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Mendel’s Law of Segregation

When reproducing, each parent can pass only one gene to their offspring, from each gene pair. This means that the offspring will inherit one gene from each parent, making a new gene pair. This is known as the Gregor Mendel’s Law of Segregation.

Image showing various generations and how traits are passed down.

This means that if your father has two genes for brown hair, and your mother has two genes for blonde hair, what will you inherit? You will receive one brown hair gene from your father, and one blonde hair gene from your mother. Your gene pair for hair color will be brown – blonde. So, what hair color will you have? Brown? or Blonde?

Mendel’s Experiment for Law of Segregation

Gregor Mendel discovered that some genes are dominant, or stronger than other genes. When there are two different kinds of genes (a tall and a short) the dominant gene will determine how the plant will grow. In pea plants, the tall gene is dominant, while the short gene is recessive. 

Mendel developed a hypothesis about how genes are passed from parents to offspring. This hypothesis, which scientists still use today, says that a pea plant (and all other life forms) have two genes, making a gene pair. They also have a gene pair for every other trait, including seed shape, seed color, or in humans, eye color, hair color skin color, and so on. When reproducing, each parent can pass only one gene to their offspring, from each gene pair. This means that the offspring will inherit one gene from each parent, making a new gene pair.

Here is chart showing Mendel's experiment, but with color. In this experiment yellow is the dominant peas are the dominant trait.

Here is chart showing Mendel’s experiment, but pea with color. In this experiment, yellow is the dominant trait and green is the recessive trait.

Gregor Mendel’s Model for Genetics

Scientists write gene pairs by using capital and lowercase letters. (TT, Tt, tt) The capital letter represents a dominant gene, while the lower case letter indicates a recessive gene.

Now, let’s return to our discussion about pea plants. In Mendel’s experiments, he put the pollen from true breeding tall pea plants (TT) on true breeding short (tt) pea plants. Remember, each parent can only pass one gene to the offspring. What will the gene pair look like for the offspring? They will all be (Tt), having one tall gene and one short gene.

Next, he took the pollen from these plants (Tt) and put it on other (Tt) plants. What will the offspring’s gene pair look like? This is a bit more tricky isn’t it.Some of the plants will inherit a (T) from the mother plant and a (T) from the father plant. Some of the offspring will inherit a (t) from the mother and a (T) from the father. Some will inherit a (T) from the mother and a (t) from the father. And some will inherit a (t) from the mother and a (t) from the father.

Illustration of the punnett square which is used in the science of genetics.

This is the punnett square which helps us map out the dominant and recessive genes. Can you figure out which gene is recessive and which is dominant?

All the (TT), (Tt), and (tT) plants will be tall. This is because the (T) is dominant. The (tt) plants will be short because there is no dominant (T) to make them grow tall. As a result, three out of every four, or 75% of the offspring will be tall, while one out of four, or 25%, will be short.

Written by: Caroline Birt