The Chromosomal Basis of Inheritance Chapter 15 The Chromosomal Basis of Inheritance

Chromosomal Theory of Inheritance Genes have specific loci on chromosomes Chromosomes undergo segregation & independent assortment (meiosis)

Chromosomal Linkage Scientist: Morgan Drosophilia melanogaster XX (female) vs. XY (male) Sex-linkage: genes on sex chromosome Linked genes: genes on same chromosome (usually inherited together)

Genetic recombination Crossing over: Genes that DO NOT assort independently of each other Genetic maps: More distance between genes = higher probability crossover occurs between = higher recombination frequency Linkage maps: Genetic map based on recombination frequencies Animation: Crossing Over

Figure 15.6 Some chromosomal systems of sex determination 44 + XY 44 + XX Parents 22 + X 22 + Y 22 + X or + Sperm Egg 44 + XX 44 + XY or Zygotes (offspring) (a) The X-Y system 22 + XX 22 + X (b) The X-0 system 76 + ZW 76 + ZZ Figure 15.6 Some chromosomal systems of sex determination (c) The Z-W system 32 (Diploid) 16 (Haploid) (d) The haplo-diploid system

Human sex-linkage SRY gene: on Y chromosome, triggers development of testes Fathers pass X-linked alleles to all daughters (no sons) Mothers pass X-linked alleles to both sons & daughters Sex-Linked Disorders: Color-blindness; Duchenne muscular dystropy; hemophilia X-inactivation: 2nd X chromosome in females condenses into a Barr body (e.g., tortoiseshell gene gene in cats)

Chromosomal errors, I Nondisjunction: homologous chromosomes do not separate properly during meiosis I or sister chromatids fail to separate during meiosis II Aneuploidy: chromosome number is abnormal Monosomy= missing chromosome Trisomy= extra chromosome (Down syndrome) Polyploidy= extra sets of chromosomes

Chromosomal errors, II Alterations of chromosomal structure: Deletion: removal of a chromosomal segment Duplication: repeats a chromosomal segment Inversion: segment reversal in a chromosome Translocation: movement of a chromosomal segment to another

Genomic imprinting Def: parental effect on gene expression Identical alleles may have different effects on offspring if they arrive via ovum or via sperm

You should now be able to: Explain the chromosomal theory of inheritance and its discovery Explain why sex-linked diseases are more common in human males than females Distinguish between sex-linked genes and linked genes Explain how meiosis accounts for recombinant phenotypes Explain how linkage maps are constructed Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings

Explain how nondisjunction can lead to aneuploidy Define trisomy, triploidy, and polyploidy Distinguish among deletions, duplications, inversions, and translocations Explain genomic imprinting Explain why extranuclear genes are not inherited in a Mendelian fashion Copyright © 2008 Pearson Education Inc., publishing as Pearson Benjamin Cummings