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Chapter 14 - Gregor Mendel and the Gene Idea A. Bergeron +AP Biology PCHS Do Now - Now Where Did I Put that Bug Spray? In fruit flies a recessive mutation in either of two independently assorting genes, brn and prp prevents the synthesis of wild-type (i.e. normal) red eye pigment. Homozygotes for either of these mutations have brownish purple eyes. But heterozygotes for both of these mutations have wild type eyes. 1) If double heterozygotes are crossed with each other what are the phenotypes and expected phenotypic ratios of the offspring? 2) Which mode of inheritance seems to be at play in this problem? 3) Where would you predict the brn and prp genes to be located in relation to one another? Hint: Let the wild type alleles of each gene be brn+ and prp+, respectively. Mendel’s Conclusions (deduced from a series of hypotheses that he tested experimentally) 1. Alternative versions of genes (i.e. alleles) account for variations in inherited characters/traits 2. An organism inherits two alleles (two versions of the same gene) from each parent (see next slide) 3. Dominant and recessive alleles for each character/trait may exist (But what does it mean to be dominant? Recessive?) 4. The alleles for each character/trait segregate during gamete production (i.e. meiosis) Two Alleles for Every Trait (most of the time…) Recall that offspring inherit one chromosome (of each pair) from Mom and Dad Together, these chromosomes make-up a homologous pair The location (i.e. locus) of a particular gene is the same on both chromosomes but the allele may be different Allele: alternative forms of the same gene Mendel’s Genetic “Laws” 1. Principle of Segregation Alleles for different traits segregate independently (as long as the alleles are on SEPARATE chromosomes) Equal probability of passing on either allele to the F2 generation (see example) Segregation of allelic pairs occurs during meiosis (anaphase I) Restoration of paired alleles occurs at fertilization Mendel’s Principle of Segregation Mendel hypothesized that there were two factors (i.e. alleles) for a particular trait Each parent passes on one allele to their offspring But why? Recall that homologous chromosomes separate into different cells during meiosis I As a result, each gamete will receive only only allele for a particular trait Useful Genetics Vocabulary 1. Homozygous - Having two identical alleles for a particular trait -Homozygotes are “true-breeding” or pure bred organisms -Can only pass on one type of allele to offspring 2. Heterozygous - Having two different alleles for a particular trait ! -Heterozygotes are often referred to as hybrids or ! carriers 3. Phenotype - An organism’s traits that are based on its genotype (Note: Phenotype is not necessarily the outward, physical appearance) 4. Genotype - The genetic make-up of an organism ! -Genotype determines phenotype Mendel’s Genetic “Principles” 2. Principle of Independent Assortment Each allele pair segregates independently of the other allele pairs during gamete formation (i.e. meiosis) Mendel’s Law of Independent Assortment (Assume genes are on separate chromosomes) Mom Dad A a A a B b b B A a A a B b b B A A a a B B b b Sex cells A A a a b b B B Independent Assortment Important Genotypic and Phenotypic Ratios Type of Parent Cross Genotypes Monohybrid Heterozygous Genotype Ratio 1:2:1 Phenotype Ratio 3:1 Dihybrid Complicated 9:3:3:1 Heterozygous (for both traits) Ratios can be predicted with a Punnett square (but Punnett squares are evil!) and there is an easier way… Do Now - Using Probability to Solve Genetics Problems What is the probability that in a trihybrid cross between two organisms with the genotypes AaBbCc and AaBbCc that the parents will produce an offspring with the genotype aabbcc? (Hint: Consider each pair of alleles as a single “coin flip”) What is the probability that the two parents will produce an offspring that is dominant for the traits controlled by genes A and C but recessive for the trait controlled by gene B? Using Probability to Solve Genetics Problems The probability of an event ranges from 0 (not going to happen) to 1 (certain to occur) The probabilities of all outcomes of an event must add up to 1 Random events are independent of one another ! -The outcome of one event does not influence the ! outcome of a previous or future event Using Probability to Solve Genetics Problems Rule of Multiplication -The probability that independent events will occur simultaneously is equal to the product of their individual probabilities Question - Determine the probability that you will role two “6’s” in a row on a single die. Three “6’s?” Seven “6’s?” Question - In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability that the offspring will be homozygous recessive? Question - What is the probability that in a trihybrid cross between two organisms with the genotypes AaBbCc and AaBbCc that the parents will produce an offspring with the genotype aabbcc? (Hint: Consider each pair of alleles as a single “coin flip”) Using Probability to Solve Genetics Problems 2. Rule of Addition (Either-or Rule) -Probability that an event will occur is the sum of the probabilities of each way that it can occur. -Useful when you want to know the probability of an event happening when there are several ways for the event to occur Question - If two parents are heterozygous for a trait, what is the probability, that their child will be