1 / 23

Extensions of Mendelian Genetics

Extensions of Mendelian Genetics. Mendelian genetics seems to be relevant to only a small set of heritable features For only a few characters there are… Only 2 versions of an allele (green or yellow) 1 gene codes for a single external character 1 allele is completely dominant to the other

teleri
Download Presentation

Extensions of Mendelian Genetics

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript


  1. Extensions of Mendelian Genetics • Mendelian genetics seems to be relevant to only a small set of heritable features • For only a few characters there are… • Only 2 versions of an allele (green or yellow) • 1 gene codes for a single external character • 1 allele is completely dominant to the other • The basic patterns of segregation & independent assortment apply to more complex patterns of inheritance

  2. Different Types of Dominance • Complete dominance • Phenotype of heterozygote & HomoD are indistinguishable • The pattern with which you are already familiar • Incomplete dominance • Phenotype of heterozygote is in between the 2 Homo phenotypes • Example: pink snapdragons

  3. Snapdragons – Incomplete Dominance

  4. So incomplete dominance does NOT provide evidence for “blending” theories

  5. Codominance • Codominance • Phenotype of heterozygote is separate & distinguishable from Homozygous Dominant & Homozygous Recessive • Example: AB blood type or Rhododendron flower

  6. Dominance & Phenotype • The observed dominance/recessiveness of alleles depends on the level of the investigation • Consider Tay-Sachs disease • Brain cells of the baby do not metabolize certain lipids • As lipids accumulate, seizures, blindness, and mental degeneration • Death occurs within a few years of conception

  7. Tay-Sachs Disease • At the Organismal level, the disease is recessive • Only children with 2 copies of the recessive trait will have the malady • Heterozygote is not afflicted – they produce some lipid-metabolizing enzyme, though not as much as in HomoD • So intermediate enzyme production • This suggests that @ the biochemical level, the disease is an example of incomplete dominance • Which is Tay-Sachs: dominance or incomplete dominance?

  8. Prevalence & Dominance • Polydactyly • Extra fingers or toes • 1 of 400 in the US • The allele for polydactyly is dominant, but rarely present • Recessive homozygotes (HomoR) are found 399 out of 400 instances

  9. Blood Typing • Only 2 alleles existed for Mendel’s peas, but this is not typical for most traits • Consider ABO blood group in humans • A refers to the “A” membrane carbohydrate & type A blood • B refers to (seriously, I’m not writing this down) • O means neither A or B carbohydrate is found • AB means both A & B are found • BUT the A and B alleles are codominant and are both expressed if an individual inherits both alleles

  10. Epistasis • A gene at one locus alters the phenotypic expression of a gene at a second locus • Example: Mouse fur color • Bb or BB = Black bb = brown • If HomoR for (C) gene [cc], then no fur color (albino or white) • Regardless of fur color specified by brown-black gene • If NOT HomoR for (c) gene [Cc or CC], then can be brown (bb) or black (Bb or BB)

  11. What is the phenotype of… • BBcc? • BbCc? • bbCC? • Bbcc? • BBCC? • bbcc?

  12. Pleiotrophy • Single gene has multiple effects • Should be unsurprising given intricate molecular and cellular interactions for development of an organism • Phenylketonuria • Mental Retardation • Reduced skin and hair pigmentation

  13. Polygenic Inheritance • The additive effect of 2 or more genes on 1 phenotypic character • Called quantitative characters since there is a continuum of gradations • Normal curve of phenotypes • Example: human skin pigmentation is determined by at least 3 separately inherited genes • AABBCC = Dark • AaBbCc = Intermediate • aabbcc = Light

  14. Pedigree Analysis • Family tree describing the interrelationships of parents & children across the generations

  15. Recessively Inherited Disease • Requires 2 copies of the recessive allele (Homozygous Recessive) to express the mality • Heterozygotes are called carriers • Normal phenotype, but may transmit disease to offspring • Examples: • Cystic Fibrosis • Tay Sachs disease • Sickle-cell disease

  16. Cystic Fibrosis • Recessive autosomal disease • Common in those of European descent • 1 of 2,500 affected, but 1 of 25 are carriers • Affects Chloride ion transport between a cell and extracellular fluid • If untreated, most die before 5th birthday • Typically, patients live until their 20s or 30s with efficacious treatment

  17. Sickle-Cell Anemia • Recessive autosomal disease • African descent • Affects Hemoglobin protein in RBCs • Low blood oxygen = hemoglobin molecules clump together forming sickle shaped RBCs • Sickle-celled RBCs clump together creating chronic vascular occlusion of small vessels • Example of incomplete dominance • Heterozygotes are usually normal but will show some symptoms during prolonged periods of reduced blood oxygenation

  18. Dominant Alleles • Most harmful alleles are recessive, but some human diseases are due to dominant alleles • Only require one copy of the allele to be expressed • Hypothesis: if there is a lethal disease carried on a dominant allele, it would have burned out its carriers by now. • UNLESS, the lethal disease carried by a dominant allele is one that affects organisms of advanced age • Like Huntington’s disease

  19. Examples of Dominant Allele Disease • Achondroplasia – form of dwarfism • Heterozygous individual = dwarf • 1 in 25,000 have achondroplasia, so 99% of the population are HomoR • Huntington’s disease • Caused by a lethal dominant allele • Degenerative disease of nervous system • Usually only affects those > 40 yrs old

  20. Genetic Testing • Pedigree analysis gives some info about risk to offspring • There are also tests to identify carriers of certain genetic diseases • Amniocentesis – amniotic fluid is removed and then cells contained in the fluid are cultured to identify certain chromosomal defects via karyotype • Chorionic villus sampling (CVS) – placental tissue is removed for same purpose as amnio, but results are available far faster, & can be performed earlier in pregnancy • Karyotype can be immediately obtained

More Related