Chapter 14: Mendel and the Gene Idea

1. Chapter 14: Mendel and the Gene Idea

2. Heredity • Heredity= passing of traits from parent to offspring • Reproductive cells are only way traits are passed to offspring • Changes in somatic cells will not be passed to next generation • Genetics= scientific study of heredity

3. What genetic principles account for the passing of traits from parents to offspring? • The “blending” hypothesis is the idea that genetic material from the two parents blends together • Kind of like blue and yellow paint blend to make green • The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes) • Heritable factors may be rearranged, but retain identity from one generation to the next • This hypothesis can explain the reappearance of traits after several generations

4. Gregor Mendel • Gregor Mendel, Austrian monk • Documented a particulate mechanism in the 1800s byexperiments with garden pea plants • 1860s- Studied inheritance patterns in pea plants • Character= heritable feature with multiple variations • Trait= each type of variation for a character • At this time, chromosomes and genes had not been discovered yet • Used the term heritable factors to represent what we call a gene

5. Gregor Mendel • Discovered the basic principles of heredity by breeding garden peas in carefully planned experiments • Advantages of pea plants for genetic study • Short-life cycle • There are many varieties with distinct characters

6. Gregor Mendel • Parents pass characterson to offspring • Pea plants= one character is flower color • Parents can pass on different traits • Pea plants= purple or white flower color

7. Gregor Mendel • Discovered the basic principles of heredity by breeding garden peas in carefully planned experiments • Advantages of pea plants for genetic study • Short-life cycle • There are many varieties with distinct heritable characters • Mating can be controlled • Each flower has sperm-producing organs (stamens) and egg-producing organ (carpel) • Cross-pollination (fertilization between different plants) involves dusting one plant with pollen from another

8. In a typical experiment, Mendel mated two different true-breeding varieties=hybridization Gregor Mendel • Mendel chose to track only those characters that occurred in two distinct alternative forms • He also used varieties that were true-breeding in P generation • Produce offspring of same variety when they self-pollinate

9. Gregor Mendel • The hybrid offspring of the P generation are called the F1 generation • When F1 individuals self-pollinate or cross- pollinate with other F1 hybrids, the F2 generation is produced

10. Gregor Mendel • F1 Generation: 100% purple flowers • Mendel discovered a ratio of about 3:1, purple to white flowers, in the F2 generation

11. Gregor Mendel • Mendel reasoned that only the purple flower factor was affecting flower color in the F1hybrids • The factor for white flowers was not diluted or destroyed because it reappeared in the F2 generation • Mendel called the purple flower color a dominant trait and the white flower color a recessive trait

12. Pea Plant Experiments • 7 Characters in Pea Plants • Crossed different varieties of each character • Hybrids • Genetic Test Cross • P generation • F1 generation • F2 generation • Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2offspring

13. Mendel’s Hypotheses Based on 4 Concepts • There are alternate forms of genes (alleles) and these account for the variation of inherited characters. • Offspring inherit 2 alleles for each character, one from each parent. • When alleles are different, one allele determines appearance (dominant allele) and other has no effect (recessive allele). • Mendel’s Law of Segregation: Allele pairs separate during gamete production and sex cells carry one allele for each inherited character.

14. 1. Alternate forms of genes (alleles) account for variation • Alleles= alternative forms of genes • Flower Color: 2 Forms

15. 2. Offspring inherit 2 alleles for each character, one from each parent. • 1 chromosome with alleles for genes are inherited from mom and one from dad • Alleles can be the same or different • Homozygous- alleles same • Heterozygous- alleles different

16. 3. When alleles are different, dominant allele masks recessive allele • Complete Dominance • Dominant Alleles (uppercase letters) • Determines appearance of offspring if present • Homozygous Dominant= PP • Recessive Alleles (lowercase letters) • Masked by dominant allele • Only determines appearance if both alleles are recessive • Homozygous recessive= pp • Heterozygous: 1 dominant, 1 recessive= Pp

17. 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 • 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 in the populationthan the dominant allele

18. Exceptions to the Rule Red = RR • Incomplete Dominance: Expression of recessive allele not completely masked in heterozygote • Snapdragon flower color • Co-dominance: Two alleles are dominant and expressed in heterozygote • Example: Blood Type: AB Pink = Rr White = rr

19. 4. Mendel’s Law of Segregation • Allele pairs separate during gamete production and sex cells carry one allele for each inherited character • Meiosis- haploid number of chromosomes • Fertilization- fusion of egg and sperm result in diploid offspring • Genotype= alleles present in offspring for character • Example: PP, pp, Pp • Phenotype= physical appearance as a result of alleles • Purple flowers, white flowers

