Mendelian Laws of Inheritance
Mendelian laws of inheritance
Mendelian laws of inheritance are statements about the way certain characteristics are transmitted from one generation to another in an organism. The laws were derived by the Austrian monk Gregor Mendel (1822–1884) based on experiments he conducted in the period from about 1857 to 1865. For his experiments, Mendel used ordinary pea plants. Among the traits that Mendel studied were the color of a plant's flowers, their location on the plant, the shape and color of pea pods, the shape and color of seeds, and the length of plant stems.
Mendel's approach was to transfer pollen (which contains male sex cells) from the stamen (the male reproductive organ) of one pea plant to the pistil (female reproductive organ) of a second pea plant. As a simple example of this kind of experiment, suppose that one takes pollen from a pea plant with red flowers and uses it to fertilize a pea plant with white flowers. What Mendel wanted to know is what color the flowers would be in the offspring of these two plants. In a second series of experiments, Mendel studied the changes that occurred in the second generation. That is, suppose two offspring of the red/white mating ("cross") are themselves mated. What color will the flowers be in this second generation of plants? As a result of these experiments, Mendel was able to state three generalizations about the way characteristics are transmitted from one generation to the next in pea plants.
Words to Know
Allele: One of two or more forms a gene may take.
Dominant: An allele whose expression overpowers the effect of a second form of the same gene.
Gamete: A reproductive cell.
Heterozygous: A condition in which two alleles for a given gene are different from each other.
Homozygous: A condition in which two alleles for a given gene are the same.
Recessive: An allele whose effects are concealed in offspring by the dominant allele in the pair.
Before reviewing these three laws, it will be helpful to define some of the terms used in talking about Mendel's laws of inheritance. Most of these terms were invented not by Mendel, but by biologists some years after his research was originally published.
Genes are the units in which characteristics are passed from one generation to the next. For example, a plant with red flowers must carry a gene for that characteristic.
A gene for any given characteristic may occur in one of two forms, called the alleles (pronounced uh-LEELZ) of that gene. For example, the gene for color in pea plants can occur in the form (allele) for a white flower or in the form (allele) for a red color.
The first step that takes place in reproduction is for the sex cells in plants to divide into two halves, called gametes. The next step is for the gametes from the male plant to combine with the gametes of the female plant to produce a fertilized egg. That fertilized egg is called a zygote. A zygote contains genetic information from both parents.
For example, a zygote might contain one allele for white flowers and one allele for red flowers. The plant that develops from that zygote would said to be heterozygous for that trait since its gene for flower color has two different alleles. If the zygote contains a gene with two identical alleles, it is said to be homozygous.
Mendel's law of segregation describes what happens to the alleles that make up a gene during formation of gametes. For example, suppose that a pea plant contains a gene for flower color in which both alleles code for red. One way to represent that condition is to write RR, which indicates that both alleles (R and R) code for the color red. Another gene might have a different combination of alleles, as in Rr. In this case, the symbol R stands for red color and the r for "not red" or, in this case, white. Mendel's law of segregation says that the alleles that make up a gene separate from each other, or segregate, during the formation of gametes. That fact can be represented by simple equations, such as:
RR → R + R or Rr → R + r
Mendel's second law is called the law of independent assortment. That law refers to the fact that any plant contains many different kinds of genes. One gene determines flower color, a second gene determines length of stem, a third gene determines shape of pea pods, and so on. Mendel discovered that the way in which alleles from different genes separate and then recombine is unconnected to other genes. That is, suppose that a plant contains genes for color (RR) and for shape of pod (TT). Then Mendel's second law says that the two genes will segregate independently, as:
RR → R + R and TT → T + T
Mendel's third law deals with the matter of dominance. Suppose that a gene contains an allele for red color (R) and an allele for white color (r). What will be the color of the flowers produced on this plant? Mendel's answer was that in every pair of alleles, one is more likely to be expressed than the other. In other words, one allele is dominant and the other allele is recessive. In the example of an Rr gene, the flowers produced will be red because the allele R is dominant over the allele r.
The application of Mendel's three laws makes it possible to predict the characteristics of offspring produced by parents of known genetic composition. The picture on page 1248, for example, shows the cross between a sweet pea plant with red flowers (RR) and one with white flowers (rr). Notice that the genes from the two parents will segregate to produce the corresponding alleles:
RR → R + R and rr → r + r
There are, then, four ways in which those alleles can recombine, as shown in the same picture. However, all four combinations produce the same result: R + r → Rr. In every case, the gene formed will consist of an allele for red (R) and an allele for "not red" (r).
The drawing at the right in the picture on page 1248 shows what happens when two plants from the first generation are crossed with each other. Again, the alleles of each plant separate from each other:
Rr → R + r
Again, the alleles can recombine in four ways. In this case, however, the results are different from those in the first generation. The possible results of these combinations are two Rr combinations, one RR combination, and one rr combination. Since R is dominant over r, three of the four combinations will produce plants with red flowers and one (the rr option) will product plants with non-red (white) flowers.
Biologists have discovered that Mendel's laws are simplifications of processes that are sometimes much more complex than the examples given here. However, those laws still form an important foundation for the science of genetics.
[See also Chromosome; Genetics ]
Mendelian Laws of Inheritance
Mendelian Laws of Inheritance
The Mendelian laws of inheritance laid down the basic principles of genetics. They state that characteristics are not inherited in a random way, but instead follow predictable, mathematical patterns. Mendelian laws were formulated by Austrian monk and botanist (a person specializing in the study of plants) Gregor Johann Mendel (1822–1887) in 1865, but went unnoticed for nearly a half century.
Before Mendel, many scientists had realized that certain traits, or characteristics, were passed on from one generation to the next, but in the middle of the nineteenth century, no one had any idea about where to begin to discover what controlled them or how or why these traits were passed on. In 1857 Mendel was able to combine his interest in both botany and mathematics by undertaking a long-term study breeding garden peas. For the next eight years, Mendel was able to conduct a thorough scientific study of how traits pass from one generation to the next.
By using ordinary garden peas—like those we eat today and call sweet peas—Mendel was able to easily breed for what are called "pure traits." This means that a self-pollinated (plants that contain both male and female reproductive organs and are able to transfer pollen between these parts) plant with pure traits will always produce offspring like itself. For example, a purebred plant that produces yellow pods will always produce yellow pods. Mendel then selected pea varieties that differed in single traits (such as height or pod color), and then he crossed them with plants that had a different trait (crossing tall plants with short, or yellow pods with green). After crossing a pure tall with a pure short, he would record the number of each type harvested and save the seeds produced by each plant for later planting, recording, and study.
While Mendel was conducting these careful experiments, neither he nor anyone else had any idea that such things as chromosomes (coiled structures in a cell's nucleus that carries the cell's heredity information) and genes (basic units of heredity) existed, although he would eventually decide that plants contained something he called "factors" and "particles of inheritance." He came to this conclusion because of the pattern of results he eventually saw. The very first thing that Mendel discovered after crossing a pure tall plant with a pure short one was that it did not result in the production of medium-size offspring. Instead, in the first generation, all the plants were tall. However, after allowing these plants to self-pollinate, he saw that the next generation produced plants that were a mix of tall and short. In fact, three-quarters were tall (which he called a dominant factor), and one-quarter were short (which he called a recessive factor). Mendel continued crossing hundreds of plants and kept careful records. Eventually he was able to state that a regular 3 to 1 ratio or pattern existed for the number of dominant versus recessive traits. This led him to realize that there must be laws or rules that make this mathematical ratio happen.
Continued work and study eventually allowed him to formulate what are now called the Mendelian laws of inheritance. He stated correctly that the characteristics of an organism are passed on from on generation to another by definite particles (which he called factors and we call genes). These genes exist in pairs, which are really different versions of the same genetic instructions. In this pair, one of the two factors comes from the male parent and the other comes from the female parent (each contributes equally). Finally, traits do not blend but remain distinct, and they combine and sort themselves out according to fixed rules. Mendel also stated
that dominant genes always have an effect on an individual, but that two recessive factors have to be present before they are expressed. Recessive factors can therefore be present in an individual but not have any effect on its characteristics.
Mendel published his findings in an obscure journal, and although his laws had laid the foundation for the new science of genetics, his work remained unknown for nearly two decades after his death. In 1900 Mendel's work was separately discovered by three different botanists (in three different countries) who realized that Mendel had discovered the laws of genetics long before they had. Although each published his own version of these laws, each man cited Mendel as the real discoverer. All three honorably stated that their work was merely a confirmation of what Mendel had accomplished in 1865.
1. (segregation) The two members of a gene pair segregate from each other during meiosis, each gamete having an equal probability of obtaining either member of the gene pair.
2. (independent assortment) Different segregating gene pairs behave independently. This second law is not universal as was originally thought, but applies only to unlinked or distantly linked pairs. At the time of Mendel, genes had not been identified as the units of inheritance: he considered factors of a pair of characters segregating and members of different pairs of factors assorting independently.