Lesson 3.6.10

3.6.10 page 3

Reflect and Connect

 

Pedigrees are a very clear way to display the inheritance of a trait within human populations. They can also be used to predict the likelihood of future offspring expressing a particular trait. Now, take out your genetic dictionary for the final time. Spend some time reviewing your terms. Add new entries on the material regarding pedigrees. Save the completed copy to your course folder, and review it when you are preparing for your Diploma exam.

 

Apply the concepts that you have studied by answering the following Diploma-style questions.

 

Diploma Connection

 

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

 

Adrenoleukodystrophy (ALD) is a rare disease of the central nervous system. ALD is characterized by the accumulation of very-long-chain fatty acids in the white matter of the brain and in the adrenal glands. Symptoms start with tantrums and other behavioural problems; then motor function, speech, and hearing are impaired; and finally blindness, mental deterioration, and death occur.

 

Hereditary diseases have diverse causes. For example, the disease mutation may be dominant or recessive, or, the mutated gene may be present on the X chromosome or on an autosome. In some cases, similar diseases are caused by mutations in two different genes. One such case is ALD, where one gene is autosomal and the other is X-linked. In both forms of inheritance, the disease mutation is recessive. Scientists continue to research the causes of ALD. The X-linked recessive form of ALD can be diagnosed prenatally.

 

  1. Describe one piece of evidence obtained from the analysis of a pedigree chart that could be used to determine whether the mode of inheritance of a human genetic disorder is X-linked or autosomal and one piece of evidence that could be used to determine whether it is recessive or dominant. Construct a pedigree of four generations that clearly illustrates one of the two types of inheritance of ALD. Clearly label where your pedigree shows evidence of X-linked recessive or autosomal recessive inheritance.

Larry and Danny Gomez, two boys known as “Wolf Boys,” have made the circus their adoptive family. Both boys perform as trampoline acrobats, and Danny also does motorcycle stunts. The boys have a condition called congenital hypertrichosis (CH), which is a very rare X-linked dominant inherited condition. CH is characterized by the growth of dark hair over the body, particularly on the face and upper torso in males. The palms of the hands, soles of the feet, and mucus membranes are not affected by this condition. A press release about the circus stated that Larry and Danny have travelled to many countries in search of a cure. When asked about the search for a cure in an interview by David Staples of The Edmonton Journal (May 14, 1997), Larry said, “I’d never take it off. I’m very proud to be who I am.” Outside the circus, the boys enjoy activities typical of most boys their age. Danny likes to play video and board games, and Larry is interested in science and is taking astronomy by correspondence.

 

The incidence of CH is very rare: only about 50 affected individuals have been reported since the Middle Ages. The incidence of this condition is considerably higher in a small Mexican village than it is in the rest of the human population. In 1984, researcher Macias-Flores studied CH in a large, five-generation Mexican family and found 19 individuals with CH. A partial pedigree showing the sampled individuals from the Macias-Flores study is shown below.

 

 

—from Staples, 1997, and Figuera et al., 1995

 

  1. Identify the genotypes for individuals II-4, II-5, III-11, III-12, IV-6, IV-7, IV-8, and IV-9 in one of the lines of inheritance on the pedigree. (Provide a key for the allele symbols you use.) Construct a Punnett square to predict the probability of individuals III-11 and III-12’s next child being a male with CH. Explain why more females than males inherit CH in generation III.

Use the following information to answer the next three questions.

 

Deaf-mutism is an autosomal recessive trait that is caused by two genes. Individuals who are homoxygous recessive for either gene will have deaf-mutism. The two genes are designated as D and E in the diagram below.

 

- from Huskey, 1998

  1. A possible genotype of individual IV-3 is

    1. ddEE

    2. ddEe

    3. DDee

    4. DdEe

  2. Individuals III-1 and III-2 are expecting their seventh child. What is the probability of this child having deaf-mutism?

    1. 0.00

    2. 0.25

    3. 0.50

    4. 0.75

Numerical Response

  1. What is the probability of a couple that are heterozygous for both genes having a child with deaf-mutism?
Reflect on the Big Picture

 

By building a pedigree of a real family, you have put into practice your knowledge of genetics. You can now visually display inheritance for other people to see and follow.

Lesson Summary

 

During this lesson you explored the following focusing question:

  • What technologies exist to help us explain and predict the inheritance of traits in breeding programs or family histories?

Pedigrees and Punnett squares are two excellent technologies that help us visually represent inheritance. They can be used to predict the probabilities of future expressions of a trait, and can also help determine the mode of inheritance of that trait.