Chapter 5: Genetics PDF
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This document presents an introduction to genetics and Mendelian inheritance, explaining concepts such as phenotype, genotype, dominant and recessive traits, and the principles of segregation and independent assortment. It describes early experiments, specifically those of Mendel with pea plants, to illustrate these concepts.
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CHAPTER V GENETICS Genetics is the study of genes and their properties. A central aim of genetics is to determine how genes relate to the physical properties of an organism. The observable characteristics of an organism – the outcomes of the expression of the gene...
CHAPTER V GENETICS Genetics is the study of genes and their properties. A central aim of genetics is to determine how genes relate to the physical properties of an organism. The observable characteristics of an organism – the outcomes of the expression of the genes – are called its PHENOTYPE. The genetic program of an individual organism is called its GENOTYPE. The genotype includes the many genes that determine the characteristics detected in the phenotype. The rules governing the transmission of heritable characteristics from generation to generation of sexually reproducing organisms were first uncovered by an Augustinian monk named Gregor Mendel in the 19th century (see CHAPTERV. PPT file Slide 2). He chose Pea Plants for his experiments and he is very fortunate in his decision due to the “good” features of pea plants (Slide 3). In a typical experiment Mendel carried out controlled matings between a chosen mother plant and a father plant (Slide 4). During choosing the mother and father plants he concentrated on “unit characters” which have two alternative forms each, therefore he called them “antagonistic pairs” (Slide 5). These antagonistic pairs have clear-cut observable forms of particular traits (Slide 6), and there was no intermediate forms – they are “either or” traits – appearing in his observations. Since pea plants are self-fertilizing, Mendel obtained “pure-breeding lines” (Slide 7) by selfing mothers and fathers first with themselves and then started his experiments. He carried out his controlled experiments for each of the 7 antagonistic pairs and these experiments produced “hybrids” (Slides 8 and 9) and since he was concentrating one “unit characteristic” at a time, these hybrids are “MONOHYBRIDS”. In a typical monohybrid cross Mendel crossed two parents with one antagonistic pairs of traits (Slide 10) and therefore he called this as the Parental Generation “Generation P”. The seeds obtained from this cross constituted the first new generation and he called them the “First Filial” Generation or F 1 Generation (Filial = sons). In the F1 generation all the progeny resembled to only one parent, so he called this characteristic as the DOMINANT trait. However the other characteristic seemed to be lost. Then, he crossed two F1 sons with each other and obtained the second generation “F2”. To his surprise, the second characteristic reappeared in low numbers (3:1 ratio) therefore he called this characteristic as the RECESSIVE trait (Slide 10). When he repeated this typical experiment for each antagonistic pairs for 7 times, he obtained the same results: All the F1 progeny were of Dominant trait In the F2 progeny, the Dominant to Recessive ratio was 3 : 1 Mendel’s experiments were carefully controlled experiments he tried to prevent many of the pith-falls of biological experiments by checking every possible factor that may give different results. One such factor or question was the –then current theory- which parent contributes most to the inherited features of the offspring? To answer this question, he carried out “Reciprocal Crosses” for each antagonistic pairs by crisscrossing the sex and character of the parents (Slide 11). The results he obtained for every seven sets of antagonistic pairs were the same, so he concluded that the offspring’s phenotype is equally determined by the parents (Slide 12). Also the - then current theory of parental traits become mixed and forever changed in offspring – is also disproved by the reappearance of the recessive traits in the F2 generation (Slide 12). After Mendel completed his Monohybrid Crosses, he proposed concepts what will be forming the foundations of Genetics in the future (Slide 13). Following these proposals, he constructed his first law: Rule of Segregation (Slide 14). The genotypic explanation of the monohybrid crosses via Punnett Square method (Slide 15) and the meiotic distribution of alleles and fertilization of gametes in monohybrid crosses (Slide 16 and 17) are obviously the interpretation of Mendel’s results with today’s knowledge. In diploid organisms, phenotype by itself would not be sufficient to give us enough information about the genetic make up. Therefore phenotype vs. genotype comparisons are required (Slide 18). For a particular