1 BASIC GENETICS 2 Gregor Mendel Austrian monk Studied science and math High school teacher and gardener Experimented on pea plants Father of Genetics ( ) We credit Mendel for forming the basis of genetics. 3 both A zygote inherits traits from both parents. Heredity is the passing of traits or characteristics from parents to offspring. 4 Sexual Reproduction Diploid zygote 2n + = 1n Haploid sperm (gamete) Haploid egg (gamete) 1n 5 So how can we predict what the offspring will look like? ? 6 Through Probability!!! likelihood is the likelihood that a specific event will happen. possibility helps understand past events and the possibility of future events. Probability Probability: 7 What are the possible outcomes when you flip a coin? compare the number of times a certain outcome can occur to the total number of possible outcomes. write it as a fraction. Probability = number of times one outcome is likely to occur number of times one outcome is likely to occur total number of all possible outcomes total number of all possible outcomes To find the probability of an event 8 Equal chance of 2 possible outcomes: Heads is one possible outcome out of a total of 2 possible outcomes. or12 Probability = What is the probability of flipping a coin and it landing heads up? What is the likelihood that a flipped coin will land heads up? So What does this have to do with genetics? tailsheads 9 We use probability to predict possible outcomes of genetic crosses. Genetic crosses involve 2 independent events Alleles contributed by one parent Because: Pp P P pp p Do not depend on Alleles contributed by the other p So 10 We multiply the separate probabilities of the two events. = X 1 4 Ppp p We combine both probabilities. Pp p What is the probability of two heterozygous purple individuals (Pp x Pp) producing a white offspring (pp)? p from one parent = 1212p from other parent = 1212The probability of these parents producing a white offspring ? One chance in four ? ? ? p 1 4 11 predict possible outcomes from genetic crosses. Probability using Pedigrees and Punnett Squares simplify analysis of genetic probabilities. PPedigrees and Punnett squares are scientific tools used to P P p p pP p P P p P p 12 Pedigree A chart of a family's history showing relationships and how a trait or disease has been inherited over many generations Unknown Gender 13 Squares represent Males. Horizontal lines show mating relationships. Circles represent Females. Vertical lines and brackets show birth relationships. Half-shaded circle or square represents a carrier of the trait. Shaded circles or squares show a person expressing the trait. Unshaded circles or squares show a person that does not express the trait. Pedigree Analysis 14 Punnett Squares A diagram that shows the possible gene combinations that might result from a genetic cross P P p p PpPP pp Pp 15 Punnett Squares The letters in a Punnett square stand for alleles (one of a number of different forms of a gene). P P p p PpPP pp Pp 16 We call the actual genetic make-up of an organism its genotype. Genotype: PP Genotype: Pp Genotype: pp 17 We call the appearance of an organism its phenotype. Genotype: PP Phenotype: Purple Genotype: Pp Phenotype: Purple Genotype: pp Phenotype: White 18 The genotype (the genes) is represented by letters such as Pp. P P p p PpPP pp Pp The phenotype (like a photograph) is represented by a description such as purple. 19 Offspring can be: An organism that has two different alleles for the same trait Homozygous for a trait or An organism that has two identical alleles for a particular trait Heterozygous for a trait Pp PP 20 Homozygous Dominant Genotype: PP Phenotype: Purple flowers Heterozygous Dominant Genotype: Pp Phenotype: Purple flowers Homozygous Recessive Genotype: pp Phenotype: White flowers More Examples 21 Dominant and Recessive Alleles When 2 different alleles for the same trait occur together, one may be expressed while the other is not expressed. Some traits are dominantwhile others arerecessive Homozygous Dominant Genotype: PP Phenotype: Purple flowers Homozygous Recessive Genotype: pp Phenotype: White flowers 22 The dominant trait will be expressed if a dominant allele is present. DDDDominant alleles overpower recessive alleles. Dominant Alleles IIIIf there is a dominant allele and a recessive allele, the dominant trait will be the one that is expressed. EEEExample: A genotype of Pp (Purple/white) shows a phenotype of Purple. 23 The recessive trait is expressed only when the dominant allele is not present. TTTThe allele that is overshadowed by a dominant allele Recessive Alleles EEEExpressed only if both alleles are recessive EEEExample: A genotype of pp (white/white) shows a phenotype of white. 