Mendel's First Law & Its Application

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  • 0:03 Gregor Mendel
  • 1:06 Alleles & Genes
  • 2:17 Dominant & Recessive
  • 3:45 Law of Segregation
  • 4:43 Application
  • 6:01 Lesson Summary
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Lesson Transcript
Instructor: Julie Zundel

Julie has taught high school Zoology, Biology, Physical Science and Chem Tech. She has a Bachelor of Science in Biology and a Master of Education.

Genetics is a fascinating and confusing world. This lesson will delve into Gregor Mendel's First Law, or the Law of Segregation, explaining what it means and how you can use it.

Gregor Mendel

Long ago, people's understanding of heredity, or how traits are passed down from parents to offspring, was a little fuzzy. At the time, they believed the traits were blended together. So if you had a giant nose and your partner had a small nose, your child would have a medium nose. Of course this isn't exactly right, but it was a reasonable explanation for the time period. Fortunately for us, the brilliant Austrian monk and scientist named Gregor Mendel came along.

Gregor Mendel was fascinated by heredity. He conducted experiments in the mid-1800s using pea plants. Pea plants, as it turns out, come in many different forms. You've got pea plants with purple flowers, some with white flowers, some with green seeds, some with yellow seeds, well, you get the idea. Mendel used all of this diversity within the pea plant to figure out some pretty neat genetic stuff including the Law of Segregation, which is also referred to as Mendel's First Law. Now I know you're eager to hear all about this law, but you're going to need some background information first, so we'll have to come back to it.

Alleles & Genes

Mendel conducted numerous experiments, but let's hone in on one in particular so we can get some vocabulary cleared up. Mendel crossed a purple flowered pea plant with a white flowered pea plant and all of the offspring were purple. That certainly didn't fit the blending ideas of the time, did it? But here's where it gets weird. When he crossed the purple flowered offspring together, the next generation had purple and white flowered plants. What? How in the world did purple flowered pea plants produce white flowered pea plants? Hmm. . . a mystery!

Today we know that traits, like purple and white flowers, are the result of different alleles, or different forms of a gene. Within your cells there is a bunch of DNA coiled up. This DNA has all of the written instructions on how to make you, well, you! If you were to check out your DNA you'd see sections that have genes and these genes tell your body to make a particular protein. Pea plants are the same. For example, pea plants have genes that tell the plant what color to make the flowers. Now some pea plants have genes to say 'make the flower purple!' and some have genes that say 'make the flower white!' The different forms of these genes are called alleles.

Dominance & Recessive

Back to our mystery. So how did those purple flowered pea plants make white flowered pea plants? Each pea plant has two alleles for flower color: one allele from each parent. In addition, there are two possibilities (or two alleles) for flower color in pea plants: purple and white. Scientists use letters to represent these alleles. Let's use an uppercase P for purple and a lowercase p for white. Here are the different combinations and consequent flower colors you'd get if those alleles came together:

  • You can have PP, which results in a purple flower
  • You can have Pp, which results in a purple flower
  • And you can have pp, which results in a white flower

You may have noticed that whenever you have an uppercase P, the flower is purple. That is because the purple allele is dominant over the white allele. Scientists represent dominant alleles with a capital letter, hence the uppercase P. This means that it covers up, or masks the white allele. The white flowers are recessive because they are covered up by the dominant allele. Recessive alleles are represented by lowercase letters, hence the p. So even though Pp is purple, it's carrying the allele for white flowers and this is why two purple plants can produce white flowered plants. Still confused? Don't worry, we'll explain further.

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