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During meiosis, the independent assortment will be made first and then cross over will be made. I am so confused, what is the difference between this two process? I looked at the diagram on internet.It is said that law of independent assortment is alleles of genes on non homologous chromosomes assort independently during gametes formation, but I don't quite understand that. So, another way for understanding this topic, independent assortment is tetrads can line up four different ways between homologous separate. Is that right. Appreciate your help.

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During meiosis, the independent assortment will be made first and then cross over will be made.

No, independent assortment occurs after crossing over. Crossing over occurs in prophase I while independent assortment occurs in metaphase I and anaphase I.

I am so confused, what is the difference between this two process?

During prophase I, a process called synapsis occurs. That is, homologous chromosomes (one from your mother and one from your father) come together and bind together, forming a tetrad. This binding is due to crossing over, which is when the genetic material on the maternal chromosome and the paternal chromosome exchange places. The maternal chromosome gives some of its genes to the paternal chromosome, and the paternal chromosome gives some of its genes to the maternal chromosome. Crossing over increases genetic variety by creating a new chromosome with a new combination of genes.

During metaphase I, the tetrads line up at the equator of the cell. During anaphase one, the tetrads are broken apart and each homologous chromosome that made each tetrad up is pulled toward one end of the cell. Look at the image below, where the blue represents paternal chromosomes and the red represents maternal chromosomes.

enter image description here

Independent assortment describes the phenomenon where the paternal and maternal chromosomes can be lined up randomly at the equator. There are many combinations. For example,

  • Both red chromosomes could face the top pole and both blue chromosomes could face the bottom pole.
  • Both blue chromosomes could face the top pole and both red could face the bottom.
  • One red on the left and one blue on the right could face the top, as shown in the picture.
  • One blue on the left and one red on the right could face the top.

This random alignment at the equator makes it so that during anaphase, random proportions of maternal and paternal chromosomes get assorted into each of the resulting daughter cells. Some cells could get more maternal chromosomes and the other would get more paternal chromosomes, and vice versa. On the other hand, they could get similar amounts. Independent assortment increases genetic variation by allowing daughter cells to each randomly receive a different proportion of paternal and maternal chromosomes.

In conclusion, crossing over and independent assortment (sometimes called random assortment) are different independent processes that both lead to an increase in genetic diversity.


References

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  • $\begingroup$ I'm interested in the randomness of the "Independent assortment" of the genetic material. How specifically is randomness achieved, to create the specific unique variation out of the 2^23=8388608 possibilities. Are certain combinations naturally more likely than others, and how does the cell make the components line up randomly at the equator? $\endgroup$ – Harald Nordgren Dec 29 '18 at 4:41

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