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

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

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

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

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

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

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

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.

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

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

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

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

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

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

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

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

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

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

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%

Another Type of Cell Division to Produce Gametes Meiosis

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

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

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

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

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.

Prophase I

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

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

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

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

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

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

Telophase II identical to Telophase of mitosis

Cytokinesis Cytokinesis separates the cells

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