Chapter 15: The Chromosomal Theory of Inheritance (5)

In recent years, geneticists have identified two to three dozen traits in mammals that depend on which parent passed along hte alleles for those traits. Such variation in phenotype depending on whether an allele is inherited from teh male or female parent is called __.

__ occurs during the formation of gametes and results in the silencing of one allele of certain genes. Because these genes are imprinted differently in sperm and eggs, a zygote expresses only one allele of an imprinted gene, either the allele inherited from the female parent or the allele inherited from the male parent.

The __ are transmitted to all the body cells during development, so either the maternal or paternal allele of a given imprinted gene is expressed in every cell of that organism. In each generation, the old imprints are "__" in gamete-producing cells, and teh chromosomes, and the chromosomes of the developing gametes are newly imprinted according to the sex of teh individual forming the gametes. In a given species, teh imprinted genes are always imprinted in the same way.

What exactly is a genomic imprint?
-- In many cases, it seems to consist of __ that are added to__ of one of the alleles. Such __ may directly silence the allele, an effect consistent with evidence that heavily methylated genes are usually inactive.
-- However, for a few genes, __ has been showm to activate expression of the allele.

__ is thought to affect only a small fraction of the genes in mammalian genomes, but most of the known imprinted genes are critical for __.

TRUE OR FALSE:
Normal development requires that embryonic cells have exactly one active copy- not zero, not two- of certain genes. The association of aberrant imprinting with abnormal development and certain cancers has stimulated numerous studies of how different genes are imprinted.
True False

Not all of a eukaryotic cell's genes are located on nuclear chromosomes, or even in teh nucleus. SOme genes are located in organelles in the cytoplasm; because they are outside the nucleus, these genes are sometimes called __ or __.

__, as well as __ and other plant plastids, contain small circular DNA molecules that carry a number of genes. These organelles reproduce themselves and transmit their genes to daughter organelles. Organelle genes are distributed to offspring according to the same rules that direct the distribution of nuclear chromosomes during meiosis, so the do not display __.

Similar material inheritance is also the rule for __ genes in most animals and plants, because almost all the mitochondria passed on to a zygote come from the cytoplasm of the egg. THe products of most mitochondrial genes help make up the protein complexes of the __ and __.

Defects in one or more of these mitochondrial proteins reduce the amount of __ teh cell can make and have been shown to cause rare human disorders. Because the parts of the body most susceptible to energy deprivation are the nervous system and the muscles, most __ diseases primarily affect these systems.
-- For ex., __ causes weakness, intolerance of exercise, and muscle deterioration. Another mitochondrial disorder is __, which can produce sudden blindness in people as young as 20 and 30. The four mutations found thus far to cause this disorder affect __ during cellular respiration, a crucial function for teh cell.

TRUE OR FALSE:
In addition to the rare diseases clearly caused by defects in mitochondrial DNA, mitochondrial mutations inherited form a person's mom may contribute to at least some cases of diabetes and heart deisease, as well as to other disorders that commonly debilitate the elderly, like Alzheimer's. In the course of a lifetime, new mutations gradually accumulate in our mitochondrial DNA, and some researchers think that these mutations play a role in the normal aging process.
True False

Wherever genes are located in the cell- in the __ or in __, their inheritance depends on the precise replciation of DNA.