Mendel
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Mendel

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Mendel Mendel Presentation Transcript

  • The Genetic Basis of Inheritance From Chromosomes to Genes
  • Chromosomal Basis of Inheritance
    • Traits are transmitted by chromosomes which contain units of heredity called genes
    • Genes are formed from DNA
  • Mendel and the History of Genetics
    • Gregor Mendel
    • Born 1822
    • Began his work at age 21
    • A monk and school teacher interested in plant breeding
    • Studied pea plants
    • Discovered the basic principles of heredity
  • Mendel’s Studies
    • Pea plants were an ideal choice for study
    • Displayed seven traits in one of two contrasting forms:
      • seed shape, seed color, seed coat color, pod shape, pod color, flower position, stem length
    • Mendel studied 30,000 plants in 7 years
      • Kept careful records & looked for mathematical patterns
  • Pure Lines
    • Mendel recognized the 2 contrasting forms as distinct varieties, or pure lines
    • In a pure line, the offspring have same trait as the parents
    • ‘ Tall’ always produced ‘tall,’ etc.
    • Called parental pure lines "P1" generation
    • First generation of offspring are F1 (for filial), then F2 and so on
    • Hybrid = An offspring produced by breeding 2 pure lines
    • What happens if you cross 2 pure lines?
  • Dominance
    • Mendel crossed the two pure lines for each trait.
    • These were the P 1 generation. 
    • All offspring of these crosses (F 1 ) had the trait of only one parent
    • The trait of the other parent disappeared in the F1 generation (but reappeared in F2)
    • Mendel hypothesized that there were 2 factors for each trait
    • Mendel called 1 factor dominant because it prevailed
  • Dominant vs. Recessive Traits
    • Mendel crossed F 1 plants with other F 1 plants, producing the F 2 generation
    • Not only did the recessive trait reappear in the F 2 generation, but in a consistent proportion:
      • 1/4 showed the recessive trait (ratio 3:1)
    • Mendel referred to the factor that was hidden in the F 1 as recessive
    • If an offspring carries 2 dominant or one dominant and one recessive factor, the offspring will appear to have the dominant trait.
    • If the offspring carries 2 recessive factors, it will appear to have the recessive trait
  • The Experiment
    • P Generation:
      • Cross 2 pure lines –
      • One white, one purple
    • F1 are all purple
      • Purple is dominant
    • Cross F1 x F1 (all purple)
      • ¼ F2 offspring are white
      • White is recessive
  • Principle of Segregation
    • Members of each pair of genes separate, or segregate , when gametes are formed.
      • Recall what we now know happens to chromosomes during meiosis
    • From Mendel:
      • For each characteristic an individual carries 2 ‘factors’
      • Each parent contributes one of its 2 factors to each offspring.
      • Chances of contributing either factor are equal.
  • Genes and Alleles
    • We now know that the heredity units are genes .
    • The separate forms of a gene that Mendel called ‘factors’ = alleles
    • One allele in a pair may prevent the other from being expressed
    • If an offspring carries 2 dominant, or one dominant and one recessive allele , the offspring will appear to have the dominant trait.
    • If an offspring carries 2 recessive alleles , it will appear to have the recessive trait.
  • Each individual carries one copy (allele) of a gene on the chromosome from their mother, and a second copy on the homologous chromosome from their father.
  • Representing Genes & Alleles
    • Generally the capitalized first letter of the dominant trait is used to represent the dominant allele
    • The recessive allele is then lower case of the same letter (e.g. T for tall, so t for short)
    • Each characteristic can be represented by a pair of letters representing the genes
    • Some multi-allele systems (more than 2 possible forms of the trait) use different conventions
    • Traits carried on sex chromosomes are written as a superscript of the X or Y chromosome 
  • Hereditary Terminology
    • Genes – The units of heredity
    • Alleles – The separate forms of a gene that Mendel called factors
    • Genotype - The set of alleles for a characteristic possessed by an organism
    • Phenotype - The appearance of the organism; the trait that is actually expressed
    • Homozygous - The 2 alleles in a pair are identical
      • An individual can be either homozygous dominant (2 identical dominant alleles) or homozygous recessive (2 identical recessive alleles)
    • Heterozygous – The 2 alleles in a pair are different
  • Genotype vs. Phenotype
  • Punnett Squares
    • A way to visualize crosses
    • Punnett square can be used to determine probability of different genotypes or phenotypes
    • Each box contains a possible combination of alleles for offspring
    • Punnett square can be used to determine probability of different genotypes or phenotypes
    • Mendel’s F1 cross can be shown as a punnett square
    • Shows 3:1 ratio
  • Determining Genotype
    • If you know the phenotype, is it possible to determine the genotype?
    • If an organism shows the recessive trait, you know the genotype.
      • It must be homozygous recessive (tt)
    • What if it shows the dominant trait?
