Week 12 Learning Outcomes Genetics: Patterns of Inheritance • Explain the scientific reasons for the success of Mendel’s experimental work. Describe Mendel’s contributions to understanding how traits are inherited and state Mendel’s laws. • Describe the expected outcomes of monohybrid crosses involving dominant and recessive alleles. • Explain the relationship between genotypes and phenotypes in dominant and recessive gene systems • Use a Punnett square and pedigrees to predict and calculate probabilities of genotypes and phenotypes in a monohybrid cross • Explain Mendel’s law of segregation and independent assortment in terms of genetics and the events of meiosis • Explain the purpose and methods of a test cross • Identify non-Mendelian inheritance patterns such as incomplete dominance, codominance, multiple alleles, and sex linkage from the results of crosses. Use monohybrid crosses and pedigrees to predict and calculate probabilities of genotypes and phenotypes. 12 Essential Vocabulary to get started… Mendel’s Pea Plant Experiments • Phenotype • Genotype • Mendel was the first person to • Pure-bred / true breeding / homozygous analyze patterns of inheritance to deduce the fundamental • Heterozygous principles of genetics • Gene (“heritable factor”) • Studied garden peas • Allele • Easily manipulated (control over fertilization) • Can self-fertilize • Reproduce quickly • Large numbers of offspring • Many distinct traits to study; most traits had only two possible variants 34 1 Traits of Mendel’s Pea Plants Self-Fertilization produces homozygous (true Characters or traits breeding or pure breeding) plants • Meiosis produces gametes (pollen grains on stamens and an egg cell within the carpel) • The plant pollinates itself (egg + pollen grain) variants Figure 9.4 56 True Breeding “Purebred” Varieties Cross- Fertilization • Pollen from one plant • Offspring inherit two identical traits used to fertilize egg of (alleles) for a character for many another plant generations • Mendel had “True-breeding” pea Purple Purple flowers flowers plants for white flowers and purple flowers All plants have purple flowers 78 2 More Genetics Terminology Results of Monohybrid Cross • Genetic “cross” • A cross to examine the – A mating between two inheritance pattern for a parent organisms, single trait producing offspring • P Generation • The F1 all look like one of the parents (purple flowers) – Parents in the genetic cross (these are purebreds) • The trait that was not seen in the F1 generation • Hybrid reappears in the F2, always – Offspring of two different in the same ratio (1/4) true-breeding organisms • F1 generation – Hybrid offspring of the P generation • F2 generation – Offspring of two F1 organisms 910 Results of Monohybrid Cross Mendel developed four hypotheses after examining the results of the monohybrid crosses: 1. Inherited characters have alternative versions that account for variations between individuals. These are called alleles. 2. An offspring inherits two alleles for each character, one from each parent. a. Two identical alleles = homozygous b. Two different alleles = heterozygous 3. If the two alleles inherited by an organism are different, the allele that determines the organism’s appearance is dominant (indicated by capital letter), and the other allele is recessive (lowercase letter) F1: 100% look like one of the two parent plants 4. A sperm or egg carries only one allele for each character because the F2: ¾ (75%) look like one parent from the P generation, while ¼ two alleles segregate (separate) during gamete production (Law of (25%) look like the other parent from the P generation Segregation). Fertilization results in two alleles for each character in the offspring. 11 12 3 Phenotype: An individual’s Check for understanding physical traits. Ex: Purple or white flowers Complete the table with the correct phrase(s) or term(s): ***Only 2 phenotypes possible genotype, heterozygous, locus, homozygous, phenotype, Genotype: The genetic alleles dominant allele, recessive allele that an individual carries Ex: PP, Pp, or pp Example Phrase(s)/term(s) ***Only 3 genotypes possible AA Position on a chromosome Punnett Square: A tool Aa used to predict the Yellow seeds outcome of a genetic cross aa Phenotype ratio A 3 purple : 1 white a Genotype ratio 1 PP : 2 Pp : 1 pp Figure 9.6 13 14 Mating of Purebred Check for understanding parents: AA x aa Complete the table with the correct phrase(s) or term(s): genotype, heterozygous, locus, homozygous, phenotype, dominant allele, recessive allele Example Phrase(s)/term(s) AA homozygous; genotype Position on a chromosome locus Aa heterozygous; genotype Yellow seeds phenotype aa homozygous; genotype A Dominant allele a Recessive allele 15 16 4 Mating of F1 generation: Aa x Aa The Rules of Probability 1. Each fertilization is an independent event. Genotype ratio for F2: • The probability of any particular offspring