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Mendel’s Laws of heredity

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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

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

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