Mendelian Genetics
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Mendelian Genetics






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Mendelian Genetics Mendelian Genetics Presentation Transcript

  • Mendelian Genetics
    • Unit I - Genetic Continuity
    • Sections 11-1, 11-2, 11-3, 14-1
    • Mr. Connors
    • Biology 12
  • Gregor Mendel’s Peas
        • Genetics is the scientific study of heredity .
        • Gregor Mendel was an Austrian monk. His work was important to the understanding of heredity.
        • Mendel carried out his work with ordinary garden peas.
  • Mendel’s Pea Plants
      • Mendel knew that:
      • the male part of each flower produces pollen, (containing sperm).
      • the female part of the flower produces egg cells.
  • Mendel’s Peas
    • During sexual reproduction, sperm and egg cells join in a process called fertilization.
    • Fertilization produces a new cell.
    • Pea flowers are self-pollinating.
    • Sperm cells in pollen fertilize the egg cells in the same flower.
    • The seeds that are produced by self-pollination inherit all of their characteristics from the single plant that bore them.
  • Mendel’s Peas
    • Mendel had true-breeding pea plants that, if allowed to self-pollinate, would produce offspring identical to themselves.
    • Mendel wanted to produce seeds by joining male and female reproductive cells from two different plants.
    • He cut away the pollen-bearing male parts of the plant and dusted the plant’s flower with pollen from another plant.
  • Cross-pollination
    • This process is called cross-pollination.
    • Mendel was able to produce seeds that had two different parents.
  • Genetic vocabulary…….
    • Punnett square: predicts the results of a genetic cross between individuals of known genotype
    • Homozygous: pair of identical alleles for a character
    • Heterozygous: two different alleles for a gene
    • Phenotype: an organism’s traits
    • Genotype: an organism’s genetic makeup
    • Testcross: breeding of a recessive homozygote X dominate phenotype (but unknown genotype)
  • Mendelian genetics
    • Character
    • (heritable feature, i.e., fur colour)
    • Trait (variant for a character, i.e., brown)
    • True-bred (all offspring of same variety)
    • Hybridization
    • (crossing of 2 different true-breds)
    • P generation (parents)
    • F 1 generation (first filial generation)
  • Genes and dominance
    • A trait is a specific characteristic that varies from one individual to another.
    • Mendel studied seven pea plant traits, each with two contrasting characters.
    • He crossed plants with each of the seven contrasting characters and studied their offspring.
  • Genes and dominance
    • Each original pair of plants is the P (parental) generation.
    • The offspring are called the F 1 , or “first filial,” generation.
    • The offspring of crosses between parents with different traits are called hybrids .
    • The F 1 hybrid plants all had the character of only one of the parents.
  • Genes and dominance
    • Mendel's first conclusion was that biological inheritance is determined by factors that are passed from one generation to the next.
    • Today, scientists call the factors that determine traits genes .
    • Each of the traits Mendel studied was controlled by one gene that occurred in two contrasting forms that produced different characters for each trait.
    • The different forms of a gene are called alleles.
    • Mendel’s second conclusion is called the principle of dominance.
  • The principle of dominance
    • The principle of dominance states that some alleles are dominant and others are recessive.
    • An organism with a dominant allele for a trait will always exhibit that form of the trait.
    • An organism with the recessive allele for a trait will exhibit that form only when the dominant allele for that trait is not present.
  • Leading to the Law of Segregation
    • Alternative versions of genes (alleles) account for variations in inherited characteristics
    • For each character, an organism inherits 2 alleles, one from each parent
    • If the two alleles differ, then one, the dominant allele, is fully expressed in the organism’s appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance
    • The alleles for each character segregate (separate) during gamete production (meiosis).
    • Mendel’s Law of Segregation
  • Segregation
      • Mendel crossed the F 1 generation with itself to produce the F 2 (second filial) generation.
      • The traits controlled by recessive alleles reappeared in one fourth of the F 2 plants.
  • Segregation
    • Mendel assumed that a dominant allele had masked the corresponding recessive allele in the F 1 generation.
    • The trait controlled by the recessive allele showed up in some of the F 2 plants.
    • The reappearance of the trait controlled by the recessive allele indicated that at some point the allele for shortness had been separated, or segregated , from the allele for tallness.
  • Segregation
    • Mendel suggested that the alleles for tallness and shortness in the F 1 plants segregated from each other during the formation of the sex cells, or gametes.
    • When each F 1 plant flowers and produces gametes, the two alleles segregate from each other so that each gamete carries only a single copy of each gene.
    • Therefore, each F 1 plant produces two types of gametes — those with the allele for tallness, and those with the allele for shortness.
