Laws of Inheritance Biology: Dominance, Segregation & Independent Assortment
You have guessed it from the title—there are laws on how we inherit traits. The process behind your beautiful brown eyes or behind your lovely curls is based on intricate biology that follows specific laws, namely laws of inheritance.
These laws were coined a long time ago by Gregor Mendel, who is largely regarded as the father of Genetics. Almost everything we know today about how traits (personality traits, physical traits and so on) that are passed on from generation to generation is, in part, due to his work on pea plants that led him to discover the fundamental laws of inheritance. His study revolved around the analysis of thousands of pea plans based on which he deduced that genes are always passed down in pairs. Specifically, he realized that genes are passed down in units of two: one unit from the mother and the other unit from the father. He was also able to help us understand how these units segregated based on the analysis of the offspring’s appearance that led to the terms “dominant trait” and “recessive trait,” referring to a gene’s ability to be expressed and visible. However, more on that later.
So, what exactly are these laws of inheritance that Mendel coined? Before we get into the details of them, it is important to briefly discuss cell division. More specifically, meiosis, or the type of cell division that starts with one cell and results in four daughter cells, each of which has exactly half the number of chromosomes (or half the genetic material) of the parent cells. In other words, let us briefly look at how genetic material is distributed as the cells divide before we can fully appreciate Mendel’s laws of the inheritance of traits.
Almost every one of us has a set number of chromosomes. Almost all of us have a set of 23 chromosome pairs or 46 chromosomes. (We say “almost” here because there are some diseases in which individuals have more than 46 chromosomes—these are beyond the scope of this post.) Out of those 23 pairs, 22 are the same in females and males. They are called autosomes. One pair is made up of sex chromosomes. More specifically, females have two X chromosomes and males have one X and one Y chromosome. In other words, females have 22 autosomes and XX, while males have 22 autosomes and XY.
These chromosome pairs and chromosomes are shuffled around during the process of meiosis. Specifically, meiosis is an umbrella term for a series of steps that each cell goes through as it divides. It takes place over the course of two rounds, meiosis I and II, and each round passes through four phases: prophase, metaphase, anaphase, and telophase. Each phase is marked by specific events that take place in preparation for proper cell division and distribution of genetic material. However, prior to the onset of meiosis, each chromosome in the parent cell doubles—it creates a copy of itself. At that point, those chromosomes are referred to as sister chromatids, joined together at the hip, so to speak, also called the centromere.
From there, meiosis starts and culminates in the creation of chromosomes that are genetically unique. The aforementioned sister chromatids join with their homologs—or the equivalents—in order to exchange genetic material. Ultimately, each parent cell generates two daughter cells, each of which has received one set of sister chromatids. Next, comes meiosis II which is very similar to meiosis I—only no chromosome duplication takes place. Rather, sister chromatids are separated, and the process culminates in a total of four daughter cells.
Now that you know a bit about cell division and how genetic material is distributed among cells, let us get back to the laws of inheritance to better understand how we inherit traits from our parents.
There are dominant and recessive traits. The former refers to genes whose traits will always be visible in the offspring. Think of the aforementioned units that are passed on from parents to offspring—half being from the mother and the other half from the father. In other words, each gene exists in two forms. These forms are also referred to as alleles. So, to paraphrase the first sentence, each gene exists in two alleles. If one allele is dominant and another is recessive, only the dominant allele will appear as a trait, whereas the recessive allele will not. According to Mendel, if one parent has two copies of the dominant allele and the other parent has two copies of the recessive allele, each one of the offspring will have a mix, namely one copy of the dominant allele and one of the recessive allele. However, all offspring will display the dominant allele trait.
The law of segregation refers to the separation of chromosomes. More specifically, according to the law, if a parent has two distinct alleles for a specific gene, and each allele is on a copy of a chromosome, these will be separated during meiosis as the chromosomes are separated. In other words, during meiosis II, the two chromosomes will be separated, which causes the two alleles to segregate as a result.
According to the law of independent assortment, Mendel’s third law, the segregation of two alleles to two daughter cells during meiosis II does not affect any other segregation of any other alleles. More specifically, the traits that are passed on via one gene will be inherited independently and with no connection to traits that are inherited based on other genes. This is because genes are located on different chromosomes and are distributed into daughter cells independently of one another.