Gregor Mendel is known as the father of modern genetics. He spent his career as an Augustinian monk with an unlikely passion for studying heritable characteristics, and he grew and studied up to 29,000 pea plants between 1856 and 1863.
In Mendel's first famous series of experiments, he established Mendel's law of segregation, which today states that every gamete, or sex cell, is equally likely to receive a given allele from the parent. (An allele is a variant of a gene; each gene usually has two, such as R for round seeds in pea plants and r for wrinkled seeds.)
Building on this work, Mendel then set about demonstrating the law of independent assortment, which states that different genes do not influence each other with regard to the sorting of alleles into gametes. There are some exceptions to the rule, as will be described.
Pea Plant Characteristics Studied
Mendel began his work by examining seven traits of pea plants that he noticed occur in two distinct variants:
- Flower color (purple or white)
- Flower position on stem (on the side or at the end)
- Stem length (dwarf or tall)
- Pod shape (inflated or constricted)
- Pod color (yellow or green)
- Seed shape (round or wrinkled)
- Seed color (yellow or green)
Pea Plant Pollination
Pea plants can self-pollinate, which is a feature Mendel needed to avoid in his work on independent assortment because he was looking specifically at the heritability of multiple traits. He therefore mainly used cross-pollination, or reproduction between different plants.
This afforded Mendel control over the specific genetic content of the plants he was breeding over time because he could be certain of the specific composition of both parents, whatever his experiments showed this to consist of.
Monohybrid vs. Dihybrid Crosses
In his early experiments, Mendel used self-pollination to breed his pea plants for only one trait (e.g., seed color). He did this using a monohybrid cross, which is the breeding of two plants with an identical hybrid genotype, such as Rr.
These plants were part of the F1 generation, with the parental (P) pea plants having the genotypes RR and rr in every case. The crossing of F1 plants with each other produces an F2 generation.
A dihybrid cross allowed Mendel to examine the inheritance of two traits at the same time, such as seed shape and pod color. These plants were crosses between parents that held copies of both alleles for each trait, and therefore had genotypes of the form RrPp.
Law of Segregation
Because Mendel saw from his monohybrid crosses that every gamete was equally likely to receive a given characteristic from the parent, thereby establishing the law of segregation, he predicted that this would manifest in multiple traits at the same time.
Mendel predicted by looking at this data that the inheritance of one characteristic did not affect the inheritance of a different one, but he had to do some more work to confirm this.
Mendel's Second Experiment
Mendel now used his pea plants to assess the results of dihybrid crosses rather than monohybrid crosses. This allowed him to determine the inheritance of multiple characteristics associated with multiple genes.
Mendel predicted that if characteristics were inherited independently of one another, these crosses would produce the four possible combinations of the two traits (e.g., for seed shape and seed color, round-yellow, round-green, wrinkled-yellow, wrinkled-green) in a fixed phenotypic ratio of 9:3:3:1, in some order. They did, accounting for small statistical fluctuations.
Mendel's Law of Independent Assortment: Definition and Explanation
The law of independent assortment states that the alleles of two (or more) different genes are sorted independently during gamete formation, implying that alleles do not affect each other or their heritability.
Were it not for certain quirks of chromosomal behavior, this law would presumably hold true under all circumstances. But different traits are in fact sometimes inherited together, as you'll see.
Dihybrid Punnett Square: Law of Independent Assortment Example
In a dihybrid Punnett square, all of the possible allele combinations of parents with identical genotypes for two traits are placed in a grid. These combinations are of the form AB, Ab, aB and ab. Thus the grid has sixteen squares, and the row and column headings are four across and four down, labeled with the above combinations.
When more than two traits are being examined at the same time, using a Punnett square begins to become very cumbersome. A trihybrid cross, for example, would require an eight-by-eight grid, which is both time-consuming and space-consuming.
Independent Assortment vs. Linked Genes
Mendel's dihybrid cross results applied perfectly to pea plants but do not completely explain heritability in other organisms. Thanks to what is known about chromosomes today, the variations from the law of independent assortment that have been observed over time can be accounted for by what is known as gene linkage.
A process often occurs in gamete formation called genetic recombination, which involves the exchange of small pieces of homologous chromosomes. In this way, genes that happen to be physically close together are transported together whenever a given form of recombination occurs, making certain linked genes heritable in groups.