An example: Gregor Mendel's genetic theory
In the middle of the nineteenth century, in Brno, in what is now the Czech Republic, a monk named Gregor Mendel developed a new theory of biological inheritance.
Most biologists of his day believed that plants and animals inherited their characteristics from their parents by a blending of genetic material, rather like the mixing of fluids. It was supposed, for example, that when the pollen from a white-flowering pea fertilized a red-flowering pea, the seeds would usually produce peas with pink flowers, because the material that made the flowers white in one plant blended with the material that made the other flowers red to produce this intermediate coloring.Mendel suggested that this theory was quite wrong. He proposed that the genetic material that offspring inherited from the germ cells of their parents persisted unchanged in the next generation. (In animals, the germ cells—or gametes—are the spermatozoa and the unfertilized eggs.) To each of the characteristics of the offspring there corresponded, he said, units of heredity that came to be called “genes.” The characteristic appearance of an organism is called its “phenotype.” The genes that affect a particular phenotypic characteristic of an organism come, according to Mendel's theory, in various types. Genes that affect the same phenotypic characteristic are called “alleles.” On Mendel's theory, when a male and female mate, they each contribute one allele of each gene to their gametes. These gametes join to form the fertilized egg, which develops into the adult organism. So while the gametes have just one allele of each gene, the new organism has two alleles of each gene once more, one from each parent. The complete collection of all the genes of an organism is called its genotype.
Let's see how Mendel's theory would work out for the genetics of the flower color of peas, assuming a much-simplified version of his theory of inheritance.
Suppose peas with red flowers have two red- making alleles for petal color, and peas with white flowers have two white-making alleles. We'll call the red-making alleles R and the white-making ones W. So when these red- and white-flowering peas are crossed, each of their offspring will get one R and one W allele. Let's suppose that this is what makes them have pink flowers.Organisms with two alleles of the same gene, like the red and the white peas, are called homozygous. The pink peas, with different alleles, are called heterozygous. If the blending theory had been correct, then crossing one of the heterozygous pink-flowering peas with a red-flowering pea should have produced offspring all of the same color. The pink-making genetic material would have blended with an equal quantity of the red-making material to produce a pea that was, say, a deeper, redder shade of pink. But what actually happened, according to Mendel, if you did cross red and pink peas, was that you got two sorts of offspring. Some were pink; others were just as red as the red parent.
His theory explained this. For, if he was right, the original alleles, R and W, were still fully present in the pink peas; their genotype was RW. When they were crossed they gave one allele to each offspring, so that half of their offspring got R and half got W. The red peas were homozygous; their genotype was RR. They could only give one R allele to each offspring. Half of the offspring of this cross between pink and red plants got two R's and half got one R and one W. The offspring had exactly the same genetic constitution, so far as petal color was concerned, as one or other of the parents. They were all either RR or RW.
In this case, the heterozygous plant was intermediate in phenotype between the parents, which were homozygous in the respective alleles.
But Mendel also proposed that some alleles had a property called “dominance” over other alleles. One allele, A, was dominant over another, a, if its presence in the genotype made an organism have the same phenotype as a homozygote both of whose alleles were A. The other allele, a, was called the “recessive” member of the pair. (By convention, we often use an upper-case letter for the dominant allele and the same letter in lower-case for the recessive. But where I name alleles with different letters, I'll use upper case.) Thus, suppose purple-making alleles, P, dominated W alleles. Then there'd be two kinds of purple pea, PW and PP, but you could tell them apart because the PP plants would produce only purple offspring when crossed with each other. So we can call the PP variety “pure-breeding purple” plants. All the offspring of a cross between a pure-breeding purple pea (PP) and a white pea (WW) would have purple flowers. Even where one allele was dominant and the other recessive, however, the recessive allele was still present. So if you crossed two purple peas that were heterozygous and each had one W allele, those offspring that got a W allele from both parents would be white. In this case the cross between two purple-flowering peas, both with genotype PW, would produce one-quarter WW offspring, which would be white.Mendel supported his theory of dominance with the results of some experiments. He showed, for example, that if you crossed pure-breed- ing purple peas with white ones, all the members of the first generation were purple. What that meant in terms of the theory, as we have just seen, was that a cross between PP and WW could produce only PW offspring, and since P dominates W, these all look like their PP parents. Then he crossed these first-generation hybrids with each other and found that some of the offspring were purple and some were white.