heterozygous as well? Question - What is the probability of tossing three coins simultaneously and obtaining two heads and one tail? Question - If pea plants with the genotypes PpYyRr and Ppyyrr are crossed, what is the probability that offspring will be recessive for at least 2 of the 3 traits? Using Math to Solve Genetics Problems (Wait…Mr. B Knows Math?) If only two alleles exist for a particular character/trait, the distribution of phenotypes among the offspring of a cross can be modeled with a binomial distribution. Question - Imagine that a couple has chosen to have 3 children. How likely is it that two of the children will be boys and one will be a girl? Let p = probability of having a male child Let q = probability of having a female child p+q=1 p = q = 1/2 Family Composition 3 boys Order of birth bbb Calculation pxpxp Prob. p3 2 boys and 1 girl bbg bgb gbb pxpxq pxqxp qxpxp 3p2q bgg gbg ggb pxqxq qxpxq qxqxp ggg qxqxq 1 boy and 2 girls 3 girls 3q2p q3 3p2q = 3 (1/2)2(1/2) = 3/8 Genetics Practice Problems Question - Calculate the probability that two parents heterozygous for the recessive allele producing albinism (a) will have one albino child in a family of five. Question - Imagine that a couple has chosen to have five children. How likely is it that three of the children will be boys and two will be girls? Assume that the ability to hear is controlled by 3 pairs (6 alleles) of independently assorting genes and that normal hearing occurs when an individual has the genotype A_ or B_. The presence of DD, however, causes deafness, no matter what other allelic combinations are present. Question: Write out all of the possible genotypes of the offspring produced from a mother with the genotype AABBDd and a father with the genotype AaBbDd using the underscore format. Question: Predict the phenotype ratios of the offspring that would be produced if the above parents were to have children. Genetics Practice Problems Flower color in pea plants is determined by two gene pairs (A/a and B/b). For the first gene pair, the ‘A’ allele is dominant to the ‘a’ allele, while for the second gene pair, the ‘B’ allele is dominant to the ‘b’ allele. If at least one dominant allele from each gene is present, the flowers are purple. All other genotypes have white flowers. 1. What is the phenotype of a plant that has the Aabb genotype? 2. Using the symbols provided above, write all possible genotypes for a plant that has purple flowers. 3. If two plants that have the AaBb genotype are crossed, what is the phenotypic ratio of the offspring? Genetics Practice Problems Fur color in mice is governed by genes concerned with producing and distributing melanin. At one gene location, a dominant allele (B) specifies dark brown and a recessive gene (b) specifies a tan coat. At another gene location, a dominant allele (C) shuts down melanin production while a recessive allele (c) allows for the deposition of melanin. 4. Show the expected phenotype ratios in the offspring resulting from crossing a true breeding (i.e. homozygous) brown mouse with an albino mouse that is heterozygous at both loci. 5. What are the possible genotypes of the offspring? Genetics Practice Problems Phenylketonuria (PKU) is an inherited disease determined by a recessive allele. If a woman and her husband are both carriers, what is the probability of each of the following? 6. All three of their children will be normal. 7. One or more of the three children will have the disease. Given two cats with the genotypes llSsdd and LlSsDd, what is the probability of the following? (Short hair is dominant to long hair, white spotted coat is dominant to not spotted, and diluted color is dominant to non-diluted color). 8. A cat having short hair 9. A cat having the genotype llssdd 10. A cat having the short hair, white-spotted, and non-diluted phenotype Do Now – Non-Mendelian Inheritance Patterns 1. What is the principle difference between codominance and incomplete dominance at the phenotypic level? At the cellular level? At the molecular level? 2. In Labrador retrievers, brown fur color is dominant over yellow fur color. A dog breeder wishes to produce brown puppies rather than yellow puppies. She has two brown males and one yellow female. What could she do to verify that her male “studs” will be more likely to produce brown puppies? What Does It Mean to Be Dominant? Recessive? Recall that genes encode the instructions for making a particular protein Dominant alleles can be thought of as the instructions for synthesizing a “normal, functional” protein ! Although there are exceptions to this rule... ! Example: Dominant negatives Typically (but not always) recessive alleles encode instructions for making a non-functional protein If a protein is non-functional, it will affect the organism’s phenotype Mendelian Genetics Extensions Incomplete Dominance Neither allele is “completely dominant” Organisms that possess the heterozygous phenotype have a phenotype which is intermediate between the two “extremes” Does not support blending hypothesis because original phenotypes will reappear if F1 organisms are crossed Genetic Disease (Tay Sachs) Tay Sachs disease is a recessively inherited disease in humans Brain cells of Tay Sachs babies lack a lipid-metabolizing enyzme. ! -Lipids accumulate in brain causing disease symptoms Heterozygotes are symptom-free ! -Normal allele appears completely dominant and Tay ! Sachs allele is recessive However, heterozygotes actually