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Mendel's Experiments

Gregor Mendel developed the model of heredity that now bears his name by experiments on various charactersitics of pea plants: height (tall vs. Short); seed color (yellow vs. Green); seat coat (smooth vs. wrinkled), etc. The following explanation uses the tall/short trait. The other traits Mendel studied can be substituted for tall and short.

Mendel started out with plants that "bred true". That is, when tall plants were self-pollinated (or cross-pollinated with others like them), plants in following generations were all tall; when the short plants were self-pollinated (or cross- pollinated with others like them) the plants in following generations were all short.

Mendel found that if true breeding Tall [T] plants are crossed (bred) with true breeding short [t] plants, all the next generation of plants, called F1, are all tall.

Next, he showed that self-pollinated F1 plants (or cross- pollinated with other F1 plants) produce an F2 generation with 3/4 of the plants tall and 1/4 short.

Q1, When Mendel put pollen from tall plants into the flowers of short plants, the seeds produced an F1 generation with all tall plants. But the short trait was not lost. How did Mendel demonstrate this?

  1. He showed that self-pollinating F1 plants produced 1/4 short and 3/4 tall plants
  2. He showed that self-pollinating F1 plants produced all medium size plants
  3. He knew that the short plants had recessive genes
  4. He showed that self-pollinating plants produced plants of all sizes from tall to short
  5. He knew that the tall gene covered up the short gene
  6. C and E are both correct

Mendel created a model that accounted for these and other data he got from his breeding experiments. The following summarizes the model's first basic feature.

Mendel's model for the F1 generation is summarized in the table at the right. The model states that each trait is controlled by a pair of hereditary packets we now call genes. One packet comes from each parent. The alleles (= forms) of the gene for height are the same in true breeding plants ( T T and t t parent plants ). Cross breeding T T with t t plants produces T t plants in the first or F1 generation. The F1 plants receive a T allele from the tall parent and a t allele from the short parent. The F1 plants are tall because the T allele is expressed and "cover up" the t allele. So the T (tall) allele is called dominant and t (short) allele is called recessive.

The diagram at the right shows how Mendel's model explains the 3:1 ratio of tall to short plants in the F2 generation. In the F1 generation each plant had one T and one t allele of the gene controlling height. Plants in the F2 generation had a 50:50 chance of getting a T or a t from each parent plant. The diagram shows that this results in 1 out of 4 plants getting only t genes and 3 plants getting at least one T gene (which makes the plant tall, because T is dominant over t)

The diagram also shows that the F2 generation actually has three kinds of plants. 1/4 are t t plants, which are short and produce only short plants in following generations in self pollinated. Of the remaining 3/4 tall plants, 1/4 are T T, which are tall and produce only tall plants in following generations if self pollinated. The remaining 2/4 get a T from one parent and at from the other. When self pollinated, they produce a pattern exactly like the F1 generation: 1 short plant for every 3 tall plants. These plants are exactly like the F1 generation.

From these and similar breeding experiments, Mendel deduced (figured out, proved logically) how traits are transmitted from generation to generation. These deductions have held up very well and form the basis of modern genetics, even though many traits and many species do not show the specific patterns of inheritance that Mendel observed.

  1. Transmission of each trait from one generation to the next is carried out by discrete units, which we now call genes. (Some people believe that Mendel already had this idea before he did the actual breeding experiments.)
  2. The gene for each trait comes in pairs, one from each parent. This deduction was confirmed by studying chromosomes, which are visible under the microscope during cell division. Chromosomes come in pairs and carry the genes, which themselves are invisible under the microscope.

    Q2. We can deduce (figure out, prove logically) from Mendel's data that heredity is transmitted by discrete units, which come in pairs, one from each parent, because

  1. you can see the individual genes for the two traits under the microscope.
  2. Mendel told us that this was true from his experimental findings with crossed pea plants
  3. a trait that disappears in the F1 generation reappears in the F2 generation from self-pollinated F1 plants (at the ratio of 1:3)
  4. a trait that blends in the second generation will separate in the third generation
  5. plants in the F2 generation don't breed true

The gene for each trait comes in different forms, which are now called alleles. Because genes come in pairs, an individual can have the same allele or different alleles in a gene-pair. If the alleles are the same, the individual is homozygous for that gene. If the alleles are different, the individual is heterozygous. In a heterozygote, one allele, the dominant, may cover up or hide the expression of the other allele, the recessive. Genes are often named by a letter (or letters) that stand for the dominant trait. The dominant allele is capitalized (eg, T for tall) and the recessive is in lower case (t for short).

