Genetics UG Unit 2 PDF

UNIT-2 : Extensions and Deviations from Mendel‟s law Mendelian deviation or exceptions or anomalies includes Incomplete dominance Codominance Lethal genes Epistasis 1. Incomplete dominance Mendel always observed complete dominance of one allele over the other for...

UNIT-2 : Extensions and Deviations from Mendel‟s law Mendelian deviation or exceptions or anomalies includes Incomplete dominance Codominance Lethal genes Epistasis 1. Incomplete dominance Mendel always observed complete dominance of one allele over the other for all the seven characters, which he studied, in garden pea. Later on cases of incomplete dominance were reported. EX: In four o clock plant (Mirabilis jalapa) there are two types of flower viz., red and white. A cross between red and white flowered plants produced plants with intermediate flower colour i.e. pink colour in F1 and a modified ratio of 1 red: 2 pink:1white in F2. Incomplete dominance…. In many cases, the intensity of phenotype produced by heterozygote is less than that produced by the homozygote for the concerned dominant allele. Therefore the phenotype of heterozygote falls between those of the homozygotes for the two concerned alleles. Such a situation is known as Incomplete or partial dominance and the dominant allele are called incompletely dominant or partially dominant. 2. Codominance In case of codominance both alleles express their phenotypes in heterozygote greater than an intermediate one. The example is AB blood group in human. The people who have blood type AB are heterozygous exhibiting phenotypes for both the IA and IB alleles. In other words, heterozygotes for codominant alleles are phenotypically similar to both parental types. The main difference between codominance and incomplete dominance lies in the way in which genes act. In case of codominance both alleles are active while in case of incomplete dominance both alleles blend to make an intermediate one. 3. Lethal genes Which causes the death of its carrier when in homozygous condition is called lethal gene. Mendel‟s findings were based on equal survival of all genotypes. Lethal genes have been reported in both animals as well as plants. Lethal genes can also be recessive, dominant, depending on the gene or genes involved.  TYPES OF GENE ACTION  Incomplete dominance and co-dominance and are called intra allele interaction. When there is a interaction occurs between different pairs of alleles influencing a character of an individual is said to be interallelic interaction or epistatic. Epistasis leads to modification of normal dihybrid or trihybrid segregation ratio in F2 generation. Epistasis: The interaction of genes at different loci that affect the same character is called epistasis. The term epistasis was first used by Bateson in 1909 to describe two different genes which control the same character, out of which one masks/suppresses the expression of another gene. Gene that masks the action of another gene is called epistatic gene, while the gene whose expression is being masked is called hypostatic gene.  EPISTATIC GENE INTERACATION Dominance: When a pair of allelomorphs / alleles are present in a heterozygous condition, one of them expresses itself completely over the other. This phenomenon is known as dominance. Epistasis: The interaction of genes at different loci that affect the same character is called epistasis. Epistatic gene: Gene that masks the action of another gene is called epistatic gene. Hypostatic gene: Gene whose expression is being masked is called hypostatic gene.  Differences between Dominance and Epistasis Dominance Epistasis It refers to the interaction of two It refers to the interaction of two or alleles of the same gene. Hence, it more gene pairs. Hence, it involves involves a single locus two or ore loci. It involves a single gene pair that It involves two gene pairs that control a single character. control a single character It is an intra-allelic interaction It is an inter-allelic interaction Allele which is expressed in the Allele which masks another heterozygous state is known as allele is known as epistatic allele dominant allele Dominant allele suppresses the Recessive allele can also mask the expression of the recessive allele expression of dominant allele Types of epistatic gene interaction Dominant epistasis (12:3:1) Recessive epitasis (9:3:4) Duplicate recessive epistasis (9:7) Duplicate dominant epistasis (15:1) Dominant and recessive inhibitory epistasis (13:3)  EPISTATIC GENE INTERACATION  Epistatic factors or dominant epitasis (12 : 3 : 1): It is an interallelic interaction in which a dominant allele at one locus can mask the expression of both alleles (dominant and recessive) at another locus. In other words, the expression of one dominant or recessive allele is masked by another dominant gene.  