Mendelian Genetics and Mendelian Laws of Heredity
The foundations of the modern science of genetics were laid by Gregor Mendel, an Austrian Monk, who carried out experiments on the inheritance of characters between generations. Mendel worked on inheritance in sweet-peas, and selected characters that bred true; that is, the characters did not blend into one another in the next generation. Characters chosen for study by Mendel included flower color (such as red versus white), plant height (tall versus dwarf), seed coat (smooth-coated seeds verses wrinkled seeds), pod length (long pods versus short pods), and so on. Mendel eventually formulated the three laws of genetics, known today as the Mendelian laws of inheritance. These are the law of segregation, the law of independent assortment, and the law of dominance. Mendel's work went unnoticed for nearly two decades after his death in 1887, but was eventually recognized widely by the scientific community.
In order to understand Mendel's three laws of inheritance, it is necessary to review the plant breeding experiments which inspired the laws. Two of the three laws involve dihybrid crosses where two different sets of traits are studied together. For example, if tall plants (controlled by two dominant alleles, TT), red-flowering (controlled by two dominant alleles, RR), sweet-peas are pollinated by dwarf (controlled by two recessive alleles, tt), white-flowering (controlled by two recessive alleles), plants, the pollen and ova will contain either TR or tr, which represent only a single allele of each set of genes. When fertilization takes place, the resulting first filial generation (F1) will all have progeny of the same outward appearance (phenotype); they will all be tall, red-flowering (TtRr) sweet-peas. The progeny of the first filial generation (the F2 generation) are also all tall, red-flowering plants, because R, the allele for red and T, the allele for tall are dominant to r and t, which are known as recessive alleles. Since sweet-peas are self-pollinating and each plant now produces gametes of four different genotypes, namely TR, Tr, tR, and tr. When the seeds resulting from the random combination of gametes with one of these genotypes, the second filial generation (F2) will have four different phenotypes: tall with red flowers, tall with white flowers, dwarf with red flowers, and dwarf with white flowers, in the proportion 9:3:3:1.
Mendel found that sweet peas with the same phenotype (purple flowers) had different genotypes which were only expressed in subsequent generations. Purple flowers were produced when the alleles contained at least one allele for purple flowers, which is dominant over the allele for white flowers. White flowers were produced when either both alleles (coded for white flowers the condition) which is the homozygous recessive allele.
During Mendel's time neither the existence of genes nor their structure and function were understood. Indeed, chromosomes remained unknown for several years after Mendel's death. Mendel's experimental plants had factors that occurred in pairs. These factors have only one member of a pair in the gametes (pollen and ova). Mendel described the three laws of inheritance that described the passage of genes from one generation to the next.
Mendel's first law of inheritance is the law of segregation. This states that genes segregate during gamete formation into their different alleles. The two members of a pair of alleles separate (segregate) into two different gametes, and exert their influence in the offspring as one of a new pair of alleles. Segregation is the result of the separate carriage of genes on chromosomes, which are not altered or blended by forming pairs. A gene for red flowers in the sweet-pea does not become diluted from having been paired with the gene for white flowers, and is passed to subsequent generations unaltered in the gametes.
The second Mendelian law of inheritance, the law of independent assortment, describes the chance distribution of alleles to the gametes (ova or spermatozoa). If an individual has two pairs of alleles, Aa and Bb, it's gametes will contain equal numbers of the four possible combinations (AB, Ab,aB, ab), with one member from each pair. Independent assortment applies only to genes lying on different chromosomes, and does not-apply to linked genes on the same chromosomes. The F2 generation shows a ratio of 9 Tall red-flowers 3 Dwarf Red-flowering, 3 Tall White-flowering, and 1 Dwarf, White-flowering. A monohybrid cross, which involves a single character, such as plant height produces 12 Tall plants, and 4 Dwarf plants, in a typical 3:1 ratio. Each pair of alleles making up a gene, whether controlling plant height or flower color behaves though each pair were the only gene present for each pair segregates independently of other characters.
The third Mendelian law of inheritance, the law of dominance, states that heredity factors (genes) work together as sets, usually as pairs of alleles. The total number of different alleles represents some of the variation available to species. Frequently, in heterozygotes (with one dominant and one recessive allele) only the dominant allele of the gene is expressed in the phenotype, the recessive allele being concealed and is not expressed.
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