1. Mendel’s Laws of heredity
Chapter 10.1
Mendel’s Laws of heredity

2. Who is Gregor Mendel? A monk in an Austrian monastery
Carried out the first important studies of heredity
First to succeed in predicting how traits are passed from parents to offspring
Bred garden pea plants to study inheritance

3. Why Mendel Chose Pea Plants
They are genetically simple
Mendel could study one trait at a time
They reproduce sexually
He could control breeding
Mendel chose which plants to cross & studied one trait at a time
They grow quickly

4. Breeding Plants Reproductive Parts:
Male = Stamen
Female = Pistil
Pollination = pollen from the anther is transferred to stigma
Pollination can be controlled

5. Traits observed Stem length Flower color Seed color Seed shape
Flower Position
Pod shape
Pod color

6. Monohybrid Crosses
Mendel carefully chose purebred (true-breeding) pea plants.
Monohybrid crosses look at one trait at a time
Example: flower color
Crossing Pea Plants:
Mendel crossed purple- flowered plants with white- flowered plants
Mendel planted the seeds, then allowed the F1 plants to self-fertilize
The resulting offspring F2 showed a 3:1 ratio of purple flowers

7. Terminology P = parental generation F1 = first Filial generation
F2 = second Filial generation

8. Rule of Unit Factors Organisms have 2 factors that control each trait
one from each parent
Alleles = different forms of a gene
Example: height may be tall or short; peas may be yellow or green, round or wrinkled.

9. Rule of Dominance
Dominant = the trait that is observed whenever it is present
Shown as CAPTAL letters
In pea plants, Tall is dominant written as T
Recessive = the trait that is hidden if a dominant trait is present
Shown as lower-case letters
In pea plants, Short is recessive  written as t

10. Mendel’s 2 laws 1. Law of Segregation
Each parent has 2 alleles that separate (segregate) during meiosis
Gametes form random pairs during fertilization
2. Law of Independent Assortment
Genes for different traits are inherited independently of one another

11. Genotypes & Phenotypes
Phenotype = the way an organism looks and behaves
Usually a description
Example: Plant height = tall or short
Genotype = the genetic combination for an organism
Usually the combination of alleles
Example: TT or Tt both produce tall plants, tt produces a short plant
 You can’t always identify the genotype from the phenotype

12. Genotypes Homozygous = two like alleles (TT or tt)
True-breeding plants are generally homozygous
Heterozygous = two different alleles (Tt)

13. Crosses Monohybrid Cross = a cross involving one trait
Dihybrid cross = a cross involving two different traits
Parents: True-breeding Round, Yellow peas & Wrinkled, Green peas
F1 generation = all Round, Yellow peas
F2 generation – F1 generation self-pollinated to produce a mixture of offspring
Yellow, round; Yellow, wrinkled; Green, round; Green, wrinkled
The F2 phenotype ratio was 9:3:3:1

14. Punnett Squares Created by Reginald Punnett in 1905
Used as a tool to predict all of the possible outcomes of a genetic cross

15. Monohybrid Cross Punnett Square is 2 x 2 Steps: set up the square
DRAW THIS
Punnett Square is 2 x 2
Steps:
set up the square
do the cross
determine offspring genotypes
determine offspring phenotypes

16. Genotype Ratios
Genotype looks at all of the possible genetic combinations
To determine ratios, look at each pair inside of the square:
TT = homozygous dominant (tall)
Tt = heterozygous
tt = homozygous recessive (short)
Genotypes:
1 TT, 2 Tt, 1 tt
Genotype Ratio = 1:2:1

17. Phenotype Ratios Phenotype looks at the trait that will be expressed
Doesn’t account for “hidden” recessive alleles
Any offspring combination that has the dominant allele will show the dominant trait.
Phenotypes:
3 Tall, 1 short
Ratio = 3:1

18. Dihybrid Cross Punnett Square is 4 x 4 Steps: Figure out the gametes
set up the square
do the cross
determine offspring genotypes
determine offspring phenotypes

19. Probability The chance of getting a certain outcome
Example: A coin has two sides – heads & tails
The probability of getting heads is ½ or 1:2
Probability in Punnett Squares: Tt x Tt
The probability of getting a tall plant is ¾ or 75%

20. Another Type of Cell Division to Produce Gametes
Meiosis

21. What is Meiosis? A process Occurs in sex cells
Only occurs in eukaryotes.
Plants
Animals
Reduces the amount of chromosomes by half
Makes gametes
Reproduction Cells
Occurs in the gonads

22. Number of Chromosomes in a Cell
Haploid: contains one set of chromosomes
N = 23
Gamete cells
Diploid: contains two sets of chromosomes
One from each parent
2n = 2(23) = 46
Humans (except for gametes)
Some plants and animals

23. Number of Chromosomes in a Cell
Homologous
Pair of chromosomes
One member obtained from the mother
The other is obtained from the father

24. Two successive nuclear divisions
Phases of Meiosis
Two successive nuclear divisions
Meiosis I (Reduction of genes)
Meiosis II (Division)
Produces 4 haploid cells

25. Meiosis I Prophase I: two events occur.
Homologues chromosomes pair up.
Crossing-over may occur at this point.
Chromatids break and may be reattached to a different homologous chromosome.

26. Prophase I

27. Metaphase I Tetrads line-up along the equator of the spindle
Spindle fibers attach to the centromere

28. Anaphase I Tetrads separate
Independent assortment of homologous chromosomes.
Drawn to opposite poles by the spindle fibers
Centromere in anaphase I remain intact

29. Telophase I
One set of (replicated) chromosomes is in each "cell.

30. Meiosis II (Similar to Mitosis)
Prophase II
Nuclear envelopes (if they formed during telophase I) dissolve
Spindle fibers reform

31. Metaphase II
spindles moving chromosomes into equatorial area and attaching to the opposite sides of the Centromere

32. Anaphase II
Centromere split and the former chromatids (now chromosomes) are segregated into opposite sides of the cell.

33. Telophase II
identical to Telophase of mitosis

34. Cytokinesis
Cytokinesis separates the cells

35. Nondisjunction,
An abnormality that occurs by the failure of replicated chromosomes to segregate during Anaphase II.
Trisomy
Extra chromosome
Monosomy
Missing a chromosome
Triploidy
Extra set of chromosomes