1. Module 6

1.12. Page 5

Lesson 2

Module 6—Mendelian Genetics: The Transmission of Traits to the Next Generation

Lesson Summary

 

During this lesson you considered the following focusing 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 alleles, 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 with 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.

 

Lesson Glossary

 

Consult the glossary in the textbook for other definitions that you may need to complete your work.

 

autosomal: refers to a trait that is due to a gene on one of the first 22 pairs of chromosomes—not the sex chromosomes; autosomal traits occur equally in males and females

 

monohybrid cross: a cross of two individuals, each of which is heterozygous for one trait (e.g., Tt X Tt; always produces a 1:2:1 genotypic ratio and 3 dominant to 1 recessive phenotypic ratio)

 

sickle cell anemia (SCA): an autosomal recessive disease that causes sickle-shaped red blood cells to form, which get caught in blood vessels, stopping blood flow to tissues

 

test cross: a cross done to determine the genotype of an individual with a dominant phenotype; the individual with the unknown genotype is mated with a homozygous recessive individual—if any offspring have the recessive phenotype, the unknown is a heterozygote