Gregor Mendel The Father of Genetics Mendel Modern genetics had its beginnings in an abbey garden, where a monk named Gregor Mendel documented a particular mechanism of inheritance. He discovered the basic principles of heredity by breeding garden peas in carefully planned experiments. Why peas? Peas are normally self-pollinating. Peas are easily grown, Peas mature quickly Peas produce many seeds Peas show several pairs of obvious contrasting traits The use of plants also allowed strict control over the mating. Pea’s contrasting traits Tall plant vs. dwarf plant Yellow seeds vs. green seeds Smooth seeds vs. wrinkled seeds Inflated pod vs. constricted pod Purple flower vs. white flower Green pod vs. yellow pod Axial flower vs. terminal flower Mendel experiments Mendel spent several years self-pollinating the pea plants in order to establish purebred plants. Mendel’s experiments These pure-breeding plants were called the P generation. Mendel had strains of peas that were pure breeding for height: the plants were either tall or dwarf He had strains that had either yellow pods or green pods, inflated pods or constricted pods. Mendel’s experiments When a pure breeding tall plant is crossed with a pure breeding dwarf plant, all the resulting plants are called hybrids. All the hybrids are called the F1 generation ( first filial generation) All of the F1 generation were tall Mendel’s experiment The same pattern was observed in each of the 7 contrasting characteristics. In the F1 generation, all of the plants showed one characteristic. Mendel called this characteristic the dominant trait The trait that was not expressed in F1 was called the recessive trait Mendel’s experiment Next, Mendel crossed the F1 generation plants. He wanted to know if the F1 plants were identical to the P generation So he crossed two of the F1 plants Mendel’s experiment The resulting plants (the F2 generation) yielded 3 tall plants and 1 dwarf plant. A ratio of 3:1 Mendel’s experiment This meant that the tall F1 plants ( the hybrids) were different than the tall P generation plants (the purebred plants) even though they were all tall. Mendel’s experiment Mendel repeated this experiment with all of the traits and the results were similar. The F2 generation displayed a phenotypic ratio of 3:1 Mendel’s explanation Units of inheritance, called factors (we now call them genes) were involved For any given characteristic there were several different forms (now called alleles) Each plants phenotype (what they look like) was determined by a pair of alleles that can be identical or different Mendel’s explanation In a hybrid plant one allele of a pair has the ability to express itself while the other one is not expressed. Recessive characteristics are expressed only when there is no dominant allele present Mendel’s explanation When gametes are produced, the members of each pair of alleles are separated into different reproductive cells. A gamete can contain only one allele of a particular characteristic. When fertilization occurs these alleles unite to give the zygote the necessary pair of alleles. Mendel’s first law The Law of Segregation Members of a pair of alleles for a given trait are segregated when gametes are formed Genotype The genotype represents the genes that exist in the organism Mendel used letters of the alphabet to represent genes. Capital letters represent the dominant allele Lower case letters represent the recessive allele Genotype A purebred tall plant would have a genotype of TT A purebred dwarf plant would have a genotype of tt Genotypes with identical alleles are called homozygous Genotypes with different alleles are called heterozygous The sex cells of a TT plant would contain only the dominant allele T The sex cells of a tt plant would contain only the recessive allele t The F1 generation would all have the genotype Tt The phenotype of all the F1 generation would be tall Mendel’s impact Mendel’s theories of inheritance, first discovered in garden peas, are equally valid for figs, flies, fish, birds and human beings. Mendel’s impact endures, not only on genetics, but on all of science, as a case study of the power of hypothesis/deductive thinking. A Punnett Square Punnett squares illustrate the possible outcomes of a particular cross (the genotype of the offspring) Mendel’s first experiment could be illustrated with the following Punnett square: TT X tt tt gametes: t TT gametes: T Tt A 2 x 2 Punnett Square The F1 generation cross is a monohybrid cross: Tt x Tt Tt gametes: T Tt gametes: T t TT t Tt tall tall Tt tt tall dwarf Phenotypic ratio = 3:1 Single Trait Analysis In humans the ability to taste PTC, T is dominant to non-tasting t. Determine the expected genotypic and phenotypic ratios resulting from a cross between a heterozygous taster and a non-taster. Tt (heterozygous taster) gametes: T t tt (non-taster) gametes: t Tt tt taster non-taster Genotype ratio: 1:1 (Tt:tt) Phenotype: 1:1 Practice Problem In humans, the allele A, for pigment formation is dominant to the allele for a, the inability to form pigment. aa individuals are albino Determine the expected genotypic and phenotypic ratios expected from a cross between two individuals heterozygous for this trait. Solving Punnett Square Problems GRASP method Given: – pigment formation (A) is dominant to the inability to form pigment (a) – aa individuals are albino – The cross is Aa x Aa Required: – The expected genotypic and phenotypic ratio of the cross Analysis: Solution: Paraphrase: Solving Punnett Square Problems GRASP method Analysis: Solution: A a A AA Aa Aa aa a Paraphrase: – The phenotypic ratio is 3:1 normal : albino – The genotypic ratio is 1 : 2: 1 AA:Aa:aa Recognizing Hybrids A geneticist crosses two parent plants that have the dominant trait of purple flowers. When the resulting seeds are planted the geneticist observes that 145 of the F1 plants have the recessive trait of yellow flowers and 430 of the F1 plants have purple flowers. How can you explain these results? What are the genotypes of the parent plants and the F1 plants? Given: F (purple flowers) is dominant to f (yellow flowers). Both of the P1 plants possess at least one F gene: F__ × F__ Required: The genotypes of the parents and F1 plants Analysis: The key to this question is the appearance of the ff (yellow plants) in the F1. We know that yellow flowers can only appear if the plants are homozygous recessive for yellow flowers (ff) Produce a Punnett square of the offspring. Let F be the dominant allele for flower colour: purple and f be the recessive allele for colour: yellow Phenotypic ratio is 430:145 gametes: gametes: Solution: The appearance of the recessive trait in the phenotype of the F1 plants can only occur if they are homozygous recessive (ff). This can only happen if both of the purple parent plants are heterozygous and each parent contributed the recessive allele to these yellow plants. In addition, recognize that the ratio of purple plants to yellow plants is approximately 3:1. This ratio indicates a monohybrid cross. Dihybrid crosses Dihybrid crosses show the genotypes of offspring using 2 different genes on two different chromosomes TtRr x TtRr hybrid tall plants with round seeds Gametes produced by independent assortment: TR, tr, Tr, tR These gametes would be present in equal numbers (eg. ¼ of the total number) Dihybrid cross TtRr x TtRr TR TR Tr tR tr Tr tR tr Dihybrid cross TtRr x TtRr TR Tr tR tr TR TTRR TTRr TtRR TtRr Tr TTRr TTrr TtRr Ttrr tR TtRR TtRr ttRR ttRr tr TtRr Ttrr ttRr ttrr The expected phenotypic ratio: 9/16 tall round plants, 3/16 tall wrinkled plants 3/16 dwarf round plants, 1/16 dwarf wrinkled plants Test cross A test cross is a cross with an individual whose genotype is being tested to a recessive individual We know that the recessive individual must be homozygous so it can only produce one type of gamete This way we can determine the genotype of the test individual.