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Alterations to Mendel. Incomplete or partial dominance Codominance Multiple alleles and Lethal alleles Gene interactions & multiple genes Epistasis and complementation Effect of environment Extranuclear inheritance Sex-linked, sex-limited, & sex-influenced

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alterations to mendel
Alterations to Mendel
  • Incomplete or partial dominance
  • Codominance
  • Multiple alleles and Lethal alleles
  • Gene interactions & multiple genes
    • Epistasis and complementation
  • Effect of environment
  • Extranuclear inheritance
  • Sex-linked, sex-limited, & sex-influenced
  • Sex determination and Gene dosage
  • Polygenics
gene dosage
Gene dosage
  • It matters how many copies of genes there are.
    • Snapdragons: heterozygous flowers are pink.
    • Multiple histone genes.
    • Too many of some genes is deleterious.
      • 3 copies of chromosome 21 = Down Syndrome
  • What about sex chromosomes? XX vs. XY
    • Y chromosomes are missing most of genes X has.
    • So, if 1 set of genes on the X is good for males, is two sets (2 X chromosomes) bad for females?
dosage compensation barr ohno and lyon
Dosage compensation: Barr, Ohno, and Lyon
  • Barr noticed that in the nucleus of females, but not males, a darkly staining body is visible.
  • Ohno hypothesized that this was an inactivated X chromosome in females so that there would only be 1 functional copy of genes, as in males.
  • Inactivated X is called a Barr body.
  • Individuals with incorrect numbers of sex chromosomes have appropriate number of Barr bodies.
    • E.g. XXX females have 2 Barr bodies
lyon hypothesis
Lyon Hypothesis
  • X chromosome inactivation takes place early in development.
  • In placental mammals, it can be either X chromosome.
    • All the descendents of that cell have the same X chromosome inactivated.
    • Results in a mosaic, patches of tissue with different lineages. Seen with X-linked traits.
      • Human females: anhidrotic epidermal dysplasia, no sweat glands; female has patches of skin w/o sweat glands, cells descended from a cell in which the X chromosome with the normal gene was inactivated.
      • G6PD alleles; Patches of color blindness
Descent of cells:

How mosaics are made.

Events during development.

Two homologous chromosomes, blue & red.

Black indicates inactivation = Barr body

formation of barr bodies 2
Formation of Barr bodies-2

Classic example: the calico cat.

One X chromosome codes for orange fur, the other for black. Cat shows characteristic mosaic patterns caused by one or the other X chromosome being inactivated.

White fur results from the effect of another gene.

molecular basis of barr body formation
Molecular basis of Barr body formation
  • Xic is a region on the X near the centromere.
  • Xic region includes a region called Xist (X inactivation specific transcript)
    • This area is transcribed, but RNA isn’t used to make a protein; it binds to the DNA of the rest of the X chromosome.
    • This promotes molecular changes that inactivate the chromosome including extensive methylation (except for XIC) and condensation of DNA (into smaller space).
  • In the OTHER X chromosome, Xic region is methylated so it will NOT be active.
Occurs in a “window” of time during development

active and inactive regions
Active and inactive regions

Red: active genes.

Black: inactive

Xic is responsible for this process; if moved to an autosome, that chromosome will be inactivated.

Besides XIC, a few other genes on the chromosome remain active. Logically, they are genes also found in the pseudoautosomal region of the Y chromosome.

polygenic traits
Polygenic Traits
  • Polygenic traits: different from multiple genes
    • Seems like it should be the same, but no
    • Also called Quantitative traits
    • Polygenic traits are different in AMOUNT not TYPE
      • Range of heights vs. purple/white
      • Traits studied by Mendel: “discrete”
    • Polygenic traits usually show continuous variation
      • Height, weight, eye color, etc.
        • Number of phenotypic classes depends on how much you subdivide.
polygenic traits 2
Polygenic Traits-2
  • Some polygenic traits are”meristic”
    • Must be integers; meristic traits must be counted
      • Number of kernels of corn can’t be continuous
  • Offspring of crosses appear blended
    • Still fit into Mendel’s notion of unit factors
    • Multiple genes, and their alleles, are additive or not
      • The total number of additive alleles determines the phenotype.
  • Usually studied using statistics
    • Distribution of traits follows bell curve
    • Mean, standard deviation, and variance
quantitative traits are mendelian
Quantitative traits are Mendelian
  • Example: red and white wheat.
    • Red results from an additive allele, “white” is the absence of of additive alleles.
    • When the F1 plants are crossed, an apparently continuous range of phenotypes is produced.

Including a “white” which is 1/16 of total. Closer view: 1:4:6:4:1

Five phenotypic classes:

4+ alleles, 3+ alleles, 2+ alleles, 1+ allele, none

continuous variation
Continuous variation
  • Traits usually quantifiable (weighing, etc.)
  • Two or more genes contribute to phenotype in an additive way.
    • Individual allele either adds to phenotype or doesn’t
  • Effect of each allele is small (but adds up)
    • Lots of incremental effects create wide range of phenotypic variation,
  • Study requires large numbers of individuals
continuous variation 2
Continuous variation-2

Variation appears continuous because these traits often affected by the environment.

note bell curve.