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) Y F 1 Generation Y R R R Y  r r r y y y Meiosis Fertilization Gametes Green-wrinkled seeds ( yyrr) All F 1 plants produce yellow-round seeds ( YyRr ) R R Y Y r r y y Meiosis R R Y Y r r y y Metaphase I Y Y RR r r y y Anaphase I r r y Y Metaphase II R Y R y y y y R R Y Y r r r r y Y Y R R yR Yr yr YR 1/41/4 1/41/4 1/41/4 1/41/4 F 2 Generation Gametes An F 1  F 1 cross-fertilization 9 : 3 : 1 LAW OF INDEPENDENT ASSORTMENT Alleles of genes on nonhomologous chromosomes assort independently during gamete formation. LAW OF SEGREGATION The two alleles for each gene separate during gamete formation. 1 2 3 3 2 1 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

5. SuttonBoveri

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)

8. EXPERIMENT P Generation (homozygous) Wild type (gray body, normal wings ) Double mutant (black body, vestigial wings)  b b vg vg b + b + vg + vg + EXPERIMENT P Generation (homozygous) Wild type (gray body, normal wings ) Double mutant (black body, vestigial wings)  b b vg vg Double mutant TESTCROSS  b + b + vg + vg + F 1 dihybrid (wild type) b + b vg + vg EXPERIMENT P Generation (homozygous) Wild type (gray body, normal wings ) Double mutant (black body, vestigial wings)  b b vg vg Double mutant TESTCROSS  b + b + vg + vg + F 1 dihybrid (wild type) b + b vg + vg Testcross offspring Eggs b + vg + b vg b + vg b vg + Black- normal Gray- vestigial Black- vestigial Wild type (gray-normal) b vg Sperm b + b vg + vg b b vgvg b + b vgvg b b vg + vg EXPERIMENT P Generation (homozygous) RESULTS Wild type (gray body, normal wings ) Double mutant (black body, vestigial wings)  b b vg vg Double mutant TESTCROSS  b + b + vg + vg + F 1 dihybrid (wild type) b + b vg + vg Testcross offspring Eggs b + vg + b vg b + vg b vg + Black- normal Gray- vestigial Black- vestigial Wild type (gray-normal) b vg Sperm PREDICTED RATIOS If genes are located on different chromosomes: If genes are located on the same chromosome and parental alleles are always inherited together: 1 1 1 1 1 1 0 0 965 944206 185 : : : : : : : : : b + b vg + vg b b vgvg b + b vgvg b b vg + vg

9. 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

10. 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

11. 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

12. 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 cinnabarbody-colorwing-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

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

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

15. 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

16. (a) Deletion (b) Duplication (c) Inversion (d) Translocation A deletion removes a chromosomal segment. A duplication repeats a segment. An inversion reverses a segment within a chromosome. A translocation moves a segment from one chromosome to a nonhomologous chromosome. ABCDEFGH ABCEFGH ABCDEFGH ABCDEFGHBC ABCDEFGH ADCBEFGH ABCDEFGH MNOPQR G MNOC HFED ABPQ R Breakage of a chromosome can lead to 4 types of changes in chromosome structure:

18. 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

19. 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

20. –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

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

22. 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)

23. Normal lgf2 allele (expressed) Normal lgf2 allele (not expressed) Wild-type mouse (normal size) Paternal chromosome Maternal chromosome A wild-type mouse is homozygous for the normal lgf2 allele. Normal lgf2 allele (expressed) Mutant lgf2 allele (not expressed) Normal size mouse Paternal Maternal Mutant lgf2 allele (expressed) Normal lgf2 allele (not expressed) Dwarf mouse Paternal Maternal 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. 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 insulin-like growth factor It appears that imprinting is the result of the methylation (addition –CH 3 ) of DNA

24. 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