In this interaction, the expression of one gene is so intense or strong that the expression of the other gene cannot be observed. Hence, such a gene interaction is also known as masking gene action. Example: Grain colour and its appearance in Sorghum bicolor  In case of Sorghum bicolor, the grain colour may be red or white with red being dominant over white.  With regard to appearance of grain, it may be pearly or chalky with pearly being dominant over chalky.  Most of the cultivated varieties have pearly grains which are shiny in appearance, while the chalky grain varieties are dull in appearance. When red grain type (WWzz) is crossed with white grain type having pearly appearance (wwZZ), the F1 (WwZz) progeny has red grain.  Whether it is pearly or chalky in appearance cannot be determined due to the predominance of red colour in seed coat. As a matter of fact, the character red grain colour masks or suppresses the other character i.e. appearance of the grain. Hence the grain colour is said to be epistatic over appearance of grain, while appearance of grain is hypostatic to grain colour. When F1 plants are selfed, the progeny segregates in the ratio of 12 red : 3 white pearly : 1 white chalky. Phenotypic ratio – 12:3:1 Generation : F1 Parents : Female x Male Phenotype : Red x White pearly Genotype : WWzz x wwZZ Gametes : Wz wZ F1 WwZz (Red )  F2 Generation Male WZ Wz wZ wz Female WZ WWZZ WWZz WwZZ WwZz (Red) (Red) (Red) (Red) Wz WWZz WWzz WwZz Wwzz (Red) (Red) (Red) (Red) wZ WwZZ WwZz wwZZ wwZz (Red) (Red) White White pearly pearly wz WwZz Wwzz wwZz wwzz (Red) (Red) White White pearly chalky  Recessive epistasis /Supplementary factors (9 : 3 : 4)  It is an interallelic interaction in which a recessive allele at one locus can mask the expression of both (dominant and recessive) alleles at another locus.  In this gene interaction, the dominant allele of one of the two genes governing a character produces a phenotypic effect. However, the dominant allele of the other gene does not produce a phenotypic effect of its own, but when it is present with the dominant allele of the first gene it modifies the phenotypic effect produced by that gene.  Thus in this gene action, the dominant allele of one gene is necessary for the development of concerned phenotype, while the other gene only modifies the expression of the first gene.  Example: Glume colour in Sorghum bicolor  In Sorghum bicolor blackish purple (P) glume colour is monogenic dominant over brown (p). Another dominant gene (Q) which is not having independent expression, modifies the expression of dominant gene (P) in its presence and gives rise to a new phenotype i.e. reddish purple (P_Q_). Homozygous recessive (ppqq) remain brown. In the absence of the dominant allele (P), the dominant factor (Q) has no effect on glume colour in the plants with brown glume colour (ppQ_).  A cross between blackish purple (PPqq) and brown (ppQQ) glume colour strains of sorghum produced plants with reddish purple glume (PpQq) in F1.Selfing of F1 plants produced progeny with reddish purple, blackish purple and brown glume colours in the ratio of 9 : 3 : 4 in F2 Phenotypic ratio – 9:3:4 Genotypic ratio – 1:2:1:2:4:2:1:2:1 Generation : F1 Parents : Female x Male Phenotype : Blackish purple x Brown Genotype : PPqq x ppQQ Gametes : Pq pQ F1 PpQq (Reddish purple)  F2 Generation Male PQ Pq pQ pq Female PQ PPQQ PPQq PpQQ PpQq ( R.P ) (R.P ) (R.P) (R.P ) Pq PPQq PPqq PpQq Ppqq (R.P ) ( B.P ) (R.P ) ( B.P ) pQ PpQQ PpQq ppQQ ppQq (R.P ) (R.P ) (Brown) (Brown) pq PpQq Ppqq ppQq ppqq ( R.P ) (B.P ) (Brown) (Brown)  Duplicate dominant genes (15 : 1):  It is an interallelic interaction in which a dominant allele at either of two loci can mask the expression of recessive alleles at two loci.  Characters showing duplicate gene action are determined by two completely dominant genes. These dominant genes produce the same phenotype whether they are alone (i.e. with the recessive allele of the other gene) or together; the contrasting phenotype is produced only when both the genes are in homozygous recessive state  Example: Awned character in rice  The awned character in rice is controlled by two dominant genes, A1 and A2. Genes A1 and A2 alone (A1_a2a2 and a1a1A2_) as well as together (A1_A2_) produce the same phenotype, awned character. The awnless phenotype is obtained only when both these genes are in the recessive state (a1a1 a2a2).  