Chapter 4 PEDIGREE ANALYSIS IN HUMAN GENETICS

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1 Chapter 4 PEDIGREE ANALYSIS IN HUMAN GENETICS Chapter Summary In order to study the transmission of human genetic traits to the next generation, a different method of operation had to be adopted. Instead of making crosses, such as would be done in experiments with peas or fruit flies, human pedigrees are studied and deductions made about the type of inheritance that is functioning with respect to a particular gene locus. These pedigrees are then used to advise couples concerned about their children being affected with a familial disorder. Pedigree charts can provide information about probability (the odds of having an affected child), but they cannot inform most couples about exactly what type of child they will have. If the trait is autosomal recessive, a pedigree chart generally shows affected children born to unaffected parents, and both sexes are equally affected. If a trait is autosomal dominant, typically one parent or the other will also have the trait, but both sexes are also affected equally. The sex chromosomes have their own modes of inheritance. X-linked and Y-linked alleles may be dominant or recessive. Fathers can pass Y-linked genes only to their sons, but pass X-linked genes only to their daughters. Males carry only one copy of the X chromosome, so X-linked recessive traits are more common among males. If a father has an X-linked dominant allele, he will pass it to all of his daughters and none of his sons; therefore, X-linked dominant traits are more common among females. Many disorders appear to have more than one pattern of inheritance. A detailed examination of individuals with these disorders often shows small differences in symptoms. Muscular dystrophy appears to be a single disorder of the muscles, but is actually a family of disorders sharing some of the same symptoms, but each different disorder may have a different genetic cause.

2 26 Chapter 4 Some disorders do not appear to follow standard Mendelian or sex-linked patterns of inheritance. Mutations of genes on the mitochondrial chromosome are only inherited from one s mother, because the sperm does not contribute mitochondria to the fertilized egg. Incomplete penetrance and variable expressivity can interfere with gene expression, making pedigree analysis difficult; for example, because of incomplete penetrance, a dominant allele can appear to skip a generation. Linked genes do not assort independently because the genes are carried on the same chromosome rather than on different (non-homologous) chromosomes Learning Objectives By the time you have finished this chapter, you should be familiar with: a. the difficulties involved in studying genetic traits in humans, relying on indirect observational methods rather than direct, experimental methods. b. the guidelines for determining whether a familiar trait is genetic. c. the expectations for the behavior of autosomal recessive traits in a pedigree, and examples of autosomal recessive traits in humans. d. the expectations for the behavior of autosomal dominant traits in a pedigree, and examples of autosomal dominant traits in humans. e. the expectations for the behavior of sex-linked traits in a pedigree. f. non-mendelian types of inheritance, including mitochondrial inheritance, incomplete penetrance, variable expressivity, and linked genes. Key Terms cystic fibrosis sickle cell anemia Marfan syndrome X-linked Y-linked hemizygous colorblindness muscular dystrophy Huntington disease expressivity

3 Pedigree Analysis in Human Genetics 27 Multiple Choice Circle the letter of the best answer. 1. Brenda and David are both heterozygous for cystic fibrosis; they should produce a. all heterozygous children b. children of four phenotypes in a 9:3:3:1 ratio c. children of two phenotypes in a 3:1 ratio d. children of four phenotypes in a 1:1:1:1 ratio 2. When X-linked traits are considered a. a man s genotype may be determined from his phenotype b. men who inherit the recessive allele of these genes will die c. a man will have whatever phenotype has father had d. many men are heterozygous e. none of the above 3. When X-linked traits are considered a. a man can inherit them from his maternal grandfather b. a man will have whatever his father had c. these traits will skip a generation, and a man will resemble his father s father d. a man will always have the same genotype as his mother d. none of the above. 4. Omar (and all other normal males) cannot be heterozygous for _?_ genes. a. dominant b. recessive c. X-linked d. large 5. Mutations in mitochondrial genes a. are usually transmitted from father to son b. are only transmitted from a mother to her male offspring c. result in a reduction in cellular energy d. only affect females

4 28 Chapter 4 6. Edwin has a mutation in a mitochondrial gene that causes a vision problem. The gamete that carried this mutation came from Edwin s a. mother b. father c. sister d. paternal grandmother 8. Two affected parents give birth to a normal child. This is an indication that the genetic trait involved is a. incompletely dominant b. autosomal dominant c. autosomal recessive d. X-linked recessive 9. The two parents discussed in question #8 a. probably both represent new mutations b. are, themselves, likely to be the children of unaffected parents c. are, themselves, likely to be the children of affected parents d. are probably both the same sex 10. The human disorder that usually results in death from lung infections is a. cystic fibrosis b. Marfan syndrome c. hypophosphatemia d. ichthyosis

5 Pedigree Analysis in Human Genetics 29 Fill-ins Supply the missing word or words to complete the statement. 1. Edwin has a mutation in a mitochondrial gene that causes a vision problem. The gamete that carried this mutation came from Edwin s. 2. A dominant allele is present in Nancy s genotype, but she does not have the dominant phenotype. This means that the allele is.