- Introduction to heredity
- Fertilization terminology: gametes, zygotes, haploid, diploid
- Alleles and genes
- Worked example: Punnett squares
- Mendel and his peas
- The law of segregation
- The law of independent assortment
- Probabilities in genetics
- Mendelian genetics
Fertilization terminology: gametes, zygotes, haploid, diploid
Fertilization occurs when a sperm cell and an egg cell (ovum) fuse, creating a zygote. Sperm and egg cells are called gametes and have half a set of chromosomes (haploid). The zygote has a full set of chromosomes (diploid). Created by Sal Khan.
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- How is it possible that the sperm cells reach the ovum and more than one get there at the same time when there is a possibility that the system was made for one sperm cell to get through? If so how can you tell why and how do you think it would affect those who are looking forward to having twins?(36 votes)
- Many sperm cells reach the ovum at similar times, but only one actually gets there first. Once the first sperm cell does meet the egg, the acrosome reaction begins, which is the process of the sperm enzymatically digesting the ovum's outer layer and fusing with its plasma membrane. Once this occurs, the ovum no longer has the ability to fuse with any other sperm because the acrosome reaction causes a cascade to occur that changes the ovum's entire cell membrane and makes it unable to fuse again.
So only one sperm ever really gets through. What happens in monozygotic twins, which is what I assume you are referring to, is that something unusual occurs AFTER fertilization. The developing embryo for some reason collapses and splits its progenitor cells very early on, leading to genetically identical offspring (usually - there is a strange exception that I don't really understand).(69 votes)
- There cant be YY?
The mother always gives X?(18 votes)
- That is correct. The mother's gametes (ova) only carry X chromosomes. The father's gametes (sperm) can either be X or Y and thus it is the sperm cell's chromosomes that determines gender.(42 votes)
- Sal said that when father contributes x and mother contributes x then the zygote is a female . But does anytime this happen that both the gametes contribute y(14 votes)
- A man has chromosomes X and Y, so he can give either X or Y.
A female has X and X, so she can only give X.
So the only combinations are the man's x, and woman's x. or the man's Y and woman's X.
People can be born with Triple X syndrome (XXX) and Klinefelter's syndrome (XXY), but these are rare diseases.(29 votes)
- Wouldn't it be more accurate to say that 23 chromosomes from each gamete come together to make 46 chromatins and 23 chromosomes, rather than saying they make 46 chromosomes?(8 votes)
- "Chromosome" essentially refers to how many centromeres there are. You may be confusing this concept with what happens during DNA replication, where there are 46 chromosomes with an extra "sister chromatid" attached to the centromere. In DNA replication you end up with 46 chromosomes and 92 chromatids.
Gametes on the other hand only have 23 centromeres with a single chromatid, therefore, only 23 chromosomes. Like the other replies have mentioned, a typical somatic cell in a human needs 23 "pairs" of chromosomes, half of which come from the father, half come from the mother.
PS I realize I am 5 years late. Hopefully this will help someone else with the same question(7 votes)
- what is the cause of down syndrome ? is it because of an extra copy of chromosome 21 ?(4 votes)
- since we normally have 23 pairs of chromosomes, Down syndrome usually results because of the following:
1. Trisomy 21. which is the cause for about 95% of all causes of down syndrome. so yes. its like you said, its an extra copy (3) instead of the usual two copies. Its caused by irregular or abnormal cell division during development.
2. Translocation down syndrome. Is when chromosome 21 becomes moved and attached to another chromosome before birth. So they have traces of additional material from chromosome 21 on the other chromosome.
