Chapter 15
Overview: Locating Genes Along Chromosomes * Mendel’s “hereditary factors” were genes * Today we can show that genes are located on chromosomes * The location of a particular gene can be seen by tagging isolated chromosomes with a fluorescent dye that highlights the gene
Mendelian inheritance has its physical basis in the behavior of chromosomes * Mitosis and meiosis were first described in the late 1800s * The chromosome theory of inheritance states: * Mendelian genes have specific loci (positions) on chromosomes * Chromosomes undergo segregation and independent assortment * The behavior of chromosomes during meiosis can account for Mendel’s laws of segregation and independent assortment *
Morgan’s Experimental Evidence: Scientific Inquiry * The first solid evidence associating a specific gene with a specific chromosome came from Thomas Hunt Morgan, an embryologist * Morgan’s experiments with fruit flies provided convincing evidence that chromosomes are the location of Mendel’s heritable factors
Morgan’s Choice of Experimental Organism * Several characteristics make fruit flies a convenient organism for genetic studies * They produce many offspring * A generation can be bred every two weeks * They have only four pairs of chromosomes * Morgan noted wild type, or normal, phenotypes that were common in the fly populations * Traits alternative to the wild type are called mutant phenotypes
Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair * In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) * The F1 generation all had red eyes * The F2 generation showed the 3:1 red:white eye ratio, but only males had white eyes * Morgan determined that the white-eyed mutant allele must be located on the X chromosome * Morgan’s finding supported the chromosome theory of inheritance
Concept 15.2: Sex-linked genes exhibit unique patterns of inheritance * In humans and some other animals, there is a chromosomal basis of sex determination
The Chromosomal Basis of Sex * In humans and other mammals, there are two varieties of sex chromosomes: a larger X chromosome and a smaller Y chromosome * Only the ends of the Y chromosome have regions that are homologous with corresponding regions of the X chromosome * The SRY gene on the Y chromosome codes for a protein that directs the development of male anatomical features * Females are XX, and males are XY * Each ovum contains an X chromosome, while a sperm may contain either an X or a Y chromosome * Other animals have different methods of sex determination * A gene that is located on either sex chromosome is called a sex-linked gene * Genes on the Y chromosome are called Y-linked genes; there are few of these * Genes on the X chromosome are called X-linked genes
Inheritance of X-Linked Genes * X chromosomes have genes for many characters unrelated to sex, whereas the Y chromosome mainly encodes genes related to sex determination * X-linked genes follow specific patterns of inheritance * For a recessive X-linked trait to be expressed * A female needs two copies of the allele (homozygous) * A male needs only one copy of the allele (hemizygous) * X-linked recessive disorders are much more common in males than in females * Some disorders caused by recessive alleles on the X chromosome in humans Color blindness (mostly X-linked) Duchenne muscular dystrophy Hemophilia
Linked genes tend to be inherited together because they are located near each other on the same chromosome * Each chromosome has hundreds or thousands of genes (except the Y chromosome) * Genes located on the same chromosome that tend to be inherited together are called linked genes * How Linkage Affects Inheritance * Morgan did other experiments with fruit flies to see how linkage affects inheritance of two characters * Morgan crossed flies that differed in traits of body color and wing size * Morgan found that body color and wing size are usually inherited together in specific combinations (parental phenotypes) * He noted that these genes do not assort independently, and reasoned that they were on the same chromosome * However, nonparental phenotypes were also produced * Understanding this result involves exploring genetic recombination, the production of offspring with combinations of traits differing from either parent
Genetic Recombination and Linkage * The genetic findings of Mendel and Morgan relate to the chromosomal basis of recombination
Recombination of Unlinked Genes: Independent Assortment of Chromosomes * Mendel observed that combinations of traits in some offspring differ from either parent * Offspring with a phenotype matching one of the parental phenotypes are called parental types * Offspring with nonparental phenotypes (new combinations of traits) are called recombinant types, or recombinants * A 50% frequency of recombination is observed for any two genes on different chromosomes
Recombination of Linked Genes: Crossing Over * Morgan discovered that genes can be linked, but the linkage was incomplete, because some recombinant phenotypes were observed * He proposed that some process must occasionally break the physical connection between genes on the same chromosome * That mechanism was the crossing over of homologous chromosomes
New Combinations of Alleles: Variation for Normal Selection * Recombinant chromosomes bring alleles together in new combinations in gametes * Random fertilization increases even further the number of variant combinations that can be produced * This abundance of genetic variation is the raw material upon which natural selection works
Mapping the Distance Between Genes Using Recombination Data: Scientific Inquiry * Alfred Sturtevant, one of Morgan’s students, constructed a genetic map, an ordered list of the genetic loci along a particular chromosome * Sturtevant predicted that the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency * A linkage map is a genetic map of a chromosome based on recombination frequencies * Distances between genes can be expressed as map units; one map unit, or centimorgan, represents a 1% recombination frequency * Map units indicate relative distance and order, not precise locations of genes * Genes that are far apart on the same chromosome can have a recombination frequency near 50% * Such genes are physically linked, but genetically unlinked, and behave as if found on different chromosomes * Sturtevant used recombination frequencies to make linkage maps of fruit fly genes * Using methods like chromosomal banding, geneticists can develop cytogenetic maps of chromosomes * Cytogenetic maps indicate the positions of genes with respect to chromosomal features * Figure 15.12
Concept 15.4: Alterations of chromosome number or structure cause some genetic disorders * Large-scale chromosomal alterations in humans and other mammals often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders * Plants tolerate such genetic changes better than animals do * Abnormal Chromosome Number * In nondisjunction, pairs of homologous chromosomes do not separate normally during meiosis * As a result, one gamete receives two of the same type of chromosome, and another gamete receives no copy
Aneuploidy results from the fertilization of gametes in which nondisjunction occurred * Offspring with this condition have an abnormal number of a particular chromosome * A monosomic zygote has only one copy of a particular chromosome * A trisomic zygote has three copies of a particular chromosome
Polyploidy is a condition in which an organism has more than two complete sets of chromosomes * Triploidy (3n) is three sets of chromosomes * Tetraploidy (4n) is four sets of chromosomes * Polyploidy is common in plants, but not animals * Polyploids are more normal in appearance than aneuploids
Alterations of Chromosome Structure * Breakage of a chromosome can lead to four types of changes in chromosome structure * Deletion removes a chromosomal segment * Duplication repeats a segment * Inversion reverses orientation of a segment within a chromosome * Translocation moves a segment from one chromosome to another * Human Disorders Due to Chromosomal Alterations * Alterations of chromosome number and structure are associated with some serious disorders * Some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals surviving to birth and beyond * These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidy * Down Syndrome (Trisomy 21) * Down syndrome is an aneuploid condition that results from three copies of chromosome 21 * It affects about one out of every 700 children born in the United States * The frequency of Down syndrome increases with the age of the mother, a correlation that has not been explained
Aneuploidy of Sex Chromosomes * Nondisjunction of sex chromosomes produces a variety of aneuploid conditions * Klinefelter syndrome is the result of an extra chromosome in a male, producing XXY individuals * Monosomy X, called Turner syndrome, produces X0 females, who are sterile; it is the only known viable monosomy in humans * Disorders Caused by Structurally Altered Chromosomes * The syndrome cri du chat (“cry of the cat”), results from a specific deletion in chromosome 5 * A child born with this syndrome is mentally retarded and has a catlike cry; individuals usually die in infancy or early childhood * Certain cancers, including chronic myelogenous leukemia (CML), are caused by translocations of chromosomes *