Genetics Review

1. Genetics Review Chapter 5

2. Mendel’s Investigations • Gregor Mendel was the first to closely examine principles of heredity. • Mendel chose peas to study inheritance because they possess several contrasting traitswithout intermediates.

3. Mendel’s Investigations • Mendel was not aware of the existence of chromosomes or genes. • It is easier to get the big picture of heredity by combining Mendel’s results with what we know about chromosomes.

4. Meiosis • Meiosisis the special type of cell division that produces eggs and sperm. • In meiosis, a diploidcell with two sets of homologous chromosomes will divide so that the daughter cells are haploidand have one set of chromosomes.

5. Meiosis • Chromosomes have replicated during interphase just as in mitosis. • Meiosis actually consists of two separate divisions. • Meiosis I – serves to separate the two versions of the chromosome (homologues). • Meiosis II– serves to separate the two replicas of each version (sister chromatids).

6. Meiosis • Because there is only one replication of DNA but two cell divisions, each of the four daughter cells is haploid– has only one set of chromosomes.

7. Fertilization • Fertilization– reestablishes the diploid chromosome number. • Union of egg and sperm produces a zygote (single cell). • Contains chromosomes of egg and sperm – 2 sets of chromosomes (diploid).

8. Meiosis I • Prophase I – Chromosomes become visible. • The 2 versions of each chromosome pair up and exchange segments. This is called crossing over. • Late in prophase, the nuclear envelope disappears.

9. Meiosis I • Metaphase I – Spindle apparatus forms. • Chromosomes line up in the middle. • Which chromosome faces which pole is random. This is called independent assortment.

10. Meiosis I • Anaphase I– The spindle is complete. • Homologues are pulled apart and move toward opposite poles. • Sister chromatids NOT separated yet. • Each pole has half as many chromosomes (one set rather than two) as the original cell. • Telophase I – the chromosomes gather at the two poles and wait for the onset of meiosis II.

11. Meiosis II • After a brief interphase in which NO DNA synthesis occurs, meiosis II begins. • Meiosis II is just like mitosis except that the sister chromatids are no longer identical due to crossing over.

12. Meiosis II • Prophase II– nuclear envelopes break down as a new spindle forms. • Metaphase II– chromosomes line up in the middle of the cell and spindle fibers bind to both sides of the centromeres.

13. Meiosis II • Anaphase II– spindle fibers contract splitting the centromeres and moving the sister chromatids to opposite poles. • Telophase II– The nuclear envelope reforms around the four sets of daughter chromosomes.

14. Meiosis II • The resulting 4 daughter cells are haploid. • No 2 cells are alike due to crossing over. • In animals, these cells develop directly into gametes(eggs & sperm). • In plants, fungi & many protists they divide mitotically to produce greater numbers of gametes.

16. Meiosis Review http://www.youtube.com/watch?v=D1_-mQS_FZ0&list=FL9N_Px072WuVorSwDfqf-9w&index=55&feature=plpp_video

17. Sex Determination • Sex chromosomes vs. autosomes • Autosomes– chromosomes present in both sexes, do not influence sex. • In humans, females have 2 X chromosomes, while males have and X and a Y.

18. Sex Determination • Some species have XX females and X males. • Others have ZZ males and ZW females. • In others, sex is determined environmentally.

19. Mendel’s Laws • Mendel’s experiments with garden peas resulted in his two laws of inheritance. • Law of segregation • Law of independent assortment

20. Mendel’s Peas • The peas can self-fertilize or outcross. • Mendel could control who the parents were. • Mendel always started with true-breedingparents. • E.g. self-fertilized white flowered parents always produced white flowered offspring.

21. Mendel’s Peas • He could cross true breeding white with true breeding purple – this is the parental generation. • Resulting in all purple offspring – this is the F1 generation.

22. Mendel’s Peas • Allowing the hybrid F1 generation to self pollinate gives the F2 generation with 3 purple: 1 white offspring. • He kept careful quantitative records that allowed him to find patterns.

23. Mendel’s Law of Segregation • Mendel’s explanation of the 3:1 ratio of purple (dominant) to white (recessive) flowers resulted in the Law of Segregation.

24. Mendel’s Law of Segregation • Alternative versions of genes account for variations in inherited characters. • Two versions of the flower color geneare purple & white. • We now call these versions alleles.

25. Mendel’s Law of Segregation • For each character, an organism inherits two alleles, one from each parent. • Mendel deduced this without knowledge of chromosomes! • If there are two different alleles present only one of them – the dominant allele – determines the appearance.

