Basics of Genetics & Mendel's Work

Ever wonder why you look like your family? That's heredity in action—the passing of traits from parents to children through DNA. Genetics is the science that studies this process.

A monk named Gregor Mendel revolutionized our understanding of inheritance in the 1800s. While working as a gardener, he noticed patterns in how pea plant traits (distinguishing features like flower color) were passed down through generations. What's amazing is that Mendel figured this out without knowing anything about DNA!

Mendel's experiments were simple but brilliant. He prevented pea plants from self-fertilizing, then carefully cross-fertilized specific plants and documented the results. He studied seven different characteristics including flower color, seed shape, and stem length.

💡 Mendel's curiosity about garden patterns led to discoveries that would eventually change our understanding of human inheritance too!

Mendel's Generations

Mendel organized his plant breeding into specific generations. He started with the "P" or parental generation, followed by their offspring (the "F₁" generation), and then the next generation (the "F₂" generation).

When Mendel crossed his pure-breeding parent plants, he made an interesting discovery. In the F₁ generation, only one parent's traits appeared! For example, when he crossed purple flowers with white flowers, all the F₁ plants had purple flowers. The white flower trait seemed to disappear.

This unexpected pattern led Mendel to develop the concept of dominance. Some traits (like purple flowers) are dominant and mask other recessive traits (like white flowers) when both are present together.

Cross-pollination was key to Mendel's work. This happens when pollen from one plant fertilizes another plant, allowing him to control which traits were being mixed together.

Dominance & Alleles

Why do some traits show up while others hide? Mendel discovered that traits can be dominant or recessive. A dominant trait is expressed when paired with a recessive trait, while recessive traits only show up when paired with another recessive trait.

The possible forms of a gene are called alleles. You receive one allele for each trait from your mom and one from your dad. These pairs determine what traits you'll display.

This led to Mendel's Law of Segregation, which states that during reproduction, the two alleles for a trait separate (segregate) and only one passes to each offspring. This explains why you might have your mom's eye color but your dad's hair type!

🧬 Think of alleles like puzzle pieces—you get one from each parent, and how they fit together determines what trait shows up!

Genotypes & Phenotypes

Your genotype is your genetic makeup—the actual alleles you possess—while your phenotype is what you physically look like. These concepts help explain why family members can look different despite sharing genes.

Genotypes come in different forms: homozygous (two identical alleles—either both dominant or both recessive) or heterozygous (one dominant and one recessive allele). For example, if we use "H" for a dominant trait and "h" for recessive, you could have HH, Hh, or hh.

The fascinating part? Different genotypes can produce the same physical appearance. Both HH and Hh might look identical because the dominant trait shows up in both, but they carry different genetic information.

Scientists use Punnett squares to predict offspring genotypes. This 2×2 grid shows all possible combinations when two parents' alleles meet. It's like a genetic calculator that shows the probability of different outcomes!

Mendel's Results & Dihybrid Crosses

When Mendel crossed his pure plants (P generation), he got predictable results. The F₁ generation was 100% heterozygous with the dominant trait showing. But the magic happened in the F₂ generation—he found a 3:1 ratio of dominant to recessive traits!

This pattern became the foundation of genetic prediction. When he looked at the F₂ generation's genetic makeup, he found 25% homozygous dominant, 50% heterozygous, and 25% homozygous recessive—perfectly matching mathematical predictions.

Sometimes traits are linked and don't separate independently. For these situations, Mendel developed the dihybrid cross. This special Punnett square tracks two different gene pairs at once (like plant height AND flower color).

📊 The 3:1 ratio in the F₂ generation was Mendel's "eureka" moment—it proved inheritance follows predictable patterns!

Dihybrid Crosses & Special Inheritance Patterns

Setting up a dihybrid cross is like solving a puzzle with more pieces. Since two different gene pairs are involved (like AaBb × AaBb), you need to account for all possible combinations each parent could pass on: AB, Ab, aB, or ab.

The "FOIL" method you learned in math class comes in handy here! Each parent can contribute four different combinations, creating a 4×4 Punnett square with 16 possible outcomes.

Not all traits follow simple dominant/recessive rules. In codominance, both dominant alleles show up together instead of one masking the other. Think of red and white spotted cows—both colors appear rather than blending or one dominating.

With incomplete dominance, the heterozygous condition creates a blend of both traits. Red flowers and white flowers might produce pink offspring—neither parent's trait completely dominates.

Incomplete Dominance & Genetic Disorders

Incomplete dominance creates blended traits when two dominants meet. Snapdragons demonstrate this perfectly: red flowers + white flowers = pink flowers in the heterozygous plants. The traits don't remain separate (like in codominance) but merge into something new.

Scientists use pedigrees to track genetic traits through family generations. These family tree diagrams help identify patterns of inheritance and predict genetic disorders.

Some people are carriers—they have one recessive disease allele but don't show symptoms. They can, however, pass the disorder to their children. Understanding carrier status is crucial in genetic counseling.

🔍 Pedigree charts are like genetic detective work—they help scientists spot patterns and predict inheritance risks across generations.