Chapter 24 Genetics and Genomics 24.1 Introduction 1. Which choice places the structures in order of increasing size? (p. 917) b. DNA base, gene, chromosome, genome 2. Discuss the origin of the 23 chromosome pairs in a diploid human cell. (p. 917) In a human, a cell contains 23 pairs of chromosomes (46 individual chromosomes). In human reproduction, a human zygote receives 46 chromosomes, however, 23 are supplied by the sperm and 23 from the egg. 3. Explain how a certain number of genes hold enough information to encode a greater number of proteins. (p. 917) The human genome is an economical information store. It includes about 24,000 protein-encoding genes. Different cell types access different subsets of the genome using the information to produce particular proteins, and in this way sculpt the hundreds of types of specialized cells in the body. Yet at the same time, the genome encodes more than 24,000 bits of information. RNA molecules can represent parts of different genes, so that the 24,000 genes actually encode 100,000 to 200,000 different proteins. 4. Explain how genes can respond to environmental factors. (p. 918) The environment includes the chemical, physical, social and biological factors surrounding an individual. A common medical condition may result from several inherited risk factors and exposure to certain environmental influences. 24.2 Modes of Inheritance 5. Which is a chromosome chart? (p. 918) a. karyotype 6. Distinguish between autosome and sex chromosome, homozygous and heterozygous, phenotype and genotype, mutant and wild type, dominant and recessive, and incomplete dominance and codominance. (p. 919) An individual who has two identical alleles of a gene is homozygous for that gene. A person with different alleles for a gene is said to be heterozygous for it. An autosome is a gene carried on a nonsex chromosome. A sex chromosome is the X or Y chromosome determining the sex of the zygote. Wild-type refers to an allele of a phenotype that is either normal or the most common expression for a particular population. A mutant is an allele that is different from the wild-type producing an uncommon phenotype. The combination of genes present in a person’s cell is its genotype. The appearance of the individual as a result of gene expression is his or her phenotype. Incomplete dominance is a condition in which the heterozygous phenotype is intermediate between that of either homozygote. In other words, neither of the alleles of the gene is completely dominant over any other allele. This can be seen in sickle cell disease. In codominance, the different alleles are both expressed. This can be seen in ABO blood types. The most drastic upset in chromosome number is an entire extra set, a condition called polyploidy. This results from formation of a diploid (rather than a normal haploid) gamete. For example, if a haploid sperm fertilizes a diploid egg, the fertilized egg is triploid, with three copies of each chromosome. Most human polyploids die as embryos or fetuses, but occasionally an infant survives for a few days, with defects in nearly all organs. However, many agriculturally important plants are polyploids. Some organs normally have a few polyploidy cells, with no adverse effects on health. Liver cells, for example, may be tetraploid (4 chromosome sets) or even octaploid (8 chromosome sets). 7. Explain how a gene might have hundreds of alleles. (p. 919) The gene consists of hundreds of nucleotides in order to build its particular product. Any one of the different, variant forms it can have is called an allele. Because the genes are paired, one allele on a gene may not necessarily be the same on its partner. 8. Which of the following is a mode of inheritance? (p. 920) d. all of the above. 24.3 Factors That Affect Expression of Single Genes 9. Explain the distinction between penetrance and expressivity. (p. 924) A genotype is incompletely penetrant if not all individuals inheriting it express the phenotype. A genotype is variably expressive if it is expressed to different degrees in different individuals. 10. A single gene disorder that produces several symptoms is _____________. (p. 924) pleiotropy 11. A single syndrome that can have more than one genetic cause exhibits (p. 924) c. genetic heterogeneity. 24.4 Multifactorial Traits 12. Define multifactorial trait. (p. 924) Characteristics molded by one or more genes plus the environment are termed multifactorial traits. 13. List three multifactorial traits. (p. 924) 1. Height 2. Skin color 3. Heart disease 14. Explain why the frequency distributions of different complex traits give very similar bell curves. (p. 926) Although the expression of a polygenic trait is continuous, we can categorize individuals into classes and calculate the frequencies of the classes. When we do this and plot the frequency for each phenotype class, a bell-shaped curve results. This curve indicating continuous variation of a polygenic trait is strikingly similar for different characteristics, such as fingerprint patterns, height, eye color, and skin color. Even when different numbers of genes are involved, the curve is the same shape. 