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Monohybrid Cross: Definition & Example

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  • 0:00 It's All in the Genes
  • 0:40 The Monohybrid Cross
  • 2:08 Dominant and Recessive
  • 5:04 Examples
  • 6:23 Lesson Summary
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Lesson Transcript
Instructor: Christine Morgan

Christine has taught college Biology and Anatomy, and has a Master's degree in Anatomy.

Some genetic characteristics are inherited in predictable patterns. In this lesson you will learn what monohybrids are and how to predict the kinds of characteristics that their offspring will show.

It's All In the Genes

Genetics - we all know it takes two parents to get things started, but after that things seem to get pretty complicated. Ever wonder why that cute puppy is the only one of the litter that looks like the mother? Or why no one in that big family down the street has their father's red hair? Many of the mechanisms of inheritance can be complex, but there are some characteristics that can be followed from one generation to the next in a fairly straightforward way. In 1865, long before our modern understanding of genetics and DNA, a scientist named Gregor Mendel discovered some of the secrets of inheritance and became the first to describe the monohybrid cross.

Gregor Mendel
Portrait of Gregor Mendel

The Monohybrid Cross

Let's take a look at the parts of the term. A cross refers to the process of fertilization and production of a new generation, and the prefix mono means 'singular'. Mendel started off by focusing his studies on singular inherited characteristics, or traits, from crosses of his pea plants.

Mendel raised hundreds of pea plants for his experiments, and he learned a lot about how they passed on traits from one generation to another. He noticed that for some traits, like flower color or seed shape, different plants would show only two forms of the trait. For example, plants had either white or purple colored flowers, or rounded or wrinkly seeds. He could also produce a pure line of plants that would only produce offspring with one form of a single trait, so that purple flowered plants always had purple flowered offspring.

A hybrid, then, is a plant that is the result of a cross between two plants that are both pure for different forms of the same trait. Gardeners do this all the time, and most of us are familiar with the term 'hybrid' from the beautiful colors that are produced in hybrid roses.

Mendel's hybrids didn't act the way roses do, though. There was no blending of colors or seed type - in fact, the offspring of these pure crosses always, 100% of the time, looked like only one of the parent plants, never the other. So Mendel decided to find out what would happen in the next generation if he crossed the monohybrids with each other - creating the monohybrid cross.

Dominant and Recessive

Mendel noticed that one of the forms of the trait occurred more often in nature - this was the form that always appeared in his first generation of hybrids. He called it the dominant form of the trait; the other was called recessive because it seemed to disappear from some generations. Although the first cross of the two pure strains always resulted in all of the offspring showing the dominant form of the trait, the second generation, the monohybrid cross, always resulted in a three to one ratio of dominant to recessive offspring.

Mendel's ratios were consistent because he used such large numbers of plants (recall that if you flip a coin a few times it may not result in a 50/50 ratio of heads to tails, but the more tosses you do, the closer you will come to that number).

Illustration of Mendel

Now this all happened before our modern knowledge of how DNA works to pass along information to every cell in every living organism. Because all living things had two parents, it made sense to Mendel that at least two copies of the information were present for each particular trait. This turned out to be true, and we now call the DNA information for a single trait a gene, and the two copies of the genes which are inherited from each parent are called alleles. For traits that are inherited according to this model (remember, not all traits are inherited this way), organisms can have three possible sets of genetic information: either two of the same alleles for one of the forms of the trait, or one each of the two different possible alleles.

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