Events in Mitosis
Provides a mechanism of mixing so as to obtain one haploid genome from each parent.
Production of gametes and their specialization
Zygotene (pairing) synapsis; the formation of the synaptomenal complex
Pachytene (recombination), rDNA amplification, histone synthesis, LONG
Diplotene (synthesis) - chiasmata (attachments) form, mRNA rRNA, LONG
Reductional division .
Mutations affecting these processes
Some genes segregate according to the sex of the organism
The pattern of sex determination depends on the organism
Flies and people XX vs XY
Worms XX vs XO
Moths and birds ZW vs ZZ
The difference can be through homogametic and heterogametic individuals or through dosage.
Bridges used nondisjunction to draw the most convincing link between genes and the chromosomes
Obtained rare red eyed sterile males (w+) or white-eyed females (wwY)
Confirmation of genotype by histology
Further testing of exceptional females which were mated to w+Y males
Chromosomes are the repository of genetic information
At least one X chromosome is essential
In Drosophila the Y chromosome is essential only for male fertility
In Drosophila being tripleX is lethal.
Consequences of having sex chromosomes signal sexual differentiation.
Genetically they lead to a different type of expression (criss-cross)
Developmentally the chromosomes need not be the repository of sex determining genes (although they may: compare the human Y to the Drosophila Y). Rather the chromosomes serve as a signal.
Because the chromosomes are the signal, individuals of one sex and only one copy of a gene while the other has two.
Therefore, a mechanism must exist to compensate for this dosage difference. In fact several mechanisms exist
Nondisjunction can occur with non-sex chromosomes. These produce aneuploid individuals
Types of changes