Mendelian genetics is the study of simple patterns of inheritance. Gregor Mendel (1822-1884), an Austrian monk was the first to publish an extensive study of how various traits are passed from parent to progeny. He studied simple garden pea plants, plotting in detail their various physical traits, and studying how combinations of various parental traits produced particular traits in the progeny. For example, he looked at true-breeding strains of garden peas which expressed such physical characteristics as stem length (short or long); seed characteristics (round or wrinkled); seed color (white or gray); seed content color (yellow or green); unripe seed pod color (green or yellow); ripe seed pod shape (inflated or constricted); flower position (attached along length or end of stem). He would choose two plants which differed in only one of these seven characteristics, and cross them. Mendel did these experiments with huge numbers of plants, examining the results of the offspring. He would then allow the offspring generation of each experiment to cross with each other, in order to examine yet another generation.
Over time, Mendel was able to hypothesize that some type of "particle" passed from parent to progeny was responsible for the physical traits expressed in subsequent generations. He even ascertained that each offspring receives two of these "particles;" one from each parent. Mendel was able to lay down the basic foundations from which modern genetic theory has been built. These "particles" were later termed alleles. One allele is contributed by each parent and the two combined alleles determine the offspring's characteristics.
Mendel's work refuted the theories put forth earlier in history by scientists who were sometimes referred to as "blenders." These scientists thought that the physical traits of each parent blended together to create the physical traits of the offspring. By these simple terms, a black dog and a white dog should give birth to gray puppies. Mendel was able to show that, instead, the physical trait of one parent tended to be expressed in the offspring. Furthermore, he noted that two parent plants could produce an offspring which expressed a trait not present in either parent. Traits that were strong enough to take precedence over another competing trait were dubbed "dominant traits," while traits which could be masked by these dominant traits were dubbed "recessive traits."
Basic tenets of Mendelian genetics include an understanding that all parents have two genes which dictate each trait. These two genes separate themselves randomly into the sex cells (eggs or sperm), so that the parent will pass only one gene for a given trait on to the offspring. The offspring receives one gene from each parent for each trait. Dominant genes take precedence over recessive genes such that an individual expresses the recessive trait only if he or she has received two recessive genes. The presence of a dominant gene for a trait always means that that trait will be expressed, and the trait of any accompanying recessive gene will be masked.
Genetics has advanced well beyond the information that Mendel was able to procure through his study of pea plants. In the early 1900s, Wilhelm Johannsen of Denmark defined the difference between "genotype" and "phenotype." Using some of Mendel's principles, Johannsen stated that genotype describes the actual genes possessed by an organism, while phenotype describes the physical appearance of an organism. In the 1920s, Thomas Hunt Morgan of Columbia University, worked with several graduate students to define the chromosome, a unit of heredity along which the very genes ("particles of inheritance") first described by Mendel are now known to be linearly arranged. Watson and Crick published brilliant work in 1953, describing the chemistry and architecture of the deoxyribonucleic acid (DNA) that composes chromosomes.
Mendel's landmark paper was published in 1866, but was essentially ignored until the early 1900s. Since that time, the brilliance and daring of Mendel's early work has been greatly appreciated. Modern genetics has Mendelian genetics as its basis.
See also Gamete.
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