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Gregor Mendel - Father of Modern Genetic Theory

Mendel and his peas are the basis of modern genetic theory. Take time to learn this building block on which genetic theory is based. It is a practical demonstration of what happens as the result of the ideas in chapter 1 and how you can use deduction to guess what might happen. Mendel was a monk that lived in the 19th Century who gained lasting fame because of the work he published based on inheritance patterns he observed in peas. He was able to deduce from his experiments how genetic material behaved even though he did not know about chromosomes.

He was careful to use pure breed pea varieties, which when mated to their own type produced offspring that resembled the parents. He then used varying characteristics, which sharply differed from each other: green or yellow round, wrinkled or smooth and tall or short. Not a complete list but what will be referred to. This enabled him to count the numbers in each group and work out the ratios between them.

In one experiment he crossed a true breeding tall plant (TT) with a true breeding short plant (tt).

T= one of the 2 possible forms a gene may be. In this case a gene that results in a tall plant

t= the alternate form of a gene. In this case it represents the gene that results in a short plant

These alternate forms are known as alleles.

All the first generation offspring were as tall as the tall parents. The offspring were then mated to each other and these produced offspring that had tall and short. He got the same result whether he bred a tall female to a short male or a short female to a tall male. The sex of the parent made no difference as to what resulted in the offspring. The factor responsible was not affected by the sex of the parents.

In another similar experiment he added pollen (male) from a line of yellow peas (YY) to the flower (female) from a green line of peas (yy). Instead of the offspring being intermediate, all of the plants in the new generation looked like one of the parents and not the other. They all had yellow peas not a blend of yellow/green. From this generation, the eggs were fertilized by pollen from the same individual. Unexpectedly, both the original colors, yellow and green, reappeared in the next generation. Whatever had made green peas could still do so even though it spent a generation within a plant with yellow peas. This did not fit with the idea that the properties of each parent blended together. Mendel also added up the numbers of yellow and green peas in each generation. In the first generations (pure bred) all yellow. In the second generation, three yellow to one green. The same ratio as the tall and short plants (3/4 yellow and 1/4 green).

Mendel also made crosses using other characteristics and found that the same ratios applied. The four ways pollen and eggs can be combined to apply (one quarter yellow to yellow, one quarter green to green and one half yellow to green) also apply to different characteristics considered together. He found plants with yellow smoothed peas crossed with green wrinkled peas did not alter his laws of color inheritance. There was no blending of color inheritance influenced by shape. He deduced that completely separate genes were involved for each characteristic. We can distinguish in the cross two sorts of individuals, true breeding individuals with both alleles the same (called homozygotes (YY) or (yy), and individuals in which the two alleles continue to segregate (heterozygotes (Yy) or carriers. The gene that we are considering is said to be homozygous or heterozygous respectively.

From this Mendel deduced the fundamental law of genetics.

What Mendel discovered can be put very simply.

The two characteristics behaved completely independently of each other. He called this the rule of independent assortment. This could be explained if it were assumed that each plant had two copies of the factor influencing the trait. We call the factor responsible a gene. More than one form of the gene can exist and we call these alternative forms alleles.

Mendel explained the results by suggesting that each plant contained two alleles which did not blend together but which remained unchanged. In the next generation the plants passed one or other allele at random into a gamete to be combined with a gamete from the other parent. The non-blending followed by separation into the next generation is the Rule of Segregation.

Summary of what the results showed:

1.   Although all the first generation offspring looked like the parents and had the same phenotype (physical appearance) they had a different genotype (genetic makeup). Unlike the parents they were not true breeding.

2.     Pea color or height, he deduced was controlled by pairs of genes. Each adult plant had two genes for pea color or height, but the pollen or egg only one. On pollination a new plant with two genes was reborn since some of the second-generation plants were green or short. The color or height was determined by what genes the plant carried. In the original pure lines the individuals carried either two yellow/tall or two green/short genes depending on the lines they came from. When breeding to pure lines, the next generation was identical to the parents.

3.     Although these plants were yellow or tall they each carried the hidden gene. The yellow gene being expressed in the appearance of the offspring that still carries the green gene. The gene for yellow being dominant to the green gene which is recessive. The gene for tallness being dominant to the recessive short gene.

4.     Both the male and female gametes must make an equal contribution to the offspring because the reverse crosses gave the same result.

5.     Since the male gamete is in the nucleus this is where the heredity material must be. In the nucleus. The material is now known as genes.

6.     Since the offspring receives genes from both parents, pairs of genes controlled color/stem height. Although the genes had come together in the first generation some of the offspring must have carried only one kind of gene because they were true breeding. Therefore, when the first generation plants produced gametes they must have only form of the gene. That is, the genes for color or height must have segregated.

7.     Mendel concluded that offspring are not the average of their parents and that inheritance is based on differences rather than similarities.


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