The study of heredity, the process by which traits are passed from parents to offspring, began with the experiments of Gregor Mendel in the mid-19th century. Mendel established the foundational principles of classical genetics, demonstrating that inheritable characteristics are passed down as distinct units, now called genes. To analyze how a single characteristic is inherited, geneticists use a specific research method known as the monohybrid cross. This type of cross allows for the prediction and observation of inheritance patterns over successive generations.
Defining the Monohybrid Cross
A monohybrid cross is a breeding experiment designed to track the inheritance of a single physical characteristic, such as flower color or plant height. The experiment typically begins with two parent organisms, referred to as the Parental (P) generation, which differ in only this one trait. These parents are chosen because they are “true-breeding,” meaning they are homozygous for the trait, possessing two identical copies of the genetic factor, or allele, for that characteristic.
Alternative versions of a gene are called alleles. An organism inherits two alleles for each trait, one from each parent. If the alleles are the same, the organism is homozygous; if they are different, it is heterozygous. In a monohybrid system, a dominant allele expresses its trait even when only a single copy is present, while a recessive allele only expresses its trait when two copies are inherited.
The initial cross involves a true-breeding parent expressing the dominant trait (homozygous dominant) and one expressing the contrasting recessive trait (homozygous recessive). The offspring of this initial cross form the First Filial (F1) generation. Every F1 individual is heterozygous, carrying one dominant and one recessive allele, and expresses only the dominant physical characteristic.
Performing the Cross Using a Punnett Square
The primary event analyzed in a monohybrid cross is the mating of two individuals from this heterozygous F1 generation. This F1 x F1 cross produces the Second Filial, or F2, generation, where the full range of inheritance possibilities is revealed. To predict the outcomes of this cross, geneticists use a simple grid diagram called a Punnett square.
The Punnett square is a visual tool that applies probability rules to predict the potential genotypes of the offspring. For a monohybrid cross, a 2×2 grid is used. Possible gametes from one parent are listed along the top, and gametes from the other parent are listed along the side. Since both F1 parents are heterozygous, each parent contributes one of two different alleles, represented by letters outside the grid.
Consider a cross between two F1 pea plants heterozygous for height, where Tall (T) is dominant and dwarf (t) is recessive. Each parent produces gametes containing either the ‘T’ or the ‘t’ allele. Filling the four boxes represents the four equally likely fertilization outcomes. The resulting F2 generation genotypes include one homozygous dominant (TT), two heterozygous (Tt), and one homozygous recessive (tt).
Understanding the Resulting Genetic Ratios
The outcomes predicted by the Punnett square are summarized using two distinct numerical measures: the genotypic ratio and the phenotypic ratio. The genotypic ratio describes the proportion of the different allele combinations, or genotypes, in the F2 offspring. In the monohybrid cross, the genotypic ratio is consistently observed as 1:2:1.
The 1:2:1 ratio represents the relative number of homozygous dominant, heterozygous, and homozygous recessive individuals in the F2 generation. In the pea plant example, this means one-quarter of the offspring are TT, one-half are Tt, and one-quarter are tt. The phenotypic ratio describes the proportion of observable physical characteristics, or phenotypes, expressed by the F2 offspring.
Since the dominant allele masks the recessive allele, both homozygous dominant (TT) and heterozygous (Tt) individuals express the dominant trait (Tall). Only homozygous recessive (tt) individuals express the recessive trait (dwarf). Therefore, the phenotypic ratio for the monohybrid cross is 3:1, representing three offspring expressing the dominant trait for every one expressing the recessive trait.
The Law of Segregation
The predictable 3:1 phenotypic ratio in the F2 generation provided the basis for Mendel’s first law of inheritance: the Law of Segregation. This law explains that during the formation of reproductive cells (gametes), the two alleles for a single trait separate from each other. Consequently, each gamete receives only one of the two alleles present in the parent organism.
The separation of these alleles occurs randomly, and the subsequent fusion of gametes during fertilization is also random, resulting in the predictable genotypic and phenotypic ratios. This physical separation of homologous chromosomes during meiosis is the cellular event that accounts for the segregation of alleles into different gametes.