1. Meiosis & Mendel’s Genetics
Chapter 6

2. Genes, chromosomes & numbers
Genes do not exist free in the nucleus of a cell; they are lined up on chromosomes & they carry your genetic information
Typically, a chromosome can contain a thousand or more genes along its length.

3. You have body cells and gametes.
Two types of cells:
1. Somatic Cells: body cells
2. Sex Cells: gametes
Gametes have half the number of chromosomes that body cells have.

4. Body cells are also called somatic cells.
sex cells (sperm)
sex cells (egg)

5. Gametes are Sex Cells Germ cells develop into gametes.
Germ cells are located in the ovaries and testes.
Gametes are sex cells: egg and sperm.
Gametes have DNA that can be passed to offspring.

6. Your cells have autosomes and sex chromosomes.
Your body cells have 23 pairs of chromosomes.
Homologous pairs of chromosomes have the same structure.
For each homologous pair, one chromosome comes from each parent.
Chromosome pairs 1-22 are autosomes.
Sex chromosomes, X and Y, determine gender in mammals.

7. Body cells are Diploid (2n)
Fertilization between egg and sperm occurs in sexual reproduction.
Diploid (2n) cells have two copies of every chromosome.
Half the chromosomes come from each parent.

8. Gametes are Haploid cells (n)
Haploid cells are called sex cells
Male gametes are called sperm
Female gamete are called eggs
When a sperm fertilizes an egg the resulting zygote then has the diploid number of chromosomes

9. Cell numbers Haploid (n) cells have one copy of every chromosome
Gametes have 22 autosomes and 1 sex chromosome

10. In the body cells of animals and most plants, chromosomes occur in pairs
Chromosome number must be maintained in animals.
Many plants have more than two copies of each chromosome.
Mitosis and meiosis are types of nuclear division that make different types of cells.
Mitosis makes more diploid cells.

11. Diploid and haploid cells
Chromosome Numbers of Common Organisms
Organism
Body Cell (2n)
Gamete (n)
Fruit fly 8 4
Garden pea 14 7
Corn 20 10
Tomato 24 12
Leopard Frog 26 13
Apple 34 17
Human 46 23
Chimpanzee 48 24
Dog 78 39
Adder’s tongue fern
1260
630

12. Meiosis
During meiosis, diploid cells undergo two cell divisions that result in haploid cells.

13. Homologous Chromosomes
The two chromosomes of each pair in a diploid cell are called homologous chromosomes.
Each pair of homologous chromosomes has genes for the same traits

14. Homologous chromosomes
On homologous chromosomes, these genes are arranged in the same order, but because there are different possible alleles for the same gene, the two chromosomes in a homologous pair are not always identical to each other. a A
Terminal
Axial
Inflated D d
Constricted T t
Short
Tall

15. Why Meiosis?
When cells divide by mitosis, the new cells have exactly the same number and kind of chromosomes as the original cells.
This kind of cell division, which produces gametes containing half the number of chromosomes as a parent’s body cell, is called meiosis

16. Why Meiosis?
Meiosis consists of two separate divisions, known as meiosis I and meiosis II.
Meiosis I begins with one diploid (2n) cell.
By the end of meiosis II, there are four haploid (n) cells
This pattern of reproduction, involving the production and subsequent fusion of haploid sex cells, is called sexual reproduction

17. Cells go through two rounds of division in meiosis.
Meiosis reduces chromosome number and creates genetic diversity.

18. Phases of Meiosis Meiosis I:
Prophase I, Metaphase I, Anaphase I & Telophase I
Meiosis II:
Prophase II, Metaphase II, Anaphase II, & Telophase II

19. Meiosis I and meiosis II each have four phases, similar to those in mitosis.
Pairs of homologous chromosomes separate in meiosis I.
Homologous chromosomes are similar but not identical.
Sister chromatids divide in meiosis II.
Sister chromatids are copies of the same chromosome.
homologous chromosomes
sister
chromatids
sister
chromatids

20. Meiosis I occurs after DNA has been replicated.
Meiosis I divides homologous chromosomes in four phases

21. Crossing over during meiosis increases genetic diversity.
Crossing over is the exchange of chromosome segments between homologous chromosomes.
occurs during prophase I of meiosis I
results in new combinations of genes

22. Meiosis II divides sister chromatids in four phases.
DNA is not replicated between meiosis I and meiosis II.

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

24. Gametogenesis is the production of gametes
It differs between females and males
Eggs contribute DNA, cytoplasm, and organelles to an embryo.
During meiosis, the egg gets most of the contents; the other cells from polar bodies.
Sperm become streamlined and motile.
Sperm primarily contribute DNA to an embryo.

