Module 6 Mendelian Genetics

3.6.10 page 2

Read

 

A pedigree is a very useful tool for tracking the inheritance of traits. It is similar to a Punnett square, but instead of showing all of the possibilities, it only shows the actual parents and children. A pedigree is like a flowchart illustrating many generations of people and their relationship to one another. By using the proper symbols, you can also indicate the presence or absence of traits, and in doing so, create a pattern that can be analyzed. While all of the patterns of inheritance that we have studied can be analyzed with pedigrees, three primary types will be studied in this lesson: autosomal dominant, autosomal recessive, and sex-linked traits. Read about tracing human genetics and creating pedigrees in your text on pages 611 – 615. Be sure to take good note of all of the possible symbols for pedigrees, which are detailed at the top of page 612.

 

 

Remember that autosomal chromosomes are all of the chromosomes that are not sex chromosomes. So anything that is not an X or a Y chromosome is an autosome. A dominant trait will show up in a pedigree by having an affected individual in every generation. This type of vertical pattern should be very clear. In addition, and autosomal trait will not show any preference to gender. There should be roughly the same number of affected males as females.

 

A recessive trait will show up in pedigrees when neither parent expresses the trait, such as an inheritable disease, however one or more of their children do. If the pedigree is long enough it may also indicate the disease showing up few generations later. Again, since this is autosomal, male and female offspring should be equally represented in the affected individuals.

 

There are a few distinguishing features of sex-linked inheritance. The first is a gender bias. For X-linked recessive traits, males will be more commonly affected than females. Remember that males give their X chromosomes to their daughters, not their sons. So on an X-linked recessive pedigree there is often an affected male who appears to have no affected offspring. In such a case, one or more of his daughters will have sons affected with the disease.

 

From your reading in the text, or from the summary presented here, you should answer the following questions for your own understanding and save your work in your course folder.

1. Distinguish between the significance of roman numerals and Arabic numerals when used in a pedigree.
2. What is autosomal inheritance?
3. How do pedigrees for autosomal recessive and X-linked recessive traits differ?
4. Can a female express an X-linked trait like hemophilia? Explain.

Practice Problems

 

Examine the sample pedigree problem on the bottom of page 614 of your textbook. Notice how they use roman numerals for generations and Arabic numerals for individuals. Also, try to follow the logic used to deduce carriers for the trait based on affected offspring and/or parents. Now, complete the four practice problems on the top of page 615. Save your work to your course folder.

 

Module 6: Lesson 10 Assignment—Lab: Creating a Real Pedigree

 

Creating pedigrees is very helpful in determining inheritance patterns in families. Here is a chance for you to determine the inheritance pattern of a single trait in your own family, or a family willing to participate in your investigation.

 

Retrieve your copy of Module 6: Lesson 10 Assignment that you saved to your computer earlier in this lesson. Complete the assignment. Save your completed assignment in your course folder. You will receive instructions later in this lesson on when to submit your assignment to your teacher.