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Genetics help? Assigning ploidy?


I just started my genetics class and I'm already lost. I have an appointment scheduled for help. I'm just trying to figure it out on my

own still.

During mitosis and meiosis you assign ploidy. I get lost when it comes to figuring out how many chromosomes and chromatids.

Anyone know of any resources off the top of your head that might help? Again not a homework question, just trying to did a resource that I understand.

Thank you :)

Esme12, ASN, BSN, RN

Specializes in Critical Care, ED, Cath lab, CTPAC,Trauma. Has 41 years experience.

This is SO NOT my forte.....I did a Google search

Here is the way I think about it: before S phase, a typical human cell contains 46 chromosomes. During S phase, all 46 chromosomes are replicated. Because they are identical copies, it is perfectly fine to say that there are now 92 (46 + 46) chromosomes in the cell. However, the term 'chromatid' is used to describe them until cell division is complete. In other words: a chromatid is just what we a call a chromosome during cell division. Also, because the chromatids are stuck together after replicating, it is also reasonable to say that there are 46 pairs of chromatids (or chromosomes) at the end of S phase.

If the cell goes through mitosis (normal cell division) those 46 pairs will separate and the two daughter cells will have 46 chromosomes.

If the cell goes through meiosis (gamete formation) each chromatid pair will pair up with another (homologous) chromatid pair to form a group of four chromatids called a tetrad (my textbook also calls this a bivalent). There should be 23 tetrads in a human cell at the beginning of meiosis. The cell then divides twice with the chromatids separating each time. This results in four cells each of which contains 23 chromosomes (46/2). These cells are called gametes. When one gamete combines with another gamete, a zygote is formed which has 46 chromosomes. The zygote then goes through mitosis to form more cells with 46 chromosomes and those cells make up the embryo.

I have been teaching genetics for four years now and I find that this concept is one of the biggest stumbling blocks for students (for obvious reasons). If it helps, whenever you see the word 'chromatid' just replace it with 'chromosome' since, for most purposes, they mean the same thing.

Brent Richards: The Oklahoma School of Science and Mathematics

Life Sciences Cyberbridge

Thank you. I looked last night through google and searched for videos in YouTube but had no luck. I signed up for a tutor because I just don't get it.

pmabraham, BSN, RN

Specializes in Hospice, Palliative Care. Has 3 years experience.

Good day, aeris99:

I found the genetic videos on https://www.youtube.com/channel/UCXllraVycgq2jwe230w1qIA/videos helpful.

In terms of the ploid part:

Euploid - 23 chromosomes for haploid, and 46 for diploid.

Polypoid is a multiple of 23 (i.e one times, two times, three times, etc.) whereby any multiple that is 3 or more results in miscarriage; ie. 69 (3 x 23) and 92 (4 x 23) where the latter is a tetraploid. See Polyploid - Wikipedia, the free encyclopedia

Aneuploid is where the number does not evenly divide by 23. see Aneuploidy - Wikipedia, the free encyclopedia. The most commonly known example of aneuploidy is Down syndrome, which is caused by an extra copy of chromosome 21.

Thank you.

la_chica_suerte85, BSN, RN

Specializes in Pediatric Hematology/Oncology.

The Scitable page from nature.com has fantastic resources for genetics (the section on chromosomes and cytogenetics should help). This is a tricky subject and it helps if you draw diagrams of the stages of meiosis/mitosis as you follow along. This is one of those things you gotta practice, practice, practice! Good luck! :)

Ploidy and the terminology are two of the biggest hurdles to overcome at the beginning! Visit the Khan Academy site, they have so e great videos explaining the terms (and they draw pictures while explaining so that helps a lot). Have fun! I loved genetics...I wish our school offered more genetics classes!

I now have a good understanding of mitosis. Meiosis is a little shaky through the mini stages of prophase.

We have had questions where we have to tell how many chromosomes and chromatids are present in the stages where n is whatever number. In this case n=3

I've already completed the assignment the best I could but I'm not sure my answers are right.

I went or a help session with the teachers assistant and 75% of the class was there and just as confused as I was.

I'll find out tonight how far off my answers are.

Thank you for the new info posted. I'll read through it today before class and try to check my work.