BIO 111 Genetic Inheritance PDF

GENETIC INHERITANCE AND INFORMATION TRANSFER  Genetics is the science of inheritance which involves transmission of characters from parents to offspring. Gregor Johann Mendel is known as the father of genetics.  Monohybrid cross A monohybrid cross is a cross betwee...

GENETIC INHERITANCE AND INFORMATION TRANSFER  Genetics is the science of inheritance which involves transmission of characters from parents to offspring. Gregor Johann Mendel is known as the father of genetics.  Monohybrid cross A monohybrid cross is a cross between two parents differing in one trait or character or where only one trait is considered. A monohybrid cross is a cross that follows only two variations on a single trait. Mendel studied seven characters in his experiments, which possessed two variants that differed from one another in ways that were easy to recognize and score. Figure 1: Mendel’s seven traits. Mendel studied how differences among varieties of peas were inherited whe varieties were crossed.  Mendel’s findings The F1 generation exhibits only one of two traits, without blending The F2 generation exhibits both traits in a 3:1 ratio The 3:1 ratio is actually 1:2:1 Figure 2: The generation is a disguised 1:2:1 ratio. Figure 3: Mendel’s monohybrid experiment  Mendel’s five-element model Parents do not transmit physiological traits directly to their offspring. Rather, they transmit discrete information for the traits, what Mendel called “factors.” We now call these factors genes. Each individual receives one copy of each gene from each parent. Genes are carried on chromosomes. Each adult individual is diploid, with one set of chromosomes from each parent. Not all copies of a gene are identical. The alternative forms of a gene are called alleles. When two haploid gametes containing the same allele fuse during fertilization, the resulting offspring is said to be homozygous. When the two haploid gametes contain different alleles, the resulting offspring is said to be heterozygous. The two alleles remain discrete. Therefore, when the individual matures and produces its own gametes, the alleles segregate randomly into these gametes. The presence of a particular allele does not ensure that the trait it encodes will be expressed. In heterozygous individuals, only one allele is expressed (the dominant one), and the other alleleis present but unexpressed (the recessive one).  The total set of alleles that an individual contains is the individual’s genotype. The physical appearance or other observable characteristics of that individual, which result from an allele’s expression, is termed the individual’s phenotype. The principle of segregation Mendel’s main conclusion is that alternative alleles for a character segregate from each other during gamete formation and remain distinct. This is commonly referred to as Mendel’s first law of heredity, or the Principle of Segregation. It can be simply stated as: The two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization.  Terminology Genetic Explanation Example Term Gene The basic unit of inheritance for a given Flower characteristic colour Allele One of a number of alternative forms of the same P p gene responsible for determining contrasting characteristics Locus Position of an allele within a DNA molecule Homozygo The diploid condition in which the alleles at a AA or aa us given locus are identical Heterozyg The diploid condition in which the alleles at a Aa ous given locus are different Phenotype The observable characteristics of an individual Purple or white Genotype The genetic constitution of an organism AA, Aa, aa Dominant The allele that is expressed in the presence of an A alternative allele Dihybrid Crosses: The Principle of Independent Assortment The inheritance of two pairs of contrasted characteristics. Since two pairs of alleles are found in the heterozygotes, this condition is known as a dihybrid inheritance’ Traits in a dihybrid cross behave independently  Consider a cross involving different seed shape alleles (round, R, and wrinkled, r) and different seed color alleles (yellow, Y, and green, y). Crossing round yellow (RR YY) with wrinkled green (rr yy), produces heterozygous F1 individuals having the same phenotype (namely round and yellow) and the same genotype (Rr Yy). Allowing these dihybrid F1 individuals to self-fertilize and produce an F2 generation. He collected a total of 556 F2 seeds from the F1 generation which showed the following characteristics: 315 round and yellow 101 wrinkled and yellow 108 round and green 32 wrinkled and green The proportions of each phenotype approximated to a ratio of 9:3:3:1. This is known as a dihybrid ratio. Figure 4: (a) Stages in the formation of F1 phenotypes from homozygous parents. This is an example of dihybrid cross since two characteristics are being considered (b) Use of Punnet square to show all possible combinations to form F2 genotypes  Mendel made two deductions from it. 1. Two new combinations of characteristics had appeared in the F2 generation. Wrinkled and yellow, and round and green. 