Module 6 Lesson 2 - 7

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Practice makes perfect, or it at least makes you a little better! Writing crosses and constructing Punnett squares is essential for any student of genetics. Here are two sample crosses with detailed solutions that you can try. 

Sample Problem 1

Height in pea plants is determined by the genes T (dominant) and t (recessive). Cross a homozygous tall pea plant with a dwarf pea plant, and determine the percentage of producing a tall plant.

Sample Problem 2

Height in pea plants is determined by the genes T and t. Cross two heterozygous tall plants, and determine the probability of producing a dwarf plant.

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One of the essential skills in genetics is learning to apply what you know to new situations. Each problem is designed to have you take a fact or discovery you have read about and apply it to a new situation.

1. In pea plants, round seeds (R) are dominant to wrinkled seeds (r). In a genetic cross of two plants that are heterozygous for the seed shape trait, what fraction of the offspring should have round seeds?


2. After mating two organisms that are heterozygous for a single trait, a phenotypic ratio of 3:1 in the offspring is expected when
   
   
   1. the alleles segregate during meiosis
   2. each allele contains two mutations
   3. the alleles are identical
   4. the alleles are incompletely dominant
   5. only recessive traits are scored


3. In Mendel's "Experiment 1", true-breeding pea plants with round seeds were crossed with true-breeding plants with wrinkled seeds. (Round (R) seeds are dominant.) Mendel collected the seeds from this cross, grew F1 generation plants, and let them self-pollinate to form a second generation. Then, he analyzed the seeds of the resulting F2 generation. The results that he obtained, and that you can predict for this experiment are in row
   
   

   	F1	F2
   a.	1/2 round	3/4 round
   b.	1/2 wrinkled	1/4 wrinkled
   c.	All round	All wrinkled
   d.	3/4 round	9/16 round
   e.	All round	3/4 round




4. What probability of green-seeded plants are yielded in the F1 generation of a genetic cross between two heterozygous pea plants having yellow seed? (Yellow (Y) seeds are dominant to green (y).)


5. When true-breeding tall (T) stem pea plants are crossed with true-breeding short (t) stem pea plants, _____ of the F1 generation plants had tall stems.
   
   1. 100%
   2. 50%
   3. 75%
   4. 25%
   5. 0%


6. To identify the genotype of yellow-seeded pea plants as either homozygous dominant (YY) or heterozygous (Yy), you could do a test cross with plants of genotype
   
   1. y
   2. Y
   3. yy
   4. YY
   5. Yy

Diploma Connection

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Answer the following questions from a previous Biology 30 Diploma Examination.

Use the following information to answer the next question.

Farmers who raise sheep for wool try not to produce offspring with black wool. Black wool is very brittle and difficult to dye; therefore, white wool is more desirable. If a farmer purchases a white ram (male), generally he will carry out a test cross to determine whether the ram is heterozygous or homozygous for white wool. White wool (W) is dominant to black wool (w).

1. If the ram (male) is heterozygous for white wool, the expected phenotypes of the offspring of the farmer's test cross are
   
   
   1. all black
   2. all white
   3. 1/2 black and 1/2 white
   4. 3/4 black and 1/4 white




Use the following information about tobiano twin colts to answer the next question.

Descriptions and Symbols Used to Represent One Type of Coat Colour in Horses

1	2	3	4
DNA sequence for coat colour	TT, Tt	T	Tobiano (white spotting pattern)
	tt	t	Not tobiano (no white spotting pattern)



2. What are the genotypes for coat colour of two horses that are predicted to produce offspring in a 1:1 genotypic ratio?
   
   
   1. Tt and tt
   2. Tt and Tt
   3. Tobiano and tobiano
   4. Tobiano and not tobiano




Use the following information to answer the next question.

Sickle cell anemia is an autosomal recessive genetic disorder. Because individuals affected by sickle cell anemia have defective hemoglobin proteins, their blood cannot transport oxygen properly. There appears to be a relationship between the incidence of malaria and sickle cell anemia. Individuals with sickle cell anemia and carriers of the sickle cell allele have some resistance to malaria. Malaria is caused by the parasite Plasmodium and is transmitted among humans by mosquitoes.



3. The probability of two carrier parents having a child with sickle cell anemia is
   
   
   1. 0.25
   2. 0.50
   3. 0.75
   4. 1.0
      




Use the following information to answer the next question.

Cystic fibrosis is the most common genetic disorder among Caucasians, affecting one in 2 000 Caucasian children. The cystic fibrosis allele results in the production of sticky mucus in several structures, including the lungs and exocrine glands. Two parents who are unaffected by the disorder can have a child with the disorder.



4. A girl and both her parents are unaffected by the disease. However, her sister is affected by cystic fibrosis. The genotypes of the mother and father are
   
   
   1. both homozygous
   2. both heterozygous
   3. homozygous and heterozygous, respectively
   4. heterozygous and homozygous, respectively