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In the previous part of our document, we focused on the topic of heredity. We mentioned that genes are responsible for passing biological information from one generation to the next. But a question came up—if genes from the older generation pass the data forward, how did the older generation get that data in the first place? The answer is that older generations duplicate their genetic material, or DNA, during the processes of mitosis and meiosis. This way, both generations end up carrying the same biological information. However, what we talked about earlier was just a simple explanation of how gene data is saved and passed on through duplication. Now it's time to go deeper into that concept. To truly understand how this works, we needed to first learn about mitosis and meiosis, which we’ve already covered. So now, we're ready to talk about genetics —the science of heredity and variation. And this story begins with a man named Mendel. ❖ Most of the time, we think that only scientists are the ones who discover new things. But here's the surprising part — Gregor Mendel , the man who laid the foundation of genetics, wasn’t a scientist at all in the way we imagine. He was actually a priest. ❖ Gregor Mendel was the one who carried out the first widely accepted research in genetics. Of course, some scientists had explored related ideas even before Mendel was born, but it was his work that truly became a game changer in the field.
(Here’s an interesting (and a little sad) fact: Mendel didn’t receive recognition for his groundbreaking work during his lifetime. Just like many other great thinkers throughout history, his contributions were only fully appreciated after his death. So, if you ever feel like your work is being ignored or overlooked, don’t lose hope. Just because others can’t see your value now doesn’t mean it’s not there. Trust in yourself and keep doing what you do. ) Before we dive into Mendel’s work, there are a few important things we need to understand first. These basic ideas will help us fully grasp the meaning and importance of his experiments and discoveries. Useful Genetic Vocabulary Character A character refers to an observable feature of an organism that can be inherited. Examples include eye color , blood type , height , and skin color. Traits
traits , but we didn’t explain how they do it. Now we can clearly understand: genes carry the instructions for making specific proteins , and those proteins are what actually control our traits and characters. In other words, it’s not the gene itself that directly creates the trait, but the protein produced from the gene’s instructions. That protein plays a role in building or influencing the physical feature we observe. For example , the gene responsible for melanin production controls skin color. This gene gives instructions to produce a protein that makes melanin, the pigment in our skin. The more melanin the protein helps produce, the darker the skin tone—a trait resulting from the expression of the gene. Alleles Now, we’re going to talk about a very interesting and important fact. As you already know, there is a gene that controls each of our characters like height, hair color, and so on. What’s even more fascinating is that for each character, there are usually two different versions of the gene that control it. These different versions of a gene are called alleles. So, the question is: How are these alleles located?
Let me explain it simply. Earlier, we talked about homologous chromosomes. Each chromosome in a homologous pair carries alleles for the same gene. These alleles are located at the same position on each chromosome, called the gene locus. Think of the gene locus as the specific “address” on the chromosome where an allele for a particular gene is found. So, the two homologous chromosomes each have an allele for the same gene at the same locus. Let’s explain this using a human cell as an example. Humans have 23 pairs of homologous chromosomes. Each pair carries alleles for various genes at corresponding loci. For instance, one chromosome from your mother and one from your father carry alleles for eye color located at the same gene locus on each chromosome. Dominant and Recessive Alleles Earlier, we said that a gene usually has two different versions , and these are called alleles. These alleles can be of two types: a dominant allele and a recessive allele. Each allele represents a different trait of the same character. For example , let’s take the gene that controls height. There are two alleles for this gene:
- The dominant allele represents the trait of being tall.
- The recessive allele represents the trait of being short. So, both alleles control the same character (height), but express different traits of that character. Now, let’s explore why these two alleles are called “dominant” and “recessive.”
When these gametes combine during fertilization, the resulting cell will have two dominant alleles for that gene. That means every cell in your body will also carry two dominant alleles for that character. Now let’s look at other possible combinations:
- If the mother’s gamete carries a dominant allele , and the father’s gamete carries the recessive allele , then your cells will have one dominant and one recessive allele. In this case, the dominant trait will appear, because it masks the effect of the recessive one.
- If both parents’ gametes carry the recessive allele , then your cells will have two recessive alleles. And in this case, the recessive trait will appear, because there’s no dominant allele to hide it. This is how different combinations of alleles happen, all thanks to the random distribution of chromosomes during meiosis — specifically due to Independent Assortment in Meiosis I. Homozygous Alleles When an organism has two identical alleles for a particular gene, it is called homozygous for that gene.
- If both alleles are dominant, it is called dominant homozygous (e.g., AA ).
- If both alleles are recessive, it is called recessive homozygous (e.g., aa ). Heterozygous Alleles When an organism has two different alleles for a gene, it is called heterozygous (e.g., Aa ), meaning one allele is dominant and the other is recessive.
Example For the gene controlling height, if an individual has two dominant alleles (AA), they are tall (dominant homozygous). If they have two recessive alleles (aa), they are short (recessive homozygous). If they have one dominant and one recessive allele (Aa), they will also be tall, because the dominant allele masks the recessive one (heterozygous). Dominant and Recessive Traits The trait controlled by the dominant allele is called a dominant trait , and the trait controlled by the recessive allele is called a recessive trait. Example: Let’s again take height as the character.
- The tall trait is controlled by the dominant allele , so it is a dominant trait.
- The short trait is controlled by the recessive allele , so it is a recessive trait. If an individual has at least one dominant allele, they will be tall. They will be short only if they inherit two recessive alleles. Genotype and Phenotype Genotype
- The genotype is the genetic makeup of an organism.
