Multiple Alleles of  a Single Gene 

Read the pages indicated by D-5 and study behavioral objectives 12 and 13  involving multiple alleles. The genetic problems discussed up to this point involve only two alleles for each gene (dominant allele and recessive allele). Example: Flower color in roses may be controlled by the gene "R" which  is dominant over "r". The two genes are "R" and "r".  Multiple alleles is where there is three or more alleles for each trait.  The example you need to learn for testing are the Human ABO blood group types. A, B and O blood groups are determined by  any combination of three alleles present as a pair. 

The three alleles are I^A = type  A, I^B = type  B and i = type  O. Also  I^A  and  I^B  are dominant over O, but A and B are codominant. Codominant means that both phenotype will show up in the individual. The "uppercase I and  lowercase i"  are used to show complete dominance and the super scripts A and B indicate codominance. 

Example: I^AI^A and I^Ai   genotypes  shows the phenotype  A 
               I^BI^B and I^Bi   genotype   shows the phenotype B 
               I^AI^B    genotype shows the phenotype  AB 
               ii genotype shows the phenotype O 

The best way to solve genetic problems involving multiple alleles  is to use Punnett squares similar to the one you did with the monohybrid problem set. 

Example: A person homozygous with type A blood marries a person with type O blood may produce children with which blood types? 

                            I^AI^A                X                    ii 

List the genetically different types of  gametes which may be produced by each parent. 

     Neither parent has the  heterozygous gene for this trait; therefore,   2ⁿ  =  2^{0     =  1} 

             P1 =  I^A  only                  X                    P2 =   i  only 
 
 
 

 2.  What does  "i"  represent? 

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 3. What does "I^Ai"  represent? 
 
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4. Give possible phenotypic blood types of the offspring when the father is (ii) and the mother (I^AI^B). 
 
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5. What genotypic blood types must parents have in order to produce offspring of blood type A and blood type O? 
 
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6. What genotypes must parents have in order to produce four different phenotypic blood types? 

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7. To produce a family of blood type A and AB, give the genotypes of the parents. 
 
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Incomplete Dominance 

Read the  pages indicated by D-6 concerning incomplete dominance and  study behavioral objective 4. These problems are  only of a slight variation from the first monohybrid problems discussed. The difference is that the phenotypes of the heterozygous condition will be intermediate between the homozygotes. Since neither allele is dominant uppercase letters are used for both types of alleles and they are differentiated by a prime ( " ' "). 

Example R = red in flower color for  snapdragons and R'  = white in flower color in snapdragons. 

Therefore RR =  red,     R'R'  = white, and RR' =  pink 

Helpful Hint:  Think (Rr) as the same as (R'R'). 

Work the following cross.    P1 = red parent        X           P2  = pink parent     (phenotype) 

                                                     RR               x              RR'                      (genotype) 
 
 

//////////////////////	R	R
R	RR	RR
R'	RR'	RR'

 Crossing a red parent with a pink parent would produce offspring which one half  would have red flowers and one half  would have pink flowers. 

 
For the following problems assume that flower color in roses  incompletely dominant over red with the heterozygous condition being pink, RR = red, RR' = pink, rr = white. 

8. What are the probable genotypes and phenotypes of F1   when both parents are pink four o'clock flowering plants? 
 
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9. State the genotypes of the parents when it was observed that the F1 had 26 pink and 23 white four o' clock plants? 
 
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10.A homozygous red (RR) bull mated with a homozygous white cow (R'R') produces a blend of red and white (roan) offspring. Give the genotypes of the offspring. 

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11. If the parent cattle gave a ratio of 1:2:1, give the genotypes of the parents. 
 
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Polygenic Inheritance 

Read the pages indicated by D-7 and examine behavioral objective 19 and 20 concerning polygenic inheritance.. These problems are similar to incomplete dominance problems above except two or more genes control a single  phenotype producing various shades of expression of this phenotypes such as short, medium short, medium, medium tall, tall  in human heights.   Since neither allele is dominant uppercase letters are used for both types of alleles and they are differentiated by a prime ( " ' "). 
 