genotype or phenotype is unaffected by previous offspring 2. Rule of Multiplication • Multiply the probabilities of each event to find the answer Phenotype ratio for F2: • Ex: What is the probability that 2 coins tossed at the same time will both land on heads? • 1/2 * 1/2 = 1/4 17 18 How to solve genetics problems: Sample Probability Problem In humans, brown eyes (B) are dominant over blue (b)*. A brown-eyed man marries a blue-eyed woman and they have three children, two of whom are brown-eyed and 1. Assign alleles for the traits in the problem one of whom is blue-eyed. Draw the Punnett square that illustrates this marriage. What is the man’s genotype? What are the genotypes of the children? What is the a. capital letter = dominant trait, lowercase letter = recessive trait probability that their fourth child will have blue eyes? b. must be the same letter (e.g. A and a, not G and y) 2. Write parent genotypes (* Actually, the situation is complicated by the fact that there is more than one gene involved in eye color, but for this example, we’ll consider only this one gene.) 3. Complete Punnett Square 4. Answer the question 19 20 5 A Parakeet Example Parents: Parents: Wild-type Wild-type Wild-type Sky-blue First- Offspring: All Wild-type generation All wild-type offspring: Matings (a) Offspring from the mating Second- of two wild-type birds generation offspring: 3 and 1 /4 /4 Wild-type Sky-blue In parakeets, the sky blue feather color is recessive to the wild-type (dominant) yellow (b) Two generations of offspring from the mating of a wild-type with a sky-blue bird feather color. Draw Punnett squares for the mating of a purebred yellow parakeet with a sky blue parakeet, and for the mating of F1 parakeets. What are the genotype and phenotype ratios of the offspring for each cross? 21 22 One allele for each trait is located on each Law of Segregation homologous chromosome Separation of homologous chromosomes in meiosis 1 results in one allele for each trait in any gamete Figure 9.7 23 24 6 Chromosomal Basis of Inheritance • Each pair of alleles Possibility 1 Possibility 2 segregates Metaphase of independently of the meiosis I other pairs during gamete formation (meiosis) Metaphase of meiosis II • Homologous pairs line up independently of one Gametes another in meiosis 1 Combination a Combination b Combination c Combination d 25 26 Mendel’s Next Question: Dihybrid cross • What happens with the inheritance of two different traits in the same cross? • Mating of organisms that differ in 2 traits • Plants that make round yellow peas (RRYY) x Plants that make wrinkled green peas (rryy) • Two possibilities: – Shape and color are inherited together (round and yellow are always inherited together (RY gametes), and green and wrinkled always inherited together (ry) – only TWO phenotypes are possible) OR – Shape and color are inherited independently of one another (round can be inherited with yellow or with green, and wrinkled can be inherited with yellow or with green – FOUR phenotypes are possible) 27 28 7 How to write the genotype for 2 traits at once: • R = round r = wrinkled • Y = yellow y = green • Heterozygous for shape = Rr • Heterozygous for color = Yy • Now put the genotypes together: 29 30 Dihybrid cross genotypes: Mendel’s Dihybrid Cross: RY rY Ry ry RRYY RrYY RRYy RrYy RY RrYY rrYY RrYy rrYy rY RRYy RrYy RRyy Rryy Ry RrYy rrYy Rryy rryy ry 31 32 8 Dihybrid cross phenotypes: Phenotypic ratio for a dihybrid cross: RY rY Ry ry RY rY Ry ry 33 34 A Dihybrid-Cross Example Phenotypes & Phenotype ratios: Draw a punnett square of this dihybrid cross, and state the phenotypes and phenotype ratio for the offspring. 35 36 9 Exception to Law of Independent Testcross to Determine an Assortment: Unknown Genotype • Alleles that are very close together on a chromosome do • A mating of: not obey the law of independent – An individual of assortment Gene A unknown • These are called linked genes and their alleles are frequently – genotype with a but not always – inherited homozygous together recessive • Do not show the expected 9:3:3:1 ratio Gene B individual Gene C 37 38 Human Genetics and Pedigrees Sample Testcross Problem • Mendel’s principles apply to the inheritance of many human traits In dogs, there is a hereditary deafness caused by a recessive gene, “d.” A kennel owner has a male dog that she wants to breed. The dog can hear, so the owner knows his genotype is either ______ or ______ . If the dog’s genotype is ______, the owner does not wish to use him for breeding so that the deafness gene will not be passed on. This can be tested by breeding the dog to a deaf female (______). Draw the Punnett squares to illustrate these two possible crosses. In each case, what percentage of the offspring would be expected to be hearing or deaf? How could you tell the genotype of this male dog? Also, using Punnett square(s), show how two hearing dogs could produce deaf offspring.