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Extensions to Mendel’s laws of inheritance

Extensions to Mendel’s laws of inheritance. Incomplete Dominance, Codominance, Multiple Alleles, and Sex-Linked Traits. AaGg – What are the possible gametes?. AG Ag aG ag. AAGg. AG Ag Why are there only two possible combinations?. AaGg x AAGg.

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Extensions to Mendel’s laws of inheritance

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  1. Extensions to Mendel’s laws of inheritance Incomplete Dominance, Codominance, Multiple Alleles, and Sex-Linked Traits

  2. AaGg – What are the possible gametes? • AG • Ag • aG • ag

  3. AAGg • AG • Ag • Why are there only two possible combinations?

  4. AaGg x AAGg • What is the probability of producing an offspring with the genotype AAGg? • AA -> ½ • Gg -> ½ • ½ x ½ = ¼

  5. Do Mendel’s Laws apply to the inheritance of all traits? • No, Mendel’s studies with peas applied to traits controlled by a single gene with two existing alleles where 1 allele is completely dominant to the other • This is not the case for MOST genes

  6. What are four extensions to Mendel’s Laws • Incomplete Dominance • Codominance • Multiple Alleles • Sex-Linked Traits

  7. Incomplete Dominance

  8. Incomplete Dominance • Heterozygotes show a “blending” or intermediate form of the homozygous forms that exist – No true dominant allele • F2 genotypic ratio is 1:2:1 (HH: Hh: hh) • Ex: Snap Dragons, Hypercholesterolemia

  9. Codominance

  10. Codominance • Two dominant alleles exist • Heterozygotes express both alleles EQUALLY • Ex: Type AB Blood, Speckled Chickens, Roan Cattle, Sickle-cell Anemia

  11. Multiple Alleles • More than two alleles exist for a particular gene, but every organism has only 2 of them • Ex: Blood Types (A B and O alleles) • A and B are codominant • A is dominant to O • B is dominant to O • O is recessive to A and B

  12. Multiple Alleles

  13. Sex-Linked Traits • Genes located on the sex chromosomes, mainly X-chromosome • Males XY; Females XX • Males will show recessively inherited characteristics more often than females. • Ex: Hemophilia, colorblindness, white eyes in flies

  14. Sex linked traits 1910 | 1933 • Genes are on sex chromosomes • as opposed to autosomal chromosomes • first discovered by T.H. Morgan at Columbia U. • Drosophila breeding • good genetic subject • prolific • 2 week generations • 4 pairs of chromosomes • XX=female, XY=male

  15. Classes of chromosomes autosomalchromosomes sexchromosomes

  16. 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) 100% red-eye female 50% red-eye male 50% white eye male F2 generation

  17. 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

  18. 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

  19. 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

  20. 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 • X chromosome • other genes/traits beyond sex determination • mutations: • hemophilia • Duchenne muscular dystrophy • color-blindness

  21. 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

  22. Channel Flipping (FLP) Catching & Throwing (BLZ-1) Self confidence (BLZ-2)note: not linked to ability gene Devotion to sports (BUD-E) Addiction to death &destruction movies (SAW-2) Air guitar (RIF) Scratching (ITCH-E) linked Spitting (P2E) Inability to express affection over phone (ME-2) Selective hearing loss (HUH) Total lack of recall for dates (OOPS) Map of Human Y chromosome? < 30 genes on Y chromosome Sex-determining Region Y (SRY)

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