- It refers to the alleles (gene forms) that the organism has inherited from its parents.
- Genotype is not always visible. Phenotype
One Phenotype — Multiple Genotypes It’s important to understand that a single phenotype can be caused by more than one genotype. This usually happens when the trait is controlled by a dominant allele. For example, let’s take the character height , where T represents the dominant allele for tall height and t represents the recessive allele for short height. A person with the genotype TT (dominant homozygous) will be tall, and a person with the genotype Tt (heterozygous) will also be tall because the dominant allele T masks the effect of the recessive allele t. Even though their genotypes are different ( TT vs. Tt ), their phenotype — being tall — is the same. This shows how one visible trait (phenotype) can come from more than one gene combination (genotype). Earlier, we mentioned that if we select a particular characteristic, like height , there can be two traits: tall and short. These traits are controlled by two alleles.
- A person with two dominant alleles will be tall. (dominant homo)
- A person with two recessive alleles will be short .(recessive homo)
- A person with one dominant and one recessive allele will still be tall (because the dominant allele masks the effect of the recessive one). (hetero) When learning Mendelian genetics , it’s important to understand how to create dominant homozygous (TT) and recessive homozygous (tt) combinations.
True breeding By self-fertilization (in plants) over several generations, organisms can become homozygous for a particular trait (ex: tall or short) , meaning they carry two identical alleles—either both dominant or both recessive. These homozygous individuals consistently produce offspring with the same trait when self-fertilized. Such organisms are called true-breeding or true- breeding homozygous because their genetic makeup is stable and predictable for that trait. This process is important in genetics because it helps create pure lines that can be used to study how traits are inherited. Example: In pea plants, height is a trait controlled by different alleles. By allowing pea plants to self-fertilize over many generations, we can produce plants that are true-breeding for height. For example, a pea plant that is true- breeding tall has two dominant alleles for height (homozygous dominant), so it always produces tall offspring when self-fertilized. Similarly, a true- breeding short pea plant has two recessive alleles for height (homozygous recessive) and always produces short offspring. Because these plants are homozygous for the height trait, their offspring will consistently show the same height, making them reliable for genetic studies. Cross-Breeding (or Cross-Pollination) Cross-breeding means mating or crossing two different true-breeding organisms that show contrasting traits. In other words, we mate two true-breeding organisms that are homozygous for the same character’s
- The ratio of traits in the F₂ generation is usually 3 tall: 1 short , showing how recessive traits can reappear. We’ll dive deeper into this when we explore Mendelian genetics and his famous experiments. Now that we understand the important genetic vocabulary, let’s dive into how Mendel discovered all of this through his simple but brilliant experiments.
What is Mendelian Genetics?
Mendelian genetics is the study of how characters are inherited based on the principles discovered by Johann Gregor Mendel in 1866. Mendel conducted experiments on the pea plant ( Pisum sativum ) , observing how characters like flower color and seed shape were passed from one generation to the next. Through his careful cross-breeding experiments, he discovered the basic rules of inheritance — how traits are
controlled by genes and how they are passed on. These principles form the foundation of what we now call Mendelian genetics. What Is a Monohybrid Cross?
- A monohybrid cross is a genetic cross between two organisms that are true-breeding (homozygous) but differ in only one character. Mendel’s first experiments were monohybrid crosses , where he studied how one character at a time (like flower color or seed shape) was inherited from parent to offspring. Mendel crossed a true-breeding pea plant with purple flowers (PP) with a true-breeding pea plant with white flowers (pp).
- The F₁ generation all had purple flowers (Pp) , showing that the purple trait is dominant.
- When these F₁ plants were crossed (Pp × Pp) , the F₂ generation showed a ratio of 3 purple: 1 white , revealing the hidden recessive trait. A monohybrid cross helped Mendel understand dominant and recessive alleles , and it led to his first law: the Law of Segregation — which we’ll explore next! Let me explain this using symbols
Monohybrid Cross and Meiosis
What Is a Punnett Diagram (Punnett Square)?
- A Punnett diagram (also called a Punnett square ) is a simple chart used in genetics to predict the possible outcomes of a cross between two organisms. It shows how alleles from each parent may combine in their offspring. This tool was developed by Reginald C. Punnett , and it’s widely used in studying inheritance patterns. Purpose of a Punnett Diagram
- To predict the genotypes (genetic combinations) of offspring.
- To predict the phenotypes (observable traits) based on genotype.
- To determine the probability of each genetic outcome.
- To help visualize how dominant and recessive alleles interact in inheritance. Example use: If two heterozygous pea plants ( Pp × Pp ) are crossed, a Punnett square helps you figure out the chance their offspring will have:
- Purple flowers (PP or Pp)
- White flowers (pp) It’s especially helpful when studying Mendelian inheritance.
Punnett Square of Monohybrid Cross
In this Punnett square, we examine a monohybrid cross between two heterozygous pea plants (Pp × Pp) for flower color. Each parent has two alleles:
- One dominant allele (P) for purple flowers
- One recessive allele (p) for white flowers When we cross these parents, their alleles separate during meiosis and combine randomly during fertilization. The Punnett square shows the four possible combinations of alleles in the offspring:
- PP: Homozygous dominant – purple flowers
- Pp: Heterozygous – purple flowers
- pp: Homozygous recessive – white flowers Genotypic ratio: 1 PP: 2 Pp: 1 pp Phenotypic ratio: 3 purple-flowered plants: 1 white-flowered plant This diagram clearly demonstrates Mendel’s Law of Segregation, where each gamete receives only one allele from each gene pair.
