Exercise 1: Dominance in Peas

    Let’s review how dominant and recessive traits exert their influence. In Mendel’s garden, peas, the allele for tall plants (T) is dominant to the allele for short plants (t).

 Questions:
1.       What is the genotype of a homozygous tall pea plant?

2.       What kinds of gametes can a homozygous tall pea plant produce?

3.       What is the genotype of a short pea plant?

4.       If a homozygous tall plant is crossed with a short plant, what are all of the possible genotypes of the
           offspring? What phenotypic proportions would you expect?

 Exercise 2: Dominance In Humans

     In humans, pigmentation of the skin is the result of a pigment known as melanin. If you don’t have the dominant form of the gene that codes for the production of melanin, you are an albino.  In humans, pigmented skin is dominant (M) to albino skin (m). -

Questions:

1.       If an albino man marries a pigmented woman whose father was an albino, what proportion of their offspring would be expected to be albino?

2.       If two pigmented people, each of whom had an albino parent marry, what proportion of their offspring would be expected to be albino? 

3.       If an albino man marries an albino woman, what proportion of their offspring would be expected to be albino?

 Exercise 3: Dominance In Corn

     In some varieties of corn, the color of corn kernels (a kernel is actually a seed and an offspring of the parent plants) is determined by a single gene. Examine the ear of com provided. Notice that there are two colors of kernels, light yellow (or almost white) and purple. Count several rows of corn kernels and determine the number of yellow and purple kernels. Then determine the genotypes of the parents.

 Color of Corn Kernels:

 Number of purple:        _____              Ratio of purple to yellow:          _____

 Number of yellow:        _____              Probable genotype of parents:   _____

Number of smooth:        _____               Ratio of smooth to wrinkled:     _____

Number of wrinkled:      _____               Probable genotype of parents:  _____

Exercise 4: Incomplete Dominance in Snapdragons

     In some cases, neither allele is dominant over the other allele, this is an example of incomplete dominance.  In this case, individuals heterozygous for the trait would exhibit an intermediate phenotype. Thus, the genotypic ratio is the same as the phenotypic ratio in cases of incomplete dominance. For example, in snapdragons, red flowered plants (RR) and white flowered plants (rr) are both homozygous. If a red snapdragon is crossed with a white snapdragon, all of the offspring have pink flowers (Rr) because the genes that code for the production of red and white pigments (R and r, respectively) are both expressed. Work through the following problems:

 Questions:

1.      In a cross between two pink snapdragons, what are the expected phenotypic and genotypic ratios of the offspring? 

2.      In a cross between a white and pink snapdragon what are the expected phenotypic and genotypic ratios of the offspring? 

3.      In a cross between a red and a pink snapdragon, what are the expected phenotypic and genotypic ratios of the offspring?

Exercise 5: Incomplete Dominance in Chickens

     In some varieties of chickens, as in snapdragons, incomplete dominance involves three colors: white (aa) blue
(Aa), and black (AA).

 Question:

1.        If a blue male chicken is mated with a blue female, what are the expected phenotypic and genotypic ratios of the offspring? 

2.      If a blue male chicken is mated with a black female, what are the expected phenotypic and genotypic ratios of the offspring?

3.      If a white male chicken is mated with a black female, what are the expected phenotypic and genotypic ratios of the offspring?

Exercise 6: Sex Linked Traits In Humans 

In humans, sex is determined by a pair of sex chromosomes the X and the Y chromosome. In females, the sex chromosomes are represented by “XX” Males have the “XY” combination because it’s the Y chromosome that determines maleness. In humans, red-green color blindness is attributed to a gene on the X chromosome (there are no sex linked traits on the Y chromosome). Such traits are said to be sex linked. Look at the following Punnett square to see how sex linked problems can be worked out.  In this case, a woman who is a carrier for the red-green color blindness gene (a) marries a man with normal color vision (A). Fill in the Punnett square and answer the following questions.

 Questions:

1.      A man whose mother was colorblind and whose father had normal color vision marries a woman with the same type of parents. What are the expected phenotypic and genotypic ratios of their sons and daughters?

Exercise 7: Dihybrid Cross

     Genetic crosses involving two sets of alleles are called dihybrid crosses. As in monohybrid crosses, the easiest way to analyze the possible results of a dihybrid cross is with the Punnett square. Make sure you correctly determine all of the possible gamete combinations first.

 Questions:

1.      In some varieties of corn, yellow kernel color is dominant to purple color and smooth seeds are dominant to wrinkled seeds. A plant that is homozygous for both yellow seeds and smooth seeds is crossed with a plant that produces purple, wrinkled seeds.  What are the genotypic and phenotyptc proportions of the offspring (i.e., the seeds)?

2.       If offspring from the above cross are mated with each other, what are the expected phenotypic and genotypic ratios of the offspring? You may want to use the Punnett square below!

3.      An ear of corn will be provided that is a result of the cross in question #2 above. Count several rows of corn kernels and determine the total number of kernels represented by each of the two colors and seed textures and record this information below. How close do the genotypic ratios follow the expected ratios?

 Color and Texture of Corn Kernels: 

 Exercise 8: Human Heredity

     In this exercise, we will concentrate on some human traits. Determine or yourself and your lab partner whether or not you express the traits described below. Next, determine your possible genotypes for these traits and fill in the table on the next page. In each case, the capital. letter represents the dominant allele. 

1.      DIMPLED CHIN. A clef tin the chin (like Kirk Douglas) is dominant (allele D).

2.      FREE EAR LOBE. In most people. The ear lobes hang free (dominant allele E), but in a person with two recessive alleles (e) the ear lobes are directly attached to the head.

 3.      WIDOWS PEAK. The presence of the dominant allele (W) results in a hairline that forms a distinct point
       on the forehead. 

4.      TONGUE ROLLING. If you can roll your tongue into a U” shape, you possess the dominant allele (T). 

5.      HITCHHIKER’S THUMB. If you can bend the distal (upper) joint of your thumb backward, you are homozygous recessive (hh) for this condition. 

6.      INTERLOCKING FINGERS. When you fold your hands, does the left thumb lay or top of the right thumb? If so, you have the dominant allele (F) for this trait. 

7.      PTC TASTING. Some persons detect a bitter taste of the chemical phenylthiocarbamide (PTC) whiles others do not taste it.  If you can taste PTC, you have at last one dominant allele (P) for this trait.   Place a PTC paper strip on the top of your tongue for 5-10 seconds. If you can taste PTC, you will know it!! 

8.      DARWIN’S EAR POINTS, If the outermost ridge of cartilage of your earlobe has “bumps,” you have the dominant allele (P) for this trait. 

9.      BIG TOE LENGTH. If your big toe is longer than your second toe, you have a dominant allele (B) for this trait. 

10.  PIGMENTED IRISES.  When a person is homozygous for the recessive allele (p), there is no pigment in the front part of the eyes and a blue layer at the back shows through resulting in blue eyes.  Any color pigment in front of the eyes (hazel, brown, green, violet) is the dominant condition. 

11.  BENT LITTLE FINGER:  A dominant allele (B) causes the upper joint of the little finger to bend inward toward the fourth finger.  Lay both hands flat on the lab table, relax your muscles, and note whether you have a bent or straight little finger. 

12.  HANDEDNESS.  Right handedness is dominant (R) to the left handedness (r). 

Exercises in Molecular Genetics: