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Mendel and Heredity


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Title: Mendel and Heredity

Mendel and Heredity
  • Chapter 8

8.1 The Origins of Genetics
  • Heredity is the passing of characters from
    parents to offspring
  • Used throughout history to alter crops and
    domestic animals
  • Gregor Johann Mendel Austrian Monk
  • Used pea plants and bred different varieties
  • Developed rules to accurately predict patterns of

Why Peas?
  • 2 characters have clearly different forms
  • Character inherited characteristic (color)
  • Trait single form of character (purple)
  • Male and female reproductive parts are in same
  • Can control fertilization
  • Flower can fertilize itself (self-fertilization)
    or can cross pollen from 1 plant to another
  • Peas are small, grow easily, mature quickly, and
    produces many seeds so results obtained quickly

Traits Expressed as Simple Ratios
  • Mendel started by looking at 1 characteristic
    (monohybrid), such as color, with 1 pair of
    contrasting traits, purple or white flowers
  • Only allowed plants to self-pollinate for many
  • True-breeding all offspring show only 1 trait
  • Parental (P) generation
  • Cross pollinated 2 P generation plants with
    contrasting traits
  • Offspring called filial (F1) generation
  • Counted numbers of each trait

  • Allowed F1 generation to self pollinate
  • Offspring called F2 generation
  • Each characterized and counted

Mendels Results
  • F1 showed only 1 form of character other had
  • When F1 self pollinates other trait reappears in
    some of F2
  • Found ratio of traits to be 3 to 1
  • 3 white flowers to 1 purple flower
  • Same ratio found for any trait he studied

8.2 Mendels Theory
  • We used to think offspring were blend of traits
  • Tall x short medium
  • Mendels experiments showed us this is not
    entirely true

Mendels Hypothesis
  • There are 2 copies of a gene, one from each
    parent, for each inherited characteristic
  • There are different versions of genes called
  • Tall or short
  • When both versions are present one may be
    dominant (completely expressed) and the other may
    be recessive (not expressed when dominant is
  • When you form gametes, alleles separate
    independently so only one allele in each gamete

Mendels Finding in Modern Terms
  • Use letters to show alleles
  • Capitol dominant (T, P, Y, etc)
  • Lower case recessive (t, p, y, etc)
  • Homozygous letters are same
  • Homozygous dominant TT, PP
  • Homozygous recessive tt, pp
  • Heterozygous letters are different
  • Tt, Pp
  • Only dominant allele is expressed

  • Genotype set of alleles
  • What you actually have
  • TT, Tt, or tt
  • Phenotype what is expressed
  • How it looks
  • Tall, Tall, or Short

Mendels Laws of Heredity
  • Law of Segregation
  • 2 alleles for a character segregate when gametes
    are formed
  • Behavior of chromosomes during meiosis
  • Law of Independent Assortment
  • 1 character does not affect another
  • Alleles of different genes separate independently
    of on another
  • Now know this only applies to genes located on
    different chromosomes or that are far apart on
    same chromosome

8.3 Studying Heredity Punnett Squares
  • Breeders want certain characteristics when they
    breed (cross) animals
  • Horticulturists produce plants with specific

  • Punnett Square
  • Used to predict outcomes
  • Shows all possible combinations of gametes
  • Put 1st parents genotype on top
  • Put 2nd parents genotype on side
  • Do the cross

  • So in Mendels F1 generation, a pure Tall plant
    bred with a pure short plant can only give 1 kind
    of offspring due to dominance of tall allele

Determining Unknown Genotypes
  • How do you know if a tall plant is homozygous or
    heterozygous? They both look tall
  • Can do a Test Cross
  • If dominant phenotype is shown with unknown
    genotype, cross it with homozygous recessive

Test Cross Results
  • If unknown is homozygous dominant, all offspring
    of test cross will have dominant trait

Test Cross Results
  • If unknown is heterozygous, offspring of test
    cross will have 2 dominant and 2 recessive

  • Can use probability calculations to predict
    results of genetic crosses
  • Probability is the likelihood a specific event
    will occur
  • Probability of 1 kind of possible outcome
    divided by total number of possible outcomes
  • We will express these as fractions
  • Chance a coin will come up heads
  • 1 head / 2 sides ½

  • DD ¼
  • Dd 2/4 or ½
  • dd ¼

Dihybrid Cross
  • Uses a Punnett Square to determine outcomes of 2
    traits at one time
  • Example Surface and Color
  • Surface RR, Rr, or rr round or wrinkled
  • Color YY, Yy, yy yellow or green
  • What are the possible combinations?
  • RY, Ry, rY, ry

  • So if you have 2 purebred homozygous parents RRYY
    and rryy and you mate them, what do you get?
  • All offspring will be RrYy
  • What if you have F1 breed?
  • Make a Punnett Square of possible gametes for
    each parent
  • What possible combos can parents offer?
  • Do you remember FOIL?
  • RY, Ry, rY, ry