20. Homologous chromosomes can have different alleles G1 S phase

21. 4. Mendel’s Law of Segregation • 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

22. Mendel’s Experimental Crosses Cross homozygous dominant plant with homozygous recessive plant P Generation PP pp

23. Mendel’s Law of Segregation

24. Genotype and Phenotype • Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition • Phenotype=physical appearance • Genotype= genetic makeup • Flower color in pea plants: • PP and Pp plants have the same phenotype (purple) but different genotypes

25. Heterozygotes • Organisms with heterozygous genotype are considered “carriers” of the recessive allele • Not a true-breeding variety • Pea Plants Flower Color= Pp • Recessive allele is present, but not “visible” in phenotype • Can be passed on to next generation

26. Punnett Square • Used to predict potential traits in offspring • 4 possible combinations of alleles • Each parent creates 2 potential gametes 2 x 2= 4 • Need to know type of gametes produced by parent • Genotype= alleles for character • Each gamete has one allele

27. Pea Plants Experiment • Cross F1 generation to produce F2 generation • Phenotype: Purple Flowers • Genotype: Heterozygous • Cross Bb X Bb • F2 generation: • 1 BB (purple) • 2 Bb (purple) • 1 bb (white)

29. Research in Genetics • Testcross • Mating between unknown genotype and homozygous recessive genotype • Use Punnett Squares to predict offspring traits and compare to actual outcomes

31. Mendel’s Law of Independent Assortment • Mendel identified his second law of inheritance by following two characters at the same time • Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters • A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently

33. Mendel’s Law of Independent Assortment • Mendel’s Law of Independent Assortment • Genes inherited independently of each other • Inheritance of one character independent from another character • This law applies only to genes on different chromosomes or those far apart on the same chromosome • Genes located near each other on the same chromosome tend to be inherited together

34. Mendel’s Law of Independent Assortment

35. Genetic 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

36. Genetic Inheritance • The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities • 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 ½ chance of carrying the dominant allele and a ½ chance of carrying the recessive allele

37. Genetic Inheritance The addition rule= probability that any one of two (or more) exclusive events will occur is calculated by adding together their individual probabilities Probability of heterozygote: ¼ + ¼ = ½ Probability of homozygote: ¼ + ¼ = ½

38. Dihybrid Crosses • A dihybrid or other multicharacter cross is equivalent to two or more independent monohybrid crosses occurring simultaneously • Crosses with 2 characters of interest • RrYy x RrYy • In gametes, alleles for one character can be paired with either allele for other character • For each parents gametes, R can be paired with Y or y • 4 possible gamete types for each parent • RY, Ry, rY, ry

39. Dihybrid Cross

40. Exceptions to the Rule • The relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied • However, the basic principles of segregation and independent assortment apply even to more complex patterns of inheritance

41. Exceptions to the Rule • 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 • When a gene has more than two alleles • When a gene produces multiple phenotypes

42. Exceptions to the Rule Red = RR • Incomplete Dominance: Expression of recessive allele not completely masked in heterozygote • Snapdragon flower color Pink = Rr White = rr

44. Exceptions to the Rule • Co-dominance: Two alleles are dominant and expressed in heterozygote • This occurs in blood types, which also has more than 2 alleles • Four phenotypes: A, B, O, AB • A and B codominant, both are dominant to O • 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 the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate • Enzyme encoded by the i allele adds neither

46. Pleiotropy • Most genes have multiple phenotypic effects, a property called pleiotropy • For example, pleiotropic alleles are responsible for the multiple symptoms of some hereditary diseases • Cystic fibrosis • Sickle-cell disease

47. Sickle-cell disease • The disease is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells • Causes cell to collapse and form sickle shape • Clogs blood vessels • In homozygous individuals, all hemoglobin is abnormal (sickle-cell) • Symptoms include physical weakness, pain, organ damage, and even paralysis

48. Sickle-Cell Disease Normal Blood Cell • Heterozygotes= produce both types of blood cells • Heterozygotes have sickle-cell trait • Usually healthy but may suffer some symptoms of disease • Sickle-cell disease affects one out of 400 African-Americans • Unusual to have high frequency of an allele with detrimental effects in homozygotes Sickle-Cell

49. Sickle-Cell Disease Normal Blood Cell • Reason: “Heterozygote Advantage” • Less susceptible to the malaria parasite, so there is an advantage to being heterozygous in areas where malaria is common • Keeps allele in population despite detrimental effects of homozygous condition Sickle-Cell

50. Epistasis • In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus • In Labrador retrievers and many other mammals, coat color depends on two genes • One gene determines the pigment color (with alleles for black or brown) • The other gene (with alleles for color or no color) determines whether the pigment will be deposited in the hair