25. Genetics
Gregor Mendel

26. Who is Gregor Mendel? The Father of Genetics
It was not until the mid-nineteenth century that Gregor Mendel, an Austrian monk, carried out important studies of heredity—the passing on of characteristics from parents to offspring.
Many in Mendel’s day thought traits were blended

27. Why he succeeded
Mendel was the first person to succeed in predicting how traits are transferred from one generation to the next.
Characteristics that are inherited are called traits.
A complete explanation requires the careful study of genetics—the branch of biology that studies heredity

28. Mendel’s Experiment
Mendel chose to use garden peas in his experiments for several reasons
Garden pea plants reproduce sexually, which means that they produce male and female sex cells, called gametes

29. Mendel’s data revealed patterns of inheritance.
Mendel made three key decisions in his experiments
1. use purebred plants
2. control breeding
3. observe seven “either-or” traits

30. Why did he choose the Garden Pea?
The male gamete formed in the pollen grain & the female gamete in the seed
This process is called fertilization and the result is a fertilized seed called a zygote
The transfer of pollen grains from a male, to a female reproductive organ in a plant is called pollination

31. Mendel used pollen to fertilize selected pea plants.
Parent generation crossed to produce 1st generation
interrupted the self-pollination process by removing male flower parts
Mendel controlled the
fertilization of his pea plants
by removing the male parts,
or stamens.
He then fertilized the female
part, or pistil, with pollen from
a different pea plant.

32. Mendel was a careful researcher
He studied only one trait at a time to control variables, and he analyzed his data mathematically.
The tall pea plants he worked with were from populations of plants that had been tall for many generations and had always produced tall offspring.
Such plants are said to be true breeding for tallness.
Likewise, the short plants he worked with were true breeding for shortness

33. Mendel drew three important conclusions
1. Traits are inherited as individual units.
2. Organisms inherit two copies of each gene, one from each parent.
3. The two copies segregate during gamete formation.
purple
white

34. Mendel’s Monohybrid Crosses
A hybrid is the offspring of parents that have different forms of a trait, such as tall and short height.
Mendel’s first experiments are called monohybrid crosses because mono means “one” and the two parent plants differed from each other by a single trait—height.

35. The First Generation - F1
Mendel selected a six-foot-tall pea plant that came from a population of pea plants, all of which were over six feet tall.
He cross-pollinated this tall pea plant with pollen from a short pea plant.
All of the offspring grew to be as tall as the taller parent.

37. Generation P1 generation: True-breeding Parents
F1 generation: 1st Generation Offspring
Two F1 plants: F2 Generation Offspring

38. Mendel allowed the resulting plants to self-pollinate.
Mendel allowed the tall plants in this first generation to self-pollinate. P1
Among the 1st generation, all plants were tall: F1
Among the 2nd generation, 3/4ths of the plants were tall and 1/4th of the plants were short: F2
Ratio of 3:1

39. Mendel Laws of Heredity
The Rule of Unit Factor:
Mendel concluded that each organism has two factors that control each of its traits.
These factors are genes and that they are located on chromosomes in alternative forms.
We call these different gene forms alleles
An organism’s two alleles are located on different copies of a chromosome—one inherited from the female parent and one from the male parent

40. Mendel Laws of Heredity
The Rule of Dominance:
Mendel called the observed traits as dominant and the disappeared traits as recessive
Tall plants are dominant (Uppercase letters) and short plants are recessive (lowercase letters).
Dominant allele is the trait shown & is always written first
Recessive allele is the hidden trait & is always written second

41. Dominant & Recessive Traits
Seed
shape
Seed
color
Flower
color
Flower
position
Pod
color
Pod
shape
Plant
height
Dominant
trait
axial
(side)
round
yellow
purple
green
inflated
tall
Recessive
trait
terminal
(tips)
wrinkled
green
white
yellow
constricted
short

