Patterns of Inheritance: Genes, Chromosomes, and Genetic Variation

1. CH. 6: Patterns of Inheritance

2. Major Concepts: • Genes are discrete sequences of DNA on chromosomes; chromosomes consist of DNA and associated proteins. • Genes are the units of inherited information. • Genes code for several RNA types; mRNA is the template for proteins. • Inheritance of genes occurs in regular patterns that can be predicted by the rules of probability. • Genetic variation, from mutation and recombination, is essential for evolution. • The products of genetic engineering give rise to ethical consideration of benefits and risks to human well-being and environmental integrity.

3. Genes Determine Biological Potential

4. Genes • Nucleotides  Genes (DNA)  Chromosomes

5. Heredity and Environment • Genetics: branch of biology that deals with inherited variation.

6. Heredity and Environment Nature vs. Nurture: • Both heredity and environment influence an individual’s development. • e.g. Siamese cats inherit genes for enzymes that produce dark pigment for fur. • The enzymes function best at temps below normal body temp., thus dark markings are at extremities: ears, face, paws, tail. • One can change coloration by keeping certain portions of cat’s body cool.

7. Heredity and Environment • The environment has a strong impact on gene expression.

8. Heredity and Environment Studies of twins: • Fraternal twins: develop from separate eggs each fertilized by separate sperm cells. • Identical twins: develop from one zygote forming two complete embryos.

9. Heredity and Environment • If a trait shows up more often in identical twins than fraternal: characteristic is probably genetic. • If a trait differs in identical twins: probably environmentally influenced.

10. Heredity and Environment • Blended inheritance: notion that mixing of parents’ genes resulted in an “averaging” of parental characteristics • no longer seriously considered, as it would not allow for passing on of traits separately to future generations, which is observed.

11. Genes • Information is stored in genes in the sequence of nucleotide bases that make up DNA: a molecular code. • The code directs the cell processes involved in development and function of cells and, thus, the entire organism. • Genes provide instructions for the structure, function and development of a cell/organism.

12. Genes • Many genes code for the synthesis of specific proteins, e.g. an enzyme, muscle protein, pigment, regulatory proteins, etc.; other genes code for various forms of RNA • Through processes of meiosis and fertilization, chromosomes are passed on from generation to generation.

13. Genes Determine Biological Potential • Heredity - passing of traits from parent to offspring • Genes – basic units of genetic info • Genetics - study of heredity • Involves predictions • referred to as probability - predicts the chances that a certain event will occur • Geneticist – one who studies heredity and the actions of genes.

14. Genes and Chromosomes • Prokaryotic chromosomes: single circular DNA molecule with little protein; generally no introns. • ~ 90% of DNA is translated.

15. Prokaryotic Chromosomes • Often have small circles of additional DNA: plasmids. • Plasmids may move from one bacterial cell to another, thereby introducing genetic variation. • Geneticists use plasmids to introduce modified genetic material into bacterial cells: genetic engineering, e.g. insulin production. • Plasmids carry genes for antibiotic resistance.

16. Genes and Chromosomes • Eukaryotic chromosomes: consist of long molecules of DNA wrapped around proteins. • Only part of the DNA codes for proteins. • Some noncoding sections of DNA consist of sequences repeating thousands of times.

17. Genes and Chromosomes • Only ~ 1.5% of human DNA codes for proteins. (We don’t know importance of rest.) • Some introns are involved with gene expression. • Repetitive sequences may serve to stabilize DNA’s bond with associated proteins. • Mutations can convert inactive DNA sequences into active genes, or inactivate functional genes  may be a source of new alleles in natural selection.

18. Genes and Chromosomes • Homologous chromosomes carry same genes, though not necessarily the same alleles for those genes.

19. Genes and Chromosomes • Chromosomes may be distinguished by their banding pattern (pattern of dye that occurs when chromosome is stained (Fig. 8.9, p. 191). Ea. chromosome has a distinctive banding pattern.

20. Genes and Chromosomes • Karyotype: (Fig. 13.11, p. 349) a display of human chromosomes arranged as homologous pairs.

21. Karyotypes • Used in genetic studies of disease to search for hereditary causes. • Members of each pair have a specific banding pattern when dyed, as the stains bind to specific regions of the chromosomes.

