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Genetic Exceptions

Genetic Exceptions. Test crosses, Incomplete D ominance, and C odominance. Test Cross. Used to determine an unknown dominant genotype. Offspring that show a dominant phenotype could have either a homozygous dominant ( RR ) or heterozygous ( Rr ) genotype…

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Genetic Exceptions

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  1. Genetic Exceptions Test crosses, Incomplete Dominance, and Codominance

  2. Test Cross • Used to determine an unknown dominant genotype. • Offspring that show a dominant phenotype could have either a homozygous dominant (RR) or heterozygous (Rr) genotype… How can the second allele be determined?

  3. Test cross example • In mice, black (R) is dominant to white (r) • Take a black mouse with an unknown dominant genotype and cross it with a homozygous recessive (rr) white mouse, then observe the color of the offspring. R ? R ? r r r r No white offspring then unknown allele is _____. If any offspring are white, the unknown allele is ______. R r

  4. Incomplete Dominance • According to Mendel’s Law of Dominance; if dominant allele is present then the dominant trait will be expressed. However...there are always exceptions to the rules!!! • In some organisms; both the dominant(R) and recessive(r)determine phenotype • Phenotype that results is a “blending” of proteins. • Alleles themselves do not combine, only the characteristics(proteins) they code for do.

  5. Example: Flower Color Red (R) Blue (r) FLASHLIGHT R R R r Rr Rr R r Rr RR r Rr rr Rr r Rr All F1= Heterozygous (Rr) F2 = 1 RR : 2 Rr: 1 rr All Heterozygous genotypes will show the blending of phenotypes. NOT MIXING ALLELES, MIXING COLORS OF PROTEINS !

  6. SNAPDRAGONS ILLUSTRATE INCOMPLETE DOMINANCE GENERALLY; MOST ‘BLENDED’ FLOWER COLORS ILLUSTRATE INCOMPLETE DOMINANCE!

  7. CO-DOMINANCE • aka: Shared Dominance. • Two dominant alleles code two different proteins. • R’ = the codominant allele • No blending of characteristics. • All dominant proteins are coded.

  8. Example 1: Coat color R= brown R’= black R R’ R R’ RR RR’ RR’ R’R’

  9. How are they similar? How are they different? They are both codominant for black and white…. The panda has a gene that codes for color placement.

  10. Example 2: Sickle Cell R= Round R’= Sickle Persons with RR’ suffer from milder symptoms than R’R’ because they have some normal cells R R’ R R’ RR RR’ RR’ R’R’

  11. NORMAL RR SICKLE CELL RR’

  12. Example 3: Blood Types • 3 alleles code for blood type: IA IBand i(type O) • A is codominant to Band both are expressed: Type AB • Ais dominant to O : Type AA or AO • B is dominant to O : Type BB or BO • Ois recessive: Type OO

  13. Taste Receptor: Phenylthiocarbamide (PTC) • The “taster” gene is incompletely dominant to the “non-taster” gene. • People with two copies of the taster gene taste the chemical compound more intensely than those with only one copy. • Chemicals similar to PTC are found in plants of the mustard family. (Broccoli, mustard, etc.) • This gene is found among many other taster genes on chromosome 7.

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