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You never have to count the results of a dihybrid
cross between heterozygotes!
  • 9 with both dominant traits
  • 3 with first dominant and second recessive
  • 3 with first recessive and second dominant
  • 1 with both recessive traits
  • So 9331

Inheritance of Traits
  • Pedigree
  • Family history that shows how a trait is
    inherited over several generations
  • Helpful in tracking genetic disorders
  • Carrier have allele for trait but show no

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Things You Can Find From A Pedigree
  • Autosomal or Sex-Linked?
  • If autosomal it will be equal in both sexes
  • If sex linked generally only found in males
  • Y linked
  • Hairy ear rims
  • X linked
  • Color-blindness
  • Hemophilia

  • Dominant or recessive
  • Autosomal Dominant every individual with
    condition will have parent with condition
  • Achondroplasia type of dwarfism
  • Huntingtons Disease brain degenerates
  • Autosomal Recessive 1, 2, or no parents with
  • Cystic fibrosis
  • Sickle cell anemia
  • Albinism

  • Heterozygous or Homozygous
  • Autosomal homozygous dominant or heterozygous
    phenotype will show dominant allele
  • Homozygous recessive will show recessive allele
  • 2 heterozygous of recessive allele dont show
    condition but can have children that do

8.4 Complex Patterns of Heredity
  • Complex Control
  • Most of the time characters display much more
    complex patterns than simple dominant-recessive
  • Characters can be influenced by several genes

Polygenic Inheritance
  • Several genes affect a character
  • These genes may be scattered along same
    chromosome or on different chromosomes
  • Determining the effect of any one gene is
  • Crossing over and independent assortment create
    many different offspring combos
  • Eye color, height, weight, hair, intelligence,
    and skin color
  • Usually gives a range of expression

Polygenic Inheritance
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Intermediate Characters
  • Incomplete dominance
  • Phenotype that is intermediate between 2 parents,
    neither is completely dominant
  • White x red pink
  • Straight hair x curly hair wavy hair

Multiple Alleles
  • Characters controlled by genes with 3 alleles
  • Humans have ABO blood types
  • IA, IB, i
  • Letters A and B refer to carbohydrates on surface
    of red blood cells
  • i has neither carbohydrate
  • IA and IB are dominant over I, but not over each
    other (codominant)
  • Still only 2 possibilities in a person

Blood Types
  • 2 forms are displayed at the same time
  • Codominance both expressed, not blended
  • IAIB both expressed
  • ii Type O

Characters Influenced by Environment
  • Plants may change color based on pH of soil
  • Arctic fox
  • Summer enzymes produce pigments for darker fur
  • Winter no enzymes, no pigments to darken fur
  • Siamese cats
  • Dark fur in cooler parts
  • Humans
  • Height related to nutrition
  • Skin color based on sun exposure
  • Twins are genetically identical, any difference
    is due to environment

Genetic Disorders
  • Proteins encoded by genes must function precisely
    for normal development and function
  • Genes may be damaged or copied wrong causing
    faulty proteins
  • Mutation changes in genetic material
  • Rare because cells try to correct errors
  • Harmful effects produced by inherited mutations
  • Many carried by recessive alleles

Sickle Cell Anemia
  • Recessive genetic disorder
  • Mutated allele produces defective form of
    hemoglobin causing red blood cells (rbc) to be
  • These rupture easily causing less O2 to be
    carried and may get stuck and cut off blood
  • Recessive allele protects heterozygous
    individuals from malaria
  • Parasites in sickle rbc die
  • Normal rbc still transport oxygen

Cystic Fibrosis
  • Most common fatal, hereditary, recessive disorder
    in Caucasions
  • 1 in 25 has at least 1 copy of defective gene
    that makes a protein needed to move chloride in
    and out of cells
  • Mucus clogs organs
  • 1 in 2,500 homozygous for cystic fibrosis
  • No cure

  • Impairs bloods ability to clot
  • Sex-linked
  • Dozen genes code for clotting proteins
  • 1 mutation on X chromosome causes Hemophilia A
  • Males only get 1 X chromosome

Huntingtons Disease
  • dominant allele on autosome
  • 1st symptoms - mild forgetfulness and
    irritability in 30s and 40s
  • Eventually lose muscle control, spasms, severe
    mental illness, and death

Treating Genetic Disorders
  • Most cant be cured
  • Genetic Counseling tells of possible genetic
    problems with offspring, may be treated if early

Phenylketonuria (PKU)
  • Lack enzyme that converts amino acid
    phenylalanine into tyrosine so it builds up in
    the body and causes severe mental retardation
  • Can be placed on phenylalenic diet

Gene Therapy
  • Replace defective genes with normal ones
  • Isolate copy of gene
  • Put working copy into a virus
  • Virus infects and puts gene in
  • Infected cells are cured
  • Still trying to get this to work