15. Give an example of how the environment can influence a multifactorial trait. (p. 926) Environmental factors can influence gene expression. For instance, nutrition affects height, and sun affects skin color. 24.5 Matters of Sex 16. Explain how genes and chromosomes determine sex. (p. 927) There are two chromosomes that determine the gender of a human, X and Y. An egg cell carries only X chromosomes and the sperm cell carries either an X or a Y chromosome. Thus, an individual who has an XX combination will be female, while an individual who has an XY combination will be male. It is the presence or absence of the Y chromosome that determines gender. On the Y chromosome, a gene called the SRY (sex-determining region of the Y) more specifically identifies whether a person will be male or female. 17. Explain why Y-linked traits are passed only from fathers to sons. (p. 928) Genes on the sex chromosomes are inherited differently than those on autosomes because the sexes differ in sex chromosome constitution. Y-linked genes are considered in three groups: those with counterparts on the X; those similar to genes on the X; and genes unique to the Y, many of which affect male fertility. Y-linked genes pass from fathers to sons. 18. Explain why the inheritance pattern of X-linked traits differs in males and females. (p. 928) Males are hemizygous for x-linked traits; that is, they can have only one copy of an x-linked gene, because they have only one X chromosome. Females can be heterozygous or homozygous for genes on the X chromosome, because they have two copies of it. A male inherits an x-linked trait from a carrier mother. These traits are more common in males than in females. A female inherits an x-linked mutant gene from her carrier mother, and/or from her father if the associated trait does not impair his ability to have children. 19. Distinguish between a sex-limited and a sex-influenced trait. (p. 928) Sex-limited traits affect structures or functions seen in only one sex and may be autosomal. Sex-influenced traits are dominant in one sex and recessive in the other. 20. Define genomic imprinting. (p. 929) About 1% of humans exhibit genomic imprinting, in which the expression of a disorder differs depending upon which parent transmits the disease-causing gene or chromosome. 24.6 Chromosome Disorders 21. State whether trisomy 21 Down syndrome is euploid, aneuploid, or polyploidy. (p. 929) Trisomy 21 Down syndrome is autosomal aneuploid. 22. In nondisjunction _________. (p. 929) c. a chromosome pair fails to separate during meiosis, and as a result a gamete has an extra or missing chromosome. 23. Describe three types of prenatal tests. (p. 931) Prenatal tests a. Maternal serum marker tests indirectly detect a small fetal liver, which can indicate a trisomy. b. Amniocentesis samples and examines fetal chromosomes in amniotic fluid. c. Chorionic villus sampling obtains and examines chorionic villus cells, which descend from the fertilized egg and therefore are presumed to be genetically identical to fetal cells. d. Fetal cell sorting obtains and analyzes rare fetal cells in the maternal circulation. 24.7 Gene Expression Explains Aspects of Anatomy and Physiology 24. Explain how gene expression profiling can add to our knowledge of anatomy and physiology. (p. 933) Identifying which genes are active and which are inactive in particular cell types, under particular conditions, has added to our understanding of physiology. Gene expression monitors the proteins that a cell produces, providing snapshots of physiology in action. 25. Explain how gene expression profiling differs from studying mutations in a single gene. (p. 933) Gene expression profiling considers suites of genes whose functioning underlies cell survival and specialization as well as how cells interact as they respond to the environment and form tissues. 26. List the steps in using gene expression profiling to compare the proteins in a skin cell from an oily part of a face to a cell from a dry part of the same face. (p. 933) Comparing gene expression profiles from the same cell type under different conditions can provide information. A cell type of interest is sampled and separated from its tissue. Its messenger RNA molecules are collected and copied using a special enzyme into DNA. A chemical tag is included that makes the DNA fluoresce under a laser scanner. Two samples of cells to be compared are each labeled with a different color. The resulting pattern of fluorescent spots seen with a laser scanner reveals which genes are expressed in the sampled cells. 27. Explain how gene expression profiling can improve the efficacy of drugs to treat cancer. (p. 934) Gene expression profiling enables medical researchers to see distinctions that their eyes cannot detect. For example, subtypes of leukemia may initially be lumped together because the cancerous white blood cells look alike. On a biochemical level, however, the cells may be quite different. Gene expression profiling can reveal these distinctions, providing information for different treatments, and thus increasing survival.