make “equal” amounts of the normal and dysfunctional enzymes ! -The presence of the normal enzyme prevents ! heterozygotes from developing the disease (i.e. gene ! dosage effect) Extensions of Mendelian Inheritance Patterns Codominance and Multiple Alleles More than one allele contributes to the phenotype of an organism however phenotype is not intermediate between two “extremes” Example: Human blood types The Dominance Spectrum Complete Dominance (A is Dominant) AA and Aa have the same phenotype Incomplete Dominance (A is Incompletely Dominant) Aa = intermediate phenotype. Phenotype is intermediate between the phenotypes of the two homozygotes Codominance (No “pure” dominance) Aa = Both alleles are equally expressed in phenotype Do Now - Cholesterol Metabolism 1. From which type of biological molecule is cholesterol derived? 2. How do you think cholesterol is transported through the body? 3. What are some of the advantages and disadvantages of cholesterol in the body? Try to be as specific as possible. 3- Endocytosis • Internalization of substances by formation of a vesicle • Types – Phagocytosis – Pinocytosis – Receptor-mediated endocytosis 3- Pinocytosis and Receptor-Mediated Endocytosis 3- HMG-CoA Reductase 3- Cholesterol Metabolism Ingested cholesterol and fatty acids cannot be transported through the bloodstream (Why?) Cholesterol molecules and fatty acids are transported in chylomicrons (“fat droplets”) Chylomicron - a small, membrane-bound vesicle composed of phospholipids and protein Chylomicrons are transported through the bloodstream to adipocytes (i.e. fat cells) where fatty acids are stored Remaining cholesterol molecules are transported to the liver Cells in the liver break down the chylomicron releasing the cholesterol Cholesterol Metabolism 3- Most dense (fatty acid composition) Chylomicron High cholesterol (food intake) Chylomicron remnant VLDL LDL Least dense HDL (scavenger) Low cholesterol 3- Cholesterol Metabolism 3- AP Biology - Genetics Quiz 1. In humans, the allele for short fingers (F) is dominant over the allele for long fingers (f). If a person with short fingers who had one parent with long fingers “mates” with a person who has long fingers and this couple have three children, what are the chances that two of their three children will have short fingers? 2. Genes A, B, and B assort independently and are dominant to their respective alleles a, b, and c. Two triple heterozygotes (AaBbCc) are crossed. What is the probability that a given offspring will be phenotypically dominant for one of the dominant traits? 3- AP Biology - Genetics Quiz 3. The recessive allele s causes Drosophila to have small wings and s+ allele results in a normal wing phenotype. This gene is known to be located on the X chromosome. a. If a small winged male is crossed with a homozygous wildtype female, what ratio of normal to small winged flies can be expected in each sex in the F1? b. If F1 flies are intercrossed (mated with each other), what F2 progeny ratios are expected? c. What progeny ratios are expected if F1 females are backcrossed to their father? 3- Testcross Used to determine the genotype of a “parent” with the dominant phenotype but an unknown genotype Dominant parent “crossed/mated” with a homozygous recessive parent Genotype of the dominant parent can be determined based on the offspring produced from the cross If offspring with ONLY the dominant phenotype are produced, the parent must have the homozygous dominant genotype If offspring with the dominant and the recessive phenotype are produced, the parent must have the heterozygous genotype Sample Testcross Epistasis Expression of one gene is dependent on the expression of a second gene Example Coat color is controlled by one set of genes (For simplicity, let’s use R for black allele and r for brown allele). In cats, the dilute gene (d) is responsible for lightening the coat color Dilute gene is recessive to dense gene (D) The presence of dense allele (D) will produce a cat with a dark coat color; absence of dense will produce a cat with lightened coat color Mendelian Genetics Extensions - Epistasis One would predict 9:3:3:1 phenotype ratio in a dihybrid cross If ratios of offspring do not adhere to predicted ratios something else must be happening Epistasis - Ability for one gene to influence the phenotype brought about by a second gene Epistasis Expression of one gene can “mask” or inhibit the expression of a second gene One human trait in which epistasis may occur is in the occurrence of deafness in a child Question: Assume that this trait is controlled by 3 pairs of genes and that normal hearing occurs when an individual has the genotype A_ or B_. The presence of DD, however, causes deafness, no matter what other gene combinations are present 1. Write out all of the possible genotypes of the offspring produced from a mother with the genotype AABBDd and a father with the genotype AaBbDd 2. Predict the phenotype ratios of the offspring that would be produced if the above parents were to have children Genetic Imprinting Does it matter which allele is inherited from each parent? In some cases, the answer is yes. Genomic imprinting can inactivate one of the two copies (i.e. alleles) of a particular gene in an organism New imprint corresponds to sex of parent Addition of methyl groups to DNA nucleotides is responsible for imprinting Genetic Imprinting Special enzymes responsible for adding methyl (e.g. -CH3) group to nitrogen base