 Using human height as an example 
          (H = short; H' = tall; controlled by two sets of genes) 
          (the numbers 1 and 2 refer to genes one H₁H₁ and two H₂H₂) 
 

 H₁H₁ H₂H₂   = short  (no primes) 
 
 H₁H₁ H₂H'₂  or  H₁H'₁ H₂H₂  = medium short (1 prime, it does not matter what gene "1 or 2" the prime  is located on) 

 H₁H₁ H'₂H'₂  or  H₁H'₁ H₂H'₂   = medium (2 primes, it does not matter what genes "1 or 2" they are located on) 
 
 H₁H'₁ H'₂H'₂  =  medium tall (3 primes, it does not matter what genes "1 or 2" they are located on) 

 H'₁H'₁ H'₂H'₂  = tall (4 primes) 
 
 

Example:  What is the phenotypic ratio for the offspring formed by the following parents? 
 

    Parent 1    H₁H'₁ H₂H'_{2                    X}                 Parent 2        H₁H'₁ H₂H'₂ 
 
 The hard part is determing the genotypes of the gametes. In the example above the gametes would be: 

Remember the gametes are haploid and you will need one  H₁  and one H₂  for each gamete. 

Parent 1 :    H₁H_{2 ,}   H₁H'_{2   ,} H'₁ H₂   and    H'₁ H'₂      gametes are haploid 
                   ____    _____   _____           ______ 
Parent 2 :    identical to parent 1    H₁H_{2 ,}   H₁H'_{2   ,} H'₁ H₂   and    H'₁ H'₂   gametes are haploid 

Set up Punnett square. 

 Answer:    1 short to 4 medium short  to 6  medium   to 4  medium  to 1 tall. 
 
 H₁H₁ H₂H₂   = short  (no primes) 
 
 H₁H₁ H₂H'₂  or  H₁H'₁ H₂H₂  = medium short (1 prime, it does not matter what gene "1 or 2" the prime  is located on) 

 H₁H₁ H'₂H'₂  or  H₁H'₁ H₂H'₂   = medium (2 primes, it does not matter what genes "1 or 2" they are located on) 
 
 H₁H'₁ H'₂H'₂  =  medium tall (3 primes, it does not matter what genes "1 or 2" they are located on) 

 H'₁H'₁ H'₂H'₂  = tall (4 primes) 
 
 
 
 
 

12.  Consider a cross between  two parents showing  the following  genotypes as to height. 
 

                  P1  =       ₁H'₁ H₂H'₂        X          H₁H₁ H₂H_{2     = P 2} 

   What are the phenotypes for these parents?. 

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13.   List the  gamete's genotypes for each parent. 
 
          P1  =              H₁H'₁ H₂H'₂        X          H₁H₁ H₂H_{2     = P 2} 

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14.   Determine the phenotype ratio for the offspring which may be produced by these parents. 

             P1  =           H₁H'₁ H₂H'₂        X          H₁H₁ H₂H_{2     = P 2} 
 
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Gene Interactions 

During the above examples (polygenic inheritance) two genes (H₁H₁ and H₂H₂) controlled one trait (height) quantitatively (tall to short). Another type of pattern is where genes determine if other genes are turned on or off.  Read the pages indicated by D-8 and study behavioral objectives 21 and 22. 

Example: In mouse fur color  the pigment melanin is controlled by two sets of genes (A and M). If the melanin distribution gene "M" is  present (MM or Mm)  the mouse's fur will have color depending of the genotype of a second gene (A). If the "A" gene  is AA or Aa than  the fur color will be gray (agouti). If the "A" gene is homozygous recessive than the fur color will be black. In other words black is recessive  to agouti only if the dominant "M" allele is present. If the melanin distribution gene (M) is present as a homozygous recessive pair (mm) than fur color will be white no matter which "A" alleles are present. 

15. What would be the fur color (phenotypes)  for the following genotypes? 

                  A. MmAa 

                  B.  Mmaa 

                  C.  mmAA 
 
 

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You would work genetic problems involving this type of gene interaction the same as you did in solving for dihybrid problems. 

In humans, hair color is controlled by two interacting genes. The same pigment, melanin, is present in both brown-haired and blond-haired individuals, but brown hair has much more of it. Brown hair (B) is dominant to blond (b). Whether any melanin can be synthesized at all depends on another gene. The dominant form (M) allows melanin synthesis; the recessive form (m) prevents melanin synthesis. 

What will be the expected proportions of phenotypes in the children of the following parents? 
 

16.     BbMm   x   bbmm 
 
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This is the end of lesson four. In the next lesson you will learn how sex is determined in mammals and some insects.  Click here to go back to the home page and lesson five Inheritance of Sex and Sex-linked Genes.   click 
 
 
 
 
 
 
 
 
 
 
 
 
 
 
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