Anonymous
Anonymous asked in Science & MathematicsBiology · 8 years ago

# Exam question help~3 sources of variation that make all humans different?

I cant find the answers to these in the book and im having trouble understanding...

1)What are three sources of variation that result in virtually every human being genetically different?

2)What is nondisjunction and relate it to triple X syndrome, Klinefelter’s syndrome, Turner’s syndrome

and Down’s syndrome (trisomy-21).

thank you!

Relevance
• 8 years ago

1. Crossing over

Crossing over is the exchange of corresponding segments of non-sister chromatids of homologous chromosomes. During prophase I (of meiosis I), homologous chromosomes synapse to form aggregates called either bivalents (bi = 2, and there are two homologous chromosomes in the aggregate) or tetrads (tetra = 4, and there are 4 chromatids in the aggregate). While the chromosomes are synapsed in prophase I, crossing over occurs. As a result, a chromatid that originally consisted of 100% maternal alleles and a chromatid that originally consisted of 100% paternal alleles become chromatids with a mixture of both maternal and paternal alleles. This generation of recombinant chromatids shuffles genetic information and increases variation.

2. Independent assortment

In metaphase I (of meiosis I), the tetrads (bivalents) line up at the center of the cell. Which pole the maternal and paternal chromosomes face is a matter of chance; and the direction the maternal and paternal chromosomes face in one tetrad has no bearing on which way the maternal and paternal chromosomes of another tetrad face. Thus, when the homologous chromosomes separate and move to opposite poles, each pole will receive a mix of maternal and paternal chromosomes. This shuffles genetic information and increases variation.

3. “Segregation”

In metaphase II (of meiosis II), chromosomes consisting of two sister chromatids (but not genetically identical due to earlier crossing over) line up at the center of the cell. Which pole the sister chromatids face is a matter of chance. When the non-identical sister chromatids are separated, one pole will receive one and the other pole will receive the other. Thus, the two poles will have genetically different copies of the same chromatid. This shuffles genetic information and increases variation.

4. Random fertilization

Because of the above processes, male animals can potentially produce millions or hundreds of millions of genetically unique sperm. And, male animals typically release millions of sperm during sex. Which one of the millions of genetically unique sperm happens to end up fertilizing the egg is largely a matter of chance. Thus, except for identical twins, siblings never have exactly the same genetic information.

Nondisjunction is the failure of either (1) homologous chromosomes in anaphase I, or (2) sister chromatids in anaphase II or mitotic anaphase, to separate. This results in one daugther cell receiving 0 copies of the chromosome and the other daughter cell receiving an extra copy.

So if the 2 sister chromatids of chromosome 21 in a cell in metaphase II fail to disjoin in anaphase II, one of the resulting gametes will have an extra chromosome 21 (2 of them, instead of 1). If that gamete then fuses with a normal gamete having the normal number of chromosome 21's (1 copy), the resulting zygote will have 3 copies of chromosome 21 .... trisomy 21, which leads to Down's syndrome.

The same general thing for triplo X syndrome, except that it is the two sister chromatids of an X chromosome that fail to disjoin and so an extra X ends up in an egg which then gets fertilized by a normal X-carrying sperm, resulting in the zygote having 3 X chromosomes.

Similar for Turner syndrome. For example, the sister chromatids of the X fail to disjoin in the mother and one egg ends up with 0 X chromosomes, and it gets fertilized by a normal sperm containing a single X, so the zygote is XO,