Mendel was very methodical. His starting point was to wonder what would happen if he cross-bred two pea plants with contrasting features – for example, plants with purple flowers and plants with white flowers. Before he carried out any breeding experiments, he self-pollinated the plants for several generations, and eventually used plants from a ‘breeding line’ that had contained only purple-flowered plants or white-flowered plants. These he called ‘true-breeding’ plants. He then cross-bred them in the following way. We will use flower color as our example, but he used the same procedure for all the contrasting characteristics.
1. He removed stamens from the flowers of the purple flowered plant (so that these flowers could not pollinate themselves).
2. He used a paintbrush to transfer pollen from the flowers of the white-flowered plant to the carpel of the purple flowers.
3. This pollinated carpel then produced a pea pod containing several pea seeds.
4. He collected and grew all the seeds from all the pods.
5. When the plants were mature, he noted the color of their flowers.
Mendel also carried out reciprocal crosses. In this case he also pollinated white-flowered plants with pollen from purple-flowered plants.
In the above cross, all the offspring (which we call the F1 or fist finial generation) have purple flowers. Mendel then allowed these purple-flowered plants to self-pollinate themselves. In the next generation (the F2 or second finial generation) he found a ratio of very nearly three purple-flowered plants for every one white flowered plant. This pattern repeated itself in all of his experiments. In each case he:
• crossed pure-breeding plants with contrasting characteristics
• found that only one of the characteristics appeared in the F1 generation (always the same one – always purple flowers, for example, never white), and
• found a ratio of 3:1 in the F 2 generation (always 3 of the one that had appeared in the F1 and 1 of the one that hadn’t).
It was this pattern that led Mendel to formulate his laws and to coin the terms dominant and recessive. He used the term dominant to describe the allele that determined the trait that appeared in the F 1 and the term recessive to describe the allele that determined the trait that did not appear in the F1. Table below summarizes the results for all Mendel’s breeding experiments.
Th overall ratio for the F 2 generation is 2.99:1. This could hardly be nearer to 3:1 and, indeed, caused some biologists to accuse him of falsifying his results. However, most believe that he was just an exceptionally meticulous experimenter. Mendel explained these results in the following way:
1. Every trait (like flower color, or seed shape, or seed color) is controlled by two ‘heritable factors’ – these are what we now call genes. Th heritable factors (genes) may take different forms (alleles).
2. If the two alleles in an individual are different, one is dominant (will be expressed in the organism’s appearance or physiology) and one is recessive (cannot be expressed unless the individual has two copies of the recessive allele). Dominant traits mask the appearance of recessive traits.
3. The only physical link between the generations is the gametes or sex cells. These must pass the genes from one generation to the next.
4. The heritable factors (alleles) separate when the gametes (sex cells) are formed; each gamete therefore contains only one allele controlling the trait. This is Mendel’s ‘law of segregation’. He also stated that the gametes (sex cells) fuse randomly at fertilization.
After further studies and experiments, Mendel also formulated another law called the ‘law of independent assortment’. This law states that the inheritance of one trait is independent of the inheritance of another. That is, the alleles of one pair segregate independently of the alleles of another pair controlling a different feature. Whilst this was true for the traits that Mendel studied in pea plants and is true for many traits in many other organisms, we now know that it is not always the case, as we shall see when we look at the phenomenon of linkage.
If we return to our example of the cross between purple-flowered plants and white-flowered plants, we can now explain what is happening in terms of segregation of alleles, random fertilization and the concepts of dominant and recessive alleles.
In the genetic diagram in Figure 3.6, the symbol P represents the dominant allele for purple flowers and p represents the recessive allele for white flowers. Both parents are homozygous. Alleles segregate and gametes contain only one of the pair. All F 1 are heterozygous, with purple flowers as P is dominant. Alleles segregate and half of the gametes receive P and half p. So, if we know the genotypes of parents, we can produce genetic diagrams like the one above to work out the possible genotypes of their offspring and the proportions in which they will occur.