1. UNIT 3
GENETICS

2. Chapter 6
Meiosis and Mendel

11. “hairy ears” (hypertrichosis)- due to holandric gene
“hairy ears” (hypertrichosis)- due to holandric gene (Y chromosome)-only occurs in males. Appears in all sons.

13. Polydactyly- having extra fingers

14. Wendy the Whippet. Double mutation of both myostatin gene.

15. A 31-year-old Chinese man (shown on right) whose body is 96 per cent coated in hair has an extra chunk of DNA that could explain his condition – called congenital generalized hypertrichosis terminalis (CGHT).

19. ‘Myths and Mutants’
Work with animals led to recognition of heritable traits in humans
Stories of heritable deformities in humans appear in myths and legends (e.g. cyclops, giants, etc.)
60 birth defects on Babylonian clay tablets (5000 years ago)
Knowledge of traits played role in shaping social customs and mores (e.g. choosing a wife/husband)

20. I. Chromosomes and Meiosis (6.1)
A. You have many types of specialized cells in your body
1. Cells can be divided into two types
a. Somatic Cells- body cells. Make up most of your body tissues and organs.

21. b. Germ Cells- cells in your reproductive organs, the ovaries and testes
1). Can develop into gametes (called sex cells)
  2). Form egg and sperm cells

22. 2. Gametes have DNA that is passed to offspring in chromosomes

23. B. Each species has characteristic number of chromosomes per cell.  
1. Chromosome number does not seem to be linked to complexity of organism.
2. Organisms differ from each other because of way genes are expressed, not because they have different genes.

24. Nucleosomes, or “beads on a string” (10-nm fiber)
Fig a
Nucleosome
(10 nm in diameter)
DNA double helix (2 nm in diameter) H1
Histone tail
Histones
DNA, the double helix
Histones
Nucleosomes, or “beads on a string” (10-nm fiber)

25. Looped domains (300-nm fiber) Metaphase chromosome
Fig b
Chromatid
(700 nm)
30-nm fiber
Loops
Scaffold
300-nm fiber
Replicated chromosome (1,400 nm)
30-nm fiber
Looped domains (300-nm fiber)
Metaphase chromosome

26. II. You cells have autosomes and sex chromosomes
A. Your body has 23 pairs of chromosomes
1. Each pair referred to as homologous pair
2. Homologous chromosomes are two chromosomes- one from father and one from mother

27. B. Autosomes- chromosome pairs 1-22 are called autosomes (are
B. Autosomes- chromosome pairs 1-22 are called autosomes (are homologous)

28. C. Sex chromosomes- pair of chromosomes
1. Directly control development of sexual characteristics
2. Very different in humans (not homologous)
    
a. X chromosome- female
b. Y-chromosome- male
Sex chromosomes

29. D. Body cells are diploid; gametes are haploid
1. sexual reproduction involves fusion of two gametes
  a. results in genetic mixture of both parents
b. Fusion of egg and sperm called fertilization
c. Egg and sperm only have half usual number of chromosomes 

30. 2. Diploid and Haploid cells
  a. Body cells are diploid (two copies of each chromosome)
  b. Gametes are haploid (have one copy of each chromosome) 

31. 3. Maintaining the correct number of chromosomes is important to survival of organisms

32. 3. Maintaining the correct number of chromosomes is important to survival of organisms
 4. Germ cells (sex cells) undergo process of meiosis to form gametes   
a. diploid cell divides into haploid cell
b. Sometimes called reduction division
Haploid cells

33. II. Process of Meiosis (6.2)
A. Cells go through two rounds of division in meiosis
1. Meiosis produces four haploid cells from one diploid cell
2. Process involves two rounds of cell division- Meiosis I and Meiosis II. 

34. B. Homologous Chromosomes and sister Chromatids
1. Need to distinguish between the two to understand meiosis
2. Homologous chromosomes- two separate chromosomes- one from mother, one from father.
a. very similar to each other- same length and carry same genes 

35. b. Each half of duplicated chromosome is called a chromatid
 b. Each half of duplicated chromosome is called a chromatid. (together called sister chromatids)
1). Homologous chromosomes divided in meiosis I
2). Sister chromatids not divided until meiosis II
Sister chromotids

36. C. Meiosis I (first of two phases)
1. Occurs after DNA has been replicated
2. Divides homologous chromosomes in four phases 

37. D. Meiosis II (second of two phases)
1. Divides sister chromatids in four phases
2. DNA is not replicated between meiosis I and meiosis II 

38. E. Meiosis differs from mitosis in significant ways.
1. Meiosis has two cell divisions while mitosis has one.
2. In mitosis, homologous chromosomes never pair up
3. Meiosis results in haploid cells; mitosis results in diploid cells. 

