Understanding PTC Taste Sensitivity: Genetic Traits and Inheritance

1. When told to, put the PTC paper on your tongue • PTC, or phenylthiourea, is an organic compound having the unusual property of either tasting very bitter, or being virtually tasteless, depending on the genetic makeup of the taster. • The ability to taste PTC is a dominant genetic trait. T = Taster t = non-taster • If you can taste, you are either TT or Tt. • Those who can not taste are tt • About 70% of people can taste PTC, varying from a low of 58% for Aborginal people of Australia to 98% for Native Americans

2. Chapter 11: Mendel and the Gene Idea • Rap (Mr . Lee) • Genetics Rap

3. The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes) • Gregor Mendel (The monk) documented a particulate mechanism through his experiments with garden peas

5. Mendel used the scientific approach to identify two laws of inheritance • Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiments during the 1860’s

6. Mendel’s Experimental, Quantitative Approach • Why pea plants for genetic study: • There are many varieties with distinct heritable features, or characters (such as flower color); character variants (such as purple or white flowers) are called traits • Mating of plants can be controlled (self-fertilizing plants) • Each pea plant has sperm-producing organs (stamens) and egg-producing organs (carpels) in the flower • Cross-pollination (fertilization between different plants) can be achieved by dusting one plant with pollen from another

7. Mendel chose to track only those characters that varied in an either-or manner • He also used varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate) • ~3:1

9. In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization • The true-breeding parents are the P generation • The hybrid offspring of the P generation are called the F1 generation • When F1 individuals self-pollinate, the F2 generation is produced

10. Mendel’s Four Basic Principles • Found that whenever he crossed F2 generations, he always obtained the 3:1 inheritance pattern • Alternative versions of genes account for variations in inherited characters. (Factors or genes for flower color in peas exist in two versions, purple or white). Today we know that a gene is a sequence of DNA nucleotides on a specific location (locus) on a particular chromosome

11. 2. For each character, an organism inherits two copies (two alleles) of a gene, one from each parent.Each somatic cell in a 2N organism has two sets of chromosomes, one set from each parent. • 3. If the two alleles at a locus differ, then one, the dominant allele, determines the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organisms appearance. When he crossed pure-breeding purple and white flowered plants, all the F1 plants had purple flowers therefore, the purple allele is the dominant allele.

12. 4. The Law of Segregation • The two alleles for a heritable character segregate (separate from each other) during gamete formation and end u p in different gametes • An egg or a sperm get only one of the two alleles that are present in the somatic cells of the parent making the gametes.

14. Mendel’s segregation model accounts for the 3:1 ratio he observed in the F2 generation of his numerous crosses • The possible combinations of sperm and egg can be shown using a Punnett square, a diagram for predicting the results of a genetic cross between individuals of known genetic makeup • A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele

15. Useful Genetic Vocabulary • An organism with two identical alleles for a character is said to be homozygous for the gene controlling that character • An organism that has two different alleles for a gene is said to be heterozygous for the gene controlling that character • Unlike homozygotes, heterozygotes are not true-breeding

16. Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition • Therefore, we distinguish between an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup • In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes • But can two different phenotypes have the same genotype? • For hand preference, if RR or Rr, could they be a righty or a left?

17. Fig. 14-6 • Phenotype • Genotype • PP • Purple • 1 • (homozygous) • Pp • 3 • Purple • (heterozygous) • 2 • Pp • Purple • (heterozygous) • pp • White • 1 • 1 • (homozygous) • Ratio 3:1 • Ratio 1:2:1

18. How can we tell the genotype of an individual with the dominant phenotype? • Such an individual must have one dominant allele, but the individual could be either homozygous dominant or heterozygous • The answer is to carry out a Testcross: breeding the mystery individual with a homozygous recessive individual • If any offspring display the recessive phenotype, the mystery parent must be heterozygous

19. The Law of Independent Assortment • Mendel derived the law of segregation by following a single character • The F1 offspring produced in this cross were monohybrids, individuals that are heterozygous for one character • A cross between such heterozygotes is called a monohybrid cross

20. Mendel identified his third and fourth laws of inheritance by following two characters at the same time • Crossing two true-breeding parents differing in two characters producesdihybridsin the F1 generation, heterozygous for both characters • A dihybrid cross, a cross between F1dihybrids, can determine whether two characters are transmitted to offspring as a package or independently

22. Using a dihybrid cross, Mendel developed the law of independent assortment • The law of independent assortment states that each pair of alleles segregates independently of each other pair of alleles during gamete formation • Strictly speaking, this law applies only to genes on different, non-homologous chromosomes • Genes located near each other on the same chromosome tend to be inherited together and are said to be linked

23. The laws of probability govern Mendelian inheritance • Mendel’s laws of segregation and independent assortment reflect the rules of probability • When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss • In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles

24. The Multiplication and Addition Rules Applied to Monohybrid Crosses • The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities – “Rule of ands” • Probability in an F1 monohybrid cross can be determined using the multiplication rule • Segregation in a heterozygous plant is like flipping a coin: Each gamete has a chance of carrying the dominant allele and a chance of carrying the recessive allele

25. Multiplication rule – Rule of ands • 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? Pp x Pp • Answer: • Probability that an egg from the F1 (Pp) will receive a p allele = 1/2. • Probability that a sperm from the F1 will receive a p allele = 1/2. • The overall probability that two recessive alleles will unite, one from the egg and one from the sperm (pp), simultaneously, at fertilization is: 1/2 X 1/2 = 1/4. • What is the probability of having a boy anda girl? • ½ x ½ = 2/4 or ¼

