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Chromosomal Basis of Inheritance

Chromosomal Basis of Inheritance . Chapter 15. 1910 | 1933. Morgan & Drosophila . Thomas Hunt Morgan - first to associate a specific gene with specific chromosome in early 20th century Drosophila breeding good genetic subject prolific 2 week generations 4 pairs of chromosomes

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Chromosomal Basis of Inheritance

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  1. Chromosomal Basis of Inheritance Chapter 15

  2. 1910 | 1933 Morgan & Drosophila • Thomas Hunt Morgan - first to associate a specific gene with specific chromosome in early 20th century • Drosophila breeding • good genetic subject • prolific • 2 week generations • 4 pairs of chromosomes • 3 pair autosomes, 1 pair sex chromosome • XX=female, XY=male

  3. Classes of chromosomes autosomalchromosomes sexchromosomes

  4. Wild Type • Morgan looked for varied traits • Discovered a single male fly with white eyes instead of the usual red • The normal character phenotype is wild type • Alt traits are mutant phenotypes

  5. Discovery of sex linkage true-breeding red-eye female true-breeding white-eye male X P Huh!Sex matters?! 100% red eye offspring F1 generation (hybrids) 50% red-eye male 50% white eye male 100% red-eye female F2 generation

  6. What’s up with Morgan’s flies? x x RR rr Rr Rr  r r R r R Rr Rr R RR Rr Doesn’t workthat way! R r Rr Rr Rr rr 100% red eyes 3 red : 1 white

  7. Genetics of Sex • In humans & other mammals, there are 2 sex chromosomes: X & Y • 2X chromosomes • develop as a female: XX • gene redundancy,like autosomal chromosomes • an X & Y chromosome • develop as a male: XY • no redundancy X Y X XX XY XX XY X 50% female : 50% male

  8. Let’s reconsider Morgan’s flies… x x XRXR XrY XRXr XRY Xr Y XR Y  XR XR XRXr XRY XRXR XRY BINGO! XR Xr XRXr XRY XRXr XrY 100% red females 50% red males; 50% white males 100% red eyes

  9. Sex-linked Gene • Sex-linked genes appear on X chrom • Females (XX) may have 2 red-eyed alleles or may be heterozygous (all have red eyes) • Males (XY) have only 1 allele and will be red eyed ONLY if they have red-eyed allele (otherwise white allele – white)

  10. Genes on sex chromosomes • Y chromosome • few genes other than SRY • sex-determining region • master regulator for maleness • turns on genes for production of male hormones • many effects = pleiotropy! • X chromosome • other genes/traits beyond sex determination • mutations: • hemophilia • Duchenne muscular dystrophy • color-blindness

  11. Ichthyosis, X-linked Placental steroid sulfatase deficiency Kallmann syndrome Chondrodysplasia punctata, X-linked recessive Hypophosphatemia Aicardi syndrome Hypomagnesemia, X-linked Ocular albinism Retinoschisis Duchenne muscular dystrophy Becker muscular dystrophy Chronic granulomatous disease Retinitis pigmentosa-3 Adrenal hypoplasia Glycerol kinase deficiency Norrie disease Retinitis pigmentosa-2 Ornithine transcarbamylase deficiency Incontinentia pigmenti Wiskott-Aldrich syndrome Menkes syndrome Androgen insensitivity Sideroblastic anemia Aarskog-Scott syndrome PGK deficiency hemolytic anemia Charcot-Marie-Tooth neuropathy Choroideremia Cleft palate, X-linked Spastic paraplegia, X-linked, uncomplicated Deafness with stapes fixation Anhidrotic ectodermal dysplasia Agammaglobulinemia Kennedy disease PRPS-related gout Lowe syndrome Pelizaeus-Merzbacher disease Alport syndrome Fabry disease Lesch-Nyhan syndrome HPRT-related gout Immunodeficiency, X-linked, with hyper IgM Lymphoproliferative syndrome Hunter syndrome Hemophilia B Hemophilia A G6PD deficiency: favism Drug-sensitive anemia Chronic hemolytic anemia Manic-depressive illness, X-linked Colorblindness, (several forms) Dyskeratosis congenita TKCR syndrome Adrenoleukodystrophy Adrenomyeloneuropathy Emery-Dreifuss muscular dystrophy Diabetes insipidus, renal Myotubular myopathy, X-linked Albinism-deafness syndrome Fragile-X syndrome Human X chromosome • Sex-linked • usually means“X-linked” • more than 60 diseases traced to genes on X chromosome

  12. Map of Human Y chromosome? < 30 genes on Y chromosome Sex-determining Region Y (SRY)

  13. Sex- linked gene • Why is crossing over not as common with the sex chromosomes? • What types of genes are on the sex chromosomes? • If there is a gene at the top of the X chromosome, how many alleles does a man have for that gene?

  14. XHXH XHXh XHXh XHXh XH XHY XHY XhY Xh male / sperm XH Y XHXH XHY XHY XH female / eggs Xh XhY XHXh sex-linked recessive Hemophilia Hh x HH XH Y carrier disease

  15. XH XHXh Xh X-inactivation • Female mammals inherit 2 X chromosomes • one X becomes inactivated during embryonic development • condenses into compact object = Barr body • which X becomes Barr body is random • patchwork trait = “mosaic” patches of black tricolor catscan only befemale patches of orange

  16. Male pattern baldness • Sex influenced trait • autosomal trait influenced by sex hormones • age effect as well = onset after 30 years old • dominant in males & recessive in females • B_ = bald in males; bb = bald in females

  17. Learning Check • If a gene for male pattern baldness is carried on the X chromosome, who in the family would pass the trait to their son? • Since females have two X chromosomes and males only have one, women end up with excess genetic info. What does the cell do to make up for this extra amount of DNA? • So, if one X chromosome is turned off, why are women not as susceptible as men to sex-linked traits such as color-blindness, balding, MD, and hemophilia?

  18. Gene linkage • Cell 1- Diploid cell w/ homologous chromosomes. traits A and B are on different chromosomes. Heterozygous. • Chance that they will produce gamete with genotype AB? aB? Ab? Or ab? (50%) • Cell 2- genes A and B are on opposite ends of the same chromosome • Chance that they will produce gamete with genotype AB? aB? Ab? Or ab? (recombination frequency will be 50% due to crossing over) • Cell 3- genes are on the same chromosome close together • Probability of recombination decreases because the genes are less likely to cross over, segregate and independently assort. They will be “linked”.

  19. Learning Check • As genes are located closer to one another, what happens to their recombination frequency?

  20. Recombination frequencies • If gene A recombines 12% of the time with gene B, but 16% of the time with gene C, which gene is closer to gene A? • If gene B and C recombine with one another 28% of the time, what is the order of genes A, B, and C along the chromosome? B A C 12 16 28

  21. Learning Check • The following is a map of four genes on a chromosome: • Between which two genes would you expect the highest frequency of recombination? • A) A and W • B) W and E • C) E and G • D) A and E • E) A and G

  22. Lab 7: Genetics (Fly Lab)

  23. Lab 7: Genetics (Fly Lab) • Description • given fly of unknown genotype use crosses to determine mode of inheritance of trait

  24. Lab 7: Genetics (Fly Lab) • Concepts • phenotype vs. genotype • dominant vs. recessive • P, F1, F2 generations • sex-linked • monohybrid cross • dihybrid cross • test cross • chi square

  25. Lab 7: Genetics (Fly Lab) • Conclusions: What is the genotype of the parent generation? Case 1 Case 2

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