When a awned rice strain with genotype A1A1A2A2 is crossed with awnless rice strain (a1a1 a2a2), the F1 (A1a1A2a2) is awned and when selfed the progeny segregates in the ratio of 15 awned : 1 awnless in F2 generation. This shows that the dominant alleles of both genes have similar affect either independently or in combination. Phenotypic ratio – 15:1 Genotypic ratio – 1:2:1:2:4:2:1:2:1 Generation : F1 Parents : Female x Male Phenotype : Awned x Awnless Genotype : A1A1A2A2 x a1a1a2a2 Gametes : A1A2 a1a2 F1 A1a1A2a2 (Awned)  F2 Generation Male A1A2 A1a2 a1A2 a1a2 Female A1A2 A1A1A2A2 A1A1A2a2 A1a1A2A2 A1a1A2a2 (Awned) (Awned) (Awned) (Awned) A1a2 A1A1A2a2 A1A1a2a2 A1a1A2a2 A1a1a2a2 (Awned) (Awned) (Awned) (Awned) a1A2 A1a1A2A2 A1a1A2a2 a1a1A2A2 a1a1A2a2 (Awned) (Awned) (Awned) (Awned) a1a2 A1a1A2a2 A1a1a2a2 a1a1A2a2 a1a1a2a2 (Awned) (Awned) (Awned) (Awnless) duplicate recessive genes (9 : 7)  It is an interallelic interaction in which recessive alleles at either of the two loci can mask the expression of dominant alleles at the two loci.  In this type of gene interaction, the production of one of the two phenotypes of a trait requires the presence of dominant alleles of both the genes controlling the concerned trait. When any one of the two or both the genes are present in the homozygous recessive state, the contrasting phenotype is produced.  Example: Corolla colour in sweet pea  In sweetpea, the development of purple coloured flowers requires the presence of two dominant genes, C1 and C2 (C1_C2_ ). When either C1 (c1c1C2_) or C2 (C1_c2c2) or both the genes (c1c1c2c2) are present in homozygous recessive condition, purple flower colour cannot be produced as a result of which white flowers are obtained.  Bateson and Punnet came across with such an example in sweet pea. When two varieties with white flowers were crossed, the resultant F1 plants were with purple coloured flowers. On selfing, in the subsequent F2 generation, 9 purple and 7 white flower types appeared Phenotypic ratio – 9:7 Genotypic ratio – 1:2:1:2:4:2:1:2:1 Generation : F1 Parents : Female x Male Phenotype : White x White Genotype : C1C1c2c2 x c1c1C2C2 Gametes : c1C2 C1c2 F1 C1c1C2c2 (Purple)  F2 Generation Male C1C2 C1c2 c1C2 c1c2 Female C1C2 C1C1C2C2 C1C1C2c2 C1c1C2C2 C1c1C2c2 Purple Purple Purple Purple C1c2 C1C1C2c2 C1C1c2c2 C1c1C2c2 C1c1c2c2 Purple White Purple White c1C2 C1c1C2C2 C1c1C2c2 c1c1C2C2 c1c1C2c2 Purple Purple White White c1c2 C1c1C2c2 C1c1c2c2 c1c1C2c2 c1c1c2c2 Purple White White White  Inhibitory gene action (13 : 3)  It is an interallelic interaction in which a dominant allele at one locus can inhibit the expression of both (dominant and recessive) alleles at the second locus.  In inhibitory gene action, one of the two completely dominant genes produces the concerned phenotype, while its recessive allele in homozygous state produces the contrasting phenotype.  The second dominant gene has no affect of its own on the concerned character. However, it has the ability to stop the expression of the dominant allele of the first gene.  As a result, when the two dominant genes are present together, they produce the same phenotype as that produced by the recessive homozygote of the first gene. The recessive allele of the second gene does not affect the development of the character in any way. Thus in inhibitory gene action, one dominant gene is capable of producing a character only when its expression is not prevented by another dominant gene known as inhibitory gene, which is denoted by “I”.  Example: Anthocyanin pigmentation in rice  A dominant gene ‘P’ produces, purple colour, while its recessive allele ‘p’ produces green colour. Another dominant gene ‘I’ does not produce any colour by itself, it only prevents the purple colour production by ‘P’, when both ‘I’ and ‘P’ are present together. The recessive allele ‘I’ does not affect in any way the anthocyanin pigmentation in rice. As a result, purple colour is present only when ‘P’ is present with the homozygous recessive state of the inhibitory locus (iiP_).  