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Mendel's law
- 1. RAJENDRA CHAVHAN MGC ARMORI Presented by Dr. R. N. Chavhan Assistant Professor in Zoology, M. G. College, Armori, District Gadchiroli (MS) email: rajendrac99@gmail.com
- 2. RAJENDRA CHAVHAN MGC ARMORI INTRODUCTION: Inheritance is the acquiring of genetic characteristics or traits from parents by their offspring. In the year 1860, Gregor Johann Mendel, the father and founder of genetics unlocked the mystery of genetics. He conducted many experiments on the pea plants (Pisum sativum) and observed their pattern of inheritance from one generation to the next generation. His investigation led to the discovery of three laws of inheritance, famously known as Mendel’s Laws of Inheritance. Mendel’s laws of inheritance 1. The Law of Dominance, 2. Law of Segregation and 3. Law of Independent Assortment These laws came into existence by the experiments on pea plants (Pisum sativum) in a variety of differing traits. Mendel started his research with monohybrid cross. Mendel observed that traits which were absent in the F1 generation had reappeared in the F2 generation. These observations led to the formulation of the Law of Dominance and the Law of Segregation.
- 3. RAJENDRA CHAVHAN MGC ARMORI Mendel’s law of dominance states that: “When parents with pure, contrasting traits are crossed together, only one form of trait appears in the next generation. The hybrid offsprings will exhibit only the dominant trait in the phenotype.”
- 4. RAJENDRA CHAVHAN MGC ARMORI Law of dominance is known as the first law of inheritance. In this law, each character is controlled by distinct units called factors, which occur in pairs. If the pairs are heterozygous, one will always dominate the other. Law of dominance explains that in a monohybrid cross between a pair of contrasting traits, only one parental character will be expressed in the F1 generation and both parental characters will be expressed in the F2 generation in the ratio 3:1. The one which is expressed in the F1 generation is called the dominant trait and the one which is suppressed is called a recessive trait. In simple words, the law of dominance states that recessive traits are always dominated or masked by the dominant trait. This law can be described by Mendel’s experiment. A monohybrid cross is a cross between the two monohybrid traits (TT and tt). Here plants which have the same characters, but differ in only one character were crossed.
- 5. RAJENDRA CHAVHAN MGC ARMORI This is a first law of inheritance. If your two alleles are different (heterozygous, e.g. Bb), the trait associated with only one of these will be visible (Dominant) while the other will be hidden (Recessive). For example B is Dominant & b is Recessive.
- 6. RAJENDRA CHAVHAN MGC ARMORI Law of Dominance: In a hybrid union, the allele which expresses itself phenotypically is the Dominant allele while the other allele which fails to express itself phenotypically is the Recessive allele. The hybrid individual shows phenotypically only the Dominant character. The law of Dominance is often described as Mendel’s First law of inheritance.
- 7. RAJENDRA CHAVHAN MGC ARMORI The law of segregation states that the alleles of a given locus segregate into separate gametes.
- 8. RAJENDR A CHAVHAN MGC ARMORI Mendel’s law of segregation states that: “During the formation of gamete, each gene separates from each other so that each gamete carries only one allele for each gene.” Law of segregation is the second law of inheritance. This law explains that the pair of alleles segregate from each other during meiosis cell division (gamete formation) so that only one allele will be present in each gamete. In a monohybrid cross, both the alleles are expressed in the F2 generation without any blending. Thus, the law of segregation is based on the fact that each gamete contains only one allele. This law is based on four basic concepts: •A gene exists in more than one form of an allele. •When gametes are produced by meiosis, the allelic pairs separate, leaving each gamete with a single allele. •Every organism inherits two alleles for each trait. •The two alleles of a pair are different, i.e., one is dominant and one is recessive.
- 9. RAJENDR A CHAVHAN MGC ARMORI Mendel’s law of segregation: a) Factors (genes) for a particular trait occur in pairs. b)For each trait, an organism inherits two genes, one from each parent 1.One trait is controlled by one gene. 2.Each gene is found in two alleles. 3.During Gamete formation each allele is segregated in one gamete. The limitations of the law of segregation Law of segregation applies only to traits that completely control a single gene pair in which one of the two alleles is overriding the other. Therefore, the law of segregation does not apply to incompletely dominant or co-dominant alleles.
