Why do we resemble our parents? The answer lies in the science of genetics, the study of heredity. I’ve always been fascinated by how traits are passed down through generations. The foundations of this entire field were laid by a 19th-century monk named Gregor Mendel, whose meticulous experiments with pea plants revealed the basic patterns of inheritance.
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These patterns, now known as Mendelian inheritance, introduced the world to the concepts of dominant and recessive genes. This guide will explore Mendel’s key principles and show how they can be used to predict the traits of offspring.
🌿 Mendel’s Pea Plant Experiments
Mendel’s genius was in his systematic approach. He chose to study simple, distinct traits in pea plants, such as flower color (purple or white) and seed shape (round or wrinkled). By carefully cross-pollinating the plants and observing the outcomes over several generations, he uncovered predictable patterns.
One of his key findings was that when he crossed a purebred purple-flowered plant with a purebred white-flowered plant, all the offspring had purple flowers. The white flower trait seemed to disappear. However, when he then bred these offspring together, the white flower trait reappeared in about 25% of the next generation. This led him to a groundbreaking conclusion.
💡 Dominant and Recessive Alleles
Mendel proposed that for each trait, an organism inherits two ‘factors’ (which we now call genes), one from each parent. These genes can exist in different versions, which we now call alleles.
- An individual with two identical alleles for a trait is called homozygous.
- An individual with two different alleles for a trait is called heterozygous.
From his experiments, Mendel deduced the principle of dominance. He concluded that in a heterozygous individual, one allele—the dominant one (like purple flowers)—can mask the effect of the other allele—the recessive one (like white flowers). The recessive trait only appears when an individual is homozygous for that allele (i.e., has two copies of it).
➕ Using a Punnett Square
To visualize and predict the outcomes of a genetic cross, we can use a simple tool called a Punnett square. I find it to be a very helpful way to understand probabilities in genetics. By placing the alleles from one parent along the top and the alleles from the other parent along the side, the boxes in the middle show all the possible combinations of alleles for their offspring.
This simple tool allows us to calculate the probable ratios of different genotypes (the combination of alleles) and phenotypes (the observable traits) in the next generation, demonstrating the predictive power of Mendel’s foundational principles.
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