Gregor Mendel, through his work on pea plants, discovered the fundamental laws of inheritance. He deduced that genes come in pairs and are inherited as distinct units, one from each parent. Mendel tracked the segregation of parental genes and their appearance in the offspring as dominant or recessive traits.
He recognized the mathematical patterns of inheritance from one generation to the next. Mendel's Laws of Heredity are usually stated as:. Parental genes are randomly separated to the sex cells so that sex cells contain only one gene of the pair. Offspring therefore inherit one genetic allele from each parent when sex cells unite in fertilization. The genetic experiments Mendel did with pea plants took him eight years and he published his results in For this same characteristic flower color , white-colored flowers are a recessive trait.
The fact that the recessive trait reappeared in the F 2 generation meant that the traits remained separate not blended in the plants of the F 1 generation. Mendel also proposed that plants possessed two copies of the trait for the flower-color characteristic, and that each parent transmitted one of its two copies to its offspring, where they came together. Moreover, the physical observation of a dominant trait could mean that the genetic composition of the organism included two dominant versions of the characteristic or that it included one dominant and one recessive version.
Conversely, the observation of a recessive trait meant that the organism lacked any dominant versions of this characteristic. So why did Mendel repeatedly obtain ratios in his crosses? To understand how Mendel deduced the basic mechanisms of inheritance that lead to such ratios, we must first review the laws of probability.
Probabilities are mathematical measures of likelihood. The empirical probability of an event is calculated by dividing the number of times the event occurs by the total number of opportunities for the event to occur.
It is also possible to calculate theoretical probabilities by dividing the number of times that an event is expected to occur by the number of times that it could occur. Empirical probabilities come from observations, like those of Mendel. Theoretical probabilities come from knowing how the events are produced and assuming that the probabilities of individual outcomes are equal. A probability of one for some event indicates that it is guaranteed to occur, whereas a probability of zero indicates that it is guaranteed not to occur.
An example of a genetic event is a round seed produced by a pea plant. When the F 1 plants were subsequently self-crossed, the probability of any given F 2 offspring having round seeds was now three out of four. In other words, in a large population of F 2 offspring chosen at random, 75 percent were expected to have round seeds, whereas 25 percent were expected to have wrinkled seeds.
Using large numbers of crosses, Mendel was able to calculate probabilities and use these to predict the outcomes of other crosses. Mendel demonstrated that the pea-plant characteristics he studied were transmitted as discrete units from parent to offspring.
As will be discussed, Mendel also determined that different characteristics, like seed color and seed texture, were transmitted independently of one another and could be considered in separate probability analyses. For instance, performing a cross between a plant with green, wrinkled seeds and a plant with yellow, round seeds still produced offspring that had a ratio of green:yellow seeds ignoring seed texture and a ratio of round:wrinkled seeds ignoring seed color.
The characteristics of color and texture did not influence each other. The product rule of probability can be applied to this phenomenon of the independent transmission of characteristics. The product rule states that the probability of two independent events occurring together can be calculated by multiplying the individual probabilities of each event occurring alone.
To demonstrate the product rule, imagine that you are rolling a six-sided die D and flipping a penny P at the same time.
The outcome of rolling the die has no effect on the outcome of flipping the penny and vice versa. There are 12 possible outcomes of this action Table 2 , and each event is expected to occur with equal probability. For example, consider how the product rule is applied to the dihybrid cross: the probability of having both dominant traits in the F2progeny is the product of the probabilities of having the dominant trait for each characteristic, as shown here:.
On the other hand, the sum rule of probability is applied when considering two mutually exclusive outcomes that can come about by more than one pathway. The sum rule states that the probability of the occurrence of one event or the other event, of two mutually exclusive events, is the sum of their individual probabilities. What is the probability of one coin coming up heads and one coin coming up tails?
This outcome can be achieved by two cases: the penny may be heads PH and the quarter may be tails QT , or the quarter may be heads QH and the penny may be tails PT. Either case fulfills the outcome. You should also notice that we used the product rule to calculate the probability of PH and QT, and also the probability of PT and QH, before we summed them. Again, the sum rule can be applied to show the probability of having just one dominant trait in the F2 generation of a dihybrid cross:.
To use probability laws in practice, it is necessary to work with large sample sizes because small sample sizes are prone to deviations caused by chance. The large quantities of pea plants that Mendel examined allowed him calculate the probabilities of the traits appearing in his F 2 generation.