When a rice strain with green colour (IIpp) is crossed with purple (iiPP) rice strain, the F1 (IiPp) is green in colour as the gene ‘I’ stops the purple colour development by ‘P’. In F2, 13 green and 3 purple are obtained. Phenotypic ratio – 13:3 Genotypic ratio – 1:2:1:2:4:2:1:2:1 Generation : F1 Parents : Female x Male Phenotype : Green x Purple Genotype : IIpp x iiPP Gametes : Ip iP F1 IiPp (Green)  F2 Generation Male IP Ip iP ip Female IP IIPP IIPp IiPP IiPp (Green) (Green) (Green) (Green) Ip IIPp IIpp IiPp Iipp (Green) (Green) (Green) (Green) iP IiPP IiPp iiPP iiPp (Green) (Green) (Purple) (Purple) ip IiPp Iipp iiPp iipp (Green) (Green) (Purple) (Green)  Pleiotropism The term pleiotropie was coined in a 1910 by Festschrift. Greek word Pleion=more and tropos= character Pleiotropy refers to the expression of multiple traits by a single gene. Pleitropy was first noticed by geneticist Gregor Mendel, who is known for his famous studies with pea plants. The study of pleitropic genes is important to genetics as it helps us to understand how certain traits are linked in genetic diseases.  In general one gene affects a single character. But many genes are known to affect more than one character such genes are known as pleiotropic genes and the condition is termed as pleiotrophy. Pleiotropism In cotton the gene Lic produces seeds without lint. This gene also causes incomplete lancinations of the leaf, reduction in boll size and fertility. An example of a pleiotropic gene in human beings is the recessive gene s which produces sickle cell anemia. These gene causes changes in two or more parts of characters, which are not related, then the gene is said to be pleiotropic gene.  Pleiotropy Examples  Sickle cell disease Sickle cell disorder results from the development of abnormally shaped red blood cells. Normal red blood cells have a biconcave, disc-like shape and contain enormous amounts of a protein called hemoglobin. Sickle cell is a result of a mutation in the beta-globin gene. This mutation results in red blood cells that are sickle- shaped, which causes them to clump together and become stuck in blood vessels, blocking normal blood flow. The single mutation of the beta-globin gene results in various health complications and causes damage to multiple organs including the heart, brain, and lungs.  Multiple alleles The word allele is a general term to denote the alternative forms of a gene or contrasting gene pair that denote the alternative form of a gene is called allele. Gene mutations may produce many different alleles of a gene. All such altered alternative forms of a gene, whose number is more than two, are called multiple alleles. Multiple alleles always occupy the same locus on the chromosome. Multiple alleles always influence the same character. Multiple Alleles; Many genes have two alternate forms but several other have more than two alternate forms. More than two alleles at the same locus give rise to a multiple allelic series. Multiple alleles can be defined as a series of forms of a gene situated at the same locus of homologous chromosomes. Important Examples of Multiple Alleles:  Wings of Drosophila  Coat Colour in Rabbit  Self-Incompetebility in Plants  Blood Groups in Man Four different alleles exist for the rabbit coat color (C) gene.  ABO blood-type system in humans The antigen-antibody relationship is one of the great specificity like that between lock and key. Each antigen and its associated antibody has a peculiar chemical configuration. Landsteiner discovered in 1900 that when the red cells of one person are placed in the blood serum of another person, the cells become clumped or agglutinated. Multiple Alleles; ABO Blood group in human Blood types vary and your immune system recognizes your own blood type as being self. Other blood types are recognized as non-self. Antigens are the markers on the cell that the immune system “looks” for to identify the cell. Antibody and Antigens Antibodies are chemicals that are a part of the immune system that attaches to the antigens. The antigen-antibody complex sends a signal to the immune system that something does not belong. The immune system tries to kill and remove the item.  