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Mendel
 

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