Module 6
1. Module 6
1.13. Lesson 3
Module 6—Mendelian Genetics: The Transmission of Traits to the Next Generation
Lesson 3—Multiple Alleles and Incomplete Dominance Crosses
© Anette Linnea Rasmussen/shutterstock
Get Focused
multiple alleles: some genes have more than two alleles; an order of dominance is given to determine phenotypes from genotypes; e.g., in a particular flower, colour alleles are red, amber, yellow, and albino in order of decreasing dominance
incomplete dominance: some genes have two alleles—neither of which are dominant; in the heterozygote the phenotype is a mixture of both alleles; e.g., if the white and black alleles are incompletely dominant, the heterozygote will be grey
Life is rarely black and white or, in the case of flowers, red and white! Instead, many physical characteristics observed in plants, or in people, have a variety of phenotypes; certainly more than the two phenotypes accounted for by Mendel’s dominant and recessive inheritance pattern.
In this lesson, as your study of genetics becomes more complex, you will discover two more ways that variety can exist. Using monohybrid crosses, you will consider multiple alleles and incomplete dominance. Multiple alleles produce the human blood types A, B, AB, and O. Incomplete dominance is common in the expression of flower colour. By examining these two new inheritance patterns, you will be able to explain some of the variety you see around you. As you explore the increased complexity of inheritance, you will find that Mendel’s laws still apply to inheritance.
In this lesson, you will explore traits that do not follow Mendel’s patterns of simple dominance but that can still be explained by his laws. You will also come to understand how genes can have more than two alternate forms.
In this lesson, the following focusing questions will be examined:
- What happens when one allele is not completely dominant over another?
- How does having more than two alleles for a gene affect the possible phenotypes for a trait?
Module 6: Lesson 3 Assignment
Your teacher-marked Module 6: Lesson 3 Assignment requires you to submit the following:
- a lab on chicken genetics for assessment
Download a copy of the Module 6: Lesson 3 Assignment to your computer now. You will receive further instructions on how to complete this assignment later in the lesson.
As you work through this lesson, remember to add new vocabulary to your flash cards.
The other questions in this lesson are not marked by the teacher; however, you should still answer these questions. The Self-Check and Try This questions are placed in this lesson to help you review important information and build key concepts that may be applied in future lessons.
The key to success in genetics is to practise the opportunities presented in questions.
You should record the answers to all the questions in this lesson and place those answers in your course folder.
While you are completing this lesson, there will be many opportunities for you to acquire, understand, and practise the concepts that are presented to you. As you complete these activities and your summary notes, you will file everything in your course folder to reference when you are preparing for exams.
Remember, you also have the option of trying additional questions from the textbook for further practice. Consult with your teacher for the answers to these questions. The Key will also provide you with many Diploma Exam-style multiple-choice, numerical-response, and written-response questions that will be an excellent review of the module. Practising your responses to these types of questions is good preparation for the Diploma Exam.