trait if the two alleles are the same than for that trait the organism is said to be HOMOZYGOUS however if the alleles are different than the organism is said to be HETEROZYGOUS. If the dominant phenotype is yellow for the seed color, than a yellow seed could either be HOMOZYGOUS DOMINANT (YY) or Heterozygous (Yy) whereas a green seed could only be HOMOZYGOUS RECESSIVE (yy) (Slide 18). Since a dominant phenotype could be either homozygous dominant or heterozygous, how could we determine this before starting an experiment? The answer to this question is very simple: By performing a TEST CROSS (Slide 19). DIHYBRID CROSSES Having satisfied himself of the validity of his Rule of Segregation, Mendel went on to study the simultaneous inheritance of two or more traits and try to understand how would these 2 pairs of alleles segregate? Towards this end he started his Dihybrid Crosses (Slide 20). In the F1 generation, his results were as predicted from his rule of segregation that is all the progeny were of dominant phenotype. When he crossed two F 1 individuals to obtain the F2 his results revealed a 9:3:3:1 phenotypic ratio. When his results are examined in a different perspective; one pair of character at a time (i.e. pea color or pea shape) still his rule of segregation ratios were obtained. For all of the 7 antagonistic pairs he carried out a total of 21 dihybrid crosses and every time he obtained the same results. Therefore he summarized his results (Slide 21) and established his 2nd Law: The Rule of Independent Assortment. The conclusion of Mendel’s two Laws can be seen in Slide 22. SEX LINKAGE Although Mendel carried out his genetic experiments with garden peas, genetics in the 20th century have relied on other experimental organisms to overcome the biological problems inherent in garden peas. One such organism is the FRUIT FLIES (Drosophila melanogaster) (Slide 23). Cells of the fruit flies have 6 autosomal chromosomes and 2 sex chromosomes (XX for females, XY for males). A scientist named T. H. Morgan (Slide 24) starting from 1906 study these organisms. He found that the inheritance pattern of certain traits did not behave according to the simple rules of Mendelian genetics. One such trait was the eye color. In Drosophila, the wild type flies have red eyes (Slide 23) however; among thousands of flies, T.H. Morgan discovered some flies having white eyes (Mutants) (Slide 25). In one experiment T. H. Morgan crossed homozygous red eyed females with white eyed males (Slide 26) and noticed that all the offspring in F 1 generation have red eyes, regardless of gender. He was relieved by obtaining this result because it was in accordance with Mendel’s rules. However, when he performed the reciprocal cross he noticed an unexpected finding rather than Mendel’s 3:1 ratio (Slide 26 and 27). This was called CRISSCROSS INHERITANCE (mothers to sons, fathers to daughters) and later on many similar cases of crisscross inheritance were reported for the inheritance of genes located on sex chromosome or SEX LINKED GENES. EXCEPTIONS OF MENDELIAN GENETICS Not all crosses between parents that have alternative forms of a hereditary trait produce the Mendelian pattern of one dominant phenotype in the F1 generation and a 3: 1 ratio of phenotypes in the F2 generation. In many plants, for example: Snapdragon (Anthrium majus), a cross between a red flower bearing plant and a white flower bearing plant yields F1 plants with only pink flowers (Slide 28) and F2 plants with a red to pink to white ratio of 1:2:1 (Slide 29). Such instances of INCOMPLETE DOMINANCE, in which “heterozygous individuals have a phenotype intermediate between the two homozygous parents”, are actually quite frequent. Another exception is the pea seed appearance. If a spotted pea and a dotted pea are crossed, all the F1 appear to be spotted and dotted. When two F1 individuals are crossed, F2 seeds with a spotted to dotted and spotted to dotted ratio of 1:2:1 is obtained (Slide 30). This phenomenon of mutual expression of both alleles (neither is dominant over the other) is called CODOMINANCE. The concepts of Incomplete Dominance and Codominance are summarized in Slide 31. Another exception, this time from humans, is the ABO blood groups. ABO blood group system involves multiple alleles of the I gene. I A and IB alleles (which code for enzymes that add corresponding sugars to lipid molecules on the red blood cell (RBC) membrane surfaces) are dominant over the I O (or ii allele) but IA and IB alleles are neither dominant nor recessive to each other (Slides 32 and slide 33): A typical example of CODOMINANCE. The presence or absence of these sugars (called “antigens”) and the corresponding recognition system for these antigens (called “antibodies”) constitutes the rules of blood transfusions (Slide 34). It should be noted at this point that, either incomplete dominance or the codominance phenomenon are still consistent with Mendel’s Rule of Segregation (Slide 35). -END OF CHAPTER V-