1. The Chromosomal Basis of Inheritance

2. Chromosomal Theory of Inheritance
Principle that states that genes are located at specific positions (loci) on chromosomes and that the behavior of chromosomes during meiosis accounts for inheritance patterns.

3. Thomas Hunt Morgan Early 20th Century
Worked with fruit flies – Drosophila melanogaster
Prolific breeders, hundreds of offspring, 2 week generation period
Only 4 pair of chromosomes, could see under light microscope
Work confirmed chromosomal theory

4. Sex-linked Genes A gene located on with the X or the Y Y linked
Very few genes on Y, inherited from father, very few disorders on Y
X linked
1,100 genes on the X chromosome
Have genes for many characters unrelated to sex
Males inherited X linked Disorder more since they only have 1 X, females have 2, so recessive allele can be masked by other X.
Hemophilia, color blindness and Duchenne Muscular Dystrophy

5. X inactivation XX
One X becomes inactivated during early embryonic development in each female cell, random process
Condensed into a compact object called a Barr Body
Tortoiseshell Cate – shows mosaicism

6. Linkage
Genes located near each other on the same chromosome tend to be inherited together in genetic crosses
Phenotypic results deviate from those expected when linked
Fig Linkage between two genes and inheritance
Morgan found a higher proportion of the combinations of traits seen in the P generation flies than would be expected if the two genes assorted independently.

7. Genetic Recombination
Production of offspring with combinations of traits that differ from those found in either P generation parent.
Ex. If 50% of all offspring are recombinants, genetics say there is a 50% frequency of recombination – the appearance of new phenotypes.
The occurrence of recombination of 50% or higher indicates that the genes are linked. This occurs through crossing over.
You can find the recombinant frequency by dividing the recombinants by the total offspring and multiply by 100. (Fig )

8. Genetic mapping
Ordered list of genetic loci along a particular chromosome
Alfred H. Sturtevant, student of Morgan, developed method of putting together a genetic map.
Predicted the farther apart two genes are, the higher probability that a crossover will occur between them and therefor the higher the recombination frequency.
Sturtevant expressed the distance between genes in map units, defining one map unit as equivalent to 1% recombination frequency.

9. Alteration of chromosome number
Nondisjunction – occurs when chromosomes fail to separate correctly during meiosis (homologous chromosomes)
Results in aneuploidy – abnormal number of a particular chromosome
Ex. Monosomy, trisomy
Polyploidy refers to more than 2 sets of chromosomes
Triploidy, tetraploidy
Common in plants, bananas are 3n, strawberries 8n

10. Chromosome alterations
Deletion
Duplication
Inversion
Translocation

11. Chromosomal Disorders – detect on Karyotype
Autosomes
Down syndrome - trisomy 21
Mosaicism syndrome – triomy 9
Edwards syndrome – trisomy 18
Patua syndrome – trisomy 13
Sex chromosomes
XXX – triplo X
XXY – Klinefelters
XO – Turners – only known viable monosomy in humans
XYY – Jacobs syndrome