Mendelian Genetics PDF

"There are no boring subjects, only disinterested minds.” - G. K. Chesterton “The most exciting phrase to hear in science, the one that heralds new discoveries, is not ‘eureka!’ but ‘that’s funny….’ -Isaac Asimov Mendelian Geneti...

"There are no boring subjects, only disinterested minds.” - G. K. Chesterton “The most exciting phrase to hear in science, the one that heralds new discoveries, is not ‘eureka!’ but ‘that’s funny….’ -Isaac Asimov Mendelian Genetics Chapter 7  Genetics Terminology Genetics: branch of biology that focuses on the inheritance of traits Heredity: transmission of traits from parent to offspring; inheritance Trait: characteristic of an individual Gene: a sequence of DNA that codes for a specific protein Allele: different versions of a gene Genes and Inheritance  Genotype vs. Phenotype Phenotype: observable features of an individual Ex.Yellow seeds, white flowers Genotype: the alleles found within a particular individual Codes for One allele from mother and one from father Only one allele expressed as the phenotype Genotype Phenotype Denoted as letters Ex: YY or Yy for yellow seeds  Characterizing the Genotype Dominant allele: allele that determines the phenotype of a heterozygous individual Denoted with a capital letter. Ex:Y or R Recessive allele: allele whose phenotype is only expressed in homozygous individuals Denoted with a lowercase letter. Ex: y or r Homozygous: having two identical alleles of a certain gene Ex:YY or yy Heterozygous: having two different alleles of a certain gene Ex:Yy or Rr Homozygous and Heterozygous  Father of Genetics Gregor Mendel (1822 – 1884) Austrian monk Inheritance of traits in pea plants Model organism Tested leading hypothesis of blended inheritance Credit for discovery came after his death  Why the pea? Model organisms: organisms that are easy to care for and can be used to make inferences about many other similar species Small Short lived Inexpensive to care for Produce large numbers of offspring Can be manipulated experimentally Life Cycle of the Pea Plant 1. Self-pollination. Pollen from the male anthers of a plant falls on the female stigma of that same plant. 2. Fertilization. Sperm from the pollen fertilize the plant's eggs, which lie inside the ovary. These eggs will develop into seeds in the ovary (peas in a pod), which represent a new plant generation. Each seed is fertilized separately. 3. Germination. Each seed can be planted and grown into a separate plant. 4. Development. Seedlings develop into mature seed plants, capable of producing their own offspring. Mendel’s Procedure: Cross Pollination 1. Before fertilization occurs, peel back the closed petals of a pea plant (in this case, one that came from a line that yielded yellow peas). Then pull out the pollen-bearing stamens with tweezers so that self-fertilization is no longer possible. 2. Next, gather pollen from a green-seed plant by dabbing its anthers with a paintbrush. 3. Finally, rub these pollen grains onto the stigma of the first plant. The results of the cross-pollination can be observed when the fertilized eggs mature into seeds in the ovary, meaning peas in a pod. Mendel’s First Test Letters represent alleles from parents Paired chromosomes = two alleles Dominant alleles = Capital letter Y Recessive alleles = lowercase letter y Green seeds absent in F1 generation  Using a Punnett square Punnett squares are used to predict the outcome of a cross between two individuals Mendel’s 2nd and 3rd Generations F1 generation self pollinated F2 generation 3:1 phenotypic ratio 1:2:1 genotypic ratio Return of green phenotype! Whaa?! F3 generation had mixed and pure lines F3 generation Mendel’s Results  Reviewing Mendel’s Results P generation crosses Father’s genotype Mother’s genotype Mother’s genotype Phenotypic ratio = 4:0 y y 4 Yellow: 0 Green Y Yy Yy Genotypic ratio = 4:0 4 Heterozygous individuals Y Yy Yy F1 generation self-fertilization Phenotypic ratio = 3:1 Father’s genotype 3 Yellow: 1 green Y y Genotypic ratio 1:2:1 Y YY Yy 1 Homozygous dominant 2 Heterozygous 1 Homozygous recessive y Yy yy  Reviewing Mendel’s Results F2 generation self-fertilized Rediscovery of pure lines  Mendel’s Law of Segregation Law of segregation: alleles of each gene segregate during formation of gametes (meiosis) Egg and sperm carry only one copy of each gene  Mendel’s Findings 1. Peas have two versions, or alleles, of each gene This is also true for many other organisms 2. Alleles do not blend together The hereditary determinants maintain their integrity from generation to generation 3. Each gamete contains one allele of each gene Law of segregation 4. Males and females contribute equally to the genotype of the offspring When gametes fuse together the offspring has one allele from each parent per gene 5. Some alleles are dominant to other alleles When dominant and recessive alleles are found together, the phenotype will be that of the dominant allele  Check Your Understanding 1. True or False: each sex cell (egg or sperm) contains two copies of each gene 2. True or False: an individual with a dominant phenotype must be homozygous dominant for that particular trait 3. True or False: recessive alleles are only expressed when both parents contribute the recessive gene  Check Your Understanding Perform a cross between a man with brown eyes who is homozygous dominant (BB) and a woman with blue eyes (bb). Brown eyes are a dominant trait and blue eyes are a recessive trait. How many of their children will have blue eyes? B B Answer: None of their children will have blue eyes. All of the b Bb Bb children will have brown eyes because they all have an allele for brown eyes (B). It just takes Bb Bb one dominant allele to have the b dominant phenotype.  