2. The ratio of each pair of allemorphic characteristics appeared in the monohybrid ratio 3:1, that is 3:1 (423 round to 133 wrinkled, 416 yellow to 140 green). On the basis of these results Mendel was able to state that the two pairs of characteristics while combining in the F1 generation, separate and behave independently from one another in subsequent generations. This forms the basis of Mendel’s second law or the principle of independent assortment which states that: any one pair of characteristics may combine with either of another or the segregation of different allele pairs is independent. The Testcross: Revealing Unknown Genotypes Mendel devised a simple and powerful procedure called the testcross. In a testcross, an individual with unknown genotype is crossed with the homozygous recessive genotype. For instance, to determine the genotype of a purple flower a testcross with a white-flowered plant can be considered. In this cross, the two possible test plant genotypes will give different results:  Alternative 1: Unknown individual is homozygous dominant (PP ) PP × pp: All offspring have purple flowers (Pp). Parental genotype PP x pp Meiosis Gametes (n) P P x p p Random fertilization F1 genotype Pp Pp Pp Pp F1 phenotype All purple  Alternative 2: Unknown individual is heterozygous (Pp) Pp × pp: ½ of offspring have white flowers (pp), and ½ have purple flowers (Pp). Parental genotype Pp x pp Meiosis Gametes (n) P p x p p Random fertilization F1 genotype Pp Pp pp pp F1 phenotype Purple White  In summary the appearance of the recessive phenotype in the offspring of a testcross indicates that the test individual’s genotype is heterozygous. Testcrosses can also be used to determine the genotype of an individual when two genes are involved. An F2 individual exhibiting both dominant traits (A_ B_) might have any of the following genotypes: AABB, AaBB, AABb, or AaBb. By crossing dominant-appearing F2 individuals with homozygous recessive individuals, it can be determined whether either or both of the traits bred true among the progeny, and thus determine the genotype of the F2 parent.  GENETIC VARIATION In genetic variation, the genes of organisms within a population change. Gene alleles determine distinct traits that can be passed on from parents to offspring. Gene variation is important to the process of natural selection. The genetic variations that arise in a population happen by chance, but the process of natural selection does not. Natural selection is the result of the interactions between genetic variations in a population and the environment. The environment determines which variations are more favorable. More favorable traits are thereby passed on to the population as a whole.  Sources of Genetic Variation 1. DNA Mutation: A mutation is a change in the DNA sequence. 2. Gene Flow: Also called gene migration, gene flow introduces new genes into a population as organisms migrate into a new environment. 3. Sexual Reproduction: a. Crossing over –  b. Independent variation/assortment – the orientation of chromatids of homologous chromosomes on equatorial spindle at metaphase I of meiosis determines the direction in which the pairs move during anaphase I. this orientation of chromatids is random. Similarly at metaphase II, orientation of pairs of chromatids is random. This random orientation and independent assortment of the chromosomes give rise to a large calculable number of different chromosome combinations in the gametes c. Random fusion of gametes – the fusion of male and female gametes is completely random. Thus any male gamete is potentially capable of fusing with any female gamete.  PHENOTYPIC VARIATION The term variation describes the difference in characteristics shown by organisms belonging to the same natural population or species. While the phenotypic appearance of any characteristic is ultimately determined by the genes controlling that characteristics, the extent to which certain characteristics develop may be more influenced by the environment. There are two forms of phenotypic differences discontinuous and continuous variation.  Discontinuous variation These are characteristics within a population with limited form of variation. Variation in this case produces individuals with clear-cut differences with no intermediates between them e.g blood groups in humans, tongue rolling in humans, wing lengths in Drosophila and sex in animals and plants. Characteristics showing discontinuous variation are usually controlled by one or two major genes which may have two or more allele forms and their phenotypic expression is relatively unaffected by the environment. This form of variation is alternatively known as qualitative inheritance.  Continuous variation Many characteristics in a population show a complete gradation from one extreme to the other without a break. This is illustrated clearly by characteristics such as mass, linear dimension, shape and colour of organs and organisms. Characteristics exhibiting continuous variation are produced by the combined effects of many genes (polygenes) and environmental factors. This form of variation is alternatively known as quantitative inheritance.  Influence of the environment The ultimate factor determining a phenotypic characteristic is the genotype. At the point of fertilization the genotype of the organism is determined but the subsequent expression of this genetic potential is greatly influenced by environmental factors during the development of the organism. For example, Mendel’s tall variety of garden pea normally attained a pair of six feet. However it will only do so if provided with adequate light, water and soil conditions. A reduction in the supply of any of these factors (limiting factor) would prevent the gene for height exerting its full effect. Hence, continuous phenotypic variation can be described as ‘the effect of cumulative effect of varying environmental factors acting on a variable genotype’. In the development of human characteristics such as personality, temperament, and intelligence, there is evidence that both nature (hereditary factors) and nurture (environmental factors) interact to varying degrees in different individuals to influence the final appearance of characteristics.  The source of phenotypic variation are majorly through sexual reproduction.