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    Hi, I'm having a hard time with the genetics chapter of A2. I have quite a few questions so if any of you know the answers to any of them I would really appreciate it.

    - What actually are chromosomes? I understand we have 23 from our Mum and 23 from our Dad but do they just consist of a load of different genes coding for different alleles? Or are 23 of them all the same and the other 23 also the same?
    - What are sex chromosomes? I get they have half the DNA in order to form diploid gametes but it says they're XX or XY so are they two chromosomes? In a karyotype all the chromosomes look like Xs so I don't understand how you can tell the difference? For sex-linkage they are found on the X chromosome but all the other chromosomes are Xs too? (Apart from the male's Y).
    - Why does crossing over (which happens in meiosis) result in the formation of gametes? Does this happen after fertilisaiton? Otherwise I don't understand because I thought it was mitosis that formed gametes and meiosis formed diploid cells?

    Thank you in advance. I just have a hard time with this topic haha.
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    (Original post by LeaX)
    Hi, I'm having a hard time with the genetics chapter of A2. I have quite a few questions so if any of you know the answers to any of them I would really appreciate it.

    - What actually are chromosomes? I understand we have 23 from our Mum and 23 from our Dad but do they just consist of a load of different genes coding for different alleles? Or are 23 of them all the same and the other 23 also the same?
    - What are sex chromosomes? I get they have half the DNA in order to form diploid gametes but it says they're XX or XY so are they two chromosomes? In a karyotype all the chromosomes look like Xs so I don't understand how you can tell the difference? For sex-linkage they are found on the X chromosome but all the other chromosomes are Xs too? (Apart from the male's Y).
    - Why does crossing over (which happens in meiosis) result in the formation of gametes? Does this happen after fertilisaiton? Otherwise I don't understand because I thought it was mitosis that formed gametes and meiosis formed diploid cells?

    Thank you in advance. I just have a hard time with this topic haha.
    XY chromosomes are names given to the chromosomes that determine gender. Males have XY females XX. It is simply a notation - other chromosomes just have numbers.

    they just consist of a load of different genes coding for different alleles.

    This is correct.

    Meiosis forms gametes. Gametes have half the chromosomes, so HAVE to be made by meiosis in order to have the correct amount of DNA.
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    (Original post by LeaX)
    Hi, I'm having a hard time with the genetics chapter of A2. I have quite a few questions so if any of you know the answers to any of them I would really appreciate it.

    - What actually are chromosomes? I understand we have 23 from our Mum and 23 from our Dad but do they just consist of a load of different genes coding for different alleles? Or are 23 of them all the same and the other 23 also the same?
    - What are sex chromosomes? I get they have half the DNA in order to form diploid gametes but it says they're XX or XY so are they two chromosomes? In a karyotype all the chromosomes look like Xs so I don't understand how you can tell the difference? For sex-linkage they are found on the X chromosome but all the other chromosomes are Xs too? (Apart from the male's Y).
    - Why does crossing over (which happens in meiosis) result in the formation of gametes? Does this happen after fertilisaiton? Otherwise I don't understand because I thought it was mitosis that formed gametes and meiosis formed diploid cells?

    Thank you in advance. I just have a hard time with this topic haha.
    yup, chromosomes are just a lot of genes (and intronic DNA) linked together to form a structure.

    Different chromosomes have different genes. But several genes from different chromosomes may work together to do one function.

    Sex chromosomes are the X and Y chromosomes, pair #23. They are called as such because they decide what gender the person is. 2 Xchromosomes are in all (as far as you need to know) females. All males (for simplicity, again) have 1X and 1Y chromosome.
    X and Y have some similar genes, and some are found only on 1 of the two.
    Y has genes that lead to masculine development.

    No, only the sex chromosome is called the Xchromosome.

    Y chromosome looks different (see right):
    Name:  y.jpg
Views: 66
Size:  24.6 KB


    No, meiosis forms gametes, mitosis forms diploid cells. Crossing-over is when bits of the homologous chromosomes (eg 2 copies of chromosome 22 in the diploin cell) break off and swap over with each other. It results in greater genetic diversity.


    Hope this helped.
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    (Original post by joostan)
    XY chromosomes are names given to the chromosomes that determine gender. Males have XY females XX. It is simply a notation - other chromosomes just have numbers.

    they just consist of a load of different genes coding for different alleles.

    This is correct.

    Meiosis forms gametes. Gametes have half the chromosomes, so HAVE to be made by meiosis in order to have the correct amount of DNA.

    (Original post by ash92:))
    yup, chromosomes are just a lot of genes (and intronic DNA) linked together to form a structure.

    Different chromosomes have different genes. But several genes from different chromosomes may work together to do one function.

    Sex chromosomes are the X and Y chromosomes, pair #23. They are called as such because they decide what gender the person is. 2 Xchromosomes are in all (as far as you need to know) females. All males (for simplicity, again) have 1X and 1Y chromosome.
    X and Y have some similar genes, and some are found only on 1 of the two.
    Y has genes that lead to masculine development.

    No, only the sex chromosome is called the Xchromosome.

    Y chromosome looks different (see right):
    Name:  y.jpg
Views: 66
Size:  24.6 KB


    No, meiosis forms gametes, mitosis forms diploid cells. Crossing-over is when bits of the homologous chromosomes (eg 2 copies of chromosome 22 in the diploin cell) break off and swap over with each other. It results in greater genetic diversity.


    Hope this helped.
    Thank you both sooo much. I have a couple more questions but don't feel obliged to answer them as you've both already helped me out so much.
    How does translocation result in Down's Syndrome? I know part of chromosome 21 breaks off and joins onto another chromosome but I'm not sure why it still counts as chromosome 21 on the karyotype when it's joined onto another chromosome?
    Also, can non-dysjunction occur in gametogenesis as well as after fertilisation?

