Mendel's Law of Segregation

<h1>Text Preview</h1> <p>Through his monohybrid cross experiments, Mendel made four key observations that led him to propose his first law of inheritance. First, Mendel noticed that each trait, such as flower color, has two alternative forms— for example, purple and white. He proposed that each trait is controlled by a different gene, and that every alternative is specified by a different form of that gene, called an allele. For flower color, Mendel assigned a capital “P” for the purple-flower allele, and a lowercase “p” for the white-flower allele. We’ll discuss the use of capital versus lowercase letters in just a minute.</p> <p>For each trait, an organism inherits two alleles, one from each parent. An individual is homozygous if both alleles are identical, while a heterozygous individual has two different alleles. The allelic composition of a gene is called the genotype. The physically observable translation of a genotype is called the phenotype. For example, a pea plant with the genotype big-P-big-P has the purple flowers phenotype, while a plant that’s small-p-small-p has white flowers. Generally speaking, true-breeding strains are homozygous.</p> <p>Mendel noticed that none of the individual traits blended in the offspring. For example, each of the F1 and F2 plants had either purple or white flowers. There were no in-between colors. For each of the seven traits studied, Mendel found that plants inherited either one or the other alternative. He never saw anything in-between. From this observation, Mendel proposed that genes are distinct factors. While they can combine in various ways, alleles remain distinct and are passed on to offspring unchanged.</p> <p>Mendel observed that traits that don’t appear in an individual are still passed on to offspring. For example, he saw only purple-flowered plants in the F1 offspring, while the F2 generation had both purple- and white-flowered plants. From this observation, Mendel proposed that there are two types of alleles: dominant and recessive. A dominant allele only needs to be present in one copy for the phenotype to be seen. For pea plants, the allele that determines purple flower color is dominant. A capital letter indicates a dominant allele. Two copies of a recessive allele are needed to see the corresponding phenotype. </p> <p>For pea flower color, the white-flower allele is recessive. A lowercase letter indicates a recessive allele. Purple flowers result from homozygous big-P-big-P or heterozygous big-P-small-P. Homozygous small-P-small-P plants have white flowers. Since his parental plants were true-breeding, Mendel reasoned they must be homozygous big-P-big-P, and small-p-small-p. The purple-flowered F1 plants were heterozygous big-P-small-P, and the purple-flowered plants in the F2 were either homozygous big-P-big-P, or heterozygous big-P-small-P, while the white-flowered plants were homozygous small-p-small-p. Mendel also observed dominant and recessive phenotypes for the other traits he studied.</p> <p>Finally, Mendel observed that the ratio of dominant to recessive phenotypes in the F2 generation was always the same. He consistently got 3 offspring with the dominant phenotype for every one offspring with the recessive phenotype. Mendel saw the 3 to 1 ratio for all seven traits. From this observation, Mendel proposed that the two alleles for each gene segregate randomly into gametes. Recall that a gamete is a sperm or egg cell. He also proposed that gametes contain one allele of each gene, and that they unite randomly. This makes sense to us because we understand meiosis. But Mendel didn’t know about meiosis!</p> <p>From his four observations, Mendel proposed his first law, the law of segregation: allele pairs separate during gamete formation and then randomly unite when gametes fuse during fertilization. A grid called a Punnett square shows how Mendel’s monohybrid cross results are predicted by the law of segregation. A Punnett square is used to diagram the expected types and ratios of each potential offspring genotype of a cross. We write the possible gametes from one parent across the top, and those of the other parent along the side. One allele goes in each box. It doesn’t matter which parent goes on which side. We fill in the boxes by copying the alleles across or down into the empty squares. This gives us the predicted frequency of each possible genotype. 100% of the offspring will be heterozygous big-P-small-P, so they’ll have purple flowers. This is exactly what Mendel observed!</p> <p>Let's use a Punnett square for the F1 generation. Self-pollination of Mendel’s F1 offspring is analogous to having two heterozygous big-P-small-P parents. Test your mastery of Mendel’s first law by filling in the progeny genotypes in the white boxes of the Punnett square. Click Submit when you’re done. Click JUMP AHEAD to skip this step.</p> <p>That’s correct!</p> <p>Sorry, that’s not right. </p> <p>We expect 1 out of 4 big-P-big-P offspring, 2 out of 4 big-P-small-P, and 1 out of 4 small-P-small-P. This translates to 3 purple offspring, for every 1 white offspring. This is exactly what Mendel saw!</p> <p>A Punnett square is one method of determining probability. To learn more about probability, see the Reference Sheet.</p> <p>When plants have the dominant phenotype, how do we know whether they’re homozygous for the dominant allele, or heterozygous? Mendel determined the genotypes of dominant pea plants by performing testcrosses, which are matings between an individual of unknown genotype and a homozygous recessive individual to determine the unknown genotype. The offspring phenotype ratios from a testcross will be different if the individual being tested is homozygous or heterozygous. We’ll use Punnet squares to compare the progeny phenotype ratios. If the tested plant is homozygous dominant, all offspring will be purple. If it's heterozygous, half of the offspring will be purple, and half will be white.</p> <p>Mendel carefully observed inheritance patterns one trait at a time. Now let’s learn what happened when he studied multiple traits at the same time!</p> <p id="TextCopyright">Copyright 2006 The Regents of the University of California and Monterey Institute for Technology and Education</p>