- 10. RAJENDRA CHAVHAN MGC ARMORI States that each pair of alleles segregates independently of the other pairs and all possible combinations of allele can occur in the resulting gametes. When a dihybrid (or a polyhybrid) forms gametes, (i) Each gamete receives one allele from each allelic pair and (ii) The assortment of the alleles of different traits during the gamete formation is totally independent of their original combinations in the parents. In other words, each allele of any one pair is free to combine with any allele from each of the remaining pairs during the formation for the gametes. This is known as the law of independent Assortment of characters. It is also referred to as Mendel’s third law of heredity
- 11. RAJENDR A CHAVHAN MGC ARMORI Mendel’s Law of INDEPENDENT ASSORTMENT
- 12. RAJENDR A CHAVHAN MGC ARMORI Explanation of the law of independent Assortment The principle of independent assortment was explained by Mendel with the help of a dihybrid cross involving characters of cotyledon colour (yellow/round) and seed shape (round/wrinkled). Mendel’s Law of INDEPENDENT ASSORTMENT Thus, the yellow round parent has the genotype (YYRR) and the green wrinked parent (yyrr). Since each parent is homozygous for both characters (colour and shape), each will produce only one type of gametes. The (YYRR) parent will produce all (YR) type gametes and the (yyrr) will produce all (yr) type gametes. Mendel crossed a true breeding variety of pea having yellow cotyledons (YY) and round seeds (RR) with another true breeding variety having green cotyledons (yy) and wrinked seeds (rr)
- 13. RAJENDR A CHAVHAN MGC ARMORI All F1 dihybrids resulting from the fusion of these gametes would be double heterozygous with (YyRr) genotype and appear yellow round. This indicated that in the dihybrid cross also in each pair, the alleles behaved exactly in the same way as in the monohybrid cross. Both the dominants (Y and R) expressed themselves in F1 while both the recessive alleles (y and r) remained hidden. Mendel’s Law of INDEPENDENT ASSORTMENT Types of Gametes formed by F 1 Dihybrid: According to Mendel, during gamete formation by the F 1 dihybrid, the alleles in both pairs Y-y and R-r first segregate from each other (Law of Segregation). Each pair segregates independently of the pair. Then the alleles enter the gametes. A gametes can receive only one allele from each pair, i.e. Y or y and R or r.
- 14. RAJENDR A CHAVHAN MGC ARMORI Similarly, a gamete that receives a factor (gene) for colour must also receive factor for shape (a factor for every character must be present in each gamete). Thus, a gamete that receives Y for colour may receive R or r for shape. This would result in (YR) and (Yr) types of gamets. Simolarly, a gamete that receives y for colour may receive R or r for shape. Mendel’s Law of INDEPENDENT ASSORTMENT This would give (yR) and (yr) types of gametes. In other words, the F 1 dihybrid would produce four types of gametes (YR), (Yr), (yR) and (yr) in equal proportions. This is the principle of independent assortment of characters. There will be four types of male gametes and four types of female gametes formed by the F 1 dihybrid.
- 15. RAJENDR A CHAVHAN MGC ARMORI 1)The inheritance of each trait is determined by “units AA” or “factors AA” (now called genes) that are passed on to descendants unchanged. 2) An individual inherits one such unit from each parent for each trait. MENDEL CAME TO FOUR IMPORTANT CONCLUSIONS FROM THESE EXPERIMENTAL RESULTS: 3) A trait may not show up in an individual but can still be passed on to the next generation. 4) The genes for each trait segregate themselves during gamete production.
- 16. RAJENDR A CHAVHAN MGC ARMORI Exception of law of dominance: 1. Incomplete dominance: - In incomplete dominance when a red snapdragon flowered plant is crossed with a white flowered plant an intermediate phenotype appears in the F1 hybrid instead of a parental phenotype. 2. Codominance: - Unlike dominance in codominance when a A (IAIAIAIA) blood group individual mates with B (IBIBIBIB) blood group individual the offsprings have blood group AB (IAIBIAIB) instead of A or B. EXCEPTION TO MENDEL’S LAW Exception of law of segregation: 1. Nondysjunction: - During meiosis homologous chromosomes/sister chromatids and hence genes may move to a common gamete violating law of segregation. Exception of law of independent assortment: 1. Linkage:- When genes are present on the same chromosome they tend to remain together and enter into the same gamete. This is the reason behind deviation of dihybrid test cross ratio from 1:1:1:1 and occurrence of parental combination in high frequencies.
- 18. RAJENDRA CHAVHAN MGC ARMORI THANK YOU VERY MUCH email: rajendrac99@gmail.com
- 19. THANK YOU SO MUCH email: rajendrac99@gmail.com