1. The Chromosomal Basis of Inheritance
Chapter 15
The Chromosomal Basis of Inheritance

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

3. 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)

4. 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

5. 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

6. 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)

8. 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

9. 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

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

11. 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

12. 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