Bloodtype Alleles Allele IA produces antigen A Genotypes Phenotypes (blood types) Allele IB produces antigen B IAIA A Allele i produces no antigen. IAi A IBIB B IBi B IAIB AB ii O Bloodtypes Antigens & Antibodies Bloodtype Antigen Antibody A A B B B A AB A and B none O none A and B Blood typing results A blood typing test card contains antibodies for Antigen A, Antigen B, Rhesus factor Antigen D, and a control field with no antibodies. If the tested blood contains the corresponding antigen to the specific antibody in the field, blood clots will be formed. All the possible results on a blood typing test card are shown in the image below. Extra chromosomal inheritance ⚫Extra chromosomal inheritance defined as non- mendelian inheritance, usually involving DNA in replicating mitochondria and some other organelles of cell. ⚫Commonly defined as transmission through cytoplasm rather than nucleus. ⚫Inheritance due to genes located in cytoplasm (plasma genes) ⚫Plasma genes are located in DNA present in mitochondria and in chloroplast.  History: 1908 –evidence for cytoplasmic inheritance was first presented by Correns in Mirabilis jalapa and Baur in Pelargonium zonale Inheritance due to genes located in cytoplasm (plasmagenes) is called cytoplasmic inheritance. In case of cytoplasmic inheritance generally the character of only one of the two parents (usually the female parent) is transmitted to the progeny. Such inheritance is also referred as extra nuclear inheritance, extrachromosomal inheritance and maternal inheritance. Inheritance of character which governed by cytoplasmic genes or plasma genes by chloroplast or mitochondrial DNA Such inheritance is also referred as cytoplasmic inheritance/ extra nuclear inheritance/extrachromosomal inheritance /maternal inheritance. Maternal effects- Inheritance of Mitochondrial DNA, Chloroplast DNA When the expression of a character is influenced by the genotype of female parent, it is referred to as maternal effect. Such characters exhibit clear-cut difference in F1 for reciprocal crosses. Maternal effects are known both in plants and animals. Ex: Coiling pattern of shell in snails and pigmentation in flour moth CHLOROPLAST INHERITANCE Specialized organelle found in higher plants. Two membranes: outer and inner membrane. They form stalk of disc at some places known as grana. Chloroplast has three parts: ⚫Envelope ⚫Matrix ⚫thyllakoids  Photosynthesis Oxygen supply Starch storage Utilize carbondioxide Synthesis of organic acid Food supply Examples of cpDNA IN M i r a b i l i s j a l a p a (a) female branch × male branch (green) (variegated) Green plants only (b) female branch × male branch (variegated) (green) Variegated plants only STRUCTURE OF MITOCHONDRIA ⚫Powerhouse of the cell. ⚫Position depends upon the requirement of energy and amino acid. ⚫ it consist of three parts: o outer and inner membrane o Cristae o matrix FUNCTIONS OF MITOCHONDRIA  Store and release calcium.  Main seat of cell respiration.  Synthesis of amino acid(glutamic and aspartic acid).  Take part in maternal inheritance.  Synthesis of several biochemicals like chlorophyll, cytochrome, alkaloid. MATERNAL EFFECT Besides chromosomes, various organelles of cytoplasm also contain DNA. The mitochondria and plastids have their own DNA and carry their genetic characters themselves. The mechanism in which cytoplasmic inclusions (e.g., alpha, beta, sigma and kappa particles) and organelles (plastids, mitochondria, centriole, etc) take part in transmission of characters from generation to generation is called cytoplasmic inheritance. Since cytoplasmic inheritance is based on cytoplasmic DNA molecules, it is also called extra chromosomal inheritance. INHERITANCE INVOLVING INFECTIOUS PARTICLE ⚫ Non mendelian inheritance is associated with infective particles like parasite, viruses. ⚫EXAMPLE: kappa particles in paramecium. ⚫T.M. sonneborn described the inheritance in paramecium aurelia. ⚫There are two strains of paramecium : killer and sensitive. ⚫Killer strain produce a toxic substance called paramecin that is lethal to other individual called sensitive. Inheritance of Kappa particles in Paramecium In Paramecium aurelia, two strains of individuals have been reported. One is called as Killer which secretes a toxic substance paramecin and the other strain in known as sensitive and is killed if comes in contact with the paramecin. In the cytoplasm of the killer strain the kappa particles (cytoplasmic – DNA) are present kappa particles are absent in sensitive strains. During this cytoplasmic exchange, the kappa particles present in the cytoplasm of the killer type enter the non-killer type and convert it into a killer type. So all the offspring produced by the exconjugants are killer type. Duration of conjugation Different results are observed when Killer strain is allowed to conjugate with sensitive strain depending on the duration of conjugation.  