Heredity

1. Heredity Chapters 13, 14, and 15

2. Meiosis and Sexual Life Cycles Chapter 12

3. An introduction to Heredity • Offspring acquire genes from parents by inheriting chromosomes. • What are genes? • Segments of DNA that each code for specific traits. • “Like begets like”, more or less • Sexual vs. Asexual Reproduction? • Asexual reproduction produces a “clone” • Sexual reproduction results in greater variation. • why?

4. Sexual Life Cycles: Alternation of Fertilization and Meiosis Human Life Cycle: • Somatic cells have 46 chromosomes • Chromosomes differ by their size, positions of the centromeres, and banding patterns. • What are karyotypes? • A display that pairs homologous chromosomes of an individual • Sex chromosomes vs. autosomes

5. Gametes – sex cells that have 22 autosomes and 1 sex chromosome • Haploid cells vs. Diploid cells • Fertilization (syngamy) is the union of two gamates resulting in a fertilized egg (zygote) • Mitosis vs Meiosis • Mitosis produces somatic cells • Meiosis produces gametes

7. What does n and 2n represent?

8. Similarities and Differences to Mitosis?

11. Meiosis • Unlike mitosis, it divides twice (meiosis I and meiosis II) • Meiosis I – homologous chromosomes separate • Meiosis II – sister chromatids separate (analogous to mitosis)

12. Prophase I • Begins like prophase of mitosis: • Nucleus disappears • Chromatin condense into chromosomes • Key difference is that Homologous chromosomes pair (synapsis) • These pairs are called tetrads (group of 4 chromatids) • Crossing Over occurs at sites called chiasmata(referred to as synaptonemal complex)

13. Metaphase I • Homologous pairs of chromosomes spread across metaphase plate • Microtubules are attached to kinetochores on one member of each pair

14. Anaphase I • Homologous tetrads uncouple and are pulled to opposite poles of cell

15. Telophase I • Chromosomes reached opposite poles • Each pole will form new nucleus with half the number of chromosomes from parent cell, but each chromosomes contain two chromatids. • At this point many species begin cytokinesis while others delay it until after meiosis II. A short interphase II may begin, but no replication of chromosomes occurs.

16. Prophase II • Nuclear Envelope disappears • Spindle develops • No chiasmata or crossing over (as in prophase I)

17. Metaphase II • Chromosomes align singly on the metaphase plate (no tetrads) • Just like mitosis, except now only half the number of chromososomes.

18. Anaphase II • Chromosomes pulled apart into chromatids • Chromatids (now chromosomes) migrate to opposite poles • Like mitosis, except with only half the chromososomes

19. Telophase II • Nuclear Envelope reappears and cytokinesis occurs • End result = 4 haploid cells (1/2 the number of chromosomes and consists of only one chromatid) • Later (in Interphase) a 2nd chromatid is replicated for each, but cell will still only have half the number of chromososomes.

21. Summary (f 13.7)

23. Genetic Variation • In meiosis there is a reassortment of genetic material called genetic recombination which originates from three events: • Crossing Over • Independent Assortment of homologues • Random joining of gametes

24. Crossing Over • During Prophase I • Nonsister chromatids of homologous chromosomes exchange pieces of genetic material (what is the result?) • Each homologue would no longer entirely represent a single parent

25. Independent Assortment of Homologues: • During metaphase I • Tetrads of homologous chromosomes separate into chromosomes that go to opposite poles. • Which chromosomes goes where depends on orientation and therefore seperation is random for each tetrad. • Maternal and Paternal chromosomes are inner mixed at each pole

26. Random Joining of Gametes • Which sperm fertilizes which egg is to a large degree a random event • Can be affected by genetic composition of the gamete (i.e. some sperm may be faster swimmers)

27. Why Do Cells Divide? (revisited) • Relative Size to surface area ratio: • When there is a large surface area relative to volume ratio, then the cell can efficiently react with the outside environment (i.e. adequate amounts of O2 can diffuse in while waste products can rapidly be eliminated) • Limited Capability of the nucleus: • Genome “controls” cell by producing needed substances which regulate activities. The capacity of the genome is limited by its finite amount of genetic material

28. Mendel and the Gene Idea Chapter 14

29. Probability Review: • The rule of multiplication:to determine the probability of two or more independent events occurring together, you merely multiply the probabilities of each event happening separately • Ex: For 2 consecutive coin tosses the chance to get heads each time is ½. So for getting two heads is ½ x ½ = ¼. Getting three heads is ½ x ½ x ½ = 1/8

30. Gene Allele Locus Homologous pair Dominant Recessive Homozygous Heterozygous Genotype Phenotype P, F1 and F2 generations Monohybrid Cross Dihybrid Cross Hybrids Testcross Terminology Review:

31. Mendel’s Discoveries • Experimental and quantitative approach to genetics brought about Mendel’s discovery (what did he use?) • Law of Segregation • Law of Independent Assortment • Dominance and Recessive

32. Mendel’s Hypothesis: • Alternative versions of genes (different alleles) account for variations in inherited characters. • For each character, an organism inherits two alleles (one from each parent). • If the two alleles differ, then one, the dominant allele if fully expressed in the organism’s appearance; the other, the recessive allele, as no noticeable effct on the organism’s appearance. • The two alleles for each character segregate during gamete production

33. Law of Segregation • Two Alleles for a character are packaged into separate gametes. • Each parent will contribute one allele for each trait to the zygote • Random segregation occurs during gamete formation • Determined by performing Monohybrid crosses • Example: a heterozygous pea plant for height would make one gamete with the trait for being tall while the other would have the trait for being short.

