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Key Concepts in Chapter 15 Chapter 15 1. Mendelian inheritance has its physical basis in the behavior of chromosomes. The Chromosomal Basis of Inheritance Dr. Wendy Sera Houston Community College Biology 1406 2. Sex-linked genes exhibit unique patterns of inheritance. 3. Linked genes tend to be inherited together because they are located near each other on the same chromosome. 4. Alterations of chromosomes number or structure cause some genetic disorders. 5. Some inheritance patterns are exceptions to standard Mendelian inheritance. © 2014 Pearson Education, Inc. Locating Genes Along Chromosomes Figure 15.1—Where are Mendel’s hereditary factors located in the cell? Mendel’s “hereditary factors” were purely abstract when first proposed Today, we can show that the factors—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 © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Locating Genes Along Chromosomes, cont. Locating Genes Along Chromosomes, cont. Cytologists had worked out the processes of mitosis and meiosis by the late 1800s, using improved techniques of microscopy The chromosome theory of inheritance states: Biologists began to see parallels between the behavior of Mendel’s proposed hereditary factors and chromosomes Around 1902, Sutton, Boveri, and others independently noted the parallels and the chromosome theory of inheritance began to form © 2014 Pearson Education, Inc. Mendelian genes have specific loci (positions) on chromosomes (singular = locus) Chromosomes undergo segregation and independent assortment The behavior of chromosomes during meiosis was said to account for Mendel’s laws of segregation and independent assortment © 2014 Pearson Education, Inc. 1 P Generation Figure 15.2—The chromosomal basis of Mendel’s F Generation laws Yellow-round seeds (YYRR) Y r R R Y Green-wrinkled seeds (yyrr) y Concept 15.1: Morgan showed that Mendelian inheritance has its physical basis in the behavior of chromosomes: Scientific inquiry r y Meiosis Fertilization y R Y Gametes 1 R All F1 plants produce yellow-round seeds (YyRr). R y r y r Y r Y LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently. Meiosis LAW OF SEGREGATION The two alleles for each gene separate. r R Y Metaphase I y r R Y y 1 1 R r R Y y r Anaphase I Y y R r Y y 2 R R 1 4 YR F2 Generation 2 r 1 yr 4 Yr An F1 × F1 cross-fertilization y y R R 1 4 yR 3 Fertilization results 3 Fertilization recombines the R and r alleles at random. 9 :3 :3 These early experiments provided convincing evidence that the chromosomes are the location of Mendel’s heritable factors y Y r r 4 R Y y r 1 r Y y Y Y Metaphase II The first solid evidence associating a specific gene with a specific chromosome came in the early 20th century from the work of Thomas Hunt Morgan :1 in the 9:3:3:1 phenotypic ratio in the F2 generation. © 2014 Pearson Education, Inc. Morgan’s Choice of Experimental Organism For his work, Morgan chose to study Drosophila melanogaster, a common species of fruit fly Several characteristics make fruit flies a convenient organism for genetic studies They produce many offspring A generation can be bred every two weeks © 2014 Pearson Education, Inc. Morgan noted wild type, or normal, phenotypes that were common in the fly populations Traits alternative to the wild type are called mutant phenotypes Figure 15.3— Morgan’s first mutant They have only four pairs of chromosomes © 2014 Pearson Education, Inc. Correlating Behavior of a Gene’s Alleles with Behavior of a Chromosome Pair © 2014 Pearson Education, Inc. Figure 15.4b—In a cross between a wild-type female fruit fly and a mutant white-eyed male, what color eyes will the F1 and F2 offspring have? P Generation In one experiment, Morgan mated male flies with white eyes (mutant) with female flies with red eyes (wild type) X X w+ w Eggs F1 Generation The F1 generation all had red eyes w+ w w+ Eggs Morgan determined that the white-eyed mutant allele must be located on the X chromosome F2 Generation Morgan’s finding supported the chromosome theory of inheritance w Sperm w+ w+ The F2 generation showed a 3:1 red to white eye ratio, but only males had white eyes © 2014 Pearson Education, Inc. X Y w+ w+ Sperm w+ w+ w+ w w w w+ © 2014 Pearson Education, Inc. 2 Concept 15.2: Sex-linked genes exhibit unique patterns of inheritance The Chromosomal Basis of Sex Morgan’s discovery of a trait that correlated with the sex of flies was key to the development of the chromosome theory of inheritance Only the ends of the Y chromosome have regions that are homologous with corresponding regions of the X chromosome, allowing the two to behave like homologues during meiosis in males In humans and some other animals, there is a chromosomal basis of sex determination: a larger X chromosome and a smaller Y chromosome Each ovum contains an X chromosome, while a sperm may contain either an X or a Y chromosome, thus