Translating once more into terms of Mendel's theory, we can say why this was. Crossing the PWs with each other would produce one- quarter PP, one-half PW, and one-quarter WW offspring. The first two genotypes would produce purple flowers, but the last one would produce white ones. So Mendel's theory got all of these cases right.Every organism has many genes, according to Mendel's theory. Since the genes persist and do not blend, once you know the genotype of the parents you should be able to predict all the possible phenotypes that could be produced by a cross. But Mendel wondered whether the genes that determined different characteristics were linked together, so that if a pea got a gene for white flowers it also got, say, the gene for hairy stems.
If the genes for different characteristics were not connected, then they would be assigned to offspring independently of each other. Suppose that the hairy-stem allele, H, dominated the smoothstem allele, S, just as P dominates W in the gene for the color of the petals. Consider a pea that was heterozygous for both petal color and stem surface. It would have, say one W and one P allele of the color gene, and one H and one S allele of the stem gene. Its genotype, then, is WP HS. If these genes were inherited independently of each other then this plant would be able to contribute four different combinations of genes to its offspring: WH, WS, PH, PS.
Suppose we crossed this plant, with genotype WP HS, with one that was homozygous for both white petals and smooth stems (i.e., of genotype WW SS), so each of its offspring got the combination W S. The resultant offspring would have one of the following four genotypes:
1: WW HS 2: WW SS
3: PW HS 4: PW SS and these genotypes should come in roughly equal numbers.
These four kinds of plant would have the following phenotypes:1: White petals, hairy stems 2: White petals, smooth stems
3: Purple petals, hairy stems 4: Purple petals, smooth stems.
If, on the other hand, W was linked somehow to S, genotype 1 would not exist: the cross would produce no white-flowering hairystemmed peas. Similarly, if P was linked to H, then genotype 4 would not exist; the cross would produce no purple-flowering smooth-stemmed peas.
In a series of experiments, Mendel showed that, in fact, for several pairs of characteristics you got all the four possible combinations. And so he proposed two laws of genetics.
Mendel's first law, the law of segregation of characteristics, says that in the gametes, there is only one allele, as opposed to the normal two in the adult organism. (So if a plant has a gene for purple petals and one for white petals, they are segregated in the gametes.)
Mendel's second law was the law of independent assortment of genes. This says that both when different genes in an organism separate to form the gametes and when they join together again to make the fertilized egg, they do so independently. As a result, genes are inherited independently. We can see what this means in practice if we consider an organism that is heterozygous for two genes. Suppose it is Aa for one gene and Bb for another. If allele A ends up in a gamete, it is just as likely to be accompanied at the other locus by B as with b. And if a male gamete has allele A, it is just as likely to fertilize an egg with allele B of the other gene as it is to fertilize one with allele b.
Because the separation of alleles and their recombination were basically random processes, Mendel's experiments were more complex than this.
His results were statistical, and by using very basic statistical ideas he was able to make rough predictions not only of the variety of phenotypes that could result from a cross, but also of their frequencies.Let's summarize the main propositions of Mendel's theory of the gene.
1) Certain aspects of the phenotype of an organism are determined by its genes. (These are the genetically determined characteristics).
2) These genes may come in various types, called alleles, which differ in the consequences that their presence has for the genetically determined characteristics.
3) Each of these genetically determined characteristics may exist in different forms—different colors of petals, for example, or textures of stem.
4) Genetically determined characteristics are produced by pairs of alleles of the gene that corresponds to them.
5) Every organism gets two alleles of each gene, one from each parent.
6) If an organism gets identical alleles from each of its parents, it is homozygous for that allele; otherwise it is heterozygous.
7) If an organism is heterozygous for an allele A, it has the genetically determined phenotypic characteristic corresponding to A, which we call the A phenotype.
8) An allele, A, must exist in one of three relations to any other allele, A*. A can either
a) be dominant with respect to A*, or
b) be recessive with respect to A*, or
c) interact with A*.
9) If A is dominant with respect to A*, then an organism that is heterozygous and has the genotype AA* will have the A phenotype.
10) If A is recessive with respect to A*, then an organism that is heterozygous and has the genotype AA* will have the A* phenotype.
11) If A interacts with A*, then an organism that is heterozygous and has the genotype AA* will have neither the A nor the A* phenotype, but some other phenotype (not necessarily intermediate between A and A*) that is determined by A and A* together.
Along with these claims about how genes behave go the laws of segregation and independent assortment of genes.
12) Segregation: Each gamete bears only one of the two
alleles of the adult organism.
13) Assortment: When two different genes separate to form
gametes and join together again to form the new genotype, they do so independently.
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