26. Rr • Rr •  • Segregation of • alleles into sperm • Segregation of • alleles into eggs • Sperm • 1/2 • 1/2 • R • r • R • R • R • 1/2 • r • R • 1/4 • 1/4 • Eggs • r • r • r • R • 1/2 • r • 1/4 • 1/4

27. Mr Anderson to the rescue! • The rule of addition states that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities – “rule of ors” • The rule of addition can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous • What is the probability of having a boy or a girl? • ½ + ½ = 2/2 or 1 • vs, What is the probability of having a boy and a girl? • ½ x ½ = 1/4

28. Addition rule – Rule of ors • Question: • In a Mendelian cross between pea plants that are heterozygous for flower color (Pp), what is the probability of the offspring being a heterozygote? Pp x Pp • Answer: • There are two ways in which a heterozygote may be produced: the dominant allele (P) may be in the egg and the recessive allele (p) in the sperm or the dominant allele may be in the sperm and the recessive in the egg. Consequently, the probability that the offspring will be heterozygous is the sum of the probabilities of those two possible ways: • Probability that the dominant allele will be in the egg with the recessive in the sperm is 1/2 X 1/2 = 1/4. • Probability that the dominant allele will be in the sperm and the recessive in the egg is 1/2 X 1/2 = 1/4. • Therefore, the probability that a heterozygous offspring will be produced is 1/4 + 1/4 = 1/2

29. Solving Complex Genetics Problems with the Rules of Probability • We can apply the multiplication and addition rules to predict the outcome of crosses involving multiple characters • A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously • In calculating the chances for various genotypes, each character is considered separately, and then the individual probabilities are multiplied together

30. For a Trihybrid • What is the chance of getting at least two recessive traits? • If parents are PpYyRrx Ppyyrr • Cross Pp x Pp (¼ PP: ½ Pp: ¼ pp) Cross Yy x yy (½Yy: ½ yy) • Cross Rr x rr(½ Rr: ½ rr) • Then apply multiplication then the addition rule • Using the multiplication rule • Using the addition rule

31. Inheritance patterns are often more complex than predicted by simple Mendelian genetics • The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied • Many heritable characters are not determined by only one gene with two alleles • However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance

32. Extending Mendelian Genetics for a Single Gene • Inheritance of characters by a single gene may deviate from simple Mendelian patterns in the following situations: • When alleles are not completely dominant or recessive – Incomplete dominance • When a gene has more than two alleles – Multiple alleles • When a gene produces multiple phenotypes - Pleiotrophy

33. Degrees of Dominance • Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical • Black and white yields either black or white • In Incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties • Blue and yellow yields green • In Codominance, two dominant alleles affect the phenotype in separate, distinguishable ways • Blue and yellow yields both blue & yellow

34. Incomplete dominance

35. CodominanceIf present, the allele will be expressed • A Roan cow • Codominant Camelia • AB Blood type RBC

36. The Relation Between Dominance and • Phenotype • A dominant allele does not subdue a recessive allele; alleles don’t interact • Alleles are simply variations in a gene’s nucleotide sequence • For any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype

37. Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in the brain. Affected children (rr) die w/i a year. • At the organismal level, the allele is recessive. If (rr), the individual cannot produce the enzyme to break down lipids. • At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant (rr’). But half the normal amount of enzymes is enough to function • At the molecular level, the alleles are codominant. If RR’,there will be equal amounts of the normal and abnormal lipase produced for lipid breakdown

39. Frequency of Dominant Alleles • Dominant alleles are not necessarily more common in populations than recessive alleles • For example, one baby out of 400 in the United States is born with extra fingers or toes (Polydactylism) • Webbed digits is dominant • Flat feet

40. The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage • In this example, the recessive allele is far more prevalent than the population’s dominant allele

41. Multiple Alleles • Most genes exist in populations inmore than two allelic forms • For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i. • The enzyme encoded by theIA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither

42. Carbohydrate • Allele • Phenotype • (blood group) • Red blood cell • appearance • Genotype • IA • A • B • IB • i • none • (a) The three alleles for the ABO blood groups • and their associated carbohydrates • IAIA or IAi • A • B • IBIB or IBi • AB • IAIB • ii • O • (b) Blood group genotypes and phenotypes

45. Extending Mendelian Genetics for Two or More Genes • Polygenic Inheritence – • Some traits may bedetermined by two or more genes

46. Polygenic Inheritance • Quantitative characters are those that vary in the population along a continuum • Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype • Skin color, eye color, height, & body size in humans are all examples of polygenic inheritance

47.  • AaBbCc • AaBbCc • Sperm • 1/8 • 1/8 • 1/8 • 1/8 • 1/8 • 1/8 • 1/8 • 1/8 • 1/8 • 1/8 • 1/8 • 1/8 • Eggs • 1/8 • 1/8 • 1/8 • 1/8 • Phenotypes: • 1/64 • 6/64 • 15/64 • 20/64 • 15/64 • 1/64 • 6/64 • Number of • dark-skin alleles: • 2 • 6 • 0 • 3 • 4 • 5 • 1

48. Inheritance of Eye Color

49. Nature and Nurture: The Environmental Impact on Phenotype • Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype • The norm of reaction (Phenotypic plasticity) is the phenotypic range of a genotype influenced by the environment. Such characters are called multifactorial because genetic and environmental factors collectively influence phenotype • For example, hydrangea flowers of the same genotype range from blue-violet (add Al & lower pH) to pink (add lime to raise pH to 6-6.5), depending on soil acidity

50. Fig. 14-14 • Neutral pH – pink Acidic - blue