The Chromosomal Theory of Inheritance: Segregation and Independent Assortment

Almost everyone has heard of Gregor Mendel and his infamous pea plants. However, fear not if you have not—we’ve got you covered. However, when it comes to the chromosomal basis of inheritance, there were several individuals at play, in addition to Mendel. Though we tend to follow diagrams like Punnett square to predict genotypes of a particular experiment on both the monohybrid cross and the dihybrid cross, as well as heredity in general, it has not always been that way. Mendel is largely considered the father of modern genetics, having discovered much of what we know about inheritance today based on his meticulous studies of nearly 30,000 plants (mostly pea plants). His work began in 1843, at a time when technology was not yet up to speed to confirm a lot of things. This is why a lot of his theories were only confirmed later by other big names in genetics such as Walter Sutton and Theodor Boveri who independently discovered that genes are found on chromosomes and published their work that confirmed Mendel’s chromosome theory of inheritance (more on that in a little bit). Another big name is Thomas Hund Morgan who studied the genetics of fruit flies and thereby confirmed Mendel’s chromosome theory of inheritance and the proposal that traits were carried on chromosomes.

So, what exactly did Mendel do? The Augustinian monk is largely to thank for our understanding of how our parents and grandparents’ traits such as facial features, uncommon hair color, or a predisposition to health problems such as cancer, end up in us. It is thanks to Mendel that we now understand that these characteristics are largely the result of a genetic basis, meaning that what makes us who we are is dependent on the genetic information that we inherit from our parents and their parents and their parents, etc. So, what if you wanted to figure out what types of genes run in the family and how they are passed on—or what your family’s inheritance patterns are? Mendel was the first to do that, except he used pea plants to delineate the principles of inheritance that gave rise to two laws, namely the Law of Segregation and the Law of Independent Assortment (more on what they are in a little bit).

From Pea Plants to People

More specifically, however, Mendel looked at traits of pea plants and how they are passed on by designing several test crosses. In other words, he crossed green peas with yellow peas and realized that the first generation offspring (F1) was only made up of yellow peas. (This led him to coin the color yellow as a dominant trait, or a trait that is more prevalent, and the color green as a recessive trait, one that is less prevalent) If he crossed the F1 offspring, however, he realized that the next offspring or generation looked much different. The majority of the F2 generation was still yellow, but a small portion was green (25% of F2 plants, to be exact). This is at the heart of his discoveries—by looking into this further and set up what he termed “test crosses,” he designed a way to determine the genetic makeup—or genotype—based on the outward characteristics or traits—or phenotype. Building on that, he came up with two laws that define how traits are passed on, which will be discussed in more detail below.

an explanation of a process called meiosis

However, before Mendel’s laws can be appreciated, we have to get into a little bit more biology first, namely an explanation of a process called meiosis. Meiosis essentially ensures that the genetic material that has come together from mom and dad after fertilization (the combination of the mother’s egg and the father’s sperm) gets properly segregated into subsequent cells. Meiosis is a process of several steps that are spread across two rounds: meiosis I and II. These two rounds, in other words, are two rounds of cell divisions. The first round creates two cells from one, two daughter cells, each of which houses half the amount of DNA of the original cell. The next round, meiosis II, or another round of cell divisions, results in four daughter cells (two from each previous daughter cells that were created) and yet another halving of the genetic material. In other words, the overall process of meiosis results in four daughter cells from one original cell each with a fourth of the genetic material of the original cell. These daughter cells are also called gametes, and genetic material that is passed on is found on chromosomes. A location of a gene on a chromosome is also called a locus. The types of genes that are passed on come in two forms also called alleles—one from the mother and one from the father. So, with this primer, you are now equipped to understand—or fully appreciate—Mendel’s laws.

Mendel’s First Law: Law of Segregation

During each round of meiosis, chromosomes are separated into daughter cells. This means that one half of the genetic material goes to one gamete and the other half goes to the other. Mendel’s first law essentially states that the alleles in one locus segregate into separate gametes. To simplify this a bit, this means that there are several types or kinds of each gene and these different gene types are passed on to different offspring.

Mendel’s Second Law: Law of Independent Assortment

The alleles of one gene separate into gametes independently of the alleles of another gene. To state this more simply, this means that each batch of genetic material (both dominant and recessive allele) is passed on in such a way that it does not affect another batch of genetic material. These are independent events meaning that the chances of separation can be viewed as the chances of flipping a coin and getting either heads or tails together. So it is simple statistics.

Final Thoughts

As groundbreaking as Mendel’s findings are, they were initially completely unnoticed by the scientific community. It was not until after his lifetime, during the 1900s, that subsequent scientists did similar research and had more advanced technological means to prove and, therefore, invalidate the traditional views of the time, and thus come full circle to Mendel’s initial proposals.

Laura Day