The Chromosomal Theory of Inheritance

X-linked Gene

Wild Type female x mutant maleWhats going to happen?

X InactivationIn 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.

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

Linked GenesGenes located on the same chromosome that tend to be inherited together.Body color and wing shapeBody color gray normal, black recessiveWing shape normal, vestigial (small)

Chromosome ProblemsDeletionDuplicationInversionTranslocation

Funky Inheritance PatternsGenomic ImprintingSometimes 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.

Organelle GenesAlmost all mitochondria come from the egg.Chloroplast genes carried in the egg as wellWhy?