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Natural Science Forum / Biology / Biology / January 2008Homologous Pairs--What Are They?
I've looked at two very introductory books on genetics (cartoons and dummies). They have good figures showing pairs of chromosomes, but don't accurately pin down the idea in my view. For example, in Genetics for Dummies, there's a figure (2.3) showing two chromatids side by side unconnected. A pair. One chromatid has an A allele and the other A. Is this the homologous pair? (I think it is.) To the right are shown a pair (call them 1 and 2 that look like an X) of, attached at the centromere, chromatids. No 1 shows an A in each chromatid, and no. 2 shows an a in each chromatid. Are these the homologous pair? I'd vote for the first arrangement, as remarked above. My guess is that the first arrangement is what is found in the nucleus prior to meiosis, and the second (1 and 2) is once meiosis has begun.  SignatureWayne Watson (Nevada City, CA) Web Page: <speckledwithStars.net> >I've looked at two very introductory books on genetics (cartoons and >dummies). They have good figures showing pairs of chromosomes, but don't [quoted text clipped - 3 lines] >chromatids side by side unconnected. A pair. One chromatid has an A allele >and the other A. Is this the homologous pair? (I think it is.) yes (Did you mean A and a?? If not, then I'm confused.) >To the right >are shown a pair (call them 1 and 2 that look like an X) of, attached at the >centromere, chromatids. No 1 shows an A in each chromatid, and no. 2 shows >an a in each chromatid. Are these the homologous pair? yes -- #1 and #2 are a homologous pair. The X-shaped #1 is one of the chromosomes you had above, but now replicated (and still attached at the centromere). If I understand your description, both show a homologous pair -- unreplicated in one case, replicated in the other. You have two sets of chromosomes, one from mom and one from dad. For chromosome #1, you have two copies, one from mom and one from dad. They are a homologous pair -- two copies of chromosome #1 (carrying the same genes, though not necessarily the same alleles of those genes). >I'd vote for the first arrangement, as remarked above. My guess is that the >first arrangement is what is found in the nucleus prior to meiosis, and the >second (1 and 2) is once meiosis has begun. yes, but they are homologous pair at any stage. bob See below. >> I've looked at two very introductory books on genetics (cartoons and >> dummies). They have good figures showing pairs of chromosomes, but don't [quoted text clipped - 5 lines] > > yes (Did you mean A and a?? If not, then I'm confused.) Yes, A and a. >> To the right >> are shown a pair (call them 1 and 2 that look like an X) of, attached at the [quoted text clipped - 22 lines] > > bob Maybe homologous is more general than I think. It seems OK to refer to an unbound male and female chromatid for the same chromosome as homologous, and in the case when they are crossed (bound). I think my confusion is that I was not accounting for sex. SignatureWayne Watson (Nevada City, CA) Web Page: <speckledwithStars.net> >Maybe homologous is more general than I think. It seems OK to refer to an >unbound male and female chromatid for the same chromosome as homologous, and > in the case when they are crossed (bound). I think my confusion is that I >was not accounting for sex. You'll only confuse yourself if you think of the chromosomes having sex or gender. In a diploid organism, one chromosome of each pair comes from each parent, but this applies to organisms that don't have males and females too, e.g. hermaphroditic animals (many invertebrates), plants, fungi, yeast, protozoa, etc. An analogy that might help you think about this is to imagine shuffling together two decks of cards, one with red backs and one with blue backs. the resulting double deck has 104 cards, and for each kind of card, e.g. the king of diamonds, there are 2 examples, one with a blue back and one with a red back. These pairs can be thought of as like homologous chromosomes in an organism with a haploid number of 52 and a diploid number of 104. As for the "bound" stuff, I think you are confusing homologous chromosomes with ones that are doubling themsleves in mitosis. To strain the card deck analogy above, if this "diploid deck" were to undergo mitosis, each card would duplicate itself, and after mitosis completes, each of the two resulting cells has an identical set of 104 chromosomes. At the stage when the two chromatids are 'bound', you would have each card doubled, but the two copies are sort of pinned together. So there would be two copies of e.g. the blue back 5 of clubs pinned together, and also two copies of the red back 5 of clubs separately pinned together. Later in the process, they are 'unpinned' and each of the two resulting cells gets a copy of each 'card' or chromosome. Ideally they are identical genetically to the original cell. In meiosis, the homologous chromosomes find each other, and undergo crossing over, mixing the contributions from the two parents to make haploid gametes for the next generation. That's why your children will have genetic material from all four of their grandparents, and if they aren't identical twins, they will each have a unique mix of traits. If you wanted to use the card deck analogy for meiosis, you'd have to use scissors and tape. I hope this helps. >I've looked at two very introductory books on genetics (cartoons and >dummies). They have good figures showing pairs of chromosomes, but don't [quoted text clipped - 10 lines] >first arrangement is what is found in the nucleus prior to meiosis, and the >second (1 and 2) is once meiosis has begun. People are diploid, we have two sets of chromosomes; one from the mother and one from the father. The two corresponding chromosomes are called homologous. They have the same genes, but what the genes "say" (what alleles they contain) can be different. So if the gene for "big nose" is on that chromosome (my favorite joke example of a gene -- there really isn't one specific gene for that trait), one of the two homologs can say "make a big nose" but the other can say "make a small nose". The technical term would be that you are heterozygous for that trait, the two alleles are different. As a separate story, when the chromosomes first become visible in mitosis, the DNA is already replicated in preparation for cell division so that each chromosome is made of two chromatids, also called "sister' chromatids, still joined together at the centromere. If the centromere is sort of near the middle of the chromosome, the two sisters make the chromosome have a sort of X shape. Later in mitosis, the sister chromatids separate so that each chromosome is made out of a single chromatid instead of out of a pair of sisters. The sisters are identical (except for mutation) because they are the copies made when the DNA replicates. Remember, the homologs are often NOT identical because they have different DNA. In your picture, sister chromatids always have the SAME allele, both either A or both a. Homologous chromosomes CAN be different so that one homolog can be A and the other a. So you are right: "1" and "2" form a homologous pair but the left side of the picture shows sister chromatids. Yes, it is very confusing. Once you catch on it seems very simple but until then you can get completely lost. Thanks. I think I'm catching on. You're right, the "big nose" approach is funny, and helpful. The cartoon book (Cartoon Guide to Genetics) introduced the homologous idea via pictures without much attention to specific attention the alleles. It left me hanging. Just looking at the cartoon gave me too many choices for possibilities, and sometimes the drawings don't carefully reveal male (dark coloring, I think) vs female (light). I could just see too many pairs in figures similar to 2.3. Sometimes a picture is not worth 1000 words. It sure is interesting how all this works, and how anyone figured it out. I peeked ahead in Dummies and noticed a kitchen demo to produce DNA . Too bad there aren't a few more such kitchen demos in the book. (There may be, but my cursory examination didn't reveal any.) >> I've looked at two very introductory books on genetics (cartoons and >> dummies). They have good figures showing pairs of chromosomes, but don't [quoted text clipped - 41 lines] > Yes, it is very confusing. Once you catch on it seems very simple > but until then you can get completely lost. SignatureWayne Watson (Nevada City, CA) Web Page: <speckledwithStars.net> |
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