Mendelian inheritance

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Gregor Mendel (1822-1884), an Austrian monk, was interested in understanding variances in plants, and between 1856 and 1863 cultivated and tested some 28,000 pea plants. His experiments brought forth two generalizations which later became known as Mendel's Laws of Heredity or Mendelian inheritance. These are described in his paper Experiments in Plant Hybridization (available at [1]) that was read to the Natural History Society of Brunn on February 8 and March 8, 1865, and was published in 1866.

Before Gregor Mendel formulated his theories of genetics in 1865 the prevailing theory of inheritance was that of blending inheritance, in which the speratozoan and egg of parent organisms contained a sampling of the parent's "essence" and that they somehow blended together to form the pattern for the offspring. This theory accounted for the fact that offspring tended to resemble both parents, but failed to show how diversity could be maintained over many generations without all members of a population eventually averaging themselves out.

Mendel proposed instead a theory of particulate inheritance, in which characteristics were determined by discrete units of inheritance that were passed intact from one generation to the next. These units would later come to be known as genes, though Mendel did not coin the term himself. Mendel based his theory on studies of inheritance patterns in garden peas (Pisum sativum), which were useful because they could be both cross-pollenated between two plants or self-pollenated with just one. Based on many years of careful, tedious breeding experiments, Mendel developed several fundamental laws of Mendelian inheritance.

Mendel's Law of Independent Assortment

The most important principle of Mendel's Law of Independent Assortment is that the emergence of one trait will not affect the emergence of another. While his experiments mixing one trait always resulted in a 3:1 ratio (Fig. 1) between dominant and recessive phenotypes, his experiments with two traits showed 9:3:3:1 ratios (Fig. 2). Mendel concluded that each organism carries two sets of information about its phenotype. If the two sets differ on the same phenotype, one of them dominates the other. That way, information can be passed on through the generations, even if the phenotype is not expressed (F1 generations, figures 1 and 2).
Figure 1 : Dominant and recessive phenotypes.
(1) Parental generation. (2) F1 generation. (3) F2 generation. Dominant (red) and recessive (white) phenotype look alike in the F1 (first) generation and show a 3:1 ratio in the F2 (second) generation
Figure 2 : Two traits (black/white and short/long hair, with black and short dominant) show a 9:3:3:1 ratio in the F2 generation. (S=short, s=long, B=black, b=white hair)
(1) Parental generation. (2) F1 generation. (3) F2 generation.
Results : 9x short black hair, 3x long black hair, 3x short white hair, 1x long white hair.

Mendel's findings allowed other scientists to simplify the emergence of traits to mathematical probability. A large portion of Mendel's spectacular findings can be traced to his proper usage of the scientific method. His choice of peas as a subject for his experiments was extraordinarily lucky. Peas have a relatively simple genetic structure. Also, Mendel could always be in control of the plants' breeding. When Mendel wanted to cross-pollinate a pea plant he needed only to remove the immature stamen of the plant. In this way he was always exactly sure of his plants' parents. Mendel made certain to start his experiments only with true breeding plants. He also only measured absolute characteristics such as color, shape, and position of the offspring. His data was expressed numerically and subjected to statistical analysis. This method of data reporting and the large sampling size he used gave credibility to his data. He also had the foresight to look through several successive generations of his pea plants and record their variations. Without his careful attention to procedure and detail, Mendel's work could not have had the impact it made on the world of genetics.

Mendel's Law of Segregation

Mendel's Law of Segregation essentially has four parts.

  1. Alternative versions of genes account for variations in inherited characters. This is the concept of alleles. Alleles are different versions of genes that impart the same characteristic. Each human has a gene that controls height, but there are variations among these genes in accordance with the specific height the gene "codes" for.
  2. For each character, an organism inherits two genes, one from each parent. This means that when somatic cells are produced from two gametes, one allele comes from the mother, one from the father. These alleles may be the same (true-breeding organisms, e.g. ww and rr in Fig. 3), or different (hybrids, e.g. wr in Fig. 3).
  3. If the two alleles differ, then one, the dominant allele, is fully expressed in the organism's appearance; the other, the recessive allele, has no noticeable effect on the organism's appearance. Today, we know several examples that disprove this "law", e.g. Mirabilis jalapa, the "Japanese wonder flower" (Fig. 3). This is called incomplete dominance. There is also codominance on a molecular level, e.g. people with sickle cell anemia, when normal and sickle-shaped red blood cells mix and prevent malaria.
  4. The two genes for each character segregate during gamete production. This is the last part of Mendel's generalization. The two alleles of the organism are separated into different gametes, ensuring variation.
Figure 3 : The color alleles of Mirabilis jalapa are not dominant or recessive.
(1) Parental generation. (2) F1 generation. (3) F2 generation. The "red" and "white" allele together make a "pink" phenotype, resulting in a 1:2:1 ratio of red:pink:white in the F2 generation.

During his experiments, Mendel encountered some traits that did not follow the laws he had encountered. These traits did not appear independently, but always together with at least one other trait. Mendel could not explain what happened and chose not to mention it in his work. Today, we know that these traits are close together on the same chromosome.

The parts of the previous version I didn't merge. Someone please have another look.

Mendel's First Law: Each adult pea plant has two genes - a gene pair - for each characteristic. The two memebers of each gene pair separate (segregate) randomly into the eggs or sperm of the plant, so that each egg or sperm contains only one member of each gene pair. The offspring therefore inherits one randomly selected gene from each parent for each characteristic.

The first law of Mendelian Genetics was easily illustrated due to the phenomenon of dominance. Certain characteristics, such as yellow seeds, were found to be "dominant" over other "recessive" characteristics, in this case over green seeds. A yellow-seeded plant crossed with a green-seeded plant produced offspring that were entirely yellow-seeded. However, when these yellow-seeded offspring were crossed with the original green-seeded parent strain (a procedure known as back-crossing), half of the plants in the second offspring generation bore yellow seeds and half bore green seeds. The following diagram illustrates these crosses, using an upper-case Y to indicate the dominant yellow characteristic and a lower-case y to indicate the recessive green characteristic. These two variants are called alleles of the gene.

YY X yy Parental generation (P)

| V Yy First generation of offspring (F1) All seeds are yellow (Y allele is dominant)

Yy X yy Second cross, F1 with green P | V Yy and yy Second generation of offspring (F2), with an equal proportion of Yy and yy

Mendel's Second Law: During the formation of sperm and egg, the segregation of alleles for one gene is independant of the segregation of alleles for another gene. This law was slightly more complex to demonstrate, requiring the statistical analysis of offspring of plants that differed in two separate characteristics. typing hands getting tired, put this demo in later