Variations of Meiosis

By: Emily Hess

Independent assortment

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Independent assortment is the process where the chromosomes move randomly to separate poles during meiosis. A gamete will end up with 23 chromosomes after meiosis, but independent assortment means that each gamete will have 1 of many different combinations of chromosomes.

This reshuffling of genes into unique combinations increases the genetic variation in a population and explains the variation we see between siblings with the same parents.

Crossing over

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When homologous chromosomes come together during prophase I of meiosis, they exchange bits of DNA with each other. This crossing-over results in new gene combinations and new chances for variety. Crossing-over is one way of explaining how a person can have red hair from his mother’s father and a prominent chin from his mother’s mother. After crossing-over, these two genes from two different people wound up together on the same chromosome in the person’s mother and got handed down together.


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Fertilization presents yet another opportunity for genetic diversity. Imagine millions of genetically different sperm swimming toward an egg. Fertilization is random, so the sperm that wins the race in one fertilization event is going to be different than the sperm that wins the next race. And, of course, each egg is genetically different too. So, fertilization produces random combinations of genetically diverse sperm and eggs, creating virtually unlimited possibilities for variation. That’s why every human being who has ever been born — and ever will be born — is genetically unique. Well, almost. Genetically identical twins can develop from the same fertilized egg, but even they can have subtle differences due to development.


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DNA polymerase occasionally makes uncorrected mistakes when copying a cell’s genetic information during DNA replication. These mistakes are called spontaneous mutations, and they introduce changes into the genetic code. In addition, exposure of cells to mutagens (environmental agents, such as X-rays and certain chemicals, that cause changes in DNA) can increase the number of mutations that occur in cells. When changes occur in a cell that produces gametes, future generations are affected.


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{Nothing’s perfect, even in the cellular world, which is why sometimes meiosis doesn’t occur right. When chromosomes don’t separate the way they’re supposed to, that’s called nondisjunction. The point of meiosis is to reduce the number of chromosomes from diploid to haploid, something that normally happens when homologous chromosomes separate from each other during anaphase I. Occasionally, a pair of chromosomes finds it just too hard to separate, and both members of the pair end up in the same gamete.

What happens next isn’t pretty. Two of the final four cells resulting from the meiotic process are missing a chromosome as well as the genes that chromosome carries. This condition usually means the cells are doomed to die. Each of the other two cells has an additional chromosome, along with the genetic material it carries.

Many times these overendowed cells simply die. But sometimes they survive and go on to become sperm or egg cells. The real tragedy, then, is when an abnormal cell goes on to unite with a normal cell. When that happens, the resulting zygote (and offspring) has three of one kind of chromosome rather than the normal two. The term scientists use for this occurrence istrisomy.

All the cells that develop by mitosis to create the new individual will be trisomic (meaning they’ll have that extra chromosome). One possible abnormality occurring from an extra chromosome is Down syndrome.}


  • Independent Assortment: The random assortment of homologous chromosomes that occurs during meiosis.
  • Crossing over: The exchange of genetic material that occurs between homolohous chromosomes during the beginning of meiosis.
  • Fertilization: The joining of egg and sperm (both haploid) to produce a diploid zygote.
  • Mutations: A change in the nitrogen bases in a cell`s DNA that results in different genetic sequences.
  • Nondisjunction: An alteration in which homologous chromosomes fail to separate normally during meiosis.