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The Chromosomal Theory of Inheritance

The Chromosomal Theory of Inheritance. X-linked Gene. Wild Type female x mutant male. What’s going to happen?. X Inactivation. In all mammalian female, one X chromosome is inactivated. (Barr body) Why? What happens if a female is heterozygous for a trait?

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The Chromosomal Theory of Inheritance

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  1. The Chromosomal Theory of Inheritance

  2. X-linked Gene

  3. Wild Type female x mutant male • What’s going to happen?

  4. X Inactivation • In all mammalian female, one X chromosome is inactivated. (Barr body) • Why? • What happens if a female is heterozygous for a trait? • Half of her cells will express one allele, the other half will express the other. • Inactivation is completely random.

  5. XIC & XIST • XIC: X-inactivation center • Sequence on the X that is responsible for inactivation • X-inactive specific transcript • Codes for an RNA that is involved in the X inactivation (no protein from this RNA) • Both chromosomes initially express XIST • Chromosome that becomes inactivated produces a lot of XIST RNA which coats the chromosome • Coating is a signal to silence the genes on this chromosome. • A few genes are never inactivated. • What about males then?

  6. Linked Genes • Genes located on the same chromosome that tend to be inherited together. • Body color and wing shape • Body color – gray normal, black recessive • Wing shape – normal, vestigial (small)

  7. Chromosome Problems • Deletion • Duplication • Inversion • Translocation

  8. Funky Inheritance Patterns • Genomic Imprinting • Sometimes a gene that is expressed depends on whether it is on the chromosome that came from the mother or the father. • During gamete formation the gene is silenced (turned off permanently) • The gene is methylated (-CH3) • When new gametes are produced, the methylation is reset.

  9. Organelle Genes • Almost all mitochondria come from the egg. • Chloroplast genes carried in the egg as well • Why?

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