Mendel's Law of Independent Assortment

Mendel's Law of Independent Assortment

In the 1860s, a monk named Gregor Mendel discovered many of the principles that govern heredity. One of these principles, now known as Mendel's law of independent assortment, states that allele pairs separate during the formation of gametes. This means that traits are transmitted to offspring independently of one another.

Mendel discovered this principle after performing dihybrid crosses between plants that had two traits, such as seed color and pod color, that differed from one another. After these plants were allowed to self-pollinate, he noticed that the same ratio of 9:3:3:1 appeared among the offspring. Mendel concluded that traits were transmitted to offspring independently.

The image above shows a true-breeding plant with the dominant traits of green pod color (GG) and yellow seed color (YY) being cross-pollinated with a true-breeding plant with yellow pod color (gg) and green seed color (yy). The resulting offspring are all heterozygous for green pod color and yellow seed color (GgYy). If the offspring are allowed to self pollinate, a 9:3:3:1 ratio will be seen in the next generation. About nine plants will have green pods and yellow seeds, three will have green pods and green seeds, three will have yellow pods and yellow seeds, and one will have a yellow pod and green seeds. This distribution of traits of typical of dihybrid crosses.

Mendel's Law of Segregation

Foundational to the law of independent assortment is the law of segregation. Mendel's earlier experiments led him to formulate this genetics principle. The law of segregation is based on four main concepts. The first is that genes exist in more than one form or allele. Secondly, organisms inherit two alleles (one from each parent) during sexual reproduction. Thirdly, these alleles separate during meiosis, leaving each gamete with one allele for a single trait. Finally, heterozygous alleles exhibit complete dominance, as one allele is dominant and the other is recessive. It is the segregation of alleles that allows for the independent transmission of traits.

Non-Mendelian Inheritance

Some patterns of inheritance do not exhibit regular Mendelian segregation patterns. In incomplete dominance, for example, one allele does not completely dominate the other. This results in a third phenotype that is a mixture of those observed in the parent alleles. An example of incomplete dominance can be seen in snapdragon plants. A red snapdragon plant that is cross-pollinated with a white snapdragon plant produces pink snapdragon offspring.

In co-dominance, both alleles are fully expressed. This results in a third phenotype that displays distinct characteristics of both alleles. For example, when red tulips are crossed with white tulips, the resulting offspring sometimes have flowers that are both red and white.

While most genes contain two allele forms, some have multiple alleles for a trait. A common example of this in humans is ABO blood type. ABO blood types have three alleles, which are represented as (IA, IB, IO).

Some traits are polygenic, which means that they are controlled by more than one gene. These genes may have two or more alleles for a specific trait. Polygenic traits have many possible phenotypes. Examples of such traits include skin color and eye color.