In short conjugation, where only genetic material is exchanged without the cytoplasmic exchange between conjugates, both sensitive and killer strains are produced In prolonged conjugation, it involves the exchange of both genetic and cytoplasmic content between conjugates. when conjugation period increases, the kappa particles move from killer strain to sensitive strain through conjugation tube converting sensitive strain to killer strain.  The Kappa particles present in cytoplasm of killer strains enter the non-killer strains and convert it into killer type ,so all the progeny produced by exconjugates will have killer strains. Paramecium become killer strains when it receive Kappa particles and it become sensitive strains when it does not receive Kappa particles. Sigma factor in Drosophila The Sigma factor in Drosophila is a classic example of non-Mendelian inheritance, which means its inheritance pattern doesn't follow the traditional Mendelian laws of inheritance. In Mendelian inheritance, traits are determined by nuclear genes that are inherited in a predictable manner, with each parent contributing one allele (copy) of the gene. However, the Sigma factor is a cytoplasmic element that's inherited solely through the maternal lineage, as it's present in the egg cytoplasm. Here's why the Sigma factor exhibits non- Mendelian inheritance: 1.Maternal inheritance: The Sigma factor is passed down from mother to offspring, but not from father to offspring. This violates the Mendelian principle of equal contribution from both parents. 2.Cytoplasmic inheritance: The Sigma factor is located in the cytoplasm, outside the nucleus, and is not encoded by a specific gene. This means it's not subject to the same rules of inheritance as nuclear genes. 3.Extra-chromosomal: The Sigma factor exists outside the chromosomal genome, so its inheritance isn't influenced by chromosomal recombination or segregation. The sigma factor influences various aspects of Drosophila biology, including:- Reproduction: Causes cytoplasmic incompatibility, leading to variation in sex ratio. Development: Affects embryonic development, potentially leading to embryonic lethality or sterility. Evolution: Contributes to the evolution of Drosophila populations by driving genetic changes and influencing the spread of specific traits. The sigma factor is an fascinating example of how non- chromosomal, maternally inherited elements can shape the biology and evolution of organisms. Shell Coiling in Limnaea In 1920, Arthur Boycott was the first to study the maternal effect of shell coiling in the snail. Arthur Boycott studied the maternal effect in Limnaea peregra. There are two types of coiling found in the Limnaea peregra: Dextral coiling: This type of coiling is towards right- handed. Sinistral coiling: This type of coiling is towards left- handed.  For the dextral coiling, a dominant gene „D‟ is responsible. If there is either “DD” or “Dd” is present in a snail, there will be dextral coiling. For the sinistral coiling, a recessive gene „d‟ is responsible. If there is “dd” gene present in a snail, there will be sinistral coiling. Ex: An extreme example of material effect is known in the snail Limnaea in this snail, the direction of coiling of its shell is controlled by a single nuclear gene D/d. The dominant allele D produces right handed coiling while its recessive allele d produces left handed coiling. The direction of shell coiling in an individual in governed by the genotype of its female parent and not by its own genotype. The shell coiling in snail is only decided by the genotype of the mother. Crossing over between DD female and dd male By this crossing over pure dextral female and a sinistral male, a zygote (Dd) will produce dextral coiling in the F1 generation. Then, the intercrossing of Dd gene will result in the formation of DD, Dd, Dd and dd genes in the F2 generation where each has dextral coiling. According to the study by Alfred Sturtevant, the dd offspring will have dextral coiling due to maternal effect or the dominant ‘D’ gene. Advantages: Extra chromosomal inheritance has some practical advantages in agriculture and can also be used in the prediction of diseases in humans for genetic reason In plants, cytoplasmic male sterility trait can be used for plant breeding. A recent clinical study involved congenital heart disease parents showed that the risk of getting heart disease in the progeny was higher if the mother is affected rather than the father.