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Module 6

1. Module 6

1.45. Page 2

Lesson 10

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

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pedigree: a picture of a family history of a genetic condition that indicates generation, gender, birth order in the generation, and parental and offspring relationships; the presence of the condition is indicated by a blackened-in symbol

 

autosomal: a condition is autosomal if the pedigree shows no significant difference in the number of each gender affected by the condition

 

autosomal dominant: these conditions appear in each generation in the pedigree, and cannot arise from two parents who did not have the condition

 

autosomal recessive: these conditions often skip a generation in a pedigree, and are indicated if two parents without the condition produce a child with the condition

A pedigree is a useful tool for tracking the inheritance of traits. A pedigree is similar to a Punnett square, but instead of showing all of the possibilities for offspring, a pedigree only shows the parents and children. A pedigree is like a flowchart illustrating many generations of people and their relationship to one another.

 

Symbols are used to indicate the presence or absence of traits, thus creating a pattern of inheritance 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, autosomal dominant, autosomal recessive, and sex-linked traits.

 

You may wish to read “Breeding Plants” and “Breeding Animals” on pages 610 to 611, but concentrate on “Human Genetics” to the end of “Human Genetic Analysis” on pages 611 to 615 of your textbook. Note the possible symbols used in pedigrees detailed at the top of page 612. These symbols are also provided to you in the Data Tables of the Diploma Exam. It is important to study the examples carefully and record information in your course folder according to your learning style.

 

This pedigree shows three generations. In the parent generation, the male is older and expresses trait. In second generation, their first child is a girl expressing the trait and mates with a male not expressing the trait. The parents’ second child is a male expressing the trait, the third child is a male not expressing who mates with a non-expressing female. The parents’ fourth child is a non-expressing female, the sixth child is a female expressing the trait, and the seventh child is a male not expressing the trait. In the third generation, the first born female expressing the trait and her mate who does not express the trait have a non-expressing son, daughter, and then a daughter with the trait. The third child in the second generation and his non-expressing mate produce a non-expressing female and male.


 

Autosomal chromosomes in humans are the 22 pairs of 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, an autosomal trait will not show any preference to gender. There should be approximately the same number of affected males as females.

 

This pedigree shows four generations. In the parent generation, neither parent expresses the trait. In the second generation, the parents have a son, another son, and a daughter. They and their mates do not express the trait. In the third generation, none of the children or their mates express the trait. In the fourth generation, cousins mate and produce a son with the trait, a daughter without the trait, and another daughter with the trait.


 

A recessive trait will express in pedigrees when neither parent expresses the trait. For example, both parents do not express an inheritable disease, but one or more of their children do. If the pedigree is long enough, the trait usually “skips” expressing in several generations. Again, since this is autosomal, male and female offspring should be equally represented in the affected individuals.

 

This pedigree shows four generations. In the parent generation, the male expresses the trait. In the second generation, the parents have a son without the trait, a daughter who carries the trait, and son without the trait, a daughter who carries the trait and mates with a male not expressing the trait, and a son without the trait who mates with a female who carries the trait. In the third generation, the second generation female carrier who mated with a male without the trait produce a female carrier who mates with a male without the trait, a female without the trait, and a male with the trait. The second generation male without the trait who mated with a female carrier produce a male with the trait, a male without the trait, and a female carrier who mates with a male with the trait. In the fourth generation, the third generation female carrier who mated with a male without the trait produce a male with the trait, a male without the trait, a female carrier , and a female without the trait. The female carrier from the third generation, who mated with a male expressing the trait, produced a male with the trait, a female carrier, a female with the trait, and a male without the trait.


 

X-linked recessive: a condition is X-linked recessive if the pedigree shows the condition is found in males significantly more than in females; carrier moms pass the allele to affected sons; affected sons cannot pass the allele to their sons, but pass it to their daughters, who in turn become carriers

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. In 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 by the disease.

 

Try This

 

To practise your understanding, do the following questions. Mastery is important. Discuss your responses with your teacher and save your work in your course folder.

 

TR 1. Distinguish between the significance of Roman numerals and Arabic numerals when used in a pedigree.

 

TR 2. What is autosomal inheritance?

 

TR 3. How do pedigrees for autosomal recessive and X-linked recessive traits differ?

 

TR 4. Can a female express an X-linked trait like hemophilia? Explain.

 

Practice Problems

 

Examine the “Sample Problem” on the bottom of page 614 of your textbook. Notice how Roman numerals are used for generations and Arabic numerals are used for individuals. Follow the logic used to deduce carriers for the trait based on affected offspring and/or parents. Mastery is important. 

 

TR 5. Complete the four “Practice Problems” on the top of page 615. Discuss your responses with your teacher. 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. In this assignment you will trace the inheritance pattern of a single trait in your family or in a family willing to participate in your investigation.

 

Complete “Thought Lab 17.2” on page 615 of your textbook. You may work alone or with a partner for this exercise.

 

Retrieve your copy of the Module 6: Lesson 10 Assignment that you saved to your computer earlier in this lesson. Add your answers to the “Thought Lab 17.2” to the document. 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.

 

Try This

 

TR 6. For further practice, complete the “Review” questions on page 617 of your textbook. Discuss your answers with your teacher as these concepts and skills are essential for your success on the Diploma Exam.