CAMPBELL BIOLOGY Reece Urry Cain Wasserman Minorsky Jackson TENTH EDITION 15 The Chromosomal Basis of Inheritance AP Biology Ms. Peplin

© 2014 Pearson Education, Inc. CHROMOSOME THEORY OF INHERITANCE Cytologists worked out the process of mitosis in 1875, using improved techniques of microscopy (technology & science connection) Biologists began to see parallels between the behavior of Mendel’s proposed hereditary factors and chromosomes in meiosis Law of segregation – separation of homologs in anaphase I Law of independent assortment – random arrangement of chromosome pairs at metaphase I

© 2014 Pearson Education, Inc. Figure 15.2b F 1 Generation Meiosis Metaphase I Anaphase I Metaphase II All F 1 plants produce yellow-round seeds (YyRr). LAW OF SEGREGATION The two alleles for each gene separate. LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently. Y R R R R y y Y r r r r Y Y Y Y Y Y yy yy y y R R R R rr r r r rr r Y Y Y Y R R R R 2 y y y y YR yr Yr yR

© 2014 Pearson Education, Inc. Figure 15.2c LAW OF SEGREGATION LAW OF INDEPENDENT ASSORTMENT F 2 Generation Fertilization recombines the R and r alleles at random. An F 1 × F 1 cross-fertilization Fertilization results in the 9:3:3:1 phenotypic ratio in the F 2 generation. 9 : 3 : 1 3 3

© 2014 Pearson Education, Inc. HISTORICAL SCIENTIFIC INQUIRY BY MORGAN 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 These early experiments provided convincing evidence that the chromosomes are the location of Mendel’s heritable factors

© 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  They have only four pairs of chromosomes  3 pairs of autosomes, 1 pair of sex chromosomes

© 2014 Pearson Education, Inc. MORGAN’S CHOICE OF EXPERIMENTAL ORGANISM Morgan noted wild type, or normal, phenotypes that were common in the fly populations Traits alternative to the wild type are called mutant phenotypes w+w+ w

© 2014 Pearson Education, Inc. MORGAN’S EXPERIMENTS: Mated male flies with white eyes (mutant) with female flies with red eyes (wild type)  The F 1 generation all had red eyes  The F 2 generation showed a 3:1 red to white eye ratio, but only males had white eyes Morgan determined that the white-eyed mutant allele must be located on the X chromosome Why?

© 2014 Pearson Education, Inc. Figure 15.4a Experiment Results P Generation F 1 Generation F 2 Generation All offspring had red eyes.

© 2014 Pearson Education, Inc. Figure 15.4b P Generation F 1 Generation F 2 Generation Conclusion Eggs Sperm X X X Y w w w w+w+ w+w+ w+w+ w+w+ w+w+ w+w+ w+w+ w+w+ w+w+ w+w+ w+w+ w w w

© 2014 Pearson Education, Inc. SEX-LINKED GENES EXHIBIT UNIQUE PATTERNS OF INHERITANCE Morgan’s discovery of a trait that correlated with the sex of flies was key to the development of the chromosome theory of inheritance In humans and some other animals, there is a chromosomal basis of sex determination

© 2014 Pearson Education, Inc. 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 XX = female XY = male

© 2014 Pearson Education, Inc. THE CHROMOSOMAL BASIS OF SEX Short segments at the ends of the Y chromosomes are homologous with the X, allowing the two to behave like homologues during meiosis in males A gene on the Y chromosome called SRY (sex-determining region on the Y) is responsible for development of the testes in an embryo

© 2014 Pearson Education, Inc. Figure 15.6a Parents (a) The X-Y system (b) The X-0 system or Zygotes (offspring) or Sperm Egg 44 + XY 44 + XX 22 + X 22 + XX 22 + Y

© 2014 Pearson Education, Inc. Figure 15.6b (c) The Z-W system (d) The haplo-diploid system 76 + ZW 76 + ZZ 32 (Diploid) 16 (Haploid)

© 2014 Pearson Education, Inc. SEX-LINKED GENES A gene that is located on either sex chromosome Y-linked genes  there are few of these X-linked genes

