Biology 205 Chromosomal Basis of Inheritance: Sex linkage and Determination
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1. State the chromosomal theory of inheritance and define basic concepts
2. Explain the X and Y chromosome system for sex determination in humans
3. Sex linkage
4. Nondisjunction of sex chromosomes
Define non disjunction.
Learn basic syndromes in humans related to non disjunction.
Mammals: Sex determined mainly by the Y chromosome
Explain the concept of dosage compensation p 61-63
- Define the Barr body and explain its significance
- Note that X chromosome inactivation is called Lyonization and it happens at about 16 days.
- List the steps in X inactivation. p 64.
- Comment: Note that X controlling element(XIST) is expressed on the inactive chromosome. It produces an RNA which prevents the inactive chromosome from being expressed.
- Give the expected number of Barr bodies for different forms of nondisjunction. See table 3.2
- Why are mammalian females often to be mosaics?
- Explain the origin of the calico cat phenotype and anhidrotic ectodermal dysplasia in humans
Other Chromosomal Systems
- Drosophila - XX is female XY is male but XXY is also female -balance autosome and X chromosome number
- C. elegans - Similar to fruit fly but here the sexes are either hermaphroditic or male. p 65
- Plants more complex - some are monoecious others dioecious
- Hymenoptera - haplodiploidy - actually individuals that are homozygous or hemizygous at a sufficient number of sex determining loci become male and individuals that are heterozygous at a sufficient number of these loci become female. Diploid males sometimes occur.
Genic systems - mating strains + Vs - mating strains as in yeast, other fungi and many protists
Environmental sex determination
Some fish and reptiles. Turtles - eggs above 32C become female, eggs below 28C become male