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CBS, CHAPTER O4, Ch, Ge, Heredity and, Evolution, Ch, Tr, Al, In this Chapter., • Accumulation of Variations During Reproduction, Heredity (Inheritance of Traits), • Mendel's Contribution towards the Inheritance of Traits, • Experiment Conducted by Mendel, • Sex-Determination, Through the process of reproduction individuals give rise to, new individuals that are similar (not same) to the parents., This similarity in progeny or offspring or child is due to, transmission of characters or traits from parents to their, Heredity (Inheritance of Traits), Traits or characteristics, which are passed on from parents to, their offspring (generation to generation) are controlled by, progeny., genes., The transfer of characters from parents to offspring is known, as heredity and the process through which characters or traits, pass from one generation to another is called inheritance., A gene is a unit of DNA which governs the synthesis of one, protein that constants a specific character of an organism., e.g. Inheritance of free or attached earlobes., Accumulation of Variations During, Reproduction, The difference in the characters among the individuals of a, species is termed as variations. These variations, are accumulated by the process of sexual reproduction., Depending upon the nature of variations, different, individuals would have different advantages, the most, important advantage of variation to a species is that it, increases the chances of its survival in a changing, (a) Free earlobe, (b) Attached earlobe, Rules for inheritance of traits Inheritance of a trait is related, to the fact that both father and mother contribute equally, towards the genetic makeup of their offspring, i.e. for each, trait two versions are available in the child., environment.

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Heterozygous A condition in which an individual has a pair of contrasting, CBSE Term Science X, Homozygous A condition in which an individual possesses a pair of, Monohybrid A hybridisation cross in which inheritance of only one pair, Chromosome A long thread like structure in the ocleus. It sppears, Some Important Terms and Definitions Used in Heredity, Terms, Definitions, 55, Experiments Conducted By Mendel, More than a century ago, Mendel worked ost the main, rules for inheritance. He performed following two, during cell division and carries genes., A functional umit of heredity. It is present on chromosome., It is a piece of DNA that codles for one protein that inturn, determines a particular character (phenotype)., The feature or characteristic of an individual like height,, colour, shape, etc., An inherited character, Le. feature, which is normally, inherited and has its detectable variant too, eg tall and, dwarf are traits of a character, Le. height., Oue of the different forms of a particular gene, occupying, Gene, experiments, Character, 1. Monohybrid Cross: Inheritance of Traits for, One Contrasting Character, * Mendel took pea plants with different charcteristics, such as height (tall and short plants)., The progeny produced from them (F-generation, plants) were all tall. Mendel then allowed F, progeny, plants to undergo self-pollination., In the F-generation, he found that all plants were not, tall, three quarter were tall and one quarter of them, were short. This observation indicated that both the, traits of shortness and tallness were inherited in, F,-generation. But, only the tallness trait was, expressed in F,-generation., • Two copies of the traits are inherited in each sexually, reproducing organism., Trait, Allele, the same position on a chromosome., Auindividual having two different alleles for the same trait., Hybrid, An allele, whose phenotype will be expressed even in the, presence of another allele of that gene. It is represented by, a capital letter, e.g. T., An allele, which gets masked in the presence of a dominant, allele and can only affect the phenotype in the absence of a, dominant gee. It is represented by a small letter, e.g. t., Dominant, allele, Recessive, allele, Genetic composition of an individual., Genotype, The expression of the genotype, which is an observable or, measurable characteristic., Crossing F hybrid with one of its parents,, e.g. Tt x tt or Tt xTT., Phenotype, Back cross, of contrasting characters is studied., cross, A cross in which inheritance of two pairs of contrasting, characters is simultaneously studied., Short, Tall, (TT) x (11), All tall offsprings, (TI), Dihybrid, cross, PXP, Mendel's experiment showing law of dominance, identical alleles controlling a given character and will, breed true for this character (e.g. occurrence of two, identical alleles for tallness in a P tall pea plant)., • TT and Tt are phenotypically tall plants, whereas tt is, a short plant. For a plant to be tall, the single copy of, T is enough. Therefore, in traits Tt, T is a dominant, trait, while 't' is a recessive trait., alleles for any one character and will not breed true for this, character (e.g, simultaneous existence of dominant and, recessive alleles in F-hybrid tall pea plant)., Reproductive cells containing only one set (haploid) of, dissimilar chromosome., Gametes, Crossing F heterozygote with homozygous recessive, parent, e.g. R hybrid tall plant (Tt) with pure dwarf, plant (tt)., Test cross, Short, Tall, (TI), Tall, Tall, (TI) x (T) F, Mendel's experiment showing law of segregation, Tall, Tall, (TT), (T), Mendel's Contribution towards the, Inheritance of Traits, • In F2-generation, both the characters are recovered,, though one of these is not seen in F, stage. During, gamete formation, the factor or allele of a pair segregate, from each other., The Austrian monk, Gregor Johann Mendel is known as Father of, Genetics. He performed many experiments on pea (Pisum sativum), plant related to hybridisation., He studied seven pairs of contrasting characters in pea plants and, only one character at a time., Thus, the phenotypic ratio is 3:1 and the genotypic ratio, is 1:2:1 for the inheritance of traits for one contrasting, character, i.e. monohybrid cross.

