Which one of the following would be different in a pair of non-identical twins?
A. alleles
B. amount of nuclear DNA
C. the total of adenine plus guanine
D. genes
E. chromosome number
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The answer to this question is A.
Firstly, what are non-identical twins? Also called dizygotic/fraternal twins, are twins that have been conceived by 2 separate sperm that penetrated 2 separate eggs. Thus, they may not share the same sex or appearance, similar to siblings that were not conceived in the same pregnancy. And like any other siblings, these twins share 50% of their genome.
With the definition of non-identical twins in mind, why is choice A the answer?
- The clue is in the definition of non-identical twins itself. Since these twins are dizygotic, they will only share 50% of their DNA, which means that their phenotype will be different from each other. As a reminder, phenotypes are the traits that we can observe such as height and eye color. Which then makes sense why these kinds of twins are called “non-identical”.
- Your job now, is to discern which among the choices are responsible for dictating this phenotype. Which you will then arrive at the answer A, which are alleles. An as you already know, alleles are variants of a gene, that gives rise to different genotypes, and whether these alleles are dominant or recessive, their combinations within these genotypes dictates how our phenotypes are expressed. It would now make sense therefore, that these twins will have different alleles, since they have different phenotypes.
What’s common among choices B to E that makes them not the correct answer for this question?
- It is safe to assume that there is no pathological error that could lead to any chromosomal abnormality, since there was no mention of such in this question. Thus, there is 1 central theme that is common among all the other choices, which is that they are essential properties/characteristics of human DNA! For example, we humans have 46 chromosomes located in our nucleus, which eliminates choices B and E. In addition, we have genes for different traits, but HOW they are expressed is determined by the specific alleles that make up the gene in the first place. For example, both twins in this question has the gene for eye color as a general trait, but the ALLELES will determine what color exactly each twin will get, thus eliminating choice D in your considerations. In terms of base pairing, the relative proportions of adenine and guanine are constant within species. Thus, this should not be in your consideration.
Therefore, the only correct answer would be choice A.
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I was just wondering, I’ve read that one of the non identical twins could have down’s syndrome - which would make the answer E. As for the alleles, is it not a possibility that although they are from a different sperm and eggs, there is a chance that they may posess the same alleles?
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Hi, so I don’t think they’d be able to have the same alleles, because during meiosis there’s genetic variation, so different phenotypes, and as for your question regarding down syndrome you’re right, the chromosome number would indeed be the different, I think thats for special cases or chromosomal disorders /disjunction, the question only asked for non identical twins without any mutation. Just my thoughts, let me know if it helps.
I do understand that during meiosis, crossing over occurs etc giving us recombinant DNA and all But there must be a slim chance like 1 in a billion chance where both the sperm cell and the oocytes contain the same types of allele since theyre both coming from the same parents. Or am I suggesting an impossible probability? I mean as for the IMAT Q I would expect that they would specify that the non identical twins are healthy in the question as well.
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i went searching on the internet because i am curious too,
In theory, it’s possible for a sperm cell and an egg (oocyte) to contain the same allele for a particular gene, especially if the parents have homozygous genotypes for that gene. However, the probability of this happening depends on various factors, including the parents’ genotypes, the specific gene in question, and the number of genes being considered.
Each parent contributes one allele for each gene to their offspring. If both parents are homozygous for a particular gene (meaning they have two identical alleles for that gene), then all their gametes (sperm and eggs) for that gene will carry the same allele.
In reality, the likelihood of both parents being homozygous for all genes and therefore having the same alleles to pass on to their offspring is extremely low. Humans have a vast number of genes, and each gene can have multiple alleles. Additionally, during meiosis (the process that produces gametes), genetic recombination and independent assortment ensure a wide variety of possible allele combinations in the offspring.
So while it’s theoretically possible for a sperm cell and an egg to carry the same allele for a particular gene, the probability of this happening is extremely low, especially when considering the entire human genome and the various combinations of alleles that can occur. hope this helps!
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