In Mendel's experiment, the parent plants were homozygous (eg, T T or t t ) for the measured trait, because they could produce only plants with that trait, if they were self-pollinated. That is, they bred true. Also, 1/4 of the F2 (2nd generation) plants in Mendel's experiments bred true as tall plants and 1/4 bred true as short plants.

The same pattern occurred for the other traits Mendel measured. For example, parent plants produced only smooth (S) or wrinkled (s) seeds when self-pollinated. The F1 generation from a cross of SS and ss plants had only smooth seeds. In the F2 generation 1/4 of plants in F2 had wrinkled (s) seeds, which produced plants that had only s seeds. This shows they were homozygous for (had only) s genes.

Another 1/4 of the F2 plants had S seeds which produced only S seeds when self-pollinated. This shows they were homozygous for S genes. The remaining 1/2 of the F2 plants produced F3 plants, 1/4 of which had wrinkled seeds and 3/4 of which had smooth seeds when they were self-pollinated. These F2 plants show the same pattern of descendant as did the F1 generation of plants.

    Q3. Match the following effects with the names of the processes that produce them.

  1. the gene controlling height comes in two forms, one producing tall plants, the other producing short plants
  2. s-seeded plants produce in the next generation only s-seeded offspring
  3. some Y-seeded plants produce in the next generation both Y- and y-seeded offspring

    Q4. An allele that is expressed even when paired with a different allele is the -------- version of that gene.

  1. normal
  2. abnormal
  3. recessive
  4. dominant
  5. chromosome

    Q5. An allele that is not expressed when paired with a different allele is the --------- version of that gene.

  1. normal
  2. abnormal
  3. recessive
  4. dominant
  5. chromosome

The fact of genetic dominance requires the distinction between phenotype, the traits individuals actually show, and "genetic potential" or genotype, the traits an individual can pass on to his/her descendants. The genotype may or may not show up in the phenotype. In Mendel's experiment, the tall and the smooth phenotypes in F1 did not reveal the genetic potential for producing short plants or plants with wrinkled seeds or, which showed up in the F2 generation.

    Q6. Bob and Sandy both have brown hair; Glenn has blond hair; Colleen has red hair. These hair colors are ---------.

  1. dominant
  2. recessive
  3. phenotypes
  4. genotypes
  5. mutations
  6. alleles

    Q7. One of Bob and Sandy's children has blond hair; this shows that Bob and Sandy's -------- had the capacity to produce blond-haired children

  1. dominant genes
  2. phenotypes
  3. genotypes
  4. mutations
  5. alleles

  1. Genes for a trait from the two parents remain separate: they do not blend. This explains the fact that short plants and plants with wrinkled seeds disappear in F1 but reappear in F2. This is Mendel's Law of Segregation.
  2. When Mendel measured two or more traits (eg, height and color) in an experiment he found that each trait was transmitted independently. For example, tall or short plants can have smooth or wrinkled seeds. This is Mendel's Law of Independent Assortment (which strictly holds only if the genes are not too close).

    Q8. Label the following with Y if it illustrates the Law of Segregation or with N if illustrates the Law of Independent assortment.

  1. Blue eyes can disappear in one generation but reappear in the next.
  2. Blue or brown eyes and blond or brown hair can appear in any of the four possible combinations
  3. Genes from each parent are independent units and do not blend in child
  4. Genes for two different characteristics (usually) are transmitted to the next generation separately

Answers: 1. A,     2. C,     3. 1. A,   3. 2. B,   3. 3. C,     4. D,     5. C,     6.     7. C,     8. 1. Y,  8. 2. N,  8. 3. Y,   8. 4. N