# What is the chance a given gene will end up in a given gamete?

Let us say a germ cell had a desired allele. This germ cell was replicated during interphase so that it had two of the desired allele. It then underwent meiosis.

My question then is, what is the chance a given allele will end up in a given gamete that will be involved in conception?

Firstly, there is a 50% chance a chromosome will be assigned to a given daughter cell. This is because of independent assortment. In addition, cross over occurs at chiasmata. Lets assume then that there was a 50% chance that an allele will be crossed over (although this probability may be far from true)

The process during meiosis is that cross-over occurs first. We have two desired alleles, each on one of two pairs of chromatid attached to each other. There is 50% chance for a given allele to cross over. For example, the chance for both desired alleles to cross over is 1/4 (1/2 x 1/2), the chance only one will cross over is 2/4 (since either of the two desired alleles can cross over).

Then there is the first independent assortment during meiosis I which produces the desired daughter cell. In this case there is a 50% chance that a given chromatid pair (containing the desired allele) will be assigned to the desired daughter cell. However, in the case where a desired allele is found in both homologs (2/4 of a chance), then the chance that at least one desired allele will be in the daughter cell is 100%.

Finally, during meiosis II, another independent assortment occurs which produces the desired gamete. The same rule applies with a 50% chance a given allele will be assigned to a given cell (in this case the gamete). However, if the desired daughter cell contains two of the desired alleles, then there is a 100% chance that the cell will have the desired allele.

In conclusion, the chance I estimated that the gamete will have at least one desired allele is 50%. Although, I'm not sure if this is correct. Anyone mind confirming this.

• "Let us say a somatic cell... then underwent meiosis" Somatic cells don't undergo meiosis. – De Novo Aug 13 '18 at 20:30
• Gametes do not undergo meiosis (at least under normal circumstances). Since you seem to be having ongoing trouble with basic concepts and terminology in genetics I strongly encourage you to take advantage of the many online resources available for making sure you have the background required for understanding this field. Suggestions to follow ... – tyersome Jun 19 '20 at 18:25
• I have found that when learning about a new area starting with a relatively accessible and reliable source like Khan Academy is very helpful. Wikipedia is also generally a good starting point and you can then check their references. Online platforms called MOOCs offer free (or very low cost) courses on a wide variety of subjects — two I am familiar with are Coursera and edX. Finally, textbooks with a good level of detail are also freely available online e.g. from NCBI. – tyersome Jun 19 '20 at 18:26