    • If the dominant trait is expressed there are 2 possibilities:
    • homozygous dominant ( TT)
    • or heterozygous  (Tt)
  • Test Crosses
    • Cross the unknown with a homozygous recessive (tt) x ?
      • This is a test cross (or backcross)
    • If the unknown is TT:
      • then crossing with tt yields all Tt
      • All offspring are tall
      • all have same phenotype and the same genotype
    • If the unknown is genotype Tt, when crossed with tt
      • 1/2 of the offspring will be Tt and 1/2 will be tt
      • so 1/2 will show a different phenotype , making it easy to tell
  • Demonstrating a Test Cross
  • Incomplete Dominance
    • In all of Mendel's cases, 1 allele was clearly dominant
      • This is not always the case
    • Some alleles show incomplete dominance
    • Heterozygotes express traits that are a blend of the phenotypes of the 2 alleles
      • red and white flowers make pink
    • Still obey the law of segregation
    • Only difference is phenotype of homozygous dominant individual is different from phenotype of heterozygous individual
  • Demonstrating Incomplete Dominance
  • Co-Dominance
    • Occurs when both alleles for a gene are expressed in a heterozygote
    • Example – Red & white produce roan color in some animals because both genes are expressed
  • Multi-allele Systems
    • Some traits are the result of more than 2 possible alleles at a locus
    • The ABO blood group is an important example
    • 3 possible alleles:
      • I A - produces A antigen;
      • anti-B antibodies in blood serum
      • I B - produces B antigen;
      • anti-A antibodies in serum
      • i - produces no antigen;
      • anti-A & anti-B antibodies in serum
    • I A & I B are co-dominant alleles
    • i is recessive
  • The ABO Blood System
  • Working with 2 Alleles
    • Crosses so far have examined only 1 characteristic = monohybrid crosses
    • It is possible to consider more than 1 characteristic at a time
      • Crosses involving 2 genes = dihybrid crosses
      • Three genes = trihybrid
    • Mendel crossed plants that differed in 2 characteristics
    • This led to Law of Independent Assortment
  • Law of Independent Assortment
    • Mendel showed that traits produced by dominant factors do not necessarily appear together
    • 2 or more pairs of genes segregate independently of one another during the formation of gametes
      • Genes are distributed to gametes independently
    • We now know this is true:
      • recall independent assortment of chromosomes from meiosis
    • If 2 traits are carried on the same chromosome independent assortment does not hold
      • If 2 traits are located close together on the same chromosome they are linked
  • Dihybrid Crosses
    • Involve 2 genes
    • Can demonstrate independent assortment
    • Cross homzygous dominant for 2 alleles with homozygous recessive for 2 alleles:
      • Y = yellow
      • R = round
    • YYRR x yyrr can produce:
      • Yellow/ round
      • Yellow/ wrinkled
      • Green/ round
      • Green wrinkled
    • Produces 9:3:3:1 ratio
  • Diagramming a Dihybrid Cross
  • Sex Linkage
    • Autosomal trait
      • a gene carried on a non-sex chromosome & present in 2 copies
    • Sex linked trait
      • a gene carried on a sex chromosome that is present in both sexes; one copy in one sex, 2 in the other
      • X chromosome in mammals
      • Z chromosome in birds (one copy in females, 2 in males)
    • Y- linked trait
      • a gene carried on the Y chromosome in humans; very rare
  • Showing Sex Linkage
    • Symbols are written as superscript of the sex chromosome:
      • X a - X chromosome carrying the recessive allele
      • X A – X chromosome carrying the dominant allele
      • No superscript is used for the normal or wild type allele
    • Suspect sex linkage when the ratios of phenotypes are different in males and females
  • Pedigrees
    • Used to determine mode of inheritance when few individuals, but several generations are involved
    • Assume genetic trait discussed is rare, so individuals marrying into the family are not assumed to carry the trait
    • Symbols:
      • O female shaded = affected
      •  male partial shading = carrier
  • Pedigree Analysis
  • Polygenic Inheritance
    • Most traits are not limited to 2 possibilities, yes or no
    • Most traits are a continuum (quantitative)
    • Examples: height, skin color
    • 2 or more genes act additively on a trait
  • Pleitropy
    • Genes have multiple phenotypic effects
    • The same gene that produces dark pigment in skin might produce dark pigment in fur
    • Pleiotropic Advantage:
    • Some genes that have one negative effect, also convey a benefit
      • Example: sickle cell trait may have provided the heterozygote an advantage against malaria
  • Sickle Cell Trait & Pleiotropy
  • Epistasis
    • A gene at one locus alters expression of a second gene
    • Example: If black (B) is dominant over brown (b) in mice, but . . .
    • A second gene (C) determines if fur pigment is made at all,
    • Then you must be (CC) or (Cc) before the mice can be either black or brown
  • Diagramming Epistasis