3.14: Mendel's Laws
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What does it mean to be dominant?
The most powerful or influential individual in a group is sometimes called dominant. In genetics, a dominant trait means nearly the same thing. A dominant trait is the most influential trait and masks the other trait.
Do you remember what happened when Mendel crossed purple flowered-plants and white flowered-plants? All the offspring had purple flowers. There was no blending of traits in any of Mendel's experiments. Mendel had to come up with a theory of inheritance to explain his results. He developed a theory called the law of segregation.
The Law of Segregation
Mendel proposed that each pea plant had two hereditary factors for each trait. There were two possibilities for each hereditary factor, such as a purple factor or white factor. One factor is dominant to the other. The other trait that is masked is called the recessive factor, meaning that when both factors are present, only the effects of the dominant factor are noticeable (Figure below). Although you have two hereditary factors for each trait, each parent can only pass on one of these factors to the offspring. When the sex cells, or gametes (sperm or egg), form, the heredity factors must separate, so there is only one factor per gamete. In other words, the factors are "segregated" in each gamete. Mendel's law of segregation states that the two hereditary factors separate when gametes are formed. When fertilization occurs, the offspring receive one hereditary factor from each gamete, so the resulting offspring have two factors.
The law of segregation predates our understanding or meiosis. Mendel developed his theories without an understanding of DNA, or even the knowledge that DNA existed. Quite a remarkable feat!
This law explains what Mendel had seen in the F1 generation when a tall plant was crossed with a short plant. The two heredity factors in this case were the short and tall factors. Each individual in the F1 would have one of each factor, and as the tall factor is dominant to the short factor (the recessive factor), all the plants appeared tall.
In describing genetic crosses, letters are used. The dominant factor is represented with a capital letter (T for tall) while the recessive factor is represented by a lowercase letter (t). For the T and t factors, three combinations are possible: TT, Tt, and tt. TT plants will be tall, while plants with tt will be short. Since T is dominant to t, plants that are Tt will be tall because the dominant factor masks the recessive factor.
In this example, we are crossing a TT tall plant with a tt short plant. As each parent gives one factor to the F1 generation, all of the F1 generation will be Tt tall plants.
When the F1 generation (Tt) is allowed to self-pollinate, each parent will give one factor (T or t) to the F2 generation. So the F2 offspring will have four possible combinations of factors: TT, Tt, tT, or tt. According to the laws of probability, 25% of the offspring would be tt, so they would appear short. And 75% would have at least one T factor and would be tall.
- One hereditary factor is dominant to the other. The dominant trait masks the recessive factor, so that when both factors are present, only the effects of the dominant factor are noticeable.
- According to Mendel's law of segregation, there are two hereditary factors for each trait that must segregate during gamete (egg and sperm) production. As a result, offspring receive one factor from each parent, resulting in two factors for each trait in the offspring.
- What is the difference between a dominant trait and a recessive trait?
- Explain the law of segregation.
- When Mendel crossed a TT tall plant with a tt short plant, what did he observe in the F1 generation? Why?
- If PP purple plants are crossed with pp white plants, what will be the possible combinations of factors if the F1 generation is allowed to self-pollinate?