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Patterns of Intermediate Inheritance Exceptions to Mendel’s Principles

Mendel’s 3 principles provide us with an important foundation in building our knowledge of genetics. However, there are many types of inheritance patterns that do not follow the principle of dominance. There are several types of intermediate inheritance patterns: Co-dominance Multiple Alleles

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Patterns of Intermediate Inheritance Exceptions to Mendel’s Principles

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  1. Mendel’s 3 principles provide us with an important foundation in building our knowledge of genetics. However, there are many types of inheritance patterns that do not follow the principle of dominance. There are several types of intermediate inheritance patterns: Co-dominance Multiple Alleles Incomplete Dominance Polygenic Traits Multifactorial Inheritance Patterns of Intermediate InheritanceExceptions to Mendel’s Principles

  2. Co-dominance • A trait for which both alleles contribute equally to the phenotype. • Example: Coat color in cows and horses • When a homozygous red cow is crossed with a homozygous white cow they produce a heterozygous cow that is red and white (roan). Genotype: WW Genotype: RR Genotype: RW

  3. A horse that is roan is a blend of red and white colors that result when a red horse is crossed with a white horse. Red (R) horses breed true; and white (W) horses breed true. What is the genotype of a red horse? RR What is the genotype of a white horse? WW What is the genotype of a roan horse? RW What is the probability that two roan horses will have at least one roan offspring? 50% What is the probability that two roan horses will have a white offspring? 25% Practice Cross - Codominance R W R W

  4. Multiple Alleles • When a gene has more than two possible alleles. • ONLY TWO alleles are inherited. • Example: Rabbit Coat Color is controlled by 4 alleles - C, cch, ch, and c Himalayan color albino chinchilla color chc or chch cc wild type color cchcch, cchch, cchc CC, Ccch, Cch, Cc

  5. Practice cross: Multiple Alleles • Practice cross: In rabbits there are four different color alleles: • C is dominant over all C = wild type color • cch is dominant to ch & c cch = chinchilla color • ch is dominant to c ch = Himalayan color • C is recessive to all c = albino • Each rabbit can inherit only two alleles. A wild colored rabbit is crossed with an albino and in the offspring there is a Himalayan rabbit.

  6. Show the cross between a wild type and albino rabbit that has a Himalayan baby. • What are the genotypes of the parents? • Parent one : cc • Parent two : Cch • What is the probability that they will have an offspring that is Himalayan? • 50% • What is the probability that they will have an offspring that is albino? • 0% • What is the probability that they will have an offspring that is wild colored? • 50% C ch Cc c ch c c ch c Cc Mult. Alleles & Co-dominance

  7. Incomplete Dominance • Results in a heterozygous phenotype that is a blend of the two homozygous phenotypes. • Examples: mirabilis (four o’clock) plants, coat color in horses • When a homozygous chestnut horse is crossed with a homozygous white horse, they produce a heterozygous tan (palomino) horse.

  8. Genotype: cc (Chestnut) Genotype: ww Genotype: cw (Palomino)

  9. In Four O’Clocks, the gene for red flowers (r) is incompletely dominant to the gene for white flowers (w). The heterozygous condition results in pink flowers. Red Plant Genotype? rr White Plant Genotype? ww Are the F1 flowers purebred or hybrid? hybrid What is the phenotype of the F1 flowers? pink Practice Cross – Incomplete Dominance r r rw rw w w rw rw

  10. Humans & Plants Polygenic • A trait that is controlled by the interaction between 2 or more genes. • Examples: skin color, eye color, height, hair color • Results in a continuum of expressed phenotypes.

  11. Multifactorial Inheritance • The phenotype is a result of an interaction between your genotype and certain environmental factors. • The expression of most all genes is influenced by environmental conditions. • Examples: • Diabetes • Height • Heart Disease • Can inherit a predisposition to all of these diseases/characteristics. • Their development (phenotype) is influenced by environmental factors such as proper nutrition, exercise, quality medical care, etc. Enviro Factors

  12. Examples in plants: • In many plants, the presence of light triggers the production of chloroplasts. • Chemicals that stimulate germination are only produced in the presence of certain temperatures. http://www.dkimages.com/discover/previews/874/30020399.JPG Review of Terms

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