The seven characteristics that Mendel evaluated in his pea plants were each expressed as one of two versions, or traits. The physical expression of characteristics is accomplished through the expression of genes carried on chromosomes.
The genetic makeup of peas consists of two similar or homologous copies of each chromosome, one from each parent. Each pair of homologous chromosomes has the same linear order of genes. In other words, peas are diploid organisms in that they have two copies of each chromosome. The same is true for many other plants and for virtually all animals.
Diploid organisms utilize meiosis to produce haploid gametes, which contain one copy of each homologous chromosome that unite at fertilization to create a diploid zygote. For cases in which a single gene controls a single characteristic, a diploid organism has two genetic copies that may or may not encode the same version of that characteristic. Gene variants that arise by mutation and exist at the same relative locations on homologous chromosomes are called alleles.
Mendel examined the inheritance of genes with just two allele forms, but it is common to encounter more than two alleles for any given gene in a natural population.
Two alleles for a given gene in a diploid organism are expressed and interact to produce physical characteristics. The observable traits expressed by an organism are referred to as its phenotype. When true-breeding plants in which one parent had yellow pods and one had green pods were cross-fertilized, all of the F 1 hybrid offspring had yellow pods. That is, the hybrid offspring were phenotypically identical to the true-breeding parent with yellow pods.
However, we know that the allele donated by the parent with green pods was not simply lost because it reappeared in some of the F 2 offspring.
Therefore, the F 1 plants must have been genotypically different from the parent with yellow pods. The P 0 plants that Mendel used in his experiments were each homozygous for the trait he was studying. Diploid organisms that are homozygous at a given gene, or locus, have two identical alleles for that gene on their homologous chromosomes.
When P 0 plants with contrasting traits were cross-fertilized, all of the offspring were heterozygous for the contrasting trait, meaning that their genotype reflected that they had different alleles for the gene being examined.
Our discussion of homozygous and heterozygous organisms brings us to why the F 1 heterozygous offspring were identical to one of the parents, rather than expressing both alleles. In all seven pea-plant characteristics, one of the two contrasting alleles was dominant, and the other was recessive. Mendel called the dominant allele the expressed unit factor; the recessive allele was referred to as the latent unit factor. We now know that these so-called unit factors are actually genes on homologous chromosome pairs.
For a gene that is expressed in a dominant and recessive pattern, homozygous dominant and heterozygous organisms will look identical that is, they will have different genotypes but the same phenotype. The recessive allele will only be observed in homozygous recessive individuals Table 4.
Several conventions exist for referring to genes and alleles. Furthermore, we will use uppercase and lowercase letters to represent dominant and recessive alleles, respectively.
Therefore, we would refer to the genotype of a homozygous dominant pea plant with violet flowers as VV , a homozygous recessive pea plant with white flowers as vv , and a heterozygous pea plant with violet flowers as Vv. When fertilization occurs between two true-breeding parents that differ in only one characteristic, the process is called a monohybrid cross, and the resulting offspring are monohybrids.
Mendel performed seven monohybrid crosses involving contrasting traits for each characteristic. On the basis of his results in F 1 and F 2 generations, Mendel postulated that each parent in the monohybrid cross contributed one of two paired unit factors to each offspring, and every possible combination of unit factors was equally likely.
To demonstrate a monohybrid cross, consider the case of true-breeding pea plants with yellow versus green pea seeds. The dominant seed color is yellow; therefore, the parental genotypes were YY for the plants with yellow seeds and yy for the plants with green seeds, respectively. A Punnett square , devised by the British geneticist Reginald Punnett, can be drawn that applies the rules of probability to predict the possible outcomes of a genetic cross or mating and their expected frequencies.
To prepare a Punnett square, all possible combinations of the parental alleles are listed along the top for one parent and side for the other parent of a grid, representing their meiotic segregation into haploid gametes. Then the combinations of egg and sperm are made in the boxes in the table to show which alleles are combining.
Each box then represents the diploid genotype of a zygote, or fertilized egg, that could result from this mating. Because each possibility is equally likely, genotypic ratios can be determined from a Punnett square. If the pattern of inheritance dominant or recessive is known, the phenotypic ratios can be inferred as well.
For a monohybrid cross of two true-breeding parents, each parent contributes one type of allele. In this case, only one genotype is possible. All offspring are Yy and have yellow seeds Figure 4. Figure 4. In the P 0 generation, pea plants that are true-breeding for the dominant yellow phenotype are crossed with plants with the recessive green phenotype.
