The Study of Heredity: Principle of Dominance & Paired Factors

What is heredity? Before we get into the principles of it, the rules of dominance, and paired factors, let’s first define heredity so that all else makes more sense.

Heredity, or heritability, is the concept of passing down of genetic material from one generation to the next. Each next generation inherits traits from the previous one by virtue of genetic material being passed on from mom and dad to their children. This transition of genetic material from one generation to the next, however, is a planned and organized process that happens over the course of several steps, each of which has a very specific task. Think of it as natural selection of the genes. As genetic material transitions from one stage to the next, there are specific rules that are followed, and these include the principle of dominance and paired factors. In other words, this specific journey of genetic material and how it is passed down is what we will cover in this post.

Gregor Mendel and His Studies

Nearly everything we know at this point about heredity and how traits are inherited is largely thanks to one man’s dedication and hard work. Gregor Mendel studied almost 30,000 pea plants and was able to line out for us the rules and regulations, by which visible traits are passed down from one generation to the next. He did this by simply observing the traits that were visible in the offspring plants when he crossed breeds that had specific characteristics. For example, he realized that when he crossed green and yellow pea plants, the first round of offspring was made up of only yellow peas. When he crossed those yellow peas among one another, their offspring had a very specific trait distribution. This, we now know, is because the color yellow is a dominant trait, while the color green is not. This relationship between dominant and recessive traits held true every time Mendel set up similar crosses. He proposed the concept of ‘unit factors’ for each trait, which we now refer to as genes. He also proposed that these unit factors are the fundamental units of heredity and travel unchanged from one generation to the next. Here, therefore, derived three principles that we know as ‘Principles of Inheritance’ today. One of the principles is the ‘principle of dominance,’ and the other two are the ‘principle of paired factors’ and the ‘principle of independent assortment.’ So, let us get into them a little.

Principle of Dominance

Going back to the yellow and green pea plants, the example that delineated the concept of dominant and recessive traits. Dominant traits are those that are expressed in the individual, while recessive ones do not follow that pattern. This is because gene types can be dominant or recessive. We say “types” because each one of us receives two types of one gene: one from the mother and the other from the father. These gene types are also called alleles. In other words, the gene for color has a yellow allele and a green allele. When yellow and green peas are crossed, the offspring are all yellow, which means that the yellow allele is dominant. However, the genotype (or the genetic blueprint) is not that simple. Each one of the parental plants has two alleles of the same kind. So the yellow plant has YY, while the green plant has yy. (Upper case is used to denote dominant alleles and lower case is used for recessive alleles.) The offspring, as we now know, is made up of all Yy heterozygotes, and since the Y allele is dominant, they are all yellow. However, once the Yy heterozygotes were crossed, their offspring (now the second generation) were a mix of green and yellow plants. This is because now some of the plants were yy homozygotes and only had recessive alleles that were expressed. In other words, the dominant allele is called dominant because one copy is enough for it to be expressed outwardly, whereas the recessive trait is only expressed when two of its alleles are present. Rest assured that while this holds true, things are not always that simple. In fact, ever since Mendel delineated the principle of dominance, subsequent researchers have indeed found that this law of dominance does not always hold true and that there are several different means by which alleles are paired and passed on. This, however, is beyond the scope of this post.

Principle of Paired Factors

We spoke a little about paired factors above but let us get into it a little bit more. The principle of pairing refers to the fact that outward traits are driven by genes that exist in pairs or alleles. Given that all of us are the product of fertilization, the combination of the mother’s egg and its genetic material and the father’s sperm and its genetic repertoire, most of us have two alleles to each gene. Each of these gene types travels together on chromosomes that are doubled and halved during the process of cell division.

Principle of Segregation

During the formation of offspring—also referred to as gametes in genetics—the alleles separate or segregate serendipitously so that each one of the gametes receives either one with an equal chance. Going back to the example of the green and the yellow peas, the first generation from the first cross of a YY and a yy parent were all made up of the Yy genotype. This is because the alleles were distributed equally among offspring during fertilization and cell division. The same is held true for the subsequent generation that was made up of green and yellow offspring. Thanks to Mendel, we now know that their genotypes were YY, Yy, and yy as a result of the heterozygous allele genotype segregating and mixing.

If you think that the process of heredity is anything but simple, you are right. However, this complexity is also incredibly beautiful as it leads to every one of us being so incredibly unique and individual in every sense of the way.

 

Laura Day
 

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