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chromosome. Each cell of the body will have two coples, CBSE Term Il Scen, 2. Dihybrid Cross : Inheritance of Traits for Two, Visible Contrasting Characters, • Mendel took pea plants with two contrasting characters, Le., one with a green round seed and the other one with a yellow Mechanism of Inheritance, sofficient protein will be produced for normal body f, If the gene for a specific protein is altered, the protein, less efficient or will not be functional at all., wrinkled seed., • When the F progeny was obtained, they had round and, yellow seeds, thus establishing that round and yellow are, dominant traits., • Both the parents contribute a copy of the same gene t, progeny. Each germ cell thus, has one set of ene, pr, Mendel then allowed the F progeny to be self-crossed, (self-pollination) to obtain F progeny. He found that seeds, were round yellow, round green, wrinkled yellow and some, were wrinkled green., • The ratio of plants with above characteristics was 9:3:3:1,, respectively (Mendel observed that two new combinations, had appeared in F,)., each chromosome, one inherited from each parent., • When two germ cells combine, they restore the normal, number of chromosomes in the progeny. This ensures, stability of the DNA of species. Such mechanism of, inheritance explains the result of Mendel's experiments, is used by all sexually and asexually reproducing organ, In F-generation, all the four characters were assorted out, independent of the others. Therefore, he said that a pair of, alternating or contrasting characters behaves independently, of the other pair. For example, seed colour is independent of different species. For example, in reptiles environment, seed coat. The independent inheritance of two separate traits, shape and colour of seeds is schematically shown below, Sex-Determination, It is the process by which sex of a newborn individual is, determined. Different strategies can determine sex in, factors such as temperature at which fertilised eggs are ke, determine sex of the offspring, The determination of sex, occurs largely by genetic control in human heings. In hn, beings, there are 23 pairs of chromosomes, out of which, 22 pairs are autosomes and one pair is sex-chromosomes, Females have a perfect pair of sex chromosome, (homogametic), but males have a mismatched pair, (heterogametic) in which one is X (normal sized) and the, other is Y-chromosome (short in size)., Parents, Mound green), (Wrrkled yelow), Gametes, Ry, Frgeneration, RYy, (Round yollow), Hence, an egg fertilised by X-chromosome carrying s, results in a zygote with XX, which becomes a female and i, egg is fertilised by Y-chromosome carrying sperm, it results, XY zygote that becomes male., Thus, the sex of the children will be determined by what, inherit from their father. A child who inherits an, X-chromosome will be a girl and one who inherits a, Y-chromosome will be a boy. The inheritance of sex in, humans is diagrammatically shown below, Fgeneration, RY R, RY, RRYY, RRY, AYY, RYy, RRYY, RRyy Rryy, Rryy, BYY, Arry, ArYy, Rryy, ryy, Ratio, Male, Female, Fa-generation 315 round yellow, 108 round green, Gametes, 101 wrinkled yollow, 3., 32 wrinkled green, 556 seeds, 16, Zygote, XX, Expression of Traits, Cellular DNA is the source of information for making, proteins in the cell. A section of DNA that provides, information for one particular protein is called a gene for that, protein. Expression of trait in body depends on the, functioning of a gene. If the gene is working normally,, Offsprings, Fernale, Male, Sex-determination in human beings