1. MENDEL AND MEIOSIS
Chapter 10

2. Zebra stripes are like fingerprints.
Every zebra’s stripe pattern is unique.
Stripes are genetically determined

3. Mendel’s Laws of Heredity
Gregor Mendel
Austrian monk who carried out studies in heredity
Chose pea plants to study
Heredity
Passing on of characteristics from parents to offspring
Traits
Inherited characteristics
Hair color, eye color

4. Why Pea Plants? Reproduce sexually Able to control pollination!
Usually self pollinate
Gametes – male and female sex cells
Fertilization – when male gamete units with female gamete
Zygote – fertilized cell, develops into a seed
Able to control pollination!
Pollination – transfer of pollen grains from male to female

5. Mendel used good scientific method
Only studied one trait at a time
Like plant height, flower color, type of seed coat
Used math to calculate data

6. Mendel’s Monohybrid Crosses
Mendel crossed a tall plant and a short plant
He called the offspring a hybrid
Offspring from parents that have different forms of a trait
“mono” meaning one trait
“hybrid” see above definition

7. Results of monohybrid cross 1st Generation

8. 2nd Generation Mendel allowed tall plants to self pollinate
¾ were tall
¼ were short
Like the short trait had reappeared from nowhere
“P” parent
“F” filial or son/daughter

9. Terms
“P1” – your parents
“F1” – you
“F2”- any children you have

10. Rule of unit factors Two factors control traits
Traits are controlled by genes
Genes are locations on chromosomes
Different forms of a gene are called “alleles”
Like two alleles for height
Possibilities
Two alleles for tall height
Two alleles for short height
One allele for tall height and one allele for short height

11. Rule of Dominance Dominant – the trait that you see
Recessive – the trait that is not expressed if dominant allele is present
Use same letter for allele
Capital letter for dominate
Lower case letter for recessive
Dominant (capital letter) is written first

12. Law of Segregation
Every individual has two alleles of each gene and when gametes are produced, each gamete receives one of these alleles

13. Phenotypes and Genotypes
Two organisms can look alike (tall pea plants) but have different underlying allele combinations.
Phenotype – the way an organism looks and behaves – PHYSICAL
Tall plant (phenotype) even though it could be TT or Tt
Genotype – the allele combination an organism contains – GENETIC
TT or Tt or tt – the actual genetic makeup of organism

14. Homozygous or Heterozygous
Homozygous – when alleles for a trait are the same
TT or tt – homozygous for tall or short
Heterozygous – when alleles for trait are different
Tt – one allele for tall height, one allele for short height

15. Practice
Give phenotype and genotype of the following (R is dominant for round seed, r is recessive for wrinkled seed) TT Tt RR Rr rr
Heterozygous or homozygous?

16. Dihybrid Cross Differing in 2 traits
Mendel paired plants that were RRYY (round and yellow) and rryy (wrinkled and green)
He found the offspring were in a definite ratio of phenotypes
9 round yellow
3 round green
3 wrinkled yellow
1 wrinkled green

17. Law of Independent Assortment
Genes for different traits are inherited independently of each other
Having the allele for a green seed color does not effect the type of seed coat or height of plant

18. Punnett Squares
A method to predict offspring outcomes based on knowledge of parent genotypes.
PROBABILITY
Punnett Squares show likelihood, not reality
Closer to actual numbers when the number of offspring is large

19. FOIL METHOD FIRST, OUTSIDE, INSIDE, LAST EXAMPLE AaBb F – AB O – Ab
I – aB
L – ab
EXAMPLE
RRYy X Rryy

20. Practice Complete a Punnett Square for the following crosses
List possible genotypes and phenotypes
Traits for flower color and plant height
P – purple flower
p – white flower
T – tall
t – short
PpTt X PPtt

21. MEIOSIS
CHAPTER 10 SECTION 2

22. Genes, Chromosomes and Numbers
Pea plants had 14 chromosomes or 7 pairs
Most chromosomes appear in pairs
One from male parent, one from female parent
Diploid
Cell that contains a diploid or 2n, number of chromosomes
Gametes contain one of each kind of chromosome
Haploid
Cell that contains one of each kind of chromosome, haploid or n

23. Chromosome Numbers Organism Body cells (2n) Gamete (n) Fruit fly 8 4
Corn 20 10
Human 46 23
Dog 78 39

24. Homologous Chromosomes
Have genes for the same trait
But may have different alleles
Mendel Pea Plants
Get one from each parent

25. Why Meiosis? Difference between Mitosis and Meiosis?
End product – Mitosis
2 identical daughter cells, with same genetic makeup as parent and the same number of chromosomes
End product – Meiosis
Gametes with half the number of chromosomes as parent
Why does this work?
When gametes unite – you are back to the right number of chromosomes

26. Meiosis Occurs in the specialized body cells of each parent
Male gametes – sperm – haploid (n)
Female gametes – egg – haploid (n)
Joining of sperm and egg – diploid (2n) number of chromosomes
Zygote then undergoes mitosis to develop into multicellular organism
Sexual Reproduction
Recombination of genetic material

27. Phases of Meiosis Meiosis I Interphase Prophase I
Chromosomes replicated, held together by centromere
Prophase I
DNA thickens
Homologous chromosomes line up
Four part structure is called a tetrad
Crossing Over
When chromosomes break and exchange genetic material
Approx 2-3 crossovers for each pair of homologous chromosomes, in humans

28. Meiosis I Metaphase I Anaphase I Telophase I
Spindle fiber pulls tetrad to middle of cell
Homologous chromosomes are side by side
Anaphase I
Homologous chromosomes separate and move to opposite ends of the cell
Centromeres are still holding sister chromatids together
Telophase I
Spindles break down
Chromosomes uncoil
Cytoplasm divides to yield two new cells
Cell is still at 2n, because of the replication of DNA

30. Meiosis II We need to get to n, or haploid, to a gamete Prophase II
Spindle forms and attach chromotids
Metaphase II
Chromatids line up randomly at equator
Anaphase II
Centromere splits, sister chromatids separate
Move to opposite poles
Telophase II
Spindle break down
Cytoplasm divides

32. End Products of Meiosis
4 haploid (n) cells are formed
These will become gametes

33. Meiosis Provides for Genetic Variation
Genetic Variation - reassortment of chromosomes and genetic information they carry by c.o. or independent segregation
Crossing Over
Provides variation by giving new combinations of alleles
Pea Plant Sperm possibilities
Seven pairs of chromosomes can each line up 2 different ways, same for eggs
2n = 2 7 = 128
Humans
2n = 223 or 8 million possible egg or sperm

34. Nondisjunction
Events usually go smoothy during Meiosis, but, sometimes, homologous chromosomes fail to separate
This is called nondisjunction
For example, when nondisjunction occurs
One gamete has an extra chromosome
One gamete is missing a chromosome
When the gamete with the extra chromosome is fertilized by a normal gamete Trisomy occurs
Now 47 not 46 chromosomes, location chromosome 21, Down Syndrome occurs

35. Turner Syndrome
Monosomy – missing a X chromosome, occurs in females

36. Polyploidy More than the usual number of chromosomes
Usually results in death for zygote in ANIMALS
But, in plants, used to plant breeders advantage
Plants are healthier, larger
Hexaploid wheat, triploid apples, polyploid chrysanthemums

37. Gene Linkage and Maps
If genes are close together, the usually are inherited together
They are said to be “linked”
All the genes on a chromosome are usually linked
Chromosomes themselves follow I.A.
Affected by Crossing Over