1. Introduction to Genetics
Chapter 6

2. Chromosomes
Each individual has two sets of chromosomes
When gametes are produced, the two sets separate & each gamete gets only one set

3. Each organism gets one set from its mom & one set from its dad
The two sets are homologous – each chromosome from the dad matches one from the mom

4. A cell that has both sets is said to be diploid (2N) (also called somatic cells, or regular body cells)
A cell that has only one set (gametes = sperm and egg) are said to be haploid (N)
Humans cells have 46 chromosomes (diploid 2N)

5. Sperm and egg cells have 23 chromosomes each (haploid N)
The process that separates the two sets of chromosomes and makes the gametes is called Meiosis

6. 22 pairs of chromosomes are numbered based on size – the largest homologous pair is #1, the smallest homologous pair is #22
These are called autosomes
The last pair are not always homologous – they are the sex chromosomes

7. The sex chromosomes are the X and Y chromosomes
Females have 2 X’s (XX)
Males have an X & Y (XY)
The X chromosome has genes related to sex development & other genes & is a normal size
Y is the smallest chromosome

8. If you look on page 169 of your text, you will see Figure 6
If you look on page 169 of your text, you will see Figure 6.1 – an actual picture of the 46 chromosomes of a human male, paired, numbered and in order
The sex chromosomes are circled

9. Now, let’s take a look at the process that starts with cells that have 46 chromosomes and makes our sex cells with only half that amount
Meiosis (my-O-sis)

10. Meiosis
Meiosis is a process of reduction division – the # of chromosomes is cut in half through the separation of the homologous chromosomes

11. Meiosis
Meiosis is similar to mitosis, but is very different in the result
There are 2 parts – Meiosis I and Meiosis II

12. Meiosis
Meiosis I separates the homologous chromosomes
Meiosis II separates sister chromatids (like mitosis)
There are 4 phases in each

13. Meiosis
The main function of meiosis is to make 4 genetically different gametes (in mitosis, we made two identical daughter cells)

15. Meiosis I
Interphase – growth, DNA replication, preparation
Prophase I – homologous chromosomes pair up to form a tetrad

16. Crossing Over
During Prophase I, a VERY IMPORTANT process called Crossing Over occurs – this ensures genetic variation in the gametes

17. Crossing Over
During crossing over, parts of one chromosome switch with corresponding parts of the other chromosome

18. Meiosis I
Metaphase I – homologous tetrads line up in middle of the cell, spindle fibers attach to chromosomes

19. Meiosis I
Anaphase I – Cell elongates, fibers pull apart the chromosomes

20. Meiosis I
Telophase I – the cell separates into 2 cells, nuclear membranes reform, cytokinesis occurs

21. Draw and explain each phase in Meiosis I
Quick Check
Draw and explain each phase in Meiosis I

22. Meiosis I
Meiosis I results in 2 haploid (N) cells (each with half the number of chromosomes as the original cell)

23. Meiosis II
Prophase II – Nuclear membranes disappear, spindle fibers appear

24. Meiosis II
Metaphase II – chromosomes line up in middle of cells

25. Meiosis II
Anaphase II – sister chromatids are pulled apart by the spindle fibers

26. Meiosis II
Telophase II – Nuclear membranes reform
Cytokinesis - Each cell separates into 2 new cells, genetically different from each other & the parent cell

27. Meiosis II
There are now 4 cells total – the gametes (sperm & eggs)

28. Egg Production
In many females, the divisions of meiosis I & II are not even – resulting in one egg cell & 3 polar bodies – only the egg cell is used in reproduction

29. Egg Production

30. Meiosis Overview

31. Gregor Mendel
Every living thing has a set of characteristics inherited from its parents
The study of this heredity is called genetics

32. Peas
Gregor Mendel was an Austrian Monk who studied science & mathematics
He was in charge of the monastery garden - peas

33. Mendel had many different types of pea plants – they were true-breeding, meaning they would produce offspring identical to themselves if they self-pollinated

34. Mendel cross-pollinated the different types of pea plants
The resulting seeds were crosses between two plants
Mendel was surprised by what he observed

35. Genes & Dominance
Mendel studied seven different traits – seed shape, color & coat color, pod shape & color, flower position and plant height

36. A trait is a specific characteristic that varies from one individual to another
Mendel crossed plants with different traits & studied their offspring

37. He called the two original plants the Parental Generation (P)
The offspring were called the first filial, or F1 generation
The offspring were called hybrids

38. People in Mendel’s time thought that offspring were a blend of their parents
Mendel did not find this to be true
In fact, the offspring had traits of only one parent!

39. From these results, Mendel drew two conclusions
1. Biological inheritance is determined by factors passed from one generation to the next

40. We call these factors genes
Each trait Mendel studied had one gene, with two different forms – for instance height has tall and short
These forms are called alleles

41. 2. The principle of dominance – some alleles are dominant and some are recessive
EX: Tall is dominant and short is recessive
Had the recessive traits disappeared?

42. Segregation
Mendel then allowed each F1 plant to self pollinate
He called these offspring the F2 generation
What do you think happened?