the male determines the sex of the offspring. A person with two X chromosomes develops as a female, while a male develops from a zygote with one X and one Y A gene on the Y chromosome called SRY (sexdetermining region on the Y) is responsible for development of the testes in an embryo X Y Other animals have different methods of sex determination © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Figure 15.6—Some chromosomal systems of sex determination 44 + XY 22 + X 44 + XX Parents or Sperm 44 + XX Egg or (b) The X-0 system 76 + ZW 44 + XY Zygotes (offspring) (a) The X-Y system 22 + XX X chromosomes have genes for many characters unrelated to sex, whereas most Y-linked genes are related to sex determination 22 + X 22 + Y Inheritance of X-Linked Genes 76 + ZZ (c) The Z-W system 32 (Diploid) 22 + X 16 (Haploid) (d) The haplo-diploid system © 2014 Pearson Education, Inc. Inheritance of X-Linked Genes, cont. X-linked genes follow specific patterns of inheritance 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 © 2014 Pearson Education, Inc. XNXN Figure 15.7—The transmission of Xlinked recessive traits Xn XNXn XNY XN XNXn XNY Sperm (a) A female needs two copies of the allele (homozygous) XNXn A male needs only one copy of the allele (hemizygous) Eggs (b) © 2014 Pearson Education, Inc. Y Eggs XN For a recessive X-linked trait to be expressed Thus, X-linked recessive disorders are much more common in males than in females XnY XNXn XNY XN Y XN XNXN XNY Xn XNXn XnY Sperm Eggs XnY Xn Y XN XNXn XNY Xn XnXn XnY Sperm (c) © 2014 Pearson Education, Inc. 3 Recessive X-Linked Disorders in Humans Some disorders caused by recessive alleles on the X chromosome in humans are: Color blindness X Inactivation in Female Mammals In mammalian females, one of the two X chromosomes in each cell is randomly inactivated during embryonic development The inactive X condenses into a Barr body Duchenne muscular dystrophy If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character Hemophilia © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. X chromosomes Allele for orange fur Early embryo: Two cell populations in adult cat: Active X Allele for black fur Cell division and X chromosome inactivation Active X Inactive X Black fur Orange fur Figure 15.8—X inactivation and the tortoiseshell cat © 2014 Pearson Education, Inc. Concept 15.3: 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 Brie Brie—A Handful of a Tortie! © 2014 Pearson Education, Inc. 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 © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. 4 How Linkage Affects Inheritance, cont. Genetic Recombination and Linkage However, nonparental phenotypes were also produced The genetic findings of Mendel and Morgan relate to the chromosomal basis of recombination Understanding this result involves exploring genetic recombination, the production of offspring with combinations of traits differing from either parent © 2014 Pearson Education, Inc. Recombination of Unlinked Genes: Independent Assortment of Chromosomes © 2014 Pearson Education, Inc. Page 300 in Textbook—shows a 50% frequency of recombination Gametes from yellow-round dihybrid parent (YyRr) 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 non-parental phenotypes (new combinations of traits) are called recombinant types, or recombinants Gametes from testcross homozygous recessive parent (yyrr) Recombination of Linked Genes: Crossing Over yr Yr yR YyRr yyrr Yyrr yyRr yr Parentaltype offspring A 50% frequency of recombination is observed for any two genes on different chromosomes © 2014 Pearson Education, Inc. YR Recombinant offspring © 2014 Pearson Education, Inc. Animation: 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 © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. 5 Sperm P generation gametes D C B A c b a d E New Combinations of Alleles: Variation for Natural Selection Egg e f F The alleles of unlinked genes are either on separate chromosomes (such as d and e) or so far apart on the same chromosome (c and f ) that they assort independently. This F1 cell has 2n = 6 chromosomes and is heterozygous for all six genes shown (AaBbCcDdEeFf ). Red = maternal; blue = paternal. D e C B A d E F Each chromosome has hundreds or thousands of genes. Four (A, B, C, F ) are shown on this one. cb a f Genes on the same chromosome whose alleles are so close together that they do not assort independently (such as a, b, and c) are said to be genetically linked. © 2014 Pearson Education, Inc. 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 © 2014 Pearson Education, Inc. 