Check Your Understanding Perform a cross between a man and a woman who are both heterozygous for eye color. Brown eyes are a dominant trait (B) and blue eyes are a recessive trait (b). What are the chances they will have a child with blue eyes? B b Answer: 25% of their children B BB Bb will have blue eyes. The only way a child will have blue eyes is if they are homozygous recessive (bb) for that trait, Bb bb b because blue eyes is the recessive allele.  Check Your Understanding Provide the genotypic ratio and phenotypic ratio for a cross between a man and women with brown hair who are both heterozygous for hair color. Blond hair is the recessive trait. B b Genotypic ratio: 1 Homozygous dominant: BB Bb B 2 Heterozygous: 1 Homozygous recessive Phenotypic ratio: 3 Brown: 1 Blond Bb bb b  Monohybrid and Dihybrid Crosses Male Genotype Monohybrid cross: a cross Female Genotype between parents that are Gene for seed color heterozygous for a single gene Phenotypic ratio = 3:1 Genotypic ratio = 1:2:1 Dihybrid cross: a genetic cross between parents that are Genes for seed heterozygous for two genes color and texture Phenotypic ratio = 9:3:3:1 Need to know this Genotypic ratio = 1:2:2:1:4:2:1:2:1 Don’t need to know this Monohybrid Cross in Humans  Test Cross Test Cross: Cross homozygous recessive with unknown genotype displaying dominant phenotype  Dihybrid Cross Yellow smooth Green wrinkled Parent generations are pure for two traits Homozygous dominant Yellow smooth Homozygous recessive Green wrinkle F1 generation is heterozygous for both traits Phenotypic ratio = 9:3:3:1 9 Yellow smooth, 3 yellow wrinkled, 3 green smooth, 1 green wrinkled Genotypic ration = 1:2:2:1:4:2:1:2:1  Mendel’s Law of Independent Assortment Hypothesis: Dependent Assortment Hypothesis: Independent Assortment Law of Independent P Generation Assortment: one trait does not influence the inheritance of another trait F1 Generation All traits are inherited independently of one another F2 Generation  Determining the Gametes for Dihybrid Cross b b B B b b B B OR h h H H h h H H b b B B b b B B H H h h h h H H b b B H B b b B B h h H H H h h Independent Assortment  Check Your Understanding Perform a Dihybrid cross between two individuals that are heterozygous for both hair color (Bb) and eye color (Ee). Black hair (B) is dominant to blond hair (b), and brown eyes (E) and dominant to blue eyes (e). Each individual’s genotype is BbEe BE Be bE be Phenotypic ratio: BBE BE BBEe BbEE BbEe (B_E_) = Black hair and Brown eyes = 9 E (B_ee) = Black hair and Blue eyes =3 Be BBEe BBee BbEe Bbee (bbE_) = Blond hair and Brown eyes = 3 bE BbEE BbEe bbEE bbEe (bbee) = Blond hair and Blue eyes =1 be BbEe Bbee bbEe bbee  Incomplete Dominance Incomplete dominance: P generation 1. The starting plants are a snapdragon heterozygous individual is an homozygous for red color (RR) and snapdragon homozygous for white color intermediate between either (rr). homozygous genotype F1 generation 2. When these plants are crossed, the resulting Rr genotype yields only enough Not blending of traits pigment to produce a flower that is pink—the only phenotype in the F1 Alleles separate again in generation. F2 generation F2 generation 3. In the F2 generation, alleles combine to produce red, pink, and white phenotypes.  Check Your Understanding What is the phenotypic ratio of a cross between a red snapdragon and a white snapdragon. Snap dragons express incomplete dominance. R is the dominant allele and r is the recessive allele. Red is dominant to white. R R Phenotypic ratio: 4 pink Rr Rr r Rr Rr r  Check Your Understanding What is the phenotypic ratio of a cross between two pink snap dragons. Snap dragons express incomplete dominance. R is the dominant allele and r is the recessive allele. Red is dominant to white. R r Phenotypic ratio: RR Rr 1: Red snap dragon R 2: Pink snap dragons 1: White snap dragon Rr rr r  Codominance Codominance: a condition in which two alleles of a given gene have different phenotypic effects, with both effects present in organisms heterozygous for the particular gene Blood type  Multiple Alleles Multiple alleles: when three or more alleles for the same gene exist within a population Increased diversity of population Ex: blood types Polygenic inheritance: interaction of several genes determine the phenotype or genetic character, each having a small additive effect on the outcome of the trait. Results in continuous variation Ex: human height, eye color, weight  Polygenic Inheritance Discrete traits: inherited traits that exhibit distinct phenotypes Pea plant traits Green or yellow, smooth or wrinkled Quantitative traits: a phenotype that depends on the cumulative actions of many genes. Quantitative traits vary among individuals to produce a continuous distribution of phenotypes. Human height, skin color  Quantitative Traits Each gene has a small additive effect on the trait resulting in a normal distribution of the trait in the population.  Quantitative Traits Quantitative traits are normally distributed Bell curve: distribution of values is symmetrical around the average Actual distribution Normal distribution  Polygenic Inheritance Ex: Skin color  Within Species Variation Within species variation is the basis for evolutionary change  Genetic Variation and Evolution Average beak depth in Galapagos finches shifted towards thicker beaks after a drought Thicker billed individuals better at eating large seeds of drought tolerant plants.  Check Your Understanding What is the phenotypic ratio and genotypic ratio of a cross between a two red roses that are both heterozygous. R is the dominant allele and r is the recessive allele. Red is dominant to white. Phenotypic ratio: Genotypic ratio: