Extended inheritance patterns

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Extended inheritance patterns

  1. 1. Extended Inheritance Patterns
  2. 2. Other Inheritance Patterns• Incomplete dominance – Heterozygosity at a locus produces a third 3 phenotype intermediate to the two homozygous phenotypes• Co-dominance – Heterozygosity at a locus produces a single unique phenotype different from either homozygous condition• Overdominance – Heterozygosity at a locus creates a phenotype that is more beneficial or more deterimental than homozygosity of either locus with any allele
  3. 3. Other Inheritance Patterns• Sex-linked – inheritance of genes on that are unique to a sex chromosomes• Sex-influenced – An allele is expressed differently in each sex. Behaving dominantly in one sex and recessively in the other• Sex-limited – An allele is only expressed in one or the other sex
  4. 4. Complete Dominance/Recessiveness• recessive allele does not affect the phenotype of the heterozygote• two possible explanations – 50% of the normal protein is enough to accomplish the protein’s cellular function – The normal gene is “up-regulated” to compensate for the lack of function of the defective allele • The heterozygote may actually produce more than 50% of the functional protein
  5. 5. Incomplete Dominance• heterozygote exhibits a phenotype intermediate to the homozygotes• Also called intermediate dominance or dosage effect• Example: – Flower color in the four o’clock plant governed by 2 alleles • CR = wild-type allele for red flower color • CW = allele for white flower color
  6. 6. Incomplete Dominance
  7. 7. 1:2:1 phenotypic ratio NOT the 3:1 ratio observed in simple Mendelian inheritance In this case, 50% of the CR protein is not sufficient to produce the red phenotypeFigure 4.2
  8. 8. Incomplete Dominance• complete or incomplete dominance can depend on level of examination
  9. 9. Multiple Alleles• The term multiple alleles is used to describe situations when three or more different alleles of a gene exist• Examples: – ABO blood – Coat color in many species – Eye color in Drosophila
  10. 10. Multiple Alleles• ABO blood phenotype is determined by multiple alleles• ABO type result of antigen on surface of RBCs – Antigen A, which is controlled by allele IA – Antigen B, which is controlled by allele IB – Antigen O, which is controlled by allele i N-acetyl- galactosamine
  11. 11. Co-dominance• Alleles IA and IB are codominant• They both encode functional enzymes and are simultaneously expressed in a heterozygous individual• Allele i is recessive to both IA and IB
  12. 12. Multiple Alleles• coat color in rabbits – C (full coat color) – cch (chinchilla pattern of coat color) • Partial defect in pigmentation – ch (himalayan pattern of coat color) • Pigmentation in only certain parts of the body – c (albino) • Lack of pigmentation
  13. 13. Multiple Alleles• Dominance hierarchy will exist for multiple alleles called an allelic series – allelic series for ABO type • IA = IB > i – allelic series for rabbit coat color alleles : • C > cch > ch > c
  14. 14. Conditional Mutations• The ch allele is a temperature-sensitive conditional mutant – The enzyme is only functional at low temperatures – Therefore, dark fur will only occur in cooler areas of the body
  15. 15. Overdominance• Overdominance is the phenomenon in which a heterozygote is more vigorous than both of the corresponding homozygotes• Example: – Sickle-cell heterozygotes are resistant to malaria – increased disease resistance in plant hybrids

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