Genetics is the study of genes, genetic variation, and heredity in living organisms. The foundation of classical genetics was established by Gregor Mendel through his experiments on pea plants, which revealed the basic rules governing the transmission of traits from parents to offspring.
Mendel’s Experiments and the Laws of Inheritance
Mendel studied seven discrete traits in the garden pea, each existing in two contrasting forms such as purple versus white flower color and round versus wrinkled seed shape. By performing controlled crosses and counting large numbers of offspring, he deduced that traits are determined by discrete heritable factors, now called genes. Mendel’s first law, the law of segregation, states that each individual carries two copies of each gene (alleles), which segregate during gamete formation so that each gamete receives only one allele. His second law, the law of independent assortment, states that alleles of different genes assort independently of one another during gamete formation, provided the genes are located on different chromosomes.
Dominance and Recessivity
When two different alleles are present in a heterozygous individual, the dominant allele determines the phenotype while the recessive allele has no noticeable effect. In a monohybrid cross between two heterozygotes, the classic genotypic ratio is 1:2:1 (homozygous dominant to heterozygous to homozygous recessive), while the phenotypic ratio is 3:1 if dominance is complete. Test crosses, in which an individual of unknown genotype is crossed with a homozygous recessive individual, can reveal whether the unknown is homozygous dominant or heterozygous based on the phenotypes of the offspring.
Punnett Squares and Probability
Punnett squares provide a visual method for predicting the genotypes and phenotypes of offspring from a genetic cross. For a monohybrid cross, a 2 × 2 grid is used, while a dihybrid cross examining two genes requires a 4 × 4 grid. The product rule of probability states that the probability of two independent events occurring together is the product of their individual probabilities, and the sum rule states that the probability of either of two mutually exclusive events occurring is the sum of their probabilities.
Codominance and Incomplete Dominance
In codominance, both alleles are fully expressed in the heterozygous phenotype, as seen in the ABO blood group system where individuals with genotype IAIB express both A and B antigens on their red blood cells. In incomplete dominance, the heterozygous phenotype is intermediate between the two homozygous phenotypes, exemplified by snapdragon flower color where red and white homozygotes produce pink heterozygotes. In both cases, the genotypic ratio of 1:2:1 is reflected in the phenotypic ratio, unlike complete dominance where the phenotypic ratio is 3:1.
Multiple Alleles and Polygenic Traits
Although each individual carries only two alleles of a given gene, more than two alleles may exist in a population. The ABO blood group system is determined by three alleles: IA, IB, and i, with IA and IB codominant to each other and both dominant to i. Polygenic traits, such as human height, skin color, and body weight, are influenced by multiple genes acting together, resulting in continuous variation that follows a normal distribution rather than discrete categories. Environmental factors further contribute to the phenotypic expression of polygenic traits.
Pedigree Analysis
Pedigrees are family trees that use standardized symbols to track the inheritance of traits through multiple generations. Autosomal dominant traits, such as Huntington disease, appear in every generation, affect males and females equally, and an affected individual has at least one affected parent. Autosomal recessive traits, such as cystic fibrosis, can skip generations, appear equally in both sexes, and often occur in families with consanguineous unions. X-linked recessive traits, such as hemophilia and color blindness, are more common in males, are transmitted from affected fathers to carrier daughters, and never pass directly from father to son.
Applications of Mendelian Genetics
Mendelian principles are the basis for genetic counseling, which assesses the risk of inherited disorders in families, and for animal and plant breeding programs that select for desirable traits. The identification of Mendelian disease genes has enabled carrier screening for conditions such as Tay-Sachs disease and sickle cell anemia, prenatal diagnosis through amniocentesis and chorionic villus sampling, and the development of gene therapy approaches for monogenic disorders.
