Drosophilia melanogaster
      Much of the initial genetics research done on the fruit fly (or on any animal!) was by Morgan
      brigher red eyes is a mutant phenotype (cn)

Sex-linked disorders

• X^cX + X^cY a child with a genetic disorder  3-4ths of the time
• Duchenne muscular dystrophy = 1/3500 lacks dystrophin (key muscle protein) on X chromosome
• Hemophilia = absence of clotting protein
  • Leopold = the first recorded person of royal lineage to have the disease

Recombinants
      offspring will be different from parents
      Which of the above two situations promotes differentiation through greater recombination of alleles and genes?

Frequency of Recombination

•       50% for any 2 genes located on different chromosomes:
             
• Parental Types
              matched phenotypes with parents
              if genes are completely linked there is a 1:1 ratio of Parental Types
              if genes are Mendelian formatted there is a 1:1:1:1 ratio of Parental Types

X Chromosome Inactivation
      An inactivated X chromosome is called a Barr body.
            If a male has a Barr body, he has 2 Xs and 1 Y (XXY); and if a female has 2 Barr bodies, she has 3 X's (XXX):
                  One in every 1000 has a third chromosome and he/she is normally taller than avg. and learns differently.
      X_IST = X inactive specific transcript
            This gene is active in the Barr body only.  It produces RNA molecule that remains in close contact with the DNA.

Aneuploidy
      abnormal gametes yield abnormal chromosome #
      polyploidy :
            multiple sets of chromosomes (common in plants)
            mosaic polyploids:
                  patches of polyploidy
      both are results of nondysjunction

How does this all fit in with Mendel?

We now know plant height is linked to pod shape.  Mendel never reported the results of dihybrid crosses that could have showed a consistency here.  We now know seed and flower color are on the same chromosome but are so far apart that the linkage isn't observed during crosses.

A Linkage Map is based on recombination frequencies.  It is NOT a picture of an actual chromosome; it is the gene sequence with the precise location of the genes.

A Cytological Map pinpoints genes by associating a mutant phenotype with a chromosome physical abnormality.

Next: "Molecular Basis of Inheritence."