Mendelian inheritance
Summary
Mendelian inheritance is the pattern of single-gene traits whose alleles segregate into separate gametes and, for unlinked genes, assort independently. Most testable: with complete dominance, Aa × Aa gives a 3:1 phenotype ratio and a 1:2:1 genotype ratio.
Detail
Meaning: Mendelian inheritance describes traits controlled by one gene with distinct alleles. Segregation: an individual has two alleles per gene, which separate in meiosis so each gamete gets one. Independent assortment: alleles of different genes enter gametes independently when the genes are on different chromosomes or far apart on one chromosome. With complete dominance, a heterozygote shows the dominant phenotype.
Equation: P(A and B) = P(A) × P(B), where P is probability (unitless) and A and B are independent events. Aa × Aa gives 1 AA : 2 Aa : 1 aa, a 3:1 phenotype ratio. AaBb × AaBb gives 9:3:3:1.
On the MCAT: Expect Punnett squares, pedigrees, and test crosses (unknown genotype × homozygous recessive; one recessive offspring shows the unknown is heterozygous). Multiply probabilities across children or genes. Two unaffected parents with an affected child suggest a recessive trait.
Common confusion: Not every inherited trait fits a 3:1 ratio. Incomplete dominance, codominance, polygenic traits, linked genes, and mitochondrial inheritance give other patterns. Ratios are probabilities for each offspring, so a small family need not match them exactly.
Sources
- Campbell Biology
- Genetics: From Genes to Genomes
- Principles of Genetics
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.