Observing Patterns in Inherited Traits Chapter 10 Before you go on Review the answers to the following questions to test your understanding of previous material. 1. Most organisms are diploid. What does this mean in terms of inheritance? 2. What is the smallest unit of inheritance, and how do they function? 3. What do you call alternate forms of a gene? How do these arise? Can there be more than two? 4. How are gametes produced, and how do you describe the genetic information that is normally contained in each? 5. Explain Independent Assortment, which describes the behavior of genes (on different chromosomes) during Meiosis. 6. Discuss linked genes, and the significance of crossing over. When and how does this happen? 1
Learning Objectives 1. Explain the process Mendel used in his experiments with garden peas, and what his results added to our understanding of inheritance. 2. Construct a Punnett square demonstrating a monohybrid cross between one homozygous dominant individual and one homozygous recessive individual. Then use this method to predict the genotypes and phenotypes of a cross between 2 of these offspring (F1 = first filial generation). Use correctly the terms: parental generation, first filial (F1) generation and second filial (F2) generation. What is the predicted phenotypic ratio in the F2 generation? 3. Construct a Punnett square demonstrating a dihybrid cross between heterozygotes for both traits. Predict the phenotypes that would result. What is the predicted phenotypic ratio? Is this a good illustration of the principle of independent assortment? 4. Discuss the effect on the phenotype for traits governed by codominant alleles, alleles which demonstrate incomplete dominance, and multiple allele systems. Provide some examples of each. 5. Differentiate between polygenic inheritance and pleiotrophy, with examples. 6. How is gender inherited, and what is the predicted phenotypic ratio? What are sex-linked genes: how are they inherited, how does one become a carrier for a sex-linked trait, and how do you predict the inheritance pattern for these genes? Model Organisms Rapid Reproduction Produce large # of offspring) Few chromosomes Ease of propagation 2
Model Organisms Organism N # genes bacteria NA 4288 yeast 16 6,022 roundworm 5 14,000 fruit fly 4 15,000 dicot plant 5 25,000 mouse 40 100,000 * human 23 100,000 pea 7 30,000 * Although not all of these code directly for the production of proteins. p. 152 Mendelian Genetics Gregor Mendel (1822-1884) A botanist and mathematician To this day, Mendel s principles accurately describe the inheritance of traits. Described the variation in sexually reproducing species. 3
The Garden Pea Plant Self-pollinating True-breeding (normally selfpollinating, so different alleles not introduced) Can be experimentally crosspollinated p. 153 Mendel s Peas p. 154 Mendel studied these 7 characters. How many traits were studied? What do these results tell us about allele dominance for these characteristics? 4
Independent Assortment p. 156 Mendel recognized units of inheritance that governed specific traits e.g. pink or white flowers We know these units as alleles Mendel concluded that the two units for the first character were to be assorted into gametes independently of the two units for the other character This is true if the genes for the two characters are located on different chromosomes e.g. flower color and pod shape are not linked In other words, homologous chromosomes separate and are sorted into gametes at random during meiosis p. 153 Genetic Terms Dominant allele (Y) will mask a recessive allele (y) that it is paired with as a result of fertilization. (Yy, YY, or yy) A pair of homologous chromosomes (one from egg; one from sperm) A gene locus (i.e. location) A pair of alleles Genotypes: If both alleles the same: homozygous If alleles different: heterozygous Three pairs of genes What is the difference between the terms phenotype and genotype? 5
Generation Parental (P) Monohybrid Cross: What did Mendel learn from this experiment? True-breeding yellow pea (pollen) True-breeding green pea (eggs) x p. 154 grow plants,cross pollinate First Filial (F1) all yellow allow to self-fertilize Second Filial (F2) 6022 yellow : 2001 green 3 : 1 Alleles Pea color alleles Y y on Homologous Chromosomes What color are the peas in this plant? When this diploid plant produces a gamete, what allele(s) might the gamete contain? If allowed to selfpollinate, and fertilization is random, what are the possible combinations of alleles in the resulting offspring, and what will their peas look like? 6
p. 153 Genotype Phenotype Yellow x Yellow YY x Yy =? Y Punnett Square of a Monohybrid cross During gamete formation, the alleles segregate from each other Y Y y YY Yy YY Yy Genotypic ratio? 1:1, YY:Yy Phenotypic ratio? 4:0, yellow: green p. 155 7
Aa X Aa Punnett Square of a Monohybrid Cross Female gametes A a Male gametes A AA Aa Genotypic ratio? 1:2:1 p. 155 a Aa aa Phenotypic ratio? 3:1 Test Cross Pea with unknown genotype Y? x yy Y y the phenotype of the offspring will reveal the genotype of the unknown parent. y y Yy Yy yy yy If offspring result in ½ yellow and ½ green what is the parental genotype? p. 155 8
Complete Dominance Unattached earlobes Widow s peak Long eyelashes Freckles Cleft chin Certain diseases: e.g. cystic fibrosis p. 153 Many traits are not expressed via complete dominance. Sex linked traits Which chromosome(s) carries these alleles? Incomplete dominance (p. 158) Is this the same as equally dominant aka co-dominant? Polygenic inheritance (p. 158) poly means. Pleiotrophy & Epistasis (p. 159) Differentiate between these forms of gene expression. Environmental controls (p. 162) Provide at least one example of how the environment can affect gene expression. 9
Sex linked genes Fathers pass sex linked alleles to all of their daughters but none of their sons. Mothers pass alleles to both. b X Y b X X b Color blindness Hemophilia Male pattern baldness p. 169, 175 Flower Color in Snapdragons: Incomplete Dominance Pink-flowered plant X Pink-flowered plant (heterozygote) (heterozygote) White-, pink-, and red-flowered plants in a 1:2:1 ratio p. 158 10
Incomplete Dominance Hair texture in Caucasians Curly hair is shaped like an elongated oval and grows at a sharp angle to the scalp. RR: curly Rr: wavy rr: straight ABO Blood Type: Co-dominance in a Multiple Allele System Type A - I A I A or I A i Type B - I B I B or I B i Identify the blood type known as the: Universal donor Universal recipient Type AB - I A I B Type O - ii p. 158 11
Polygenic Inheritance & Continuous Variation Genes A, B, C each contribute a unit of darkness. AABBCC: very dark AaBbCc: intermediate darkness aabbcc: very light p. 160 The cumulative effect of multiple genes on one phenotype. The cumulative effect of multiple genes on one phenotype. Pleiotropy Alleles at a single locus may have effects on two or more traits Classic example is the effects of the mutant allele at the beta-globin locus that gives rise to sickle-cell anemia Cell shape and resistance to malaria Why has this disease not been eliminated by natural selection? p. 159 12
Epistasis: gene expression dependent upon two or more genes. Melanin BB = black lab Bb = black lab bb = brown lab How much melanin? EE = full deposition Ee = full deposition ee = no deposition p. 160 Epistasis cont. BBEE, BbEE, BBEe, or BbEe = black Must have at least one dominant allele at both loci for black pigment to be deposited BBee, Bbee, or bbee = yellow Pigment is not deposited bbee = chocolate bbee = chocolate Brown pigment is deposited 13
Don t for get the world around you Acidity of the soil changes the color of hydrangea flowers. The expression of some genes is heavily influenced by environmental factors. p. 162 Don t for get the world around you Being in the sun darkens our skin Ratio of red : white blood cells are affected by exercise, injury, and illness. Siamese cat and Himalayan rabbit: fur color is dependent upon temperature. 14