Advanced GeneticsHypertrichosis

• Women have twice the number of X chromosomes as men do – how can this be?

• This means that they have twice the “gene dosage” for genes on the X chromosome as men.

• In order to correct this, one X chromosome is “turned off” and turned into a “Barr Body.”

• The Barr Body shows up as a dark spot within the nucleus during interphase.

• Cells which have the same X chromosome inactivated tend to group together in adult females, forming patches.

• This means that different patches of the body will express different phenotypes.

• For example, hair colour in cats is determined by a gene on the X chromosome.

• The gene has two alleles: XO (orange) and XB (black).

• For males:–XOY will be orange.–XBY will be black.

• For females:–XOXO will be orange.–XBXB will be black.

• What about females with the genotype XOXB?

• Since different patches of skin will have different X chromosomes inactivated– some patches will express the orange

phenotype– some will express the black phenotype

• The result will be calico:

• Do male calico cats exist?• In order to be calico, a cat must have an XOXB

genotype.

• Only way a male can have this is if they have the

genotype XOXBY

• This means they have –Klinefelter Syndrome

– polysomy X– 2 copies of the X chromosome (remember XXY)

• The same thing happens in women who have anhydrotic ectodermal dysplasia… patches of their bodies expressing the affected X chromosome have no sweat glands, hair, or teeth. For example:

Barbara McClintock• Discovered the method by

which “crossing over” of chromosomes or genetic recombination occurs.

• She did this despite facing considerable sexism, in a field where women were almost universally excluded.

• She also discovered that some elements of the chromosome (called “transposons”) can “jump” from one chromosome to another.

• For this work, Dr. McClintock won the 1983 Nobel Prize in Medicine.

• These transposons exist all over the place in nature, especially in simple genomes like those of bacteria.

• Additionally, bacteria sometimes have circular segments of DNA called “plasmids” which they can “inject” into other bacteria to transmit genetic information.

• For example, look at the animation on this web page.

• This mechanism allows bacteria to transmit antibiotic resistance or other dangerous traits to each other – this can often occur in areas with a large variety of bacteria, such as hospitals.

• We can also make use of similar elements of DNA to “repair” genetic abnormalities before the fetus has developed a disorder.

• This is done by replacing the defective gene with a working copy of the gene – in other words, an undesirable allele

is taken out and a desirable allele is put in

• For thousands of years before DNA was even known to exist, human beings engaged in a form of “genetic engineering”.

1. Selective breeding:– Animals which possess desirable traits are encouraged

to breed, while those which do not are prohibited from breeding….examples????

2. Inbreeding:– Animals from the same “family” are interbred to

strengthen desirable characteristics.– Gene pool becomes limited….examples????

• Both forms of breeding can “limit the gene pool”

Controlled Breeding!!

• Using just these two methods, humans have made some amazing changes.

• This:

• Became this:

• And this:

• Became this: