2. 4 Chapter 14~ Mendel & The Gene Idea

3. The Origins of Genetics 4 Heredity: the passing of traits from parents to offspring 4 Gregor Mendel did experiments with garden peas 100+ years ago in a monastery in Vienna. 4 1 st to develop rules that accurately predict patterns of heredity. 4 Mendel Animation Mendel Animation

4. Mendelian genetics (characters and traits) 4 Gene- DNA segment that codes for particular protein production (heritable feature, i.e., fur color) 4 Allele- variant for a gene (blue vs. brown eyes) 4 P generation (parents) 4 F generation (filial generation: offspring)

5. Crossing pea plants 1 5 4 3 2 Removed stamens from purple flower Transferred sperm- bearing pollen from stamens of white flower to egg- bearing carpel of purple flower Parental generation (P) Pollinated carpel matured into pod Carpel (female) Stamens (male) Planted seeds from pod Examined offspring: all purple flowers First generation offspring (F 1 ) APPLICATION By crossing (mating) two true-breeding varieties of an organism, scientists can study patterns of inheritance. In this example, Mendel crossed pea plants that varied in flower color. TECHNIQUE When pollen from a white flower fertilizes eggs of a purple flower, the first-generation hybrids all have purple flowers. The result is the same for the reciprocal cross, the transfer of pollen from purple flowers to white flowers. TECHNIQUERESULTS

6. Mendel’s Work with Pea Plants 4 1 st experiments were monohybrid crosses: involve one trait like flower color or seed shape 4 Used true breeding parents: plants that always showed one form of the trait

7. 4 When cross-pollinating two true breeding parent plants (one purple & one white) 4 F 1 : the offspring from the cross, only showed the purple flower form of the trait 4 F 1 generation was allowed to self-pollinate and the F 2 generation had the traits in a ratio of 3:1  purple:white P Generation (true-breeding parents) Purple flowers White flowers  F 1 Generation (hybrids) All plants had purple flowers F 2 Generation Mendel discovered

8. 4 Mendel observed the same pattern –In many other pea plant characters

9. Mendel’s Theory 1. For each inherited trait, an individual has two copies, one from each parent 2. There are alternative versions of genes, alleles 3. When two different alleles occur together, one trait may be expressed while the other is not. Dominant: trait expressed if one or two alleles were present. Recessive: only expressed if two alleles are present.

10. First, alternative versions of genes 4 Account for variations in inherited characters, which are now called alleles Allele for purple flowers Locus for flower-color gene Homologous pair of chromosomes Allele for white flowers

11. 4. Alleles for each trait separate independently when forming gametes. Gametes only carry one allele for each trait (haploid). 4 Homozygous: when an individual has two of the same alleles for a trait. 4 Heterozygous: when an individual has two different alleles for a trait

12. 4 Genotype: the set of alleles that an individual has for a gene (e.g. PP, Pp, pp) 4 Phenotype: the physical appearance of a trait (purple vs. white flowers) 4 The phenotype is determined by the genotype –PP = purple –Pp = purple –pp = white

13. Phenotype versus genotype 3 1 1 2 1 Phenotype Purple White Genotype PP (homozygous) Pp (heterozygous) Pp (heterozygous) pp (homozygous) Ratio 3:1 Ratio 1:2:1

14. Leading to the Law of Segregation 4 The alleles for each character segregate (separate) during gamete production (meiosis). 4 Mendel’s Law of Segregation 4 Punnett square: predicts the results of a genetic cross between individuals of known genotype

15. P Generation F 1 Generation F 2 Generation Pp P p P p P p PpPp PP pp Pp Appearance: Genetic makeup: Purple flowers PP White flowers pp Purple flowers Pp Appearance: Genetic makeup: Gametes : F 1 sperm F 1 eggs 1/21/2 1/21/2  Each true-breeding plant of the parental generation has identical alleles, PP or pp. Gametes (circles) each contain only one allele for the flower-color gene. In this case, every gamete produced by one parent has the same allele. Union of the parental gametes produces F 1 hybrids having a Pp combination. Because the purple-flower allele is dominant, all these hybrids have purple flowers. When the hybrid plants produce gametes, the two alleles segregate, half the gametes receiving the P allele and the other half the p allele. 3 : 1 Random combination of the gametes results in the 3:1 ratio that Mendel observed in the F 2 generation. This box, a Punnett square, shows all possible combinations of alleles in offspring that result from an F 1  F 1 (Pp  Pp) cross. Each square represents an equally probable product of fertilization. For example, the bottom left box shows the genetic combination resulting from a p egg fertilized by a P sperm.

16. The testcross  Dominant phenotype, unknown genotype: PP or Pp? Recessive phenotype, known genotype: pp If PP, then all offspring purple: If Pp, then 1 ⁄ 2 offspring purple and 1 ⁄ 2 offspring white: p p P P Pp pp Pp P p pp APPLICATION An organism that exhibits a dominant trait, such as purple flowers in pea plants, can be either homozygous for the dominant allele or heterozygous. To determine the organism’s genotype, geneticists can perform a testcross. TECHNIQUE In a testcross, the individual with the unknown genotype is crossed with a homozygous individual expressing the recessive trait (white flowers in this example). By observing the phenotypes of the offspring resulting from this cross, we can deduce the genotype of the purple-flowered parent. RESULTS 4 Testcross: breeding of a recessive homozygote to a dominate phenotype (but unknown genotype)

17. The Law of Independent Assortment 4 Law of Segregation involves 1 character. What about 2 characters? 4 Monohybrid cross vs. dihybrid cross 4 The two pairs of alleles segregate independently of each other.