26. Mendel’s Law of Segregation • Mendel’s Law of Segregation– the two alleles for a heritable character separate during gamete formation and end up in different gametes. • Each egg or sperm will contain either one of the two alleles, but not both!

27. Genetic Terms • Homozygous– both alleles are the same. • PP homozygous dominant – purple flowers. • pp homozygous recessive – white flowers. • Heterozygous– two different alleles. • Pp heterozygous, shows dominant, purple color.

28. Genetic Terms • Genotype– the alleles that are actually present. • PP, Pp, pp • Phenotype– the physical appearance of the organism. • Purple or white flowers.

29. Genetic Terms • Monohybrid cross– crossing two individuals that are heterozygous for one particular trait. • Pp X Pp • Dihybrid cross– crossing two individuals that are both heterozygous for two separate traits. • YyTt X YyTt

30. The Testcross • Given a purple flowered pea plant with unknown parents, we will cross it to a homozygous recessive (white) individual to determine its genotype.

31. The Law of Independent Assortment • Following two traits at once: • Yellow (Y) vs. green (y) • Tall (T) vs. short (t) • Cross true-breeding yellow, tall (YYTT) with true-breeding green, short (yytt) to get F1 individuals that are dihybrids (het for both traits – YyTt).

32. The Law of Independent Assortment • Each pair of alleles separates independentlyof other pairs during gamete formation. • At least as long as the pairs of alleles are on separate chromosomes.

33. Complexities • Mendel was fortunate to have chosen a simple system for study. • In reality, there are a number of complicating factors.

34. The Spectrum of Dominance • The traits that Mendel examined showed complete dominance. • The heterozygotes looked just like homozygous dominant individuals.

35. The Spectrum of Dominance • Codominanceoccurs when both alleles affect the phenotype in separate, distinguishable ways. • Both phenotypes are expressed. • Not an intermediate. • AB blood types

36. The Spectrum of Dominance • In incomplete dominance, the phenotype of a heterozygote appears to be intermediate to, or distinct from, the homozygous dominant and homozygous recessive conditions.

37. Multiple Alleles • Most genes actually have more than two different alleles. • Human Blood Type – 3 different alleles. • IA, IB, i • IAIA, IAi Type A blood • IBIB, IBi Type B blood • IAIB Type AB blood • ii Type O blood

38. Pleiotropy • Pleiotropyis a property where a gene has more than one effect on the phenotype of an organism. • The gene that causes sickle cell disease also conveys some resistance to malaria.

39. Epistasis • Epistasis(from the Greek word for stopping) – one gene can alter the phenotypic expression of another gene.

40. Polygenetic Inheritance • Polygenetic inheritance- Some traits have more than one gene contributing to a phenotype – like skin color in humans. • Alleles have a cumulative effect.

41. Environmental Effects • Some traits can be affected by the environment. • Exposure to sunlight affects skin color in humans. • Nutrition affects height in humans. • Soil acidity affects color in hydrangea flowers.

42. Mendel & Modern View of Heredity • Mendel’s fundamental principles of heredity can be expanded to understand the more complex issues. • These principles can be applied to any living organism.

43. The Chromosomal Theory of Inheritance • Genes have specific positions (loci) on chromosomes. • Chromosomes undergo independent assortment and segregation.

44. Sex-Linked Genes • Genes located on the sex chromosomes (X or Y – usually X) are called sex-linked genes. • Fathers pass on a sex-linked genes only to daughters (sons only receive the Y).

45. Sex-Linked Genes • Color-blindness is a sex-linked trait in humans. • Much more common in males. • Males only need to inherit one recessive allele • Females need to inherit two – one from each parent.

46. Experimental Evidence • T.H. Morgan’s experiments provided the first evidence of the association between a specific gene & chromosome.

47. Experimental Evidence • White eyes in fruit flies are recessive to red eyes. • White eyes found only in males in F2 • Eye color gene located on X chromosome.

48. Autosomal Linkage • Linked genesare located close together on the same chromosome. • They are usually inherited together. • They do not follow the rule of independent assortment!

49. Linked Gene Experiment • Two recessive mutant traits: • Black rather than grey bodies • Vestigial rather than normal size wings • Parental phenotypes: • Grey, normal wings • Black, vestigial wings

50. Linked Gene Experiment • Testcross produced mostly the parental phenotypes.