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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

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Gene dosage

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  1. 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?

  2. 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

  3. 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

  4. Descent of cells: How mosaics are made. Events during development. Two homologous chromosomes, blue & red. Black indicates inactivation = Barr body

  5. 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. http://www.petstreetmall.com/merchant/Embroidery/Cat/CalicoCatBody.gif.jpe

  6. 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.

  7. Occurs in a “window” of time during development http://bioweb.wku.edu/courses/biol566/Images/PlathF2.jpg

  8. 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.

  9. 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.

  10. 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

  11. 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

  12. Five phenotypic classes: 4+ alleles, 3+ alleles, 2+ alleles, 1+ allele, none

  13. Summary of polygenic idea

  14. 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

  15. Continuous variation-2 Variation appears continuous because these traits often affected by the environment. note bell curve.

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