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 © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Meiosis I Figure 15.13— Meiotic nondisjunction Nondisjunction Video: Nondisjunction in Mitosis Meiosis II Nondisjunction Gametes n+1 n+1 n−1 n−1 n+1 n−1 n n Number of chromosomes (a) Nondisjunction of homologous chromosomes in meiosis I © 2014 Pearson Education, Inc. (b) Nondisjunction of sister chromatids in meiosis II © 2014 Pearson Education, Inc. 6 Polyploidy A tetraploid (4n) mammal (Kangaroo rat) 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 Plant Examples: Bananas = 3n; Strawberries = 8n © 2014 Pearson Education, Inc. 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 Figure 15.14—Alterations of chromosome structure (c) Inversion (a) Deletion A B C D E A B C E F G B C D A D A B C D E F G H C B E F G H (d) Translocation F A G H B C D A duplication repeats a segment. B C B E An inversion reverses a segment within a chromosome. H (b) Duplication A Human Disorders Due to Chromosomal Alterations A H A deletion removes a chromosomal segment. Translocation moves a segment from one chromosome to another © 2014 Pearson Education, Inc. F G C D E F G E F G H M N O P Q R A translocation moves a segment from one chromosome to a nonhomologous chromosome. H M N O C D E F G H A B P Q R © 2014 Pearson Education, Inc. Down Syndrome (Trisomy 21) Alterations of chromosome number and structure are associated with some serious disorders Down syndrome is an aneuploid condition that results from three copies of chromosome 21 Some types of aneuploidy appear to upset the genetic balance less than others, resulting in individuals surviving to birth and beyond It affects about one out of every 700 children born in the United States These surviving individuals have a set of symptoms, or syndrome, characteristic of the type of aneuploidy © 2014 Pearson Education, Inc. The frequency of Down syndrome increases with the age of the mother, a correlation that has not been explained © 2014 Pearson Education, Inc. 7 Figure 15.15—Down syndrome 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 (or XXXY or XXXXY) Monosomy X, called Turner syndrome, produces X0 females, who are sterile; it is the only known viable monosomy in humans Other types of nondisjunction: XYY, XXX © 2014 Pearson Education, Inc. Klinefelter’s Syndrome—usually XXY © 2014 Pearson Education, Inc. XXXXY, Klinefelter’s Syndrome © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Turner Syndrome—XO 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 severely intellectually disabled and has a catlike cry; individuals usually die in infancy or early childhood © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. 8 Cri du Chat Syndrome Disorders Caused by Structurally Altered Chromosomes, cont. Certain cancers, including chronic myelogenous leukemia (CML), are caused by translocations of chromosomes © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. Normal chromosome 9 Normal chromosome 22 Figure 15.16— Translocation associated with chronic myelogenous leukemia (CML) Reciprocal translocation Translocated chromosome 9 Concept 15.5: Some inheritance patterns are exceptions to standard Mendelian inheritance There are two normal exceptions to Mendelian genetics One exception involves genes located in the nucleus (genomic imprinting), and the other exception involves genes located outside the nucleus (extranuclear genes found in mitochondria and chloroplasts.) In both cases, the sex of the parent contributing an allele is a factor in the pattern of inheritance Translocated chromosome 22 (Philadelphia chromosome) © 2014 Pearson Education, Inc. Inheritance of Organelle Genes Extranuclear genes (or cytoplasmic genes) are found in organelles in the cytoplasm Mitochondria, chloroplasts, and other plant plastids carry small circular DNA molecules Extranuclear genes are inherited maternally because the zygote’s cytoplasm comes from the egg © 2014 Pearson Education, Inc. Mitochondrial Disorders in Humans Some defects in mitochondrial genes prevent cells from making enough ATP and result in diseases that affect the muscular and nervous systems. For example: Mitochondrial myopathy—weakness, intolerance of exercise, muscle deterioration Leber’s hereditary optic neuropathy—sudden blindness in young people as young as 20s and 30s—affects oxidative phosphorylation (4 different mutations cause the disorder) © 2014 Pearson Education, Inc. © 2014 Pearson Education, Inc. 9 Sex-linked Genetics Problem #1 Sex-linked Genetics Problem #2 A man with hemophilia (a recessive, sex-linked condition) has a daughter of normal phenotype. She marries a man who is normal for the trait. Red-green color blindness is caused by a sexlinked recessive allele. A color-blind man marries a woman with normal vision whose father was colorblind. What is the probability that a daughter of this mating will be a hemophiliac? That a son will be a hemophiliac? If the couple has four sons, what is the probability that all four will be born with hemophilia? © 2014 Pearson Education, Inc. What is the probability that they will have a colorblind daughter? What is the probability that their first son will be color-blind? © 2014 Pearson Education, Inc. 10