AAMC 11 No.107 /Chromosome Inactivation Sex Determination
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In humans, males have XY chromosomes and females have XX chromosomes (one of which is inactivated). You have to apply the same concept here. Since the question states that MALES are born with two copies of R, one of the R chromosomes would become inactive in MALES to compensate for the imbalance.
Didn't know this...Ave maria, help me for this exam.
I thought it's Y chromosome that makes someone male. Why is necessary to inactivate the X chromosome in females if it determines female sex?
So you can fulfill the questions requirements - to create a system that allows 50/50 male/female.
RR * RS = majority female
R*RS = half half.
i think what futuredoc said is irrelevant. but yeah the deactivated X chromosome is called a barr body.
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You don't want a superfemale, so the X chromosome inherited from the father (via epigenetic imprinting mechanisms) with the XIST gene, in the X-Inactivation Center (XIC) will transcribe RNA products that wrap ITSELF in RNA, condensing it into an inactivated Barr Body. This process is called lyonization.
Of course, lyonization is random in human females: they are mosaics. To explain this, the X-inactivation reverses itself in the inner cell mass after the blastocyst stage, and then randomly picks one X to be Xa (the active) and all the rest (normally only one other X) to be Xi, or inactive. This process is irreversible. The phenomenon can be easily observed in tortoiseshell cats:
That said, XXX superfemales aren't dysfunctional human beings. They will have two Barr bodies.
Also, it's worth nothing that there are other sex-determining systems, such as the http://en.wikipedia.org/wiki/ZW_sex-determination_system in birds, where females are heterogametic (ZW) and males are homogametic (ZZ). There's also the http://en.wikipedia.org/wiki/Haplodiploid_sex-determination_system in ants and bees, wherein the unfertilized hymenopteran egg develops into a haploid male, and the fertilized hymenopteran egg develops into a diploid female. Note that male hymenopterans have no father, and cannot have sons!
S***, I literally yelped out loud when I saw this picture...Goodness gracious. Can you umm maybe remove it...I'm a sensitive girl.. Thanks, but this is way too much information Morsetlis.
What I really don't understand is the logic behind the punnet square producing "unequal" female: male ratio.
What I really don't understand is the logic behind the punnet square producing "unequal" female: male ratio.
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X-Inactivation is not present in the female germline cells. There is no "unequal" female : male ratio unless you're talking about hymenopterans, and in those cases I hope it should be obvious.
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One of the X needs to be inactivated so there isn't double the genetic material. The Y chromosome has very little actually on it. It's the X's that carry most of the useful stuff.
Interesting that this concept was indirectly tested (novel situation) on my actual MCAT. Glad I knew about barr bodies and the reason for them.
Interesting that this concept was indirectly tested (novel situation) on my actual MCAT. Glad I knew about barr bodies and the reason for them.
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So the inactivation of one of the X chrom in females takes place during embryonic development. My question is does every cell in a grown female have an inactivated barr body present?
I would think not because cells undergoing mitosis would not be able to replicate the heterochromatin.
Also thinking that if one of the X chrom's is inactivated then how is there XX when female undergo Meiosis 1 producing primary oocytes?
My lack of understanding of this subject is obvious, but i knew you guys could answer it faster then I could look it up.
I would think not because cells undergoing mitosis would not be able to replicate the heterochromatin.
Also thinking that if one of the X chrom's is inactivated then how is there XX when female undergo Meiosis 1 producing primary oocytes?
My lack of understanding of this subject is obvious, but i knew you guys could answer it faster then I could look it up.
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The lyonization process is reversed in Germline cells (of females) which are suspended in Prophase I of Meiosis, until puberty (and then you know what happens).
In somatic cells, the reversal of lyonization happens in the S phase, before mitosis and separation of the
chromosomes to each daughter cells. In the S phase, variations of levels of specific proteins called cyclins and association of these cyclins with a protein called Cdk2 (cyclin-dependent kinase) will cause the chromosomes to decondense fully and allow DNA replication by DNA polymerase. After G2 and mitosis, if the cell returns to a resting (G0) state, the same X sex chromosome (being paternal or maternal for a female or only maternal for the male) will recondense and form a barr body.
But.. this is too much info. Just know that it happens.
(And, yes, multiple professors CAN confuse you if they don't know what they're talking about, especially since the process CAN be irreversible in certain species, at certain stages, in certain tissues. A lot of what we know about genetics comes from animal experimentation.)
In somatic cells, the reversal of lyonization happens in the S phase, before mitosis and separation of the
chromosomes to each daughter cells. In the S phase, variations of levels of specific proteins called cyclins and association of these cyclins with a protein called Cdk2 (cyclin-dependent kinase) will cause the chromosomes to decondense fully and allow DNA replication by DNA polymerase. After G2 and mitosis, if the cell returns to a resting (G0) state, the same X sex chromosome (being paternal or maternal for a female or only maternal for the male) will recondense and form a barr body.
But.. this is too much info. Just know that it happens.
(And, yes, multiple professors CAN confuse you if they don't know what they're talking about, especially since the process CAN be irreversible in certain species, at certain stages, in certain tissues. A lot of what we know about genetics comes from animal experimentation.)
