, independent assortment, and dominance.
Mendel's central insight was that inherited traits are carried by discrete units — now called genes — rather than blending like fluids. Each organism carries two alleles for a given gene, one from each parent. The combination of alleles is the genotype, and the observable trait it produces is the phenotype. An organism with two identical alleles is homozygous; one with two different alleles is heterozygous, and in simple dominance the dominant allele masks the recessive one.
Three laws organize the framework. The law of segregation states that the two alleles for a gene separate during gamete formation, so each gamete carries only one — the physical basis is the separation of homologous chromosomes in meiosis I. The law of independent assortment states that alleles for different genes are distributed to gametes independently, which holds for genes on different chromosomes or far apart on the same one. The law of dominance states that in a heterozygote, the dominant allele determines the phenotype.
Punnett squares make the predictions concrete. Crossing two heterozygotes (Aa × Aa) yields a 1:2:1 genotypic ratio and a 3:1 phenotypic ratio, so a quarter of offspring show the recessive trait. A dihybrid cross between two heterozygotes (AaBb × AaBb) produces the familiar 9:3:3:1 phenotypic ratio. A testcross — crossing a dominant-phenotype individual with a homozygous recessive — reveals whether the unknown parent is homozygous or heterozygous.
Real inheritance frequently departs from these ratios. Incomplete dominance produces an intermediate phenotype, codominance expresses both alleles fully (as in AB blood type), multiple alleles exist for many genes, polygenic traits involve many loci, and genes located near each other on the same chromosome are linked and violate independent assortment. The MCAT tests Mendelian concepts in the biology and biochemistry section, expecting you to compute cross ratios, interpret pedigrees, apply the Hardy-Weinberg framework to populations, and recognize when a pattern indicates a non-Mendelian mechanism such as X-linked or mitochondrial inheritance.
Key takeaways
- Mendelian genetics treats traits as controlled by discrete alleles rather than blending inheritance.
- The law of segregation says paired alleles separate during gamete formation, reflecting meiosis I.
- The law of independent assortment applies to genes on different chromosomes or far apart on the same one.
- A monohybrid cross of two heterozygotes gives a 3:1 phenotypic ratio; a dihybrid cross gives 9:3:3:1.
- Incomplete dominance, codominance, polygenic traits, and linkage all produce non-Mendelian patterns.