20. Genetic Variation 4 Independent assortment: the homologous chromosomes distribute randomly to the gametes 4 Crossing over and independent assortment increase the number of genetic combinations 4 Genetic variation is important for evolution –Alleles (different versions of a genes) lead to different traits being expressed.

21. The Multiplication and Addition Rules Applied to Monohybrid Crosses 4 The multiplication rule –States that the probability that two or more independent events will occur together is the product of their individual probabilities

22. Probability in a monohybrid cross 4 Can be determined using this rule  Rr Segregation of alleles into eggs Rr Segregation of alleles into sperm R r r R R R R 1⁄21⁄2 1⁄21⁄2 1⁄21⁄2 1⁄41⁄4 1⁄41⁄4 1⁄41⁄4 1⁄41⁄4 1⁄21⁄2 r r R r r Sperm  Eggs

23. The rule of addition 4 States that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities

24. Solving Complex Genetics Problems with the Rules of Probability 4 We can apply the rules of probability –To predict the outcome of crosses involving multiple characters

25. 4 A dihybrid or other multicharacter cross –Is equivalent to two or more independent monohybrid crosses occurring simultaneously 4 In calculating the chances for various genotypes from such crosses –Each character first is considered separately and then the individual probabilities are multiplied together

26. 4 Inheritance patterns are often more complex than predicted by simple Mendelian genetics –The relationship between genotype and phenotype is rarely simple

27. Extending Mendelian Genetics for a Single Gene 4 The inheritance of characters by a single gene –Complete dominance Occurs when the phenotypes of the heterozygote and dominant homozygote are identical 4 May deviate from simple Mendelian patterns

28. Heredity Can Be Complicated 4 Codominance: both traits are displayed 4 Multiple alleles: genes with three or more alleles. Example; blood types. 4 Incomplete dominance: when both alleles have an influence over the phenotype

29. The human blood group ABO 4 Codominance –Two dominant alleles affect the phenotype in separate, distinguishable ways 4 Multiple alleles: more than 2 possible alleles for a gene (Ex: human blood types)

30. Single gene genetics (examples) 4 Incomplete dominance: appearance between the phenotypes of the 2 parents. (Ex: snapdragons)

31. Multiple gene genetics Pleiotropy: genes with multiple phenotypic effect. Ex: sickle-cell anemia 4 Epistasis: a gene at one locus (chromosomal location) affects the phenotypic expression of a gene at a second locus. Ex: mice coat color

32. Multiple gene genetics 4 Polygenic traits: several genes influence one trait. Examples: eye color, height, weight, hair and skin color

33. Nature and Nurture: The Environmental Impact on Phenotype 4 Another departure from simple Mendelian genetics arises –When the phenotype for a character depends on environment as well as on genotype

34. 4 The norm of reaction –Is the phenotypic range of a particular genotype that is influenced by the environment

35. Studying Heredity Pedigree: a family history that shows a trait over several generations Autosomal traits appear in both sexes equally (on autosomes) Sex-linked trait: a trait whose allele is located on a sex chromosome.

36. Pedigree Analysis A pedigree –Is a family tree that describes the interrelationships of parents and children across generations

37. Inheritance patterns of particular traits Can be traced and described using pedigrees Ww ww Ww wwWw ww Ww WW or Ww ww First generation (grandparents) Second generation (parents plus aunts and uncles) Third generation (two sisters) Ff ffFf ff Ff ff FF or Ff ff FF or Ff Widow’s peak No Widow’s peak Attached earlobe Free earlobe (a) Dominant trait (widow’s peak) (b) Recessive trait (attached earlobe)

38. Recessively Inherited Disorders Many genetic disorders –Are inherited in a recessive manner Recessively inherited disorders –Show up only in individuals homozygous for the allele Carriers –Are heterozygous individuals who carry the recessive allele but are phenotypically normal

39. Cystic Fibrosis Symptoms of cystic fibrosis include –Mucus buildup in the some internal organs –Abnormal absorption of nutrients in the small intestine

40. Sickle-Cell Disease Sickle-cell disease –Affects one out of 400 African-Americans –Is caused by the substitution of a single amino acid in the hemoglobin protein in red blood cells Symptoms include –Physical weakness, pain, organ damage, and even paralysis

41. Dominantly Inherited Disorders One example is achondroplasia –A form of dwarfism that is lethal when homozygous for the dominant allele

42. Huntington’s disease Is a degenerative disease of the nervous system Has no obvious phenotypic effects until about 35 to 40 years of age

43. Multifactorial Disorders Many human diseases –Have both genetic and environment components Examples include –Heart disease and cancer

44. Pedigrees –Can also be used to make predictions about future offspring (genetic counseling) Other tests also available –Amniocentesis –Chorionic villi sampling (CVS)

45. (a) Amniocentesis Amniotic fluid withdrawn Fetus PlacentaUterus Cervix Centrifugation A sample of amniotic fluid can be taken starting at the 14th to 16th week of pregnancy. (b) Chorionic villus sampling (CVS) Fluid Fetal cells Biochemical tests can be Performed immediately on the amniotic fluid or later on the cultured cells. Fetal cells must be cultured for several weeks to obtain sufficient numbers for karyotyping. Several weeks Biochemical tests Several hours Fetal cells Placenta Chorionic viIIi A sample of chorionic villus tissue can be taken as early as the 8th to 10th week of pregnancy. Suction tube Inserted through cervix Fetus Karyotyping and biochemical tests can be performed on the fetal cells immediately, providing results within a day or so. Karyotyping