Mendelian Genetics — Principles of Inheritance & 3 Laws

Quick History Lesson:

Gregor Mendel was an Austrian monk and scientist who lived in the 1800s. He is remembered as the founder of modern genetics for his laws of inheritance, which are based on the results of his experiments with pea plants.

Although Mendel published his work, it didn't get recognized for its importance during his lifetime. It was only once his papers were rediscovered in the early 20th century that scientists realized his findings applied to and explained many observed patterns of inheritance. And the rest is history.

Mendel and his pea plants come up a lot for anyone studying either biology or genetics (Punnett squares, anyone?), but the key principles of Mendelian inheritance can be broken down into Mendel's three laws of inheritance:

Law of Independent Assortment

The law of independent assortment says that genes for different traits segregate independently of each other. It means that separate traits are separately inherited. This is because during meiosis the chromosomes line up randomly before the cell divides, allowing for gamete formation.

Law of Dominance

The law of dominance says that there are dominant and recessive traits. Dominant traits are defined as whichever phenotype is expressed in an organism that is heterozygous for the trait.

Law of Segregation

The law of segregation says that everyone has two versions (called alleles) for each trait—one from each parent—and that these alleles segregate randomly (see independent assortment) during meiosis.

A Brief Disclaimer:

Don't forget that Mendel's laws don't always work.

Mendel studied the results of crossing different traits in pea plants, which show relatively simple inheritance patterns. Luckily for Mendel, he didn't have to deal with any funky things like incomplete dominance, sex-linked genes, or polygenic traits in his pea plants. Although Mendel's laws work, they don't perfectly apply in all situations. See non-Mendelian inheritance for more.

(Video) Gregor Mendel: Great Minds

by SciShow

In this video, Hank from SciShow talks about Mendel and his studies in genetics. Look out for when he mentions the Law of Segregation and the Law of Independent Assortment.

Summary of Mendelian Inheritance

Gregor Mendel did many experiments with pea plants while looking at different traits (like round seeds vs. wrinkled seeds, and red flowers vs. white flowers).

Mendel’s experiments in the 19th century led him to three conclusions:

(1) Law of Segregation: In sexual reproduction, each gamete gets just one of the parent’s alleles. The parent has two alleles for each gene, but each egg or sperm cell only gets one. Alleles are different versions of a gene.

(2) Law of Dominance: There are dominant alleles and recessive alleles. Dominant alleles aren’t more common than recessive alleles, they are just expressed more than recessive alleles. That’s because to have a dominant phenotype, you only need one of your two alleles to be dominant. To have the recessive phenotype, your genotype has to include two copies of the recessive allele.

(3) Law of Independent Assortment: Alleles for different traits are on different parts of chromosomes. In other words, each gene has its own locus, and different genes are inherited separately so that one gene won’t influence another. For example, hair color doesn’t affect height and height doesn’t affect hair color.

Was Mendel Right?

It's important to realize that genetics doesn't always follow Mendelian inheritance patterns.

Mendel's Experiments and Laws: Dominance, Separation, and Independence

Mendel the monk's pea plant experiments paved the way for his Law of Independent Assortment. So, what did he do? What did he learn? And what did we learn from him?

1. The Law of Dominance: After crossing two pea plants with opposite traits and getting offspring that all looked identical to one another and to one parent, Mendel came up with a new question. Might those offspring still have the ability to pass on the other parent's traits? He crossed two of the offspring together and—aha—he found that some of these new offspring did look like the other grandparent. This information paved the way for our understanding of dominant alleles and recessive alleles today.

2. The Law of Segregation: When Mendel saw that those second-generation pea plants could still pass on the traits of their grandparents, he wondered how. He hypothesized that the things (alleles) that carry traits end up separating from each other when they're being passed on. In other words, Mendel said that alleles are separated in the parent to be passed on to the offspring. That way each kid gets one allele from each parent.

3. The Law of Independent Assortment: This is where Mendel worked on his dihybrid crosses. Mendel did some math and statistics here and found that a specific phenotypic ratio was present when you crossed pea plants and looked at two traits at once. Because of the math, he was able to hypothesize that if those genes didn't sort independently, you wouldn't see that same ratio.