1. Chromosomes and Inheritance

2. The Chromosomal Theory of Inheritance
Sutton and Boveri… 1902
Genes have specific loci on chromosomes and chromosomes undergo segregation and independent assortment

3. Mendel makes Sense
Knowledge of meiosis and molecular genetics prove that Mendel was way beyond his time.

4. Fruit flies
Thomas Hunt Morgan– chromosomal studies on Drosophilia melanogaster (fruit flies)
Produce lots of offspring and able to reproduce quickly
Four pairs of chromosomes that can be seen easily under a light microscope
Wild type- normal characteristics found in majority of the natural population

5. Chromosome Linkage
XX (female) vs. XY (male)… only males have the mutant white eye gene
Sex-linkage: genes located on a sex chromosome

6. Gene Linkage Double mutant flies- black and vestigial
F2 generation doesn’t have the phenotypes that are expected when there is independent assortment
Conclusion: black and vestigial genes don’t assort independently… they are linked
Linked genes: genes located on the same chromosome that tend to be inherited together

7. Genetic recombination
Parental types
Same pheno as parents
Recombinants
Have new combination of genes
Genes are not always linked because crossing over occurs.
Genetic maps
Ordered list of genes on a chromosome
Linkage maps
Genetic map based on recombination frequencies
The further apart 2 genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency

8. Sex Determination In humans: X-O Z-W Haplo-diploid
XX= female, XY= male
SRY gene: gene on Y chromosome that triggers the development of testes
X-O
Grasshoppers, cockroaches, etc
XX= female, XO= male
Z-W
Birds, some fish and insects
Egg determines sex
ZW= female, ZZ= male
Haplo-diploid
Bees and ants
Females result from fertilized eggs; males are haploid (eggs never fertilized by a male)

9. Sex-linkage
Fathers= pass X-linked alleles to all daughters only (but not to sons)
Mothers= pass X-linked alleles to both sons & daughters
Sex-Linked Disorders:
Color-blindness
Duchenne muscular dystropy (MD)
Male pattern baldness
Hemophilia

10. X Inactivation
2nd X chromosome in females becomes almost completely inactive
Condenses into a Barr body
e.g., tortoiseshell (calico) gene gene in cats

11. Alterations of Chromosome Structure

12. 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

13. Down Syndrome Trisomy 21 3 copies of chromosome 21
1 in 700 children born in U.S.
Chromosome 21 is one of the smallest human chromosome but still causes severe effects
Frequency correlates with the age of the mother

14. Nondisjunction of Sex Chromosomes
Produces a variety of distinct conditions in humans:
XXY = Klinefelter’s syndrome male
XXX = Trisomy X female
XYY = Jacob’s syndrome male
XO = Turner syndrome female

15. “Male” Sex Disorders Kleinfelter’s Kleinfelter’s Syndrome XXY male
1 in every 2000 live births
have male sex organs, but are sterile
Feminine characteristics
tall
normal intelligence
Jacob’s Syndrome
XYY Males
1 in 1000 live male births
extra Y chromosome
somewhat taller than average
more active and aggressive
slight learning disabilities
delayed emotional immaturity
normal sexual development
Kleinfelter’s
XYY

16. “Female” Sex Disorders
Turner syndrome
Monosomy X or X0
1 in every 5000 births
Varied degree of effects
webbed neck
short stature
Sterile
Trisomy X
XXX
1 in every 2000 live births
produces healthy females
Girl with Turner Syndrome

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

18. Inheritance of Organelle Genes
Genes can be inherited through more than just chromosomes within the nucleus
Mitochondria also contains DNA
Inherited only from mother
Important for electron transport and ATP synthesis