Lesson 9 — Pedigree Analysis


Self-Check

The pedigree below shows blood type inheritance. Determine the mode of inheritance that best accounts for the transmission of this trait. Determine the blood types of individual I-4 and I-6 as well as genotypes of individual I-1 and I-3.



Can blood tests be used to prove that a man is definitely the father of a child?


I-4 can be A or AB. To determine the blood type of I-4, we need to examine the blood types of subsequent generations. The children of I-4 have blood types A and AB. Because I-3 is blood type B, the A allele must come from I-4. Blood types that can pass on the A allele are A and AB. Without any additional information, we cannot determine whether individual I-4 is of blood type A or AB.

I-6 is blood type A with genotype Ai. One of I-6's children, individual II-8, has blood type O. This means I-6 has an i allele that was passed to II-8. The other child, II-7, has blood type A with genotype Ai. Because I-5 can only pass the i allele, the A allele must have come from I-6. Therefore, I-6's genotype is Ai.

I-1 has genotype Ai. I-1 has three children, one of whom has blood type B (II-4). Because I-1 has blood type A, the B allele must have come from I-2. For II-4 to have blood type B, her genotype has to be BB or Bi. Because I-1 has blood type A, her genotype must be Ai to pass the i allele to II-4.

I-3 has genotype Bi. He has two children, II-5 and II-6, with blood types A and AB. Because I-3 has blood type B, he can only pass the B allele if his genotype is BB. He can also pass the i allele if his genotype is Bi. Because II-5 is blood type A, I-3 must have passed the i allele and individual II-5 did not receive the B allele.

Blood tests cannot be used to prove that a man is definitely the father of a child. Blood tests can identify only the instances where a child cannot be the biological child of a man. For example, two individuals with blood type O cannot have a child that has blood type A. It cannot be used to confirm the paternity of a child.

Diploma Connection

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


    Use the following information for the next question.

    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 in which 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 illustrates clearly one of the two types of inheritance of ALD. Label clearly where your pedigree shows evidence of X-linked recessive or autosomal recessive inheritance.



    2. Use the following information for the next question.

    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 remainder 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.



     

    Figuera, L., Pandolfo, M., Dunne, P. et al. Mapping of the congenital generalized hypertrichosis locus to chromosome Xq24–q27.1. Nat Genet 10, 202–207 (1995). https://doi.org/10.1038/ng0695-202.  Used under the Fair Dealings Provision of the Canadian Copyright Act. 


  2. 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.




    3. 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 homozygous recessive for either gene will have deaf-mutism. The two genes are designated as D and E in the diagram below.

    - from Huskey, 1998



  3. A possible genotype of individual IV-3 is

    1. ddEE

    2. ddEe

    3. DDee

    4. DdEe


  4. 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


    5. Numerical Response

  5. What is the probability of a couple that are heterozygous for both genes having a child with deaf-mutism?


Diploma Question Answers


    • more males will have the condition because they have only one X chromosome
    • males are getting the allele from the mother
    • no male carriers
    • females tend to be carriers as opposed to having the condition

    • recessive - condition appears to skip generations
    • dominant - condition appears to every generation

    • Pedigree charts will vary.

  2. Let XCH represent allele with condition.

    Let Xch represent normal allele.


     II-4  XCHY
     II-5  XchXch
     III-11  XchY
     III-12  XCHXch
     IV-6  XCHXch
     IV-7  XchY
     IV-8  XCHXch
     IV-9  XchXch


    		 XCH 
    		 Xch
     Xch  XCHXch  XchXch
     Y  XCHY  XchY

    Only one of the four offspring is both a male and has CH. Therefore, the probability is 0.25.

    All daughters receive one X chromosome from the father who can only donate an X chromosome with the condition. Because CH is an X-linked dominant condition, all daughters are affected due to the affected X chromosome from their father. Because their mother is unaffected, she passes the unaffected X chromosome to all her sons, and the sons receive a Y chromosome from their affected father. All sons will be unaffected.


  3. D

  4. A

  5. 7/16 or 0.44

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