Basics of Genetics

GENETICSMENDEL’S LAWS OF INHERITANCEGregor Mendel , conducted hybridisation experiments on garden peas for seven years (1856-1863) and proposed the laws of inheritance in living organisms.It was for the first time that statistical analysis and mathematical logic were applied to solve the problems in biology.Pea plants had the following chacracteristics:Produces many offsprings in one cross.Self pollination and self fertilization is possible.The life cycle was completed in one season.Hybrids were perfectly fertile.Many varieties with well-defined characters were available i.e. presence of seven pairs of contrasting characteristics.IDEAS USEDHis experiments had a large sampling size, which gave greater credibility to the data that he collected.Mendel investigated characters in the garden pea plant that were manifested as two opposing traits, e.g., tall or dwarf plants, yellow or green seeds. This allowed him to set up a basic framework of rules governing inheritance, which was expanded on by later scientists.EXPERIMENTMendel conducted artificial pollination/cross pollination experiments using several true-breeding pea lines.A truebreeding line is one that, having undergone continuous self-pollination, shows the stable trait inheritance and expression for several generations.Mendel selected 14 true-breeding pea plant varieties, as pairs which were similar except for one character with contrasting traits.Some of the contrasting traits selected were smooth or wrinkled seeds, yellow or green seeds, smooth or inflated pods, green or yellow pods and tall or dwarf plants.INHERITENCE OF ONE GENEThe cross between pea plants differing in single pair of contrasting characters is called Monohybrid Cross.MONOHYBRID CROSSPARENTAL TALL X DWARF F1 GENERATION TALL SELFING TALL X TALL F2 GENERATION TALLTALL TALL DWARFMendel carried out an experiment where he crossed tall and dwarf pea plants to study the inheritance of one gene.He collected the seeds produced as a result of this cross and grew them to generate plants of the first hybrid generation.This generation is also called the Filial 1 progeny or the F1.He observed that all the F1 progeny plants were tall, like one of its parents; none were dwarf.He made similar observations for the other pairs of traits – he found that the F1 always resembled either one of the parents, and that the trait of the other parent was not seen in them.He then self-pollinated the tall F1 plants and found that in the Filial 2 generation some of the offspring were ‘dwarf ’; the character that was not seen in the F1 generation was now expressed.The proportion of plants that were dwarf were 1/4th of the F2 plants while 3/4th of the F2 plants were tall.The tall and dwarf traits were identical to their parental type and did not show any blending, that is all the offspring were either tall or dwarf, none were of in between height. Only one of the parental traits was expressed in the F1 generation while at the F2 stage both the traits were expressed in the proportion 3:1.The character which expresses itself (i.e., tall) in F1 generation is called by Mendel as Dominant character.The characters which remained unexpressed or latent had been called Recessive character.The contrasting traits did not show any blending at either F1 or F2 stage.Mendel proposed that something was being stably passed down, unchanged, from parent to offspring through the gametes,over successive generations. He called these things as ‘factors’.These were later called as GENES.Genes, are the units of inheritance. They contain the information that is required to express a particular trait, in an organism.Genes which code for a pair of contrasting traits are known as alleles, i.e., they are slightly different forms of the same gene.The capital letter is used for the trait expressed at the F1 stage and the small alphabet for the other trait.Mendel also proposed that in a true breeding, tall or dwarf pea variety the allelic pair of genes for height are identical or homozygous, TT and tt, respectively.TT and tt are called the genotype of the plant while the descriptive terms tall and dwarf are the phenotype.He proposed that in a pair of dissimilar factors, one dominates the other (as in the F1 ) and hence is called the dominant factor while the other factor is recessive.In case of Tt, T (for tallness) is dominant over t (for dwarfness), that is recessive.TT and tt are homozygotes and cross between TT and tt is a monohybrid cross.PUNNET SQUARE :It was developed by a British geneticist, Reginald C. Punnett. It is a graphical representation to calculate the probability of all possible genotypes of offspring in a genetic cross. The possible gametes are written on two sides, usually the top row and left columns. All possible