34. Law of Independent Assortment • Each pair of alleles segregates into gametes independently • Factors for different characteristics are distributed to reproductive cells independently. • Determined by performing Dihybrid crosses • Example: seed shape and seed color are inherited independently of each other.

35. Punnet Square Practice: • Round Seeds are dominant over wrinkled seeds. Cross: Pure round seeds with Pure wrinkled seeds. • Green pods are dominant over yellow pods. Cross: Hybrid green pods with Hybrid Green pods. • Axial flowers are dominant over terminal flowers. Cross: Hybrid axial flowers with Pure axial flowers.

36. Colored seed coats are dominant over white seed coats. Cross:Hybrid colored seeds with Hybrid colored seeds. • Horned cattle is dominant over the hornless condition. Cross: Pure hornless with Hybrid Horned. • Black fur is dominant over white fur in guinea pigs. Cross: Hybrid black with Hybrid Black. • Long wings are dominant over curly wing in fruit flies. Show all the different crosses that can produce hybrid long wing individuals.

37. In Sheep, black wool is recessive to white wool. What happens when you mate a black ram to a heterozygous ewe? Use W to represent dominant white, w for the recessive black allele. • What is the genotype of the ram? • What is the genotype of the ewe? • What are the genotypes of the offspring? • What is the genotypic ratio of the offspring? • What are the phenotypes of the offspring?

38. Cross a heterozygous black female angus to a heterozygous bull (B = black; b = red) • What is the genotype of the female angus? • What is the genotype of the bull? • What are the genotypes of the offspring? • What is the genotypic ratio of the offspring? • What are the phenotypes of the offspring?

39. In cattle, the polled gene (P) is dominant over the horned gene (p). A polled cow with genotype (Pp) is mated to a horned bull. ½ of the offspring were polled and ½ were horned? • What is the genotype of the bull? • Whare the genotypes of the offspring?

40. Cross a heterozygous polled black angus bull (BbPp) to a heterozygous polled black angus cow (BbPp). • Use a punnet square to determine genotype and phenotype of offspring.

41. If plant heterozygous for all three characters “self-fertilizes”, what proportion of the offspring would be expected to be as follows? (Use probability rules instead of a huge punnet square) • Homozygous for the three dominant traits? • Homozygous for the three recessive traits? • Heterozygous for the three recessive traits? • Homozygous for axial and tall, heterozygous for seed shape?

42. What is the probability that each of the following pairs of parents will produce the indicated offspring? (assume independent assortment of all gene pairs) • AABBCC x aabbcc  AaBbCc • AABbCc x AaBbCc  AAbbCC • AaBbCc x AaBbCc  AaBbCc • aaBbCC x AABbcc  AaBbCc

43. Extending Mendelian Genetics: • “it was brilliant (or lucky) that Mendel chose pea plant characters because they turned out to have relatively simple genetic basis: each character studied was determined by one gene, for which there are only two alleles, one completely dominant to the other • The relationship between genotype and phenotype is rarely so simple…

44. Incomplete Dominance • True dominant and recessive behaviors are not exhibited • The heterozygous condition produces a blending of the individual expressions of the two alleles. • EX: Snapdragons: R = red flowers; r = white flowers and Rr = Pink flowers Sometimes both alleles are written with the same upper-case or lower-case letter but with a prime or a superscript number (R and R’ ) H = Straight Hair; H1 = Curly Hair; HH1 = intermediate phenotype expressed

45. Co-Dominance • A type of incomplete dominance • Both inherited alleles are completely expressed • EX: M and N blood types produce molecules that appear on the surface of human red blood cells. M produces 1 type of blood, while N produces another type. Those with MN produce both types of molecules

46. Three important points of Dominance/recessivness relationships: • Range from complete dominance, through various degrees of incomplete dominance, to co-dominance. • Reflect mechanisms by which specific alleles are expressed in phenotype and do not involve the ability of one allele to subdue another at the level of the DNA. • Do not determine the relative abundance of alleles in a population.

47. Multiple Alleles • Genes that exist in populations in more than two forms • EX: A, B, O blood types • This refers to a type of carbohydrate found on the surface of red blood cells; type O is the absence of the carbohydrate. • How does this connect with things we’ve learned earlier?

48. Connecting to blood transfusions • What would happen if an individual with type B or O blood was given type A blood? • Their immune system would identify the wrong carbohydrate as a foreign substance (antigens) and produce antibodies to attack which results in agglutination (clumping of the blood cells which could lead to death). • AB is the universal recipient • O is the universal donor

49. Epitstasis • One gene affects the phenotypic expression of a second gene • Frequently occurs in expression of pigmentation • One gene turns on (or off) the production of pigment while a second controls either the amount of pigment produced or the color of pigment. • If the first gene turns it off, then the second gene has no effect, regardless of what it codes for. • EX: Mouse fur color (f14.11)

50. Pleiotropy • Single gene has more than one phenotypic expression • EX: Gene in pea plants for round (R) or wrinkled (r) seeds also influences the phenotypic expressions of starch metabolism and water absorption. Rounds seeds code for greater conversion of glucose to starch, while wrinkled seeds have more uncoverted glucose. Higher concentration of glucose then effects osmotic gradient… • Many disease-causing genes exhibit pleiotropy (sickle-cell anemia