Module 6 Mendelian Genetics

3.6.2 page 5

Lesson Summary

 

During this lesson you were to focus on the following essential questions:

• What are the simple principles of single trait inheritance?

• How can genotype be determined from phenotype?

Mendel worked with seven contrasting traits in peas. Each trait was either dominant or recessive; there was no blending in his work. You have learned that dominant traits will be expressed in either homozygous dominant or heterozygous plants, while recessive traits can only be seen in homozygous recessive plants. You have also learned how to predict the genotypes and phenotypes of offspring using a Punnett square.

 

Since dominant alleles can hide recessive ones, breeders and farmers are most interested in working with homozygous or true breeding individuals. This ensures the continuation of desired traits. Since genotypes can not be seen, but only inferred from probabilities, setting up a test cross is an excellent way to determine genotype. In a test cross an individual with the recessive phenotype is crossed to an individual with unknown genotype displaying the dominant phenotype. The results of the cross will indicate if the unknown was either heterozygous or homozygous. If all resultant offspring express the dominant trait, then the parent of unknown genotype is homozygous dominant. However, if half of the offspring express the recessive trait, then the parent of unknown genotype is heterozygous.

 

Diploma Connection

 

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

 

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, he will generally 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 is heterozygous for white wool, the expected phenotypes of the offspring of the farmer’s test cross would be:
   
   
   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 between humans by mosquitoes.

3. The probability of two carrier parents having a child with sickle cell anemia is:
   
   
   1. 25%
   2. 50%
   3. 75%
   4. 100%

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