This cross produces F 1 heterozygotes with a yellow phenotype. Punnett square analysis can be used to predict the genotypes of the F 2 generation. Therefore, the offspring can potentially have one of four allele combinations: YY , Yy , yY , or yy Figure 4. Notice that there are two ways to obtain the Yy genotype: a Y from the egg and a y from the sperm, or a y from the egg and a Y from the sperm.
For example, one allele results in round seeds, and another allele specifies wrinkled seeds. One of the most impressive things about Mendel's thinking lies in the notation that he used to represent his data.
Mendel's notation of a capital and a lowercase letter Aa for the hybrid genotype actually represented what we now know as the two alleles of one gene : A and a. Moreover, as previously mentioned, in all cases, Mendel saw approximately a ratio of one phenotype to another. When one parent carried all the dominant traits AA , the F 1 hybrids were "indistinguishable" from that parent. However, even though these F 1 plants had the same phenotype as the dominant P 1 parents, they possessed a hybrid genotype Aa that carried the potential to look like the recessive P 1 parent aa.
After observing this potential to express a trait without showing the phenotype, Mendel put forth his second principle of inheritance: the principle of segregation. According to this principle, the "particles" or alleles as we now know them that determine traits are separated into gametes during meiosis , and meiosis produces equal numbers of egg or sperm cells that contain each allele Figure 5.
Mendel had thus determined what happens when two plants that are hybrid for one trait are crossed with each other, but he also wanted to determine what happens when two plants that are each hybrid for two traits are crossed.
Mendel therefore decided to examine the inheritance of two characteristics at once. Based on the concept of segregation , he predicted that traits must sort into gametes separately. By extrapolating from his earlier data, Mendel also predicted that the inheritance of one characteristic did not affect the inheritance of a different characteristic.
Mendel tested this idea of trait independence with more complex crosses. First, he generated plants that were purebred for two characteristics, such as seed color yellow and green and seed shape round and wrinkled. These plants would serve as the P 1 generation for the experiment. In this case, Mendel crossed the plants with wrinkled and yellow seeds rrYY with plants with round, green seeds RRyy. From his earlier monohybrid crosses, Mendel knew which traits were dominant: round and yellow.
So, in the F 1 generation, he expected all round, yellow seeds from crossing these purebred varieties, and that is exactly what he observed. Mendel knew that each of the F 1 progeny were dihybrids; in other words, they contained both alleles for each characteristic RrYy. He then crossed individual F 1 plants with genotypes RrYy with one another. This is called a dihybrid cross.
Mendel's results from this cross were as follows:. Next, Mendel went through his data and examined each characteristic separately. He compared the total numbers of round versus wrinkled and yellow versus green peas, as shown in Tables 1 and 2. The proportion of each trait was still approximately for both seed shape and seed color. In other words, the resulting seed shape and seed color looked as if they had come from two parallel monohybrid crosses; even though two characteristics were involved in one cross, these traits behaved as though they had segregated independently.
From these data, Mendel developed the third principle of inheritance: the principle of independent assortment. According to this principle, alleles at one locus segregate into gametes independently of alleles at other loci. Such gametes are formed in equal frequencies. More lasting than the pea data Mendel presented in has been his methodical hypothesis testing and careful application of mathematical models to the study of biological inheritance.
From his first experiments with monohybrid crosses, Mendel formed statistical predictions about trait inheritance that he could test with more complex experiments of dihybrid and even trihybrid crosses.
This method of developing statistical expectations about inheritance data is one of the most significant contributions Mendel made to biology. But do all organisms pass their on genes in the same way as the garden pea plant?
The answer to that question is no, but many organisms do indeed show inheritance patterns similar to the seminal ones described by Mendel in the pea. In fact, the three principles of inheritance that Mendel laid out have had far greater impact than his original data from pea plant manipulations. To this day, scientists use Mendel's principles to explain the most basic phenomena of inheritance. Mendel, G. Strachan, T. Mendelian pedigree patterns. Human Molecular Genetics 2 Garland Science, Chromosome Theory and the Castle and Morgan Debate.
Discovery and Types of Genetic Linkage. Genetics and Statistical Analysis. Thomas Hunt Morgan and Sex Linkage. Developing the Chromosome Theory. Genetic Recombination.
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