42. Mendel Laws of Heredity
The Law of Independent Assortment:
States that genes for different traits—(ex: seed shape and seed color)—are inherited independently of each other

43. Mendel Laws of Heredity
The Law of Segregation
every individual has two alleles of each gene and when gametes are produced, each gamete receives one of these alleles.
The way an organism looks and behaves is called its phenotype
The allele combination an organism contains is known as its genotype
An organism’s genotype can’t always be known by its phenotype

44. Genotype & Phenotype
Genotype refers to “gene type” and describes the possible gene combinations.
¼ TT, 2/4 Tt, ¼ tt
Phenotype refers to “gene expression” and describes in words the gene combinations
½ Tall plants
½ Short plants

45. The same gene can have many versions.
A gene is a piece of DNA that directs a cell to make a certain protein.
Each gene has a locus, a specific position on a pair of homologous chromosomes.

46. An allele is any alternative form of a gene occurring at a specific place on a chromosome.
Each parent donates one allele for every gene.
Homozygous describes two alleles that are the same
Heterozygous describes two alleles that are different

47. Homozygous & Heterozygous
Homozygous: An organism is homozygous for a trait if its two alleles for the trait are the same
Traits: TT or tt would be the only possibilities
Heterozygous: An organism is heterozygous for a trait if its two alleles for the trait differ from each other
Traits: Tt

48. Phenotype & Genotype
Law of segregation
Tt ´ Tt cross
Two organisms can look alike but have different underlying allele combinations.
Genotype:
G=1/4 TT, 2/4 Tt, 1/4tt
Ratio: 1:2:1
Phenotype:
P=3/4 Tall, ¼ Short
Ratio: 3:1 F1
Tall plant
Tall plant T t T t F2
Tall
Tall
Tall
Short T T T t T t t t 3 1

49. Punnett Square
In 1905, Reginald Punnett, an English biologist, devised a shorthand way of finding the expected proportions of possible genotypes in the offspring of a cross called the Punnett Square
If you know the genotypes of the parents, you can use a Punnett square to predict the possible genotypes of their offspring.

50. Monohybrid crosses
Heterozygous
tall parent
The Punnett square is a grid system for predicting all possible genotypes resulting from a cross. t T T t Tt T TT Tt T Tt tt Tt tt t t

51. The boxes represent the possible gametes of each parent.
The boxes show the possible genotypes of the offspring.
The Punnett square yields the ratio of possible genotypes and phenotypes.

52. A monohybrid cross involves one trait.
Monohybrid crosses examine the inheritance of only one specific trait.
Purple is dominant to White
Cross a Homozygous purple with a white
Genotype: 4/4 Ff
Phenotype: 4/4 Purple

53. Purple is dominant to white
Cross 2 heterozygous purple flowers
G: ¼ FF, 2/4 Ff, ¼ ff
1:2:1
P: ¾ Purple
¼ white 3:1

54. Using more than one set of variables to determine genetics
Dihybrid Test Crosses
Using more than one set of variables to determine genetics

55. Mendel’s Dihybrid Crosses
Mendel performed another set of crosses in which he used peas that differed from each other in two traits rather than only one.
Such a cross involving two different traits is called a dihybrid cross.

58. The First Generation
Mendel took true-breeding pea plants that had round yellow seeds (RRYY) and crossed them with true-breeding pea plants that had wrinkled green seeds (rryy).
He already knew the round-seeded trait was dominant to the wrinkled-seeded trait.
He also knew that yellow was dominant to green.

59. Punnett Square of Dihybrid Cross
Gametes from RrYy parent
Dihybrid crosses RY Ry rY ry
RRYY
RRYy
RrYY
RrYy
A Punnett square for a dihybrid cross will need to be four boxes on each side for a total of 16 boxes. RY
RRYy
RRYy
RrYy
Rryy Ry
Gametes from RrYy parent
RrYY
RrYy
rrYY
rrYy rY
RrYy
Rryy
rrYy
rryy ry

60. The first generation Ratio: 9:3:3:1 round yellow x wrinkled green
Dihybrid Cross P1
Round yellow
Wrinkled green
All round yellow F1 9 3 3 1 F2
Round yellow
Round green
Wrinkled yellow
Wrinkled green
Ratio: 9:3:3:1