1. Meiosis and Genetics Jaime Crosby, CHS

2. Diploid and Haploid Cells
Body cells of animals and most plants, chromosomes occur in pairs
One chromosome in each pair comes from the male parent and one from the female parent
A cell with two of each kind of chromosome is called diploid cell (2n)
Organisms produce gametes that contain one of each kind of chromosome; called a haploid cell (n)

3. Homologous Chromosomes
The two chromosomes of each pair in a diploid cell are known as homologous chromosomes
Each pair of chromosomes has genes for the same traits
Because there are different possible alleles for the same gene, the two chromosomes in a homologous pair are not always identical

4. Why Meiosis?
Meiosis allows offspring to have the same number of chromosomes as their parents
Produces gametes containing half the number of chromosomes as a parent’s body cell
Occurs in specialized body cells of each parent that produce gametes
Male gamete called sperm
Female gamete called egg
Fertilized egg called zygote

5. Phases of Meiosis
During meiosis chromosomal activity is different than that during mitosis
Mitosis produces cells that are genetically the same
Meiosis produces cells that can be genetically different

6. Meiosis: Step by Step: IPMATPMAT
Interphase I-replication of chromosomes
Prophase I-homologous chromosomes form a four part structure called tetrad, can exchange genetic material
Metaphase I-spindle fibers attach to centromere of each chromosome and pull chromosomes to the center equator of cell
Anaphase I-separation of tetrads to opposite sides of the cell
Telophase I-chromosomes uncoil, cytoplasm divides, two new cells formed with half the chromosome number

7. Meiosis II: Step by Step
Prophase II-spindle fiber forms, attach to chromosomes
Metaphase II-chromosomes are pulled to center of cell
Anaphase II-centromere of each chromosome splits as chromosomes are pulled to poles of cell
Telophase II-nucleus reforms, cytoplasm splits
Four haploid cells are formed, become gametes

8. Mitosis versus Meiosis

9. Genetic Recombination; Crossing over
Meiosis provides for the shuffling of the chromosomes, providing many possible genetic combinations
Typically two or three crossovers per chromosome occur during meiosis

10. Cool Facts
The number of different kinds of sperm or eggs a person can produce is more than 8 million
When fertilization occurs 70 trillion different zygotes are possible
Experimentation has allowed us to make XL RED tomatoes, broccoflower, seedless watermelon, etc.

11. Mistakes in Meiosis
The failure of homologous chromosomes to separate properly during meiosis is called nondisjunction
Trisomy is when a gamete with an extra chromosome is fertilized by a normal gamete, the zygote has an extra chromosome; will develop Down syndrome
Monosomy is when a gamete with a missing chromosome fuses with a normal gamete during fertilization, the zygote lacks a chromosome; most do not survive

12. Meiosis Example II

13. Mendel and Meiosis

14. Gregor Mendel
Gregor Mendel, who many consider the father of genetics, was an Austrian monk in the mid-nineteenth century
Began to breed garden pea plants so that he could study the inheritance of their characteristics

15. Mendel’s Study
Mendel conducted the first important studies of heredity
Heredity is the passing on of characteristics from parents to offspring
Genetics is the study of heredity
Characteristics that are inherited are called traits

16. Mendel’s Choices
Mendel chose the pea plants he used in experiments carefully
They reproduce sexually, having male and female gametes or sex cells
Male gamete is pollen grain
Female gamete is the ovule, located in the pistil

17. Transfer of pollen grains to the pistil is called pollination
When male and female gametes combine, called fertilization
Peas normally reproduce by
self-pollination
If Mendel needed to cross two different plants he would open the petals, remove anthers, and dusted the pistil of the plant he wanted to cross it with.

18. Mendel’s Monohybrid Crosses
Mendel crossed “pure bred” tall plants with “pure bred” short plants to produce what he called hybrids
Hybrid is the offspring of parents that have different forms of a trait, such as tall or short height
Monohybrid means a cross of only one trait

19. Mendel selected a tall plant and cross pollinated with a short pea plant
Planted seeds
All resulting offspring were tall
As if short pea plant had not been parent
First Generation

20. Second Generation
Next, Mendel allowed the tall plants to self pollinate, form seeds, and planted
Counted more than 1000 plants
Found ¾ were as tall as the tall plants, ¼ were short as the short plants
Shortness had reappeared
Ratio of 3 tall to 1 short plant (3:1)

21. Generational Labels
Original parents, “true tall” and “true short”, are known as the P1 generation (P stands for parent)
The offspring of the parent plants are known as the F1 generation (F stands for filial)
Their offspring become the F2 generation

22. The Rule of Dominance Refer to the Mendel’s experimentation
Trait that was observed in the F1 generation is dominant
Choose the letter of the dominant trait to represent the allele choices
Trait that disappeared is recessive
Tall allele is dominant to the allele for short plants, short is recessive
For a single trait, a single letter is used to represent (capital represents dominant trait and lower case represents recessive trait)

23. The Law of Segregation/ Independent Assortment
Mendel concluded that each
tall plant in the F1 generation
carried one dominant allele for tallness and one unexpressed recessive allele for shortness
F1 generation plants receive one allele from the tall parent, one from short parent
During fertilization, these gametes randomly pair to produce four combinations of alleles.
These traits are all inherited independently from each other x

24. Phenotypes and Genotypes
Tt x TT
Two organisms can look alike but have different gene combinations
Phenotype is the way an organism looks and behaves; phenotype of tall plant is tall, no matter what gene it contains
Genotype is the actual gene combination an organism contains; genotype for a pure tall plant is TT, for non-pure tall plant it is Tt

25. Homozygous vs. Heterozygous
An organism is heterozygous for a trait if its two alleles differ from each other
A tall plant that has one allele for tall and one for short (Tt) is heterozygous
An organism is homozygous for a trait if its two alleles for the trait are the same
TT is homozygous for tall
tt is homozygous for short

26. 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
This type of cross is known as a dihybrid cross x

27. Dihybrid Cross: F1 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)
All seeds produced in the F1 generation were yellow round seeds
Round and yellow were dominant traits

28. Dihybrid Cross: F2 Generation
Mendel allowed F1 plants to self pollinate
All possible combinations were found in F2 generation
Definite phenotypic ratio present
9 round green
3 round yellow
3 wrinkled green
1 wrinkled yellow
CALLED THE PRODUCT RULE

29. Incomplete Dominance
When one trait is not completely dominant over the other. Example:
red + white snapdragons = PINK

30. Punnett Squares T T TT TT T t Tt Tt

31. Punnett Squares
Method of finding the expected proportions of possible genotypes in the offspring of a cross
Monohybrid crosses
Cross Gg x gg, G = green, g = blue
Cross Rr x Rr, R = round, r = wrinkled; record genotypic and phenotypic ratio
Dihybrid crosses

32. Probability
Punnett squares show the likelihood that a possible combination of gametes will occur
Reality is that results will follow the rules of chance
Flipping a coin gives you a possibility of heads or tails
1:2 or 1/2 chance of landing on heads
1:2 or 1/2 chance of landing on tails