3. Mosaic Down syndrome. This is the final and most rare form. A child has some cells with an extra copy of Chromosome 21. Hence the name mosaic since its a mixture of both. Caused by irregular cell division.(18 votes)
- Are there other species with the higher number of chromosomes than we have? If yes, what does it mean? Are they somehow more complicated than we are or what?(9 votes)
- Plants usually have the largest number of chromosomes. For example, fern has 1260 chromosomes. I would not say that the fern is more complicated than we are, therefore it seems that there is no correlation between the number of chromosomes and complexity of the organism.(3 votes)
- how can they differenciate an 'x' chromosome with a 'y' chromosome?do they look different under a microscope?(5 votes)
- They certainly do! Take a look at this pic
At the bottom right is the X and Y chromosomes. The Y is quite a bit smaller than the X.(7 votes)
- Can anyone help me understand basically why only one sperm cell enters the egg, thus preventing an insane amount of fetal formation? Also, does this process occur in all of nature or in other physiological processes?(4 votes)
- This is because there are certain special chemicals in both the sperm and the egg to ensure this happens. As soon as a properly activated "sperm comes in contact with the egg, it releases some chemicals which makes the egg also release some chemicals. All these bring about what is called the Zona reaction and the Cortical reaction. What this essentially means is that, these chemicals change the properties of the surrounding layers of the egg cell (called zona pellucida) whcih makes it impenetrable for any more sperms. Hence the first sperm to reach and release the chemicals in an appropriate manner is the only one allowed to enter.
This process is not perfect, and hence there are several cases where more than one sperm has entered into the ovum.(7 votes)
- Why are there two terms for reproductive cells (haploid & gamete) and body cells (diploid & zygote)? What is the difference between each group of terms?(2 votes)
- "Haploid" refers to any cell that has 23 chromosomes (half of the total 46). "Gametes" are specifically sex cells that have 23 chromosomes. "Diploid" refers to any cell that has all 46 chromosomes. "Zygote" is the result of two gamete (haploid) cells fusing, and becoming a diploid cell.(7 votes)
- What if it is y an y.. will at be a female or male.. and how do people get twins.. How r the cells different(3 votes)
- It is not possible to have two y's since your mother(female) will always carry an x. Parents recieve twins when two eggs get fertilized or a single fertilized egg splits into two embryos.(1 vote)
Voiceover: So let's talk a little bit about how we all came into being. What we see right over here, this is a picture of a sperm cell fusing with an egg cell. So that's a sperm cell and this is an egg cell or we could call this an ovum. And even this scene depicted right over here, this is the end of an epic competition because this sperm cell is one of one of two to three hundred million that is vying for this ovum. So there's two to three hundred million of these characters and they're all vying for this ovum and the one that you see that's about to fuse for it, this is the winner of this incredibly - remember two to three hundred, 200 million to 300 million sperm are trying to get here so this is a major victory and to some degree we should all feel pretty good about ourselves because we are all the by-product of that one in 200 to 300 million sperm cells that won this race getting to our mother's ovum. So the sperm cell came from our father and the egg cell, this is all happening inside of our mothers, the egg cell is from our mother. Now, once this happens, let's talk a little bit about the terminology. So once these two fuse, or the process of them fusing, we call that fertilization. Fertilization. And it produces a cell that then differentiates into all of the cells of our body, so you can imagine that this is an important process. So let's make sure that we understand the different terminology, the different words for the different things that are acting in this process. So each of these sex cells, I guess we could say, the sperm cell and the ovum, these are each called gametes. So this right over here is a gamete and the ovum is a gamete, the egg cell is also a gamete. And as we'll see, each gamete has half the number of chromosomes as your body cells or most of the somatic cells of your body so outside of your sex cells that might be in your ovaries or your testes, depending on whether you're male or female, these have half the number so let's dig a little bit deeper into what I mean there. So let's just do a blow up of this sperm cell right over here, so a blow up of a sperm cell and I'm not going to draw it to scale, you see the sperm cell is much smaller than the egg cell but just to get a sense, so let me draw the nucleus of this sperm cell, so just like that. If we're talking about a human being, and I'm assuming you are a human being, so that might be of interest to you, this will have 23 chromosomes from your father so let's do them. One, two, three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22 