3MendelModern 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.
4Why peas? Peas are normally self-pollinating. Peas are easily grown, Peas mature quicklyPeas produce many seedsPeas show several pairs of obvious contrasting traitsThe use of plants also allowed strict control over the mating.
5Pea’s contrasting traits Tall plant vs. dwarf plantYellow seeds vs. green seedsSmooth seeds vs. wrinkled seedsInflated pod vs. constricted podPurple flower vs. white flowerGreen pod vs. yellow podAxial flower vs. terminal flower
7Mendel experiments Mendel spent several years self-pollinating the pea plants in orderto establish purebredplants.
8Mendel’s experimentsThese 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 dwarfHe had strains that had either yellow pods or green pods, inflated pods or constricted pods.
9Mendel’s experimentsWhen 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
11Mendel’s experimentThe 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 traitThe trait that was not expressed in F1 was called the recessive trait
12Mendel’s experiment Next, Mendel crossed the F1 generation plants. He wanted to know if the F1 plants were identical to the P generationSo he crossed two of the F1 plants
13Mendel’s experimentThe resulting plants (the F2 generation) yielded 3 tall plants and 1 dwarf plant.A ratio of 3:1
14Mendel’s experimentThis 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.
15Mendel’s experimentMendel repeated this experiment with all of the traits and the results were similar.The F2 generation displayed a phenotypic ratio of 3:1
16Mendel’s explanationUnits of inheritance, called factors (we now call them genes) were involvedFor 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
17Mendel’s explanationIn 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
18Mendel’s explanationWhen 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.
19Mendel’s first law The Law of Segregation Members of a pair of alleles for a given trait are segregated when gametes are formed
20Genotype The genotype represents the genes that exist in the organism Mendel used letters of the alphabet to represent genes.Capital letters represent the dominant alleleLower case letters represent the recessive allele
21Genotype A purebred tall plant would have a genotype of TT A purebred dwarf plant would have a genotype of ttGenotypes with identical alleles are called homozygousGenotypes with different alleles are called heterozygous
22The 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 tThe F1 generation would all have the genotype TtThe phenotype of all the F1 generation would be tall
24Mendel’s impactMendel’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.
25A Punnett SquarePunnett 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 tttt gametes:tTT gametes:TTt
26A 2 x 2 Punnett SquareThe F1 generation cross is a monohybrid cross: Tt x TtTt gametes:TtTTTttttalldwarfPhenotypic ratio =3:1
27Single Trait AnalysisIn 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:tTTttttasternon-tastertt (non-taster) gametes:Genotype ratio: 1:1 (Tt:tt)Phenotype: 1:1
28Practice Problem In humans, the allele A, for pigment formation is dominant to the allele for a, the inability toform pigment.aa individuals are albinoDetermine the expected genotypic andphenotypic ratios expected from a crossbetween two individuals heterozygous forthis trait.
29Solving Punnett Square Problems GRASP method Given:pigment formation (A) is dominant to the inability to form pigment (a)aa individuals are albinoThe cross is Aa x AaRequired:The expected genotypic and phenotypic ratio of the crossAnalysis:Solution:Paraphrase:
30Solving Punnett Square Problems GRASP method Analysis:Solution:Paraphrase:The phenotypic ratio is 3:1 normal : albinoThe genotypic ratio is 1 : 2: 1 AA:Aa:aaA aAAAaA aAaaa
31Recognizing HybridsA geneticist crosses two parent plants that have the dominant trait ofpurple flowers.When the resulting seeds are planted the geneticist observes that 145 ofthe F1 plants have the recessive trait of yellow flowers and 430 of the F1plants have purple flowers.How can you explain these results? What are the genotypes of the parentplants and the F1 plants?Given: F (purple flowers) is dominant to f (yellow flowers). Both of the P1plants possess at least one F gene: F__ × F__Required: The genotypes of the parents and F1 plantsAnalysis: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.
32Let F be the dominant allele for flower colour: purple and f be the recessive allele for colour: yellowPhenotypic ratio is 430:145gametes:gametes:
33Solution:The appearance of the recessive trait in thephenotype of the F1 plants can only occur ifthey are homozygous recessive (ff). This canonly happen if both of the purple parent plantsare heterozygous and each parent contributedthe recessive allele to these yellow plants. Inaddition, recognize that the ratio of purpleplants to yellow plants is approximately 3:1.This ratio indicates a monohybrid cross.
34Dihybrid crossesDihybrid crosses show the genotypes of offspring using 2 different genes on two different chromosomesTtRr x TtRr hybrid tall plants with round seedsGametes produced by independent assortment: TR, tr, Tr, tRThese gametes would be present in equal numbers (eg. ¼ of the total number)
37Test crossA test cross is a cross with an individual whose genotype is being tested to a recessive individualWe know that the recessive individual must be homozygous so it can only produce one type of gameteThis way we can determine the genotype of the test individual.