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Linked genes—on the same chromosome —tend to be inherited together

Linked genes—on the same chromosome —tend to be inherited together Model organism: Drosophila melanogaster —fruit fly Wild type—most common phenotype (+) Mutant phenotype—any other phenotype Drosophila genes are usually named after the mutant phenotype. Body color b + = gray

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Linked genes—on the same chromosome —tend to be inherited together

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  1. Linked genes—on the same chromosome —tend to be inherited together Model organism: Drosophila melanogaster—fruit fly Wild type—most common phenotype (+) Mutant phenotype—any other phenotype Drosophila genes are usually named after the mutant phenotype

  2. Body color b+ = gray b = black Wing shape vg+ = normal vg = vestigial What ratio of phenotypes would be expected from the F1 cross?

  3. When there are more parental phenotypes produced than recombinant phenotypes, that suggests that the genes are linked. Recombination frequency = # of recombinant offspring total offspring Map units represent relative distances between genes on a chromosome. 1 map unit is equivalent to a 1% recombination frequency x 100

  4. Linkage map—shows relative locations (a linear map) of genes on a chromosome We can use recombination frequencies between each gene pair to figure out where the genes are located. b—cn 9% cn—vg 9.5% b—vg 17%

  5. The X and Y chromosomes have short segments that are homologous so that they can separate in meiosis. SRY = sex-determining region of the Y chromosome --required for development of the testes --codes for a protein that regulates other genes involved in sex determination Sex-linked genes—genes located on either the X or the Y chromosome

  6. Most Y-linked genes help determine sex, but there are many X-linked genes that are unrelated to sex determination. Females have 2 copies of the X chromosome, so they need 2 recessive alleles to exhibit the recessive phenotype. XAXA XAXa XaXa Males have only 1 X chromosome, so they only need 1 recessive allele to exhibit the recessive phenotype. XAY XaY

  7. Males inherit X-linked traits from their mothers. • Examples of X-linked traits: • Hemophilia • Red-green color blindness • Male-pattern baldness • Duchenne muscular dystrophy

  8. X Inactivation—most of one X chromosome in females becomes inactivated during early embryonic development. Barr body—inactivated X chromosome in each cell condensed so that most genes aren’t expressed The Barr bodies are reactivated in the cells in the ovaries that undergo meiosis to form eggs.

  9. The X chromosome that becomes inactivated in each cell is random and independent of any other cell. Females are mosaics of two types of cells. If a female is heterozygous for a sex-linked trait, about half of her cells will express the X from mom and the other half will express the X from dad. Tortoiseshell cat

  10. Nondisjunction—when homologous chromosomes or sister chromatids don’t separate properly during meiosis, causing an abnormal number of chromosomes in a gamete

  11. Trisomy 21—Down’s Syndrome—usually results from nondisjunction in meiosis I • Characteristic facial features • Short stature • Correctible heart defects • Susceptibility to respiratory infection • Mental development delays • Sterility (often)

  12. Aneuploidy of sex chromosomes is more common than aneuploidy of autosomes. XXY—Klinefelter Syndrome—phenotypically male but sterile XYY—tend to be somewhat taller than average, fertile XXX—phenotypically female, at risk for learning disabilities, slightly taller than average, fertile XO—Turner Syndrome—phenotypically female, sterile

  13. Deletions of chromosomes cause severe problems. Ex. Cri du chat (Cry of the cat) syndrome— deletion in chromosome 5 Chromosomal translocation occurs when a fragment from one chromosome attaches to another, nonhomologous, chromosome. Ex. chronic myelogenous leukemia (CML)

  14. Genomic Imprinting— • when phenotype differs depending on whether an autosomal gene is inherited from mom or dad • occurs during gamete formation • the zygote expresses only one allele of an imprinted gene (maternal or paternal) • the imprints are passed down to new cells during mitosis • in each generation, “old” imprints are erased in gamete-producing cells, and are imprinted according to the sex of the individual • imprints often consist of methyl groups attached to the DNA

  15. Insulin-like growth factor (Igf2)—only the paternal gene is expressed When a mouse is heterozygous, the phenotype is dependent on whether the normal allele came from mom or dad.

  16. Angelman syndrome—deletion or inactivation of genes on maternally inherited chromosome 15 (paternal copy is imprinted and silenced) Prader-Willi syndrome—deletion or inactivation of genes on paternally inherited chromosome 15 (maternal copy is imprinted and silenced) Extranuclear DNA (from mitochondria and other organelles) is maternally inherited

  17. If there’s a difference between observed and expected results in a genetic cross, we use the χ2 test to see if the difference is significant. Null hypothesis—assume the difference is NOT significant χ2 [ ] = Σ (o-e)2 o = observed e = expected e Degrees of freedom = # of expected phenotypes - 1

  18. Use table with p = .05 (~95%) If χ2 is < critical value, then null hypothesis is accepted  difference is not significant If χ2 is > critical value, then null hypothesis is rejected  difference is significant

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