1. The Chromosomal Basis of Inheritance AP Biology Chapter 15 http://genetics-education-partnership.mbt.washington.edu/cool/images/chromi_sm.jpg

2. Chromosome Theory of Inhertiance Locus: location of a gene on a chromosome Allele: different form of a gene Chromosome: – Segregation – Independent Assortment http://www.nwcreation.net/articles/images/genelocus.JPG

3. The Chromosomal Basis of Mendel’s laws Figure 15.2 Yellow-round seeds (YYRR) Green-wrinkled seeds (yyrr) Meiosis Fertilization Gametes All F 1 plants produce yellow-round seeds (YyRr) P Generation F 1 Generation Meiosis Two equally probable arrangements of chromosomes at metaphase I LAW OF SEGREGATION LAW OF INDEPENDENT ASSORTMENT Anaphase I Metaphase II Fertilization among the F 1 plants 9: 3 : 1 1414 1414 1414 1414 YR yr yR Gametes Y R R Y y r r y R Y yr R y Y r R y Y r R Y r y rR Y y R Y r y R Y Y R R Y r y r y R y r Y r Y r Y r Y R y R y R y r Y F 2 Generation Starting with two true-breeding pea plants, we follow two genes through the F 1 and F 2 generations. The two genes specify seed color (allele Y for yellow and allele y for green) and seed shape (allele R for round and allele r for wrinkled). These two genes are on different chromosomes. (Peas have seven chromosome pairs, but only two pairs are illustrated here.) The R and r alleles segregate at anaphase I, yielding two types of daughter cells for this locus. 1 Each gamete gets one long chromosome with either the R or r allele. 2 Fertilization recombines the R and r alleles at random. 3 Alleles at both loci segregate in anaphase I, yielding four types of daughter cells depending on the chromosome arrangement at metaphase I. Compare the arrangement of the R and r alleles in the cells on the left and right 1 Each gamete gets a long and a short chromosome in one of four allele combinations. 2 Fertilization results in the 9:3:3:1 phenotypic ratio in the F 2 generation. 3

4. Thomas Hunt Morgan & Fruit Flies Drosophila melanogaster – 2 week generations – 4 Chromosomes (3 autosomes & one pair of sex chromosomes) – Easy to breed Wild Type, what exists in nature has a + superscript – W + red eyes (wild type) – W white eyes (mutant) – Sex linked: on X chromosome in F 2 generation, only males showed the white eyes If not sex linked, ½ White eyes female & ½ white eyes male – If it weren’t sex linked, the F 2 white eyed flies would include females http://www.humansystemstherapeutics.com/fruitfly.gif http://www.iq.poquoson.org/2001vasol/eocbio/e ocbio0115_.png

5. CONCLUSION Since all F 1 offspring had red eyes, the mutant white-eye trait (w) must be recessive to the wild-type red-eye trait (w + ). Since the recessive trait—white eyes—was expressed only in males in the F 2 generation, Morgan hypothesized that the eye-color gene is located on the X chromosome and that there is no corresponding locus on the Y chromosome, as diagrammed here. P Generation F 1 Generation F 2 Generation Ova (eggs) Ova (eggs) Sperm X 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

6. Sex Linked Genes Carried on the X chromosome – Heterozygous females are carriers they can pass allele to offspring w/out showing symptoms Females get disease if they inherit both recessive X alleles – Men are more likely to have diseases because they only have 1 X chromosome Duchenne Musculare dystrophy Color Blindness ALD Hemophilia

7. Recall: Crossing Over Causes recombination in linked genes Occurs during prophase I of meiosis between non- sister chromatids of homologous chromosomes – Recombinant frequency

8. Linked Genes Each chromosome contains hundreds or thousands of genes Genes on the same chromosomes are usually inherited together – linked gene – Not inherited together if crossing over takes place

9. Parental Types vs. Recombinants Parental Type - The offspring's phenotype resembles the parents Recombinant – Offspring’s Genotype is different from parental & Offspring’s genotype is new combination of parental genes 50% each = non linked – Caused by Independent assortment – genetic recombination Less than 50% recombinants = linked genes

10. Linkage Mapping The further apart 2 genes are on a chromosome the greater the chance of crossing over – greater recombinant frequency Map unit = recombinant frequency A frequency below 50% indicates that 2 genes are carried on the same chromosome aka linked

11. Linkage Mapping 1% recombination frequency = 1 map unit Recombination Frequencies A/B = 19% B/C = 4% A/D = 12% B/D = 23% What is the correct order of the genes? BACD

12. X Inactivation In mammals only one X chromosome is expressed in somatic cells Second X condenses to become a barr body Barr bodies are reactivated during gamete formation Tortoiseshell Cats – Female: orange fur where one X chromosome is expressed, black fur where other X is expressed

13. Sex Chromosomes X-Y system – mammals – XX = female – XY = male X-O system – some insects – grasshoppers, cockroaches – XX = female – X = male (sperm contained no sex chromosome Z-W system – birds, fish, some insects – ZW = female (determines sex) – ZZ = male Haplo-diploid system – bees and ants – no sex chromosomes – Diploid – females – Haploid – males (haploid, parthenogenic development) Figure 15.9b–d 22 + XX 22 + X 76 + ZZ 76 + ZW 16 (Haploid) 16 (Diploid) (b) The X–0 system (c) The Z–W system (d) The haplo-diploid system

14. Nondisjunction Homologous chromosomes do not separate in anaphase of meiosis Result is aneuploidy – Trisomy – 3 chromosomes n + 1 – Monosomy – 1 chromosome n – 1 – Polyploidy – 3 or more chromosomes

15. Chromosomal Alterations A B CD E FG H Deletion A B C E G H F A B CD E FG H Duplication A B C B D E C F G H A A MN OPQR B CD EFGH B CDEFGH Inversion Reciprocal translocation A BPQ R M NOCDEF G H A D CBEFH G

16. Chromosomes Alterations TypeExplanationExample 1. DeletionRemoval of a chromosome segment ABCDE  ABDE 2. DuplicationRepetition of a chromosomal segment ABCDE  ABBCDE 3. InversionReversal of a chromosome segment ABCDE  ABDCE 4. TranslocationMovement of a segment on one chromosome to another - nonhomologous ABCDE  FGCDE FGHIJ  ABHIJ

17. Full Chromosomal Disorders Syndromes – Down Syndrome – Trisomy 21 – Kleinfelters – XXY Sterile – Trisomy X – XXX – Turner syndrome – XO Sterile

18. Altered Chromosomal Disorders Cri du Chat – Deletion on chromosome 5 Leukemia – CML – Reciprocal translocation between chromosome 9 and 22

19. Genomic Imprinting Genes on autosomal chromosomes that are expressed depending on whether they come from the mother or father Insulin growth factor – only the paternal gene is activated

20. Organelle Inheritance Chloroplasts are inherited through the cytoplasm from the egg NOT the pollen Mitochondria are also passed in the cytoplasm of the egg – Not carried in sperm cells