© 2014 Pearson Education, Inc. INHERITANCE OF X-LINKED GENES X chromosomes have genes for many characters unrelated to sex, whereas most Y-linked genes are related to sex determination 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)

© 2014 Pearson Education, Inc. Figure 15.7 (a) (b) (c) Sperm Eggs XNXNXNXN XnYXnY XNXnXNXn XNXnXNXn XNYXNY XNYXNY XNYXNY XnYXnY XNYXNY XnYXnY XNYXNY XnYXnY XNXNXNXN XNXnXNXn XNXnXNXn XNXnXNXn XNXnXNXn XnXnXnXn XNXN XNXN XNXN XNXN XnXn XNXN XnXn XnXn XnXn Y Y Y

© 2014 Pearson Education, Inc. 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

© 2014 Pearson Education, Inc. 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 If a female is heterozygous for a particular gene located on the X chromosome, she will be a mosaic for that character

© 2014 Pearson Education, Inc. LINKED GENES 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

© 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

© 2014 Pearson Education, Inc. Figure 15.9 Experiment Results P Generation (homozygous) Wild type (gray body, normal wings) Double mutant (black body, vestigial wings) F 1 dihybrid testcross Wild-type F 1 dihybrid (gray body, normal wings) Homozygous recessive (black body, vestigial wings) Testcross offspring Eggs Sperm Wild type (gray-normal) Black- vestigial Gray- vestigial Black- normal b + vg + b vgb + vgb vg + b vg b + b vg + vg b b vg vgb + b vg vgb b vg + vg b + b + vg + vg + b + b vg + vg b b vg vg PREDICTED RATIOS Genes on different chromosomes: Genes on the same chromosome: 1 : 1 : 1 : 1 1 : 1 : 0 : : 944 : 206 : 185

© 2014 Pearson Education, Inc. MORGAN’S RESULTS: 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  they were on the same chromosome (linked) Nonparental phenotypes were also produced  genetic recombination: the production of offspring with combinations of traits differing from either parent

© 2014 Pearson Education, Inc. Figure 15.UN01 Most offspring F 1 dihybrid female and homozygous recessive male in testcross or b + vg + b vg

© 2014 Pearson Education, Inc. RECOMBINATION OF LINKED GENES: CROSSING OVER Morgan discovered that genes can be linked, but the linkage was incomplete, because some recombinant phenotypes were observed some process must occasionally break the physical connection between genes on the same chromosome  crossing over of homologous chromosomes

© 2014 Pearson Education, Inc. RECOMBINATION OF UNLINKED GENES: INDEPENDENT ASSORTMENT OF CHROMOSOMES 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

© 2014 Pearson Education, Inc. Gametes from yellow-round dihybrid parent (YyRr) Gametes from testcross homozygous recessive parent (yyrr) Parental- type offspring Recombinant offspring yyRr YyRr Yyrryyrr YR yr Yr yR yr

© 2014 Pearson Education, Inc. 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

© 2014 Pearson Education, Inc. P generation (homozygous) Wild type (gray body, normal wings) b + vg + Double mutant (black body, vestigial wings) Wild-type F 1 dihybrid (gray body, normal wings) b + vg + b vg

© 2014 Pearson Education, Inc. Figure 15.10b Wild-type F 1 dihybrid (gray body, normal wings) F 1 dihybrid testcross Homozygous recessive (black body, vestigial wings) Meiosis I Meiosis I and II Meiosis II Recombinant chromosomes Eggs b + vg + b vg b + vg b vg + b vg Sperm b vg

© 2014 Pearson Education, Inc. Figure 15.10c Meiosis II Recombinant chromosomes Eggs b + vg + b vg b + vg b vg + Sperm b vg Testcross offspring Parental-type offspring Recombinant offspring Recombination frequency × 100 = 17% = 965 Wild type (gray-normal) 944 Black- vestigial 206 Gray- vestigial 185 Black- normal b vg b + vg + b + vg b vg recombinants 2,300 total offspring

© 2014 Pearson Education, Inc. NEW COMBINATIONS OF ALLELES: VARIATION FOR NATURAL 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