22. Karyotypes • White blood cells frequently used for such studies: • They can be made to divide easily. • Grow well in culture. • Chemicals interrupt cell cycle at metaphase. Why? • Cells are placed on microscope slide and treated with water to spread chromosomes apart. • Stains cause the banding pattern to appear. • Unique banding of each chromosome pair enables researchers to detect missing or extra chromosome parts and extra chromosomes themselves. • Have also helped with mapping of genes on chromosomes.

23. Genes and Chromosomes Karyotype: • Allows one to count and identify chromosomes, and spot any unusual, missing or extra chromosomes fairly quickly.

24. Genes and Chromosomes • Human karyotype made up of 22 pairs of autosomes, chromosomes that are the same in ♂ & ♀, and one pair of sex chromosomes, chromosomes that are different in ♂ & ♀. • ♀: XX; ♂: XY

25. Probability • Probability: an area of mathematics that predicts the chances that a certain event will occur. • Using the rules of probability, one can predict the most probable outcome of randomly ordered events; the actual outcome, however, may not match the prediction, i.e. the prediction is simply that: a prediction, not a guarantee. • Investigation 13A: Probability, pp. 748 – 49 (Lab write-up due ___)

26. Gregor Mendel • 1822 – 1884 • Austrian monk • Studied science & math at the University of Vienna • Formulated the laws of heredity in the early 1860's • Did a statistical study of  traits in garden peas over an eight year period • Mendel’s work led to the concept of the gene - Mendelian Genetics

27. Why garden peas (Pisum sativum)? • Can be grown in a small area • Produce lots of offspring • Produce pure plants when allowed to self-pollinate several generations (true-breeding) • Can be artificially cross-pollinated

28. Garden pea flowers contain both male & female reproductive parts • Self-pollination (pollinates itself) • Cross-pollination (collect pollen from flowers of one pea plant & transfer to another)

29. Mendel and the Idea of Alleles • 4 ways experiments were unique: • Looked at only one trait at a time • Used large numbers • Combined results of many identical experiments • Analyzed results using rules of probability • Thus, Mendel was able to see patterns of inheritance

30. Mendel and the Idea of Alleles • Mendel studied 22 simple traits of  pea plants (e.g. seed color & shape, pod color & shape, etc.). • Mendel traced the inheritance of individual traits & kept careful records of numbers of offspring. • He used his math principles of probability to interpret results. • Mendel studied pea traits, each of which had a dominant & a recessive form.

31. Inheritance of Alleles • Trait: any characteristic that can be passed from parent to offspring • Allele: one of two or more possible forms of a gene (e.g. dominant & recessive) • Dominant: an allele that masks the presence of another allele of the same gene in a heterozygous organism, represented by capital letter, e.g. B • Recessive: a trait (allele) whose expression is masked (hidden) in a heterozygous organism, represented by lower-case letter, e.g. b • Genotype: genetic makeup of an organism; gene combination for a trait (e.g. RR, Rr, rr) • Phenotype: observable appearance or trait determined by the genotype; the physical feature resulting from a genotype (e.g. tall, short)

32. Inheritance of Alleles • Homozygous: The condition (genotype) in which both alleles are the same form, e.g. RR, rr; can produce only one type of gamete; also called “pure.” • Heterozygous: The condition in which two alternate forms (alleles) of a gene are contained within the organism, e.g. Rr; also called “hybrid.” • Monohybrid cross: a cross (mating) involving a single trait. • Dihybrid cross: a cross (mating) involving two traits. • Punnett Square: graphic tool (grid) used to solve genetics problems.

