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Chapter 15

Chapter 15. The Chromosomal Basis of Inheritance. Chromosomal genetics. Part I- Sex linkage Part II: Linkage Part III: Chromosomal aberrations. Dihybrid Pea Cross. seed color: yellow (dominant) green (recessive). seed shape: round (dominant) wrinkled (recessive). P Generation.

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Chapter 15

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  1. Chapter 15 The Chromosomal Basis of Inheritance

  2. Chromosomal genetics • Part I- Sex linkage • Part II: Linkage • Part III: Chromosomal aberrations

  3. Dihybrid Pea Cross seed color: yellow (dominant) green (recessive) seed shape: round (dominant) wrinkled (recessive)

  4. P Generation Yellow-round seeds (YYRR) Green-wrinkled seeds (yyrr) y Y r  R r R Y y • The chromosome theory of inheritance • states that: • Mendelian genes have specific loci (positions) on chromosomes • It is the chromosomes that undergo segregation • and • independent assortment Meiosis Fertilization r y Y R Gametes All F1 plants produce yellow-round seeds (YyRr) F1 Generation R R y y r r Y Y LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. Meiosis r r R R Metaphase I y Y y Y 1 1 r r R R Anaphase I Y y Y y Metaphase II r R R r 2 2 y Y y Y y Y Y y Y y y Y Gametes r R r r R r R R 1/4 1/4 1/4 1/4 yr yR YR Yr F2 Generation An F1  F1 cross-fertilization 3 3 : 1 : 3 9 : 3

  5. Sutton Boveri

  6. Linked genes Each chromosome has 100-1000’s of genes (except Y chromosome) Genes located on the same chromosome that tend to be inherited together are called linked genes Morgan did 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 (genes are not on sex chromosome)

  7. EXPERIMENT EXPERIMENT EXPERIMENT EXPERIMENT P Generation (homozygous) P Generation (homozygous) P Generation (homozygous) P Generation (homozygous) Wild type (gray body, normal wings) Wild type (gray body, normal wings) Wild type (gray body, normal wings) Wild type (gray body, normal wings) Double mutant (black body, vestigial wings) Double mutant (black body, vestigial wings) Double mutant (black body, vestigial wings) Double mutant (black body, vestigial wings)     b b vg vg b b vg vg b b vg vg b b vg vg b+ b+ vg+ vg+ b+ b+ vg+ vg+ b+ b+ vg+ vg+ b+ b+ vg+ vg+ F1 dihybrid (wild type) F1 dihybrid (wild type) F1 dihybrid (wild type) Double mutant Double mutant Double mutant TESTCROSS TESTCROSS TESTCROSS    b+ b vg+ vg b+ b vg+ vg b b vg vg b b vg vg b+ b vg+ vg b b vg vg Testcross offspring Testcross offspring b+ vg+ b vg b+ vg b+ vg+ b vg b+ vg b vg+ b vg+ Eggs Eggs Black- normal Black- normal Wild type (gray-normal) Wild type (gray-normal) Black- vestigial Gray- vestigial Black- vestigial Gray- vestigial b vg b vg Sperm Sperm b b vg+vg b b vg+vg b+b vg+ vg b b vgvg b+b vgvg b+b vg+ vg b b vgvg b+b vgvg PREDICTED RATIOS 1 If genes are located on different chromosomes: 1 : : : 1 1 If genes are located on the same chromosomeand parental alleles are always inherited together: 1 0 : 1 : 0 : 965 : : 185 : 944 206 RESULTS

  8. From the results, Morgan reasoned that body color and wing size are usually inherited together in specific combinations (parental phenotypes) because the genes are on the same chromosome However, nonparental phenotypes were also produced Understanding this result involves exploring genetic recombination, production of offspring with combinations of traits differing from either parent

  9. 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 calledrecombinant types, or recombinants A 50% frequency of recombination is observed for any two genes on different chromosomes

  10. Recombination of Linked Genes: Crossing Over • Morgan discovered that genes can be linked, but the linkage was incomplete, as evident from recombinant phenotypes • Morgan proposed that some process must sometimes break the physical connection between genes on the same chromosome • That mechanism was the crossing over of homologous chromosomes

  11. linkage map- is a genetic map of a chromosome (an ordered list of genetic loci) based on recombination frequencies • Distances between genes can be expressed as map units; one map unit, or centimorgan, represents a 1% recombination frequency • the farther apart 2 genes are, the higher the probability that a crossover will occur between them, and the higher the recombination frequency body-color cinnabar wing-size • 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

  12. Recombination frequencies can be used to make linkage maps of fruit fly genes • portray the order of genesalong a chromosome, but does not accurately portray the precise location of those genes chromosome II Using methods like chromosomal banding, geneticists can develop cytogenetic maps, indicate the positions of genes with respect to chromosomal features

  13. Alterations of chromosome # • Large-scale chromosomal alterations often lead to spontaneous abortions (miscarriages) or cause a variety of developmental disorders • 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 all gametes will be abnormal ½ gametes will be abnormal/ normal

  14. aneuploidy- result of fertilization of gametes in which nondisjunction occurred, offspring have an abnormal # of a particular chromosome • trisomic zygote- three copies of a particular chromosome • monosomic zygote- has only one copy of a particular chromosome • polyploidy- an organism has more than 2 complete sets of chromosomes

  15. 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 M N O P Q R A B P Q R M N O C D E F H Breakage of a chromosome can lead to 4 types of changes in chromosome structure: A deletion removes a chromosomal segment. (a) Deletion A duplication repeats a segment. (b) Duplication An inversion reverses a segment within a chromosome. (c) Inversion H A translocation moves a segment from onechromosome to a nonhomologous chromosome. (d) Translocation G

  16. 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

  17. Down Syndrome • Down syndrome (trisomy 21) is a condition that results from three copies of chromosome 21 • It affects about one out of every 700 children born in the US • The frequency of Down syndrome increases with the age of the mother, a correlation that has not been explained

  18. 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

  19. Disorders Caused by Structurally Altered Chromosomes • Certain cancers, including chronic myelogenous leukemia (CML), are caused by translocations of chromosomes

  20. Some inheritance patterns are exceptions to the standard chromosome theory • There are two normal exceptions to Mendelian genetics that involve genes located: 1. in the nucleus 2. outside the nucleus • mitochondria • plastids (chloroplasts)

  21. insulin-like growth factor Normal lgf2 allele (expressed) Paternal chromosome Maternal chromosome • For a few mammalian traits, the phenotype depends on which parent passed along the alleles for those traits • Such variation in phenotype is genomic imprinting • involves the silencing of certain genes that are “stamped” with an imprint during gamete production Wild-type mouse (normal size) Normal lgf2 allele (not expressed) A wild-type mouse is homozygous for the normal lgf2 allele. Normal lgf2 allele (expressed) Paternal Maternal Normal size mouse Mutant lgf2 allele (not expressed) Mutant lgf2 allele (expressed) Paternal Maternal It appears that imprinting is the result of the methylation (addition –CH3) of DNA Normal lgf2 allele (not expressed) Dwarf mouse When a normal lgf2 allele is inherited from the father, heterozygous mice grow to normal size. But when a mutant allele is inherited from the father, heterozygous mice have the dwarf phenotype.

  22. Inheritance of Organelle Genes • Extranuclear genes are genes found in organelles in the cytoplasm • The inheritance of traits controlled by extranuclear genes depends on the maternal parent because the zygote’s cytoplasm comes from the egg • The first evidence of extranuclear genes came from studies on the inheritance of yellow or white patches on leaves of an otherwise green plant • 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

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