and for the 23rd one, that's going to be your sex-determining chromosome so if your father contributes an x, you are going to be female, if your father contributes a y, you are going to be male. So these are the chromosomes in the male gamete or I guess I should say the gamete that your father's contributing, the sperm. So this is a gamete right over here and that's going to fuse with the egg, the ovum that your mother is contributing and once again, I'm not drawing that to scale. So this is the egg, and let me draw it's nucleus. So that's it's nucleus, once again none of this is drawn to scale. And your mother is also going to contribute 23 chromosomes. So one, two, three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22 and then she will contribute an x chromosome for the sex determining so your sex determining chromosomes are going to be xy, you're going to be male, if this was xx, you're going to be female so this is also a gamete here. So a gamete is the general term for either a sperm or an egg. Now once these two things are fused, what do we have? Once they're fused, then we're going to have you could say a fertilized egg but we are going to call that a zygote so let me draw that. I'm going to do this in a new color, and I'm running out of space and I want this all to fit on the same screen so I'll draw it not quite at scale and so let me draw the nucleus of the zygote, I'm going to make the nucleus fairly large so that we can focus on the chromosomes in it, once again none of this is drawn to scale. So you're going to have the 23 chromosomes from your father, so let me do that. One, two, three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22 and 23, and then the 23 chromosomes from your mother. One, two, three, four, five, six, seven, eight, nine, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20, 21, 22 and 23 so you got that x chromosome from your mother. And as you might have notice, I've drawn them in pairs so you now have a total, let me make it clear, you have 23 chromosomes here, 23 chromosomes in the sperm, you have 23 chromosomes in the egg and now you have 46 chromosomes in the fertilized egg, 46 chromosomes, and now that we have a full contingent of chromosomes and then this cell can now keep replicating, keep splitting and differentiating into all of what makes you, you, we call this right over here, we call this a zygote. So one way to think about it, the gametes are the sex cells that have half the number of chromosomes and the zygote is the cell that's now ready to differentiate into an actual organism that has double the number or that has a full contingency of chromosomes, that has 46 chromosomes, and you see that I've made them in pairs and these pairs, we call these homologous pairs and in each of these pairs, this is a pair of homologous chromosomes. So what does that mean? Well that means that in general, these two chromosomes, you got one from your father, one from your mother, they code for the same things, they code for the same proteins but there are different variants of how they code for those proteins, those traits that you have so gross oversimplification is, let's say that there is a gene on, that one from your father that helps code for hair color well there would be a similar, there would be another variant of that gene on the chromosome from your mother that helps code for hair color as well. So these are homologous chromosomes, these two chromosomes code, in general, for the same things and so the zygote now has, you could say it has 46 chromosomes or you could say it has 23 pairs of homologous chromosomes. And this is, once again, this is the case for human beings. If we're talking about some other species, instead of 23 pairs of homologous chromosomes or 46 chromosomes in total, you might be talking about 10 pairs of homologous chromosomes with 20 chromosomes in general. Now to help biologists, to help clarify when they're talking about the number of chromosomes for a given species, they introduce two words, haploid and diploid. And haploid is referring to when you have half the full contingency of chromosomes. So for human beings, the haploid number is 23. So, this is the haploid number, it is 23. For another species it would be something else and haploid is based on the prefix "hapl", that's the prefix for single, so you have a single member I guess you could think of it, of each of the pairs and now you have both of each pair, you have both chromosomes in each pair or you have the full contingency and this 46 chromosomes, this is called the diploid number for humans. The diploid number right over here. And when people talk in general, and we will speak in general when we start talking about mitosis and meiosis for a given species they will refer to the haploid number, they will refer to the haploid number as n chromosomes and they will refer to the diploid number as just twice that, as two n chromosomes. So hopefully this gets you familiar with some of the vocabulary around fertilization and haploid and diploid and zygotes and gametes and also makes you feel a little bit better about yourself that, just to exist, at least half of your chromosomes had to win an incredibly competitive race.