    Thanks.
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    (Original post by LeaX)
    Hi, I'm having a hard time with the genetics chapter of A2. I have quite a few questions so if any of you know the answers to any of them I would really appreciate it.

    - What actually are chromosomes? I understand we have 23 from our Mum and 23 from our Dad but do they just consist of a load of different genes coding for different alleles? Or are 23 of them all the same and the other 23 also the same?
    - What are sex chromosomes? I get they have half the DNA in order to form diploid gametes but it says they're XX or XY so are they two chromosomes? In a karyotype all the chromosomes look like Xs so I don't understand how you can tell the difference? For sex-linkage they are found on the X chromosome but all the other chromosomes are Xs too? (Apart from the male's Y).
    - Why does crossing over (which happens in meiosis) result in the formation of gametes? Does this happen after fertilisaiton? Otherwise I don't understand because I thought it was mitosis that formed gametes and meiosis formed diploid cells?

    Thank you in advance. I just have a hard time with this topic haha.
    Each chromosome represents a long DNA molecule. We obtain 23 from our mother and 23 from our father. Each set of chromosomes from a single parent contains 23 different chromosomes, each of which contain a particular array of genes in a particular order. For each chromosome (excluding sex chromosomes) we have two copies (one from mother and father). These copies are referred to as homologous chromosomes (e.g. Chr1 from mother and Chr1 from father are homologues of each other). Homologous chromosomes contain all of the same genes but may contain allelic variants of any particular gene. It is not correct to say that genes code for alleles; genes code for proteins (or non-coding RNA) and alleles represent sequence variations of a particular gene.

    Sex chromosomes -- somebody else has done this so I'll leave it. Different chromosomes are distinguished by their length and also by the fact that each has a distinct banding pattern when stained, which uniquely identifies each chromosome.

    Crossing over does not result in the formation of gametes per se but is just something that always occurs during gametogenesis. Crossing over occurs when the replicated homologous chromosomes are in synapsis, so happens before meiosis I is even complete. In humans, mitosis forms diploid cells whereas meiosis forms the haploid gametes.

    As for Down's syndrome, that can be caused by non-disjunction or resulting from a translocation. Down's syndrome is trisomy 21. For this to happen, one of the gametes needs to have two copies of chromosomes 21 (or large portion of it). It's quite difficult to explain how translocation of chromosome 21 can lead to trisomy 21 without the use of a diagram. Think of meiosis occurring in a cell in which there is one normal chromosome 21 and one chunk of chromosome 21 fused (i.e. translocated) to another autosome. The fusion chromosome contains the centromere of the non-21 autosome but not the centromere of chromosome 21. The translocation effectively bypasses the normal rules of homologous chromosome segregation because the two chromosome 21 homologues do not undergo synapsis during meiosis. So during independent assortment, you could get the complete Chr21 going to one cell, and then the Chr21-fusion going to the same cell. This is because a chunk of Chr21 is piggybacking onto the centromere of a different autosome that segregates independently of the complete Chr21.

    So for the gamete in question you would have one complete chromosome 21 and then one large fragment of Chr21 fused to another chromosome. As soon as this gamete fuses with a "normal" gamete then you will have (partial) trisomy 21.
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    (Original post by Ashnard)
    Each chromosome represents a long DNA molecule. We obtain 23 from our mother and 23 from our father. Each set of chromosomes from a single parent contains 23 different chromosomes, each of which contain a particular array of genes in a particular order. For each chromosome (excluding sex chromosomes) we have two copies (one from mother and father). These copies are referred to as homologous chromosomes (e.g. Chr1 from mother and Chr1 from father are homologues of each other). Homologous chromosomes contain all of the same genes but may contain allelic variants of any particular gene. It is not correct to say that genes code for alleles; genes code for proteins (or non-coding RNA) and alleles represent sequence variations of a particular gene.

    Sex chromosomes -- somebody else has done this so I'll leave it. Different chromosomes are distinguished by their length and also by the fact that each has a distinct banding pattern when stained, which uniquely identifies each chromosome.

    Crossing over does not result in the formation of gametes per se but is just something that always occurs during gametogenesis. Crossing over occurs when the replicated homologous chromosomes are in synapsis, so happens before meiosis I is even complete. In humans, mitosis forms diploid cells whereas meiosis forms the haploid gametes.

    As for Down's syndrome, that can be caused by non-disjunction or resulting from a translocation. Down's syndrome is trisomy 21. For this to happen, one of the gametes needs to have two copies of chromosomes 21 (or large portion of it). It's quite difficult to explain how translocation of chromosome 21 can lead to trisomy 21 without the use of a diagram. Think of meiosis occurring in a cell in which there is one normal chromosome 21 and one chunk of chromosome 21 fused (i.e. translocated) to another autosome. The fusion chromosome contains the centromere of the non-21 autosome but not the centromere of chromosome 21. The translocation effectively bypasses the normal rules of homologous chromosome segregation because the two chromosome 21 homologues do not undergo synapsis during meiosis. So during independent assortment, you could get the complete Chr21 going to one cell, and then the Chr21-fusion going to the same cell. This is because a chunk of Chr21 is piggybacking onto the centromere of a different autosome that segregates independently of the complete Chr21.

    So for the gamete in question you would have one complete chromosome 21 and then one large fragment of Chr21 fused to another chromosome. As soon as this gamete fuses with a "normal" gamete then you will have (partial) trisomy 21.
    Thank you so so much!
 
 
 
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