  • Alleles separate during gamete formation
  • Punnett Squares
      • The gene combinations that might result from a genetic cross can be determined by drawing a diagram known as a Punnett square.
      • Punnett squares can be used to predict and compare the genetic variations that will result from a cross.
  • Punnett Squares
    • A capital letter represents the dominant allele for tall.
    • A lowercase letter represents the recessive allele for short.
    • In this example,
    • T = tall
    • t = short
  • Punnett Squares
    • Gametes produced by each F1 parent are shown along the top and left side.
    • Possible gene combinations for the F2 offspring appear in the four boxes.
  • Punnett Squares
    • Organisms that have two identical alleles for a particular trait are said to be homozygous.
    • Organisms that have two different alleles for the same trait are heterozygous .
    • Homozygous organisms are true-breeding for a particular trait.
    • Heterozygous organisms are hybrid for a particular trait.
  • Punnett Squares
    • All of the tall plants have the same phenotype , or physical characteristics.
    • The tall plants do not have the same genotype , or genetic makeup.
    • One third of the tall plants are TT , while two thirds of the tall plants are Tt.
  • Punnett Squares
        • The plants have different genotypes ( TT and Tt ), but they have the same phenotype (tall).
  • Probability and Segregation
        • One fourth (1/4) of the F2 plants have two alleles for tallness ( TT ).
        • 2/4 or 1/2 have one allele for tall ( T ), and one for short ( t ).
        • One fourth (1/4) of the F2 have two alleles for short (tt).
  • Probability and Segregation
    • Because the allele for tallness ( T ) is dominant over the allele for shortness (t), 3/4 of the F 2 plants should be tall.
    • The ratio of tall plants ( TT or Tt ) to short (tt) plants is 3:1.
    • The predicted ratio showed up in Mendel’s experiments indicating that segregation did occur.
  • Probabilities predict averages
        • Probabilities predict the average outcome of a large number of events.
        • Probability cannot predict the precise outcome of an individual event.
        • In genetics, the larger the number of offspring, the closer the resulting numbers will get to expected values.
  • The Law of Independent Assortment
    • Law of Segregation involves 1 character. What about 2 (or more) characters?
    • Monohybrid cross vs. dihybrid cross
    • The two pairs of alleles segregate independently of each other.
    • Mendel’s Law of Independent Assortment
  • The Two-Factor Cross: F 1
        • Mendel crossed true-breeding plants that produced round yellow peas (genotype RRYY ) with true-breeding plants that produced wrinkled green peas (genotype rryy ).
        • All of the F 1 offspring produced round yellow peas ( RrYy ).
  • Independent Assortment
    • The alleles for round ( R ) and yellow ( Y ) are dominant over the alleles for wrinkled ( r ) and green ( y ).
  • The Two-Factor Cross: F 2
        • Mendel crossed the heterozygous F 1 plants ( RrYy) with each other to determine if the alleles would segregate from each other in the F 2 generation.
        • RrYy × RrYy
  • Independent Assortment
    • The Punnett square predicts a 9 : 3 : 3 :1 ratio in the F2 generation.
  • Independent Assortment
      • In Mendel’s experiment, the F 2 generation produced the following:
        • some seeds that were round and yellow
        • some seeds that were wrinkled and green
        • some seeds that were round and green
        • some seeds that were wrinkled and yellow
    • The alleles for seed shape segregated independently of those for seed color. This principle is known as independent assortment .
    • Genes that segregate independently do not influence each other's inheritance.
  • Independent Assortment
    • Mendel's experimental results were very close to the 9 : 3 : 3 : 1 ratio predicted by the Punnett square.
    • Mendel had discovered the principle of independent assortment.
    • The principle of independent assortment states that genes for different traits can segregate independently during the formation of gametes.
    • Independent assortment helps account for the many genetic variations observed in plants, animals, and other organisms.
  • Non-single gene genetics, I
    • Incomplete dominance : appearance between the phenotypes of the 2 parents. Ex: snapdragons
    • Codominance: two alleles affect the phenotype in separate, distinguishable ways. Ex: Tay-Sachs disease
    • Multiple alleles: more than 2 possible alleles for a gene. Ex: human blood types
  • Non-single gene genetics, II
    • Pleiotropy: genes with multiple phenotypic effect. Ex: sickle-cell anemia
    • Epistasis: a gene at one locus (chromosomal location) affects the phenotypic expression of a gene at a second locus. Ex: mice coat color
    • Polygenic Inheritance: an additive effect of two or more genes on a single phenotypic character Ex: human skin pigmentation and height
  • Human disorders
    • The family pedigree
    • Recessive disorders: •Cystic fibrosis •Tay-Sachs •Sickle-cell
    • Dominant disorders: •Huntington’s
    • Testing: •amniocentesis •chorionic villus sampling (CVS)