39. F. Haploid cells develop into mature gametes
1. gametogenesis- production of mature gametes

40. 2. Differs between the sexes
a. Males produce 4 equal sperm cells
b. Females produce one large egg and smaller polar bodies that are eventually broken down  

41. Is it Mitosis, Meiosis or both??
No pairing of homologs occurs
Two divisions
Four daughter cells produced
Associated with growth and asexual production
One division
Daughter cells are not identical to the parent cell
Involves duplication of chromosomes
Daughter cells are diploid
Chromosome number is maintained
Daughter cells are haploid
Daughter cells are identical to parent cell
Associated with sexual reproduction
Two daughter cells produced
Produces gametes
Starts with a diploid cell

42. III. Mendel and Heredity (6.3)
A. Mendel laid the groundwork for genetics
1. Traits are distinguishing characteristics that are inherited.
2. Genetics is the study of biological inheritance patterns and variation.
3. Gregor Mendel showed that traits are inherited as discrete units.
4. Many in Mendel’s day thought traits were blended. 

43. B. Mendel’s data revealed patterns of inheritance
1. Mendel studied plant variation in a monastery garden
2. Mendel made three key decisions in his experiments
a. Control over breeding
b. Use of purebred plants
c. Observation of “either- or” traits (only appear two alternate forms)

44. 3. Experimental design
a. Mendel chose pea plants because reproduce quickly and could control how they mate 

45. b. Crossed purebred white-flowered with purebred purple-flowered pea plants.
1). Called parental, or P generation
2). Resulting plants (first filial or F1 generation) all had purple flowers 

46. c. Allowed F1 generation to self-pollinate
  1). Produced F2 generation that had both plants with purple and white flowers)
2). Trait for white had been “hidden”, it did not disappear. 

47. d. He began to observe patterns- Each cross yielded similar ratios in F2 generation (3/4 had purple, and 1/4 white)   

48. 4. Mendel made three important conclusions
  a. Traits are inherited as discrete units (explained why individual traits persisted without being blended or diluted over successive generations)

49. b. Two other key conclusions collectively called the law of segregation
1). Organisms inherit two copies of each gene, one from each parent
2). Organisms donate only one copy of each gene in their gametes (two copies of each gene segregate, or separate, during gamete formation

51. IV. Traits, Genes, and Alleles (6.4)
A. The same gene can have many versions
1. gene- a “piece” of DNA that provides a set of instructions to a cell to make a certain protein.

52. Most genes exist in many forms (called alleles)
You have two alleles for each gene` 

53. 2. Homozygous- means two of same allele
3. Heterozygous- two different alleles 

54. B. Genes influence the development of traits
1. Genome- is all the organisms genetic material
2. Genotype- refers to genetic makeup of a specific set of genes
3. Phenotypes- physical characteristics of organism (wrinkled or round seeds)

55. C. Dominant and Recessive Alleles
1. Dominant alleles- allele that is expressed when two different alleles or two dominant alleles are present (use capital letter to represent)

56. 2. Recessive alleles- only expressed if have
2. Recessive alleles- only expressed if have two copies of recessive present (use small-case letter to represent)
3. Homozygous dominant = TT
4. Heterozygous = Tt
5. Homozygous recessive = tt 

57. D. Alleles and Phenotypes
1. Both homozygous dominant and heterozygous genotypes yield a dominant phenotype.
2. Most traits occur in a range and do not follow simple dominant-recessive patterns

58. V. Traits and Probability (6.5)
A. Punnett squares illustrate genetic crosses
1. Used to predict possible genotypes resulting from a cross
a. Axes of grid represent possible gamete genotypes of each parents
b. Boxes show genotypes of offspring
c. Can determine ratio of genotypes in each generation

59. Punnett Square parents gametes Dominant Allele Possible offspring
Recessive allele
homozygous
heterozygous

60. B. Monohybrid cross involves one trait
1. Homozygous dominant X Homozygous recessive
Genotypic ratio
100% Ff
Phenotypic ratio
100% purple

61. 2. Heterozygous X Heterozygous
Genotypic ratio
1:2:1
Phenotypic ratio
3:1

62. 3. Heterozygous X Homozygous recessive
Genotypic ratio
1:1
Phenotypic ratio

63. C. Test Cross- a cross between organism with an unknown genotype and an organism with a recessive phenotype

64. D. Dihybrid cross involves two traits
1. Mendel also conducted dihybrid crosses- wondered if both traits would always appear together or if they would be expressed independently of each other
2. Mendel discovered phenotypic ratio in F2 generation as always 9:3:3:1 regardless of combination traits he used

66. 3. Mendel’s dihybrid crosses led to his second law,the law of independent assortment.
 4. The law of independent assortment states that allele pairs separate independently of each other during meiosis 

67. E. Heredity patterns can be calculated with probability
1. probability - the likelihood that a particular event will happen
2. Probability applies to random events such as meiosis and fertilization 

68. VI. Meiosis and Genetic Variation (6.6)
A. Sexual reproduction creates unique gene combinations
1. Sexual reproduction creates unique combination of genes
a. independent assortment of chromosomes in meiosis
b. random fertilization of gametes

69. 2. 223 possible sperm or egg cells produced
223 X 223 = about 70 trillion different combinations of chromosomes

70. B. Crossing over during meiosis increases genetic diversity
1. crossing over - exchange of chromosome segments between homologous chromosomes during Prophase I of Meiosis I
2. Results in new combination of genes 

71. C. Linked genes - genes located on the same chromosome inherited
C. Linked genes - genes located on the same chromosome inherited together.
1. Closer together they are high chance of inheriting together
2. If genes far apart, crossing-over may separate them
3. Gene linkage used to build genetic map of many species