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How do you write 23 using only the number 2? 34 using only the number 3? 56 using only the number 5? 100 using only the number 9? How do you write 23 using only the number 2? 34 using only the number 3? 56 using only the number 5? 100 using only the number 9? By using fractions. 22+2/2=23, 33+3/3=34, 55+5/5=56, 99+9/9=100 Genetics A study of inheritance Basic Genetics Gregor Mendel: curious about physical characteristics of plants Physical Characteristics = traits Heredity = passing of traits from one parent to another Gregor Mendel Father of modern genetics Researched with pea plants Developed ideas of dominance and trait segregation Phenotypes & Genotypes Phenotype = physical characteristics = visible traits Genotype = genetic makeup or allele combinations Homozygous = two identical alleles for a trait Heterozygous = two different alleles for a trait – the different forms of a gene Genotype – combination of alleles Phenotype – organism’s appearance Alleles dominant – two dominant alleles Homozygous recessive – two recessive alleles Heterozygous – one dominant, one recessive allele Homozygous Dominant and Recessive Alleles Genes = factors that control traits Allele = different form of a gene Ex: Pea = stem height - one allele for short and one allele for tall Individual alleles control the inheritance of traits Some alleles are dominant and some recessive Dominant allele: trait always show Recessive allele: masked, covered up when dominant allele is present Ex: In pea plants, the tall stem allele is dominant over the short stem Phenotype Phenotype Physical characteristics Phenotype Notice the similarities: Facial structure Eyes Smile Ears Nose Neck Genotype Phenotype Physical characteristics Genotype Genes we inherit from our parents Mendel’s Experiments Purebred plants: always have the same form of a trait as the parent; Ex: Purebred short pea = short pea Mendel cross purebred tall plants with purebred short plants Parent plants = P generation Offspring = F1 generation F1 Generation = all tall F2 Generation = 3 tall and 1 short Mendel’s Crosses P generation had two identical alleles for stem height Purebred tall pea = 2 alleles for tall Purebred short pea = 2 alleles for short F1 generation = received one allele for tall and one allele for short F1 plants = hybrids = had two different alleles for a trait Mendelian Genetics Mendel studied a number of characteristics in pea plants including: •Height - short or TALL •Seed color - green or YELLOW •Seed shape - wrinkled or ROUND Mendelian Genetics Mendel studied a number of characteristics in pea plants including: •Height - short or TALL •Seed color - green or YELLOW •Seed shape - wrinkled or ROUND •Seed coat color - white or GRAY Mendelian Genetics Mendel studied a number of characteristics in pea plants including: •Height - short or TALL •Seed color - green or YELLOW •Seed shape - wrinkled or ROUND •Seed coat color - white or GRAY •Pod shape - constricted or SMOOTH •Pod color - yellow or GREEN Mendelian Genetics Mendel studied a number of characteristics in pea plants including: •Height - short or TALL •Seed color - green or YELLOW •Seed shape - wrinkled or ROUND •Seed coat color - white or GRAY •Pod shape - constricted or SMOOTH •Pod color - yellow or GREEN •Flower position - terminal or AXIAL Mendelian Genetics We will work with the following three: •Height - short or TALL •Seed color - green or YELLOW •Seed shape - wrinkled or ROUND •Seed coat color - white or GRAY •Pod shape - constricted or SMOOTH •Pod color - yellow or GREEN •Flower position - terminal or AXIAL Mendel & Probability Mendel = 1st scientist to recognize the principles of probability can be used to predict the results of a genetic cross Punnett Square = a chart that shows all possible combinations of alleles that can result from a genetic cross Predicting Inheritance To determine the chances of inheriting a given trait, scientists use Punnett squares and symbols to represent the genes. UPPERCASE letters dominant genes. lowercase letters recessive genes. are are used used to represent to represent Predicting Inheritance For example: T = represents the gene for TALL in pea plants t = represents the gene for short in pea plants So: TT & Tt both result in a TALL plant, because T is dominant over t. t is recessive. tt will result in a short plant. Remember there are two genes for every trait! One from each parent. Predicting Inheritance Mendels’ Principle of Dominance Some genes (alleles) are dominant and others are recessive. The phenotype (trait) of a dominant gene will be seen when it is paired with a recessive gene. For example: T = represents the gene for TALL in pea plants t = represents the gene for short in pea plants So: TT & Tt both result in a TALL plant, because T is dominant over t. t is recessive. tt will result in a short plant. Remember there are two genes for every trait! Predicting Inheritance Let’s cross a totally dominant tall plant (TT) with a short plant (tt). Each plant will give only one of its’ two genes to the offspring or F1 generation. TT x tt T T t t Predicting Inheritance Mendels’ “Law” of Segregation Each gene (allele) separates from the other so that the offspring get only one gene from each parent for a given trait. Let’s cross a totally dominant tall plant (TT) with a short plant (tt). Each plant will give only one of its’ two genes to the offspring or F1 generation. TT x tt T T t t Tt x tt Predictions? 50 ___% Tall ___% Short 50 Punnett Squares Tt Tt The genes from one parent go here. The genes from the other parent go here. Tt Tt Punnett Squares T T t Tt Tt t Tt Tt Punnett Squares T T t Tt Tt t Tt Tt Punnett Squares T T t Tt Tt t Tt Tt Punnett Squares T T t Tt Tt t Tt Tt Punnett Squares T T t Tt Tt t Tt Tt Punnett Squares t t T T Tt Tt Tt Tt F1 generation Interpreting the Results The genotype for all the offspring is Tt. The genotype ratio is: Tt - 4/4 The phenotype for all the offspring is tall. The phenotype ratio is: tall - 4/4 Punnett Squares T t T ?? ?? t ?? ?? Punnett Squares T t T t TT Tt Tt tt F2 generation Punnett Squares Next, give the genotype and phenotype T (F2 generation). t ratios of the offspring T TT Tt t Tt tt Punnett Squares Genotype ratio: TT - 1 T t T TT Tt t Tt tt Punnett Squares Genotype ratio: TT - 1, Tt - 2 T t T TT Tt t Tt tt Punnett Squares Genotype ratio: TT - 1, Tt - 2, tt - 1 T t T TT Tt t Tt tt Punnett Squares Genotype ratio: TT - 1, Tt - 2, tt - 1 T t T TT Tt t Tt tt Phenotype ratio: Tall - 3 Punnett Squares Genotype ratio: TT - 1, Tt - 2, tt - 1 T t T TT Tt t Tt tt Phenotype ratio: Tall - 3, short - 1 Punnett Squares This is a monohybrid T cross. We worked with t only one trait. The height of the plant. T TT Tt t Tt tt Punnett Squares This is a monohybrid T cross. We worked with t only one trait. The height of the plant. T TT Tt t Tt tt Dihybrid Crosses: Crosses that involve 2 traits. For these crosses your punnet square needs to be 4x4 In any case where the parents are heterozygous for both traits (AaBb x AaBb) you will get a 9:3:3:1 ratio. If you cross other combinations, you will need to do a square. Try RrYy x rryy Determine all possible combinations of alleles in the gametes for each parent. Half of the gametes get a dominant S and a dominant Y allele; the other half of the gametes get a recessive s and a recessive y allele. Both parents produce 25% each of SY, Sy, sY, and sy. List the gametes for Parent 1 along one edge of the punnett square. List the gametes for Parent 2 along one edge of the punnett square. Fill out the squares with the alleles of Parent 1. Fill out the squares with the alleles from Parent 2. The result is the prediction of all possible combinations of genotypes for the offspring of the dihybrid cross, SsYy x SsYy. There are 9 genotypes for spherical, yellow seeded plants. They are: SSYY (1/16) SSYy (2/16) SsYY (2/16) SsYy (4/16) Two recessive alleles result in green seeded plants. There are 2 genotypes for spherical, green seeded plants. They are: SSyy (1/16) Ssyy (2/16) Two recessive s alleles result in dented seeded plants. There are 2 genotypes for dented, yellow seeded plants. They are: ssYY (1/16) ssYy (2/16) A ssyy plant would be recessive for both traits. There is only 1 genotypes for dented, green seeded plants. It is: ssyy (1/16) A phenotypic ratio of 9:3:3:1 is predicted for the offspring of a SsYy x SsYy dihybrid cross. 3 spherical, green 9 spherical, yellow 1 dented, green 3 dented, yellow