24 Pea plants have purple and white alleles for flower color. TTTThe allele for purple flowers is dominant and the allele for white flowers is recessive. IIIIf the allele for purple flowers is present, the plant will produce purple flowers. Dominant and Recessive Alleles 25 Complete Dominance P P p p PpPP pp Pp 1/4 (25%) Homozygous Offspring (Genotype PP) (Phenotype Purple) 2/4 (50%) Heterozygous Offspring (Genotype Pp) (Phenotype Purple) 1/4 (25%) Homozygous Offspring (Genotype pp) (Phenotype White) 2 Heterozygous Parents (Genotype Pp) (Phenotype Purple) would produce 26 P P p p PpPP pp Pp 1/4 (25%) Homozygous Offspring (Genotype PP) (Phenotype Purple) 2/4 (50%) Heterozygous Offspring (Genotype Pp) (Phenotype Purple) 1/4 (25%) Homozygous Offspring (Genotype pp) (Phenotype White) 2 Heterozygous Parents (Genotype Pp) (Phenotype Purple) would produce Complete Dominance 27 P P p p PpPP pp Pp 1/4 (25%) Homozygous Offspring (Genotype PP) (Phenotype Purple) 2/4 (50%) Heterozygous Offspring (Genotype Pp) (Phenotype Purple) 1/4 (25%) Homozygous Offspring (Genotype pp) (Phenotype White) 2 Heterozygous Parents (Genotype Pp) (Phenotype Purple) would produce Complete Dominance 28 P P p p PpPP pp Pp 2 Heterozygous Parents Genotype Pp Phenotype Purple would produce 1/4 (25%) Homozygous Offspring (Genotype PP) (Phenotype Purple) 2/4 (50%) Heterozygous Offspring (Genotype Pp) (Phenotype Purple) 1/4 (25%) Homozygous Offspring (Genotype pp) (Phenotype White) Complete Dominance 29 Lets try some examples with Mendels pea plants. gg GG Gg Green Yellow GenotypePea pod colorPhenotype This is complete dominance because yellow is hidden in a heterozygous genotype. 30 A cross of a homozygous green pea plant and a heterozygous green pea plant would yield: Gg GG G g G G Gg all offspring with a phenotype of green, but (50%) heterozygous (Gg) and (50%) homozygous (GG). 31 A cross of a homozygous yellow pea plant and a heterozygous green pea plant would yield: gg Gg G g g g gg (50%) offspring with a phenotype of green, genotype Gg and (50%) with a phenotype of yellow, genotype gg. 32 Lets add another trait. rr RR Rr Round Wrinkled GenotypePea seed shapePhenotype This is complete dominance because wrinkled is hidden in a heterozygous genotype. 33 GGrr A cross of two pea plants, both with heterozygous green seed pods and heterozygous round seeds would yield: GR GgRr Gr gR gr GRGrgRgr GGRR GGRr GgRR GgRr GGRr GgRr Ggrr GgRR GgRr ggRR ggRr GgRr Ggrr ggRr ggrr There is only one chance in 16 that both recessive traits will be expressed. 34 Complex Patterns of Heredity 35 Incomplete Dominance Produce a heterozygous offspring with a blended phenotype BUT THE ALLELES REMAIN DISTINCT; ONLY THE PHENOTYPE APPEARS BLENDED. Two homozygous parents with different phenotypes red + white= pink r r + w w= r w 36 Incomplete Dominancerr w w rwrwrwrw rwrwrwrw Crossing homozygous parents to produce F 1 generation THE ALLELES REMAIN DISTINCT; ONLY THE PHENOTYPE APPEARS BLENDED. 37 Incomplete Dominancerw r w rrrwrw rwrwww Crossing heterozygous F 1 generation to produce F 2 gen. THE ALLELES REMAINED DISTINCT; THE ORIGINAL PHENOTYPES REAPPEAR IN THE F 2 GENERATION 38 Codominance Both alleles in the heterozygote express themselves fully. Example: Blood types 39 Codominance A person homozygous for Type A blood And a person homozygous for Type B blood Will produce a child that will demonstrate both Type A and Type B blood (Type AB) 40 Incomplete Dominance Traits are blended (red + white = pink) Codominance Both traits are expressed (A + B = AB) In summary: 41 Pleiotropy A single gene affects more than one trait. For example, sickle-cell disease results from one gene, but it has numerous effects on the body. 42 Polygenic Traits A trait controlled by more than one gene. Eye color is an example of a polygenic trait. 43 Says Dr. Rick Sturm, the IMB researcher who led the study: "... the model of eye colour inheritance using a single gene is insufficient to explain the range of eye colours that appear in humans. We believe instead that there are two major genes -- one that controls for brown or blue, and one that controls for green or hazel -- and others that modify this trait." (Credit: Image courtesy of University of Queensland) 44 Pleitropy Single gene affects more than one trait. Polygenic Trait Single trait controlled by more than one gene. In summary: 45 Carrier is heterozygous for the trait. does not express the trait. can pass the trait to offspring. An individual who carries a recessive trait X C X c For Example: This female is heterozygous for colorblindness. She has normal vision. She can pass the trait on to her son who will be colorblind. 46 Without sexual reproduction, we would have very little genetic variation. 47 With sexual reproduction, there is great variety in the appearance of offspring. 48 Issues in Biology Nature vs. Nurture Issue Studies have been done on identical twins that have been separated at birth What had a greater influence on their development what they inherited or the environment in which they were raised? 49 Paula Bernstein (left) and Elyse Schein are identical twins who were separated as infants and adopted by different families. Here at age 7.