combinations are represented in boxes below in the squares, which generates a square output form.A Punnett square is used to understand a typical monohybrid Cross conducted by Mendel between true-breeding tall plants and true-breeding dwarf plants.TEST CROSSMendel crossed the tall plant from F2 with a dwarf plant. This he called a test cross.F2Tt Tt Tt tt Tall Tall Tall DwarfTest cross Tt Xtt F2 progeny dwarf TtTtttTttt t tIn a typical test cross an organism (pea plants here) showing a dominant phenotype (and whose genotype is to be determined) is crossed with the recessive parent instead of self-crossing.Test cross is used to determine if an individual exhibiting a dominant trait is homozygous or heterozygous for that trait.Test crosses determine the genotype of an individual with a dominant phenotype.LAWS OF INHERITANCE :Based on his observations on monohybrid crosses Mendel proposed two general rules to consolidate his understanding of inheritance in monohybrid crosses.These rules are called the Principles or Laws of Inheritance.LAW OF DOMINANCELAW OF SEGREGATIONLAW OF DOMINANCECharacters are controlled by discrete units called factors.Factors occur in pairs.In a dissimilar pair of factors one member of the pair dominates (dominant) the other (recessive).used to explain the expression of only one of the parental characters in a monohybrid cross in the F1 and the expression of both in the F2.It also explains the proportion of 3:1 obtained at the F2.LAW OF SEGREGATIONThis law is based on the fact that the alleles do not show any blending and that both the characters are recovered as such in the F2 generation though one of these is not seen at the F1 stage. Though the parents contain two alleles during gamete formation, the factors or alleles of a pair segregate from each other such that a gamete receives only one of the two factors.INCOMPLETE DOMINANCE When experiments on peas were repeated using other traits in other plants, it was found that sometimes the F1 had a phenotype that did not resemble either of the two parents and was in between the two. The inheritance of flower colour in the dog flower(snapdragon or Antirrhinum sp.) is a good example to understand incomplete dominance.In a cross between true-breeding red-flowered (RR) and truebreeding white-flowered plants (rr), the F1(Rr) was pink. RR xrr Red whiteGametes RrF1 generetaion Rr PinkSelf cross Rr XRr Rr RRRRr rRrrr RR : Rr : rr 1 : 2 : 1When the F1 was self-pollinated the F2 resulted in the following ratio 1 (RR) Red : 2 (Rr) Pink : 1 (rr) White.the genotype ratios were exactly as we would expect in any mendelianmonohybrid cross, but the phenotype ratios had changed from the 3:1.R was not completely dominant over r and this made it possible to distinguish Rr as pink from RR (red) and rr (white).CO- DOMINANCEF1 resembled either of the two parents (dominance) or was in-between (incomplete dominance). But,in the case of co-dominance the F1 generation resembles both parents.different types of red blood cells that determine ABO blood grouping in human beings is a good example of co-dominance.ABO blood groups are controlled by the gene I. The plasma membrane of the red blood cells has sugar polymers that protrude from its surface and the kind of sugar is controlled by the gene. The gene (I) has three alleles IA, IB and i. The alleles IA and IB produce a slightly different form of the sugar while allele i doesn’t produce any sugar. Because humans are diploid organisms, each person possesses any two of the three I gene alleles. IA and IB are completely dominant over i, in other words when IA and i are present only IA expresses (because i does not produce any sugar), and when IB and i are present IB expresses.But when IA and IB are present together they both express their own types of sugars: this is because of co-dominance. BASIC DEFINITIONSAllele - one alternative form of a given allelic pair; tall and dwarf are the alleles for the height of a pea plant; more than two alleles can exist for any specific gene, but only two of them will be found within any individual.Homozygote - an individual which contains only one allele at the allelic pair; for example TT is homozygous dominant and tt is homozygous recessive.Heterozygote - an individual which contains one of each member of the gene pair; for example is the Tt heterozygote.Genotype - the specific allelic combination for a certain gene or set of genes.Backcross - the cross of an F1 hybrid to one of the homozygous parents. Tt x tt or Tt x TT. backcross is a cross to a fully recessive parent.Testcross - the cross of any individual to a homozygous recessive parent; used to determine if the individual is homozygous dominant or heterozygous.Monohybrid cross - a cross between parents that differ at a single gene pair.