43. The recessive traits reappeared!
About ¼ of the offspring had the recessive trait
Mendel assumed the F1 plant had 2 alleles for the trait (1 dominant, 1 recessive)

44. The dominant allele masked the recessive allele
But since the F2 gen. had some short plants, the two alleles must have been separated from each other

45. How did that separation occur?
Mendel assumed that the alleles were separated from each other during the formation of sex cells (gametes)

46. When gametes are formed, the alleles for each gene separate from one another
Each gamete gets only one copy of the gene, not both
That is how the recessive trait reappeared!

47. Why did Mendel’s F1 generation show traits of only one parent?
Quick Check
Why did Mendel’s F1 generation show traits of only one parent?

48. Probability Probability is the likelihood of an event occurring
Flipping a coin – 50% heads, 50% tails
Probability can be used to predict the outcomes of genetic crosses

49. Punnett Squares
A Punnett Square is a diagram used to determine the expected results from a genetic cross

50. In drawing a Punnett Square, we use letters to represent alleles
Capital letters are used for dominant alleles
Lower case letters are used for recessive alleles

51. An organism with 2 dominant alleles is said to be homozygous dominant
An organism with 2 recessive alleles is homozygous recessive

52. An organism with 1 dominant and 1 recessive allele is heterozygous
An organism’s physical appearance is called the phenotype

53. An organism’s genetic makeup is called genotype
How can 2 plants have the same phenotype but different genotypes?

54. Independent Assortment
Genes that segregate independently do not affect one another
For instance, a pea plant can get a yellow color but still be wrinkled or vice versa

55. Incomplete Dominance
Some alleles are neither dominant or recessive!
The heterozygous phenotype is a blend of the two homozygous forms (pink flowers in some species)

56. Co-Dominance
Some alleles do not blend but are both expressed in the heterozygous form – a speckled black & white chicken is heterozygous

57. Multiple Alleles
Some genes have more than 2 alleles
Each individual can still only have 2 total, but there is more variety – fur color in rabbits

58. Multiple Alleles – Rabbit Fur
The alleles are – B, B’, b’, & b
Brown fur = BB, BB’, Bb’, Bb
Chinchilla = B’B’, B’b’, B’b
Himalayan = b’b’,b’b
Albino = bb only

59. Multiple Alleles – Rabbit Fur
Ex: If a brown rabbit (Bb) mates with a chinchilla rabbit (B’b’) what are the possible offspring?

60. Multiple Alleles – Rabbit Fur
Ex: If a chinchilla rabbit (B’b) mates with a himalayan rabbit (b’b), what are the possible offspring?

61. Multiple Alleles – Blood Type
Human blood type exhibits multiple alleles & there are 4 possible phenotypes
Alleles = A, B, & O
A & B are co-dominant
O is recessive

62. Multiple Alleles – Blood Type
Blood Type A – AA or AO
Blood Type B – BB or BO
Blood Type AB – AB only
Blood Type O – OO only
(Note – we are not talking about + or – blood types)

63. Multiple Alleles – Blood Type
EX: A woman with blood type AB marries a man with blood type O. What are their possible offspring (phenotype and genotype)?

64. Multiple Alleles – Blood Type

65. Multiple Alleles – Blood Type

68. Polygenic Traits
Some traits are controlled by more than one gene
Skin color in humans is controlled by at least 4 different genes – range of colors from light to dark

69. Quick Check
A heterozygous blue haired, weeble mates with an identical weeble. What is the phenotypic ratio in the weeblets? (pink hair is recessive)

70. Dihybrid Crosses
So far, we have been talking about monohybrid crosses (only 1 trait)
Often we look at more than 1 trait when we are considering a genetic cross.

71. Dihybrid Crosses
So for pea plants, we can look at flower color and plant height.
The idea is the same, but the square is bigger (4x4)
Lets do an example….

72. Dihybrid Crosses
A short purple pea plant (hhPp) is mated with a tall white pea plant (Hhpp).
1st, determine the gametes – they can only have 1 of each letter (hP, hP, hp, hp) &
(Hp, Hp, hp, hp)

73. Dihybrid Crosses
Use the gametes for the top (hP, hP, hp, hp) & side
(Hp, Hp, hp, hp) of the square
Fill in the 16 boxes

74. Dihybrid Crosses
EX: A heterozygous tall/purple plant (HhPp) is crossed with a homozygous short/white plant (hhpp). What are the possible offspring?

75. Where are Genes Located?
Gregor Mendel did not know where the genes were located in the cell
We now know that genes are located on the chromosomes

76. Gene Linkage
On any given chromosome, there can be hundreds or thousands of genes
Genes on different chromosomes separate independently

77. Gene Linkage
However, if two genes are on the same chromosome, they are said to be linked – meaning they do not separate independently

78. Gene Linkage
If genes are linked, they will usually be inherited together
EX: if the genes for blue hair & blue eyes are linked in weebles, a weeble that has blue hair will usually have blue eyes too!

79. Gene Linkage
We can figure out where the genes are on a chromosome by determining how often they are NOT inherited together – gene map

80. Gene Linkage
If two genes on the same chromosome are not close together, they will separate and recombine more often due to crossing over

81. Gene Linkage
If two genes are very close together on a chromosome, they will rarely separate and recombine due to crossing over