© 2014 Pearson Education, Inc. MAPPING THE DISTANCE BETWEEN GENES USING RECOMBINATION DATA The farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency genetic map, an ordered list of the genetic loci along a particular chromosome

© 2014 Pearson Education, Inc. LINKAGE MAPS A linkage map is a genetic map of a chromosome based on recombination frequencies  Distances between genes can be expressed as map units, or centimorgan, represents a 1% recombination frequency Map units indicate relative distance and order, not precise locations of genes

© 2014 Pearson Education, Inc. Figure Chromosome Results Recombination frequencies 9% 9.5% 17% b cn vg

© 2014 Pearson Education, Inc. Figure Mutant phenotypes Wild-type phenotypes Short aristae Maroon eyes Black body Cinnabar eyes Vestigial wings Down- curved wings Brown eyes Long aristae (appendages on head) Red eyes Gray body Red eyes Normal wings Red eyes

© 2014 Pearson Education, Inc. 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

© 2014 Pearson Education, Inc. 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

© 2014 Pearson Education, Inc. Figure Meiosis I Nondisjunction

© 2014 Pearson Education, Inc. Figure Meiosis I Meiosis II Nondisjunction Non- disjunction

© 2014 Pearson Education, Inc. Figure Meiosis I Meiosis II Nondisjunction Non- disjunction Gametes Number of chromosomes (a) Nondisjunction of homo- logous chromosomes in meiosis I (b) Nondisjunction of sister chromatids in meiosis II n + 1 n nn − 1

© 2014 Pearson Education, Inc. 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. 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

© 2014 Pearson Education, Inc. ANEUPLOIDY OF SEX CHROMOSOMES XXX females and XYY males are healthy, with no unusual physical features Klinefelter syndrome (XXY male)  Sterile  Mental retardation Turner syndrome (X0 female)  who are sterile  it is the only known viable monosomy in humans

© 2014 Pearson Education, Inc. POLYPLOIDY 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

© 2014 Pearson Education, Inc. ALTERATIONS OF CHROMOSOME STRUCTURE Breakage of a chromosome can lead to four types of changes in chromosome structure 1.Deletion removes a chromosomal segment 2.Duplication repeats a segment 3.Inversion reverses orientation of a segment within a chromosome 4.Translocation moves a segment from one chromosome to another

© 2014 Pearson Education, Inc. Figure (a) Deletion (c) Inversion (b) Duplication (d) Translocation A deletion removes a chromosomal segment. An inversion reverses a segment within a chromosome. A duplication repeats a segment. A translocation moves a segment from one chromosome to a nonhomologous chromosome. A B C D E F G H A B C E F G H A B C D E F G H A B C B C D E F G H A B C D E F G H A D C B E F G H A B C D E F G H M N O P Q R M N O C D E F G H A B P Q R Alterations of Chromosome Structure

© 2014 Pearson Education, Inc. DISORDERS CAUSED BY STRUCTURALLY ALTERED CHROMOSOMES The syndrome cri du chat (“cry of the cat”), results from a specific deletion in chromosome 5  severely intellectually disabled 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

© 2014 Pearson Education, Inc. Figure Normal chromosome 9 Normal chromosome 22 Reciprocal translocation Translocated chromosome 9 Translocated chromosome 22 (Philadelphia chromosome)

© 2014 Pearson Education, Inc. EXCEPTIONS TO STANDARD MENDELIAN INHERITANCE There are two normal exceptions to Mendelian genetics  One exception involves genes located in the nucleus, and the other exception involves genes located outside the nucleus In both cases, the sex of the parent contributing an allele is a factor in the pattern of inheritance

© 2014 Pearson Education, Inc. 1. GENOMIC IMPRINTING The phenotype depends on which parent passed along the alleles for those traits Involves the silencing of certain alleles depending on which parent passes them on during gamete formation  Found on autosomes Improper imprinting = abnormal embryonic development & possible cancer formation

© 2014 Pearson Education, Inc. 2. 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 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 and Leber’s hereditary optic neuropathy