33. Inheritance of Alleles

34. Inheritance of Alleles • A Punnett Square:

35. Inheritance of Alleles • A Punnett Square:

36. The dominant (shows up most often) gene or allele is represented with a capital letter, & the recessive gene with a lower case of that same letter (e.g. B, b) • Mendel’s traits included: a. Seed shape ---  Round (R) or Wrinkled (r)         b. Seed Color---- Yellow (Y) or  Green (y)         c. Pod Shape --- Smooth (S) or wrinkled (s)         d. Pod Color ---  Green (G) or Yellow (g)         e. Seed Coat Color ---  Gray (G) or White (g)         f. Flower position --- Axial (A) or Terminal (a)         g. Plant Height --- Tall (T) or Short (t)         h. Flower color --- Purple (P) or white (p)

38. Mendel’s Experiments (cont.) • 1st: Mendel tested each strain of plant he used to ensure that it was true-breeding (homozygous), i.e. genetically true, producing offspring identical to themselves generation after generation. • 2nd: He worked with strains that were true-breeding in all but one characteristic, e.g. tall vs. short plant form; green vs. yellow seeds, round vs. wrinkled seeds, etc. • This allowed him to follow the pattern of inheritance of one trait at a time from generation to generation, e.g. round vs. wrinkled seeds:

39. Mendel’s Experiments (cont.) • Parental generation (P1): Crossed round seed-producing plants with wrinkled seed-producing plants. • First filial generation (F1): All offspring of the above cross produced round seeds. • Second filial generation (F2): ¾ produced round seeds; ¼ produced wrinkled seeds. • Thus, wrinkled seeds seemed to disappear in one generation (F1), then reappear in the next (F2). • Mendel called the round seed condition, “dominant,” and the wrinkled seed condition, “recessive.” • He surmised that the recessive form of a trait could only be manifest when the individual inherited that trait from both parents, i.e. received two “doses” of the trait. • This is known as Mendel’s principle of dominance.

40. Mendel’s Experiments (cont.)

41. Mendel’s Experiments (cont.)

42. Mendel’s Experiments (cont.) • Alleles are not dominant or recessive; only their effects on a trait are. • At the molecular level, the genotype, both alleles are present. • At the level of the organism, the phenotype, the effects of one allele (the dominant form) may mask those of the other (recessive form) allele. • Mendel repeated this type of experiment for other traits (outlined in Fig. 6.10, p. 179). • He calculated the ratio of dominant to recessive forms for each trait and it was always essentially the same: the dominant form appeared in approx. ¾ of the F2 plants, while the recessive form appeared in ¼ of the F2 plants. Thus, the ratio was always3:1. • (See p. 179)

43. Inheritance of Alleles • Mendel did not know about genes. He referred to dominant and recessive “factors” to describe the results of his experiments. • He did not know where these “factors” were located in cells. • Hypothesized that only one copy of a factor went into each sperm or ovum, i.e. if a parent were true-breeding for round seeds, for example, all its gametes would have the “round-seed factor.” Similarly for “wrinkled-seed factor.” • Offspring of a round-seed by wrinkled seed cross would have one factor of each from each parent: the principle of segregation.

44. Inheritance of Alleles • Genotype: genetic makeup of an organism; made up of the alleles for any gene, e.g. RR, Rr, rr. • Phenotype: the observable appearance or trait that is determined by the genotype, e.g. Three distinct genotypes: • BB  phenotype: purple flowers (dominant) • Bb  phenotype: purple flowers (dominant) • bb  phenotype: white flowers (recessive)

45. Inheritance of Alleles

46. Inheritance of Alleles • Mendel also worked with plants that varied in two traits at a time, e.g. round vs. wrinkled seeds and yellow vs. green seed color. • Dihybrid cross: a cross that results in offspring that are heterozygous for two (di) traits. • See Fig. 13.14, p. 353; a Punnett square • F2 Phenotypes occur in 9:3:3:1 ratio (9/16, 3/16, 3/16, 1/16). • Each trait individually displays the 3:1 ratio of a monohybrid cross. • The genes for the various traits separate independently from one another . . . • Principle of independent assortment: alleles for one trait segregate independently of alleles for the other trait during gamete formation.

47. Dihybrid Cross

48. Dihybrid Cross

49. Dihybrid Cross

50. Patterns of Inheritance • Testcross: a cross between an organism with an unknown genotype and an organism with the recessive phenotype • The recessive phenotype individual must be homozygous recessive, thus one knows the genotype. • The resulting Punnett Square results will allow you to determine the genotype of the “unknown” parent. • Quick Lab: Using a Testcross (p. 185)