1. Lecture #6 Date ________ 4 Chapter 15~ The Chromosomal Basis of Inheritance

2. The Chromosomal Theory of Inheritance 4 Genes have specific loci on chromosomes and chromosomes undergo segregation and independent assortment

3. Chromosomal Linkage 4 Morgan 4 Drosophilia melanogaster 4 XX (female) vs. XY (male) 4 Sex-linkage: genes located on a sex chromosome 4 Linked genes: genes located on the same chromosome that tend to be inherited together

4. Genetic recombination 4 Crossing over Genes that DO NOT assort independently of each other 4 Genetic maps The further apart 2 genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency 4 Linkage maps Genetic map based on recombination frequencies

5. Human sex-linkage 4 SRY gene: gene on Y chromosome that triggers the development of testes 4 Fathers= pass X-linked alleles to all daughters only (but not to sons) 4 Mothers= pass X-linked alleles to both sons & daughters 4 Sex-Linked Disorders: Color-blindness; Duchenne muscular dystropy (MD); hemophilia 4 4 X-inactivation: 2nd X chromosome in females condenses into a Barr body (e.g., tortoiseshell gene in cats)

6. Chromosomal errors, I 4 Nondisjunction: members of a pair of homologous chromosomes do not separate properly during meiosis I or sister chromatids fail to separate during meiosis II 4 Aneuploidy: chromosome number is abnormal Monosomy~ missing chromosome Trisomy~ extra chromosome (Down syndrome) Polyploidy~ extra sets of chromosomes

7. Aneuploidy of Sex Chromosomes 4 XXY-Klinefelter syndrome-male sex organs, sterile, breast enlargement 4 XYY-males, taller than average 4 XXX-females, healthy 4 X0-Turner syndrome, only viable monosomy, females, sterile

8. Chromosomal errors, II 4 Alterations of chromosomal structure: 4 Deletion: removal of a chromosomal segment 4 Duplication: repeats a chromosomal segment 4 Inversion: segment reversal in a chromosome 4 Translocation: movement of a chromosomal segment to another

9. Genomic imprinting 4 Def: a parental effect on gene expression, expression of traits dependent on which parent passed them down 4 Identical alleles may have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm. 4 Fragile X syndrome: higher prevalence of disorder and retardation in males