Laws of Mendel on Herdity, Dominance and Segregation


Since Mendel‘s time. scientists have added to his fundamentals and have linked such processes 11% meiosis as well as mitosis to his findings. Modern genetics has explored crossing over. non disjunction. incomplete dominance. linkage, and the like. Our current knowledge of these area has even engendered the field of genetic counseling. Use of the Punnett square is assessed on the exam. and you should know how to utilize this tool in calculating answers in genetics. particularly Mendelian genetics. The student should know these processes and Mendel’s laws in detail, be capable of comparing and contrasting mitosis and meiosis (a favorite essay question)‘ know the differences between spermatogenesis and oogenesis, and be able to perform monohybrid crosses. test crosses. and dihybrid crosses. The student should also be comfortable with the basics of Mendel‘s work and be able to transition to modern geneticsvwith COnflanCQ Finally. the study of the patterns of inheritance will be discussed from a variety of levels and is a theme that actually runs through most of the study ofthis area. '

Mendel

The “father” of genetics and a nineteenth-century monk trained in mathematics, Mendel conducted years of studies on the traits of pea plants as a hobby. He theorized that certain factors-known today as genes-controlled the passing on of these traits from one generation to the next. His laws are the foundation of genetics and are the principles upon which we base our current knowledge of inheritance. His techniques were precise, and his knowledge of mathematics was an essential asset to his conclusions. In discussing terminology, you should be aware that not all terms listed emanated from Mendel’s original work. A large measure of them are “post-Mendelian” but involve either clarification of his work or the discovery of new principles that dictated new terms. We begin with a discussion of his laws; meiosis is treated separately but is obviously an integral part of these laws and, in fact, embodies what Mendel discovered.

Law of Dominance

First and foremost, Mendel noticed that certain traits did not have a halfway point or a blending affect (see incomplete Dominance). Such was the case with the trait for height in peas in a monohybrid cross or mating for one trait. Mendel’s conclusion was that in the expression of the trait for height in peas, tall pea plants dominate over short pea plants; Mendel called them factors, and we now call them genes or alleles. When a pea plant that is homozygous, or pure, for tallness is crossed with a pea plant that is homozygous for shortness, it appears that the offspring produced in the F l, or first filial generation, have lost the recessive allele. This is an indicatioa that one trait is dominant, and the other (seemingly vanished) trait is recessive. Eventually. Mendel came to conclude that the recessive trait was being masked. In truth we know today that. although each offspring receives one gene from each parent (the P1 generation) for a trait, only one gene works to express that trait. In the case of a homozygous individual, it doesn’t matter which one because they are identical. In a heterozygous individual, it will be the dominant allele. Thus. the effect of the Law of Dominance was a powerful one, illuminating the relationship between dominant and recessive alleles. It should be noted that farmers and agriculturists were aware of this relationship to a certain extent before Mendel but used it informally in a hit-or-miss manner. Mendel codified the relationship, and, although his work sat on a library shelf for more than three decades, it nonetheless had a powerful effect on the world when it was rediscovered.

Law of Segregation

Prior to the potential for mating. certain cells go through a process we now know as meiosisDuring this time. the alleles ultimately separate from each other and end up in separate gametes or se* cells.





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