Which of the following cells contains an odd number of chromosomes? (A) A human skin cell, (B) the muscle cells of birds, (C) the nerve cells of a reptile, (D) a plant ovum, egg cell, or (E) the kidney cells of mammals.
You might have noticed that all of the organisms listed in these answer options, humans, birds, reptiles, plants, and mammals, are all eukaryotic organisms. Almost every cell in eukaryotic organisms contains a membrane-bound nucleus that contains genetic information in the form of chromosomes. These chromosomes can either exist in a haploid state or in a diploid state.
A haploid cell is a cell that only has a single set of chromosomes, and it’s often simply represented as n. The only cells that are haploid are called gametes, otherwise known as reproductive cells or sex cells. In most eukaryotes, these gametes are either an egg cell, sometimes known as an ovum, or a sperm cell. In humans specifically, each of these gametes will contain 23 chromosomes. These gametes need to be haploid as when they fuse together in fertilization, they form a zygote with a full set of 46 chromosomes. This zygote is a diploid cell as it contains two full sets of chromosomes, which is commonly represented as 2n.
As we mentioned in humans, the number of chromosomes in a diploid cell will be 46 chromosomes. All other body cells will eventually arise from this diploid zygote. And most body cells that aren’t gametes will have a diploid number of chromosomes. For example, skin cells are diploid, as our muscle cells and nerve cells. Even the cells that make up our kidneys are diploid cells, as are almost every other cell in a eukaryotic organism’s body. We’ve already mentioned that in humans, a diploid cell will usually contain 46 chromosomes. But it’s worthwhile noting that different species often have different chromosome numbers in their cells nuclei to humans.
For example, some species of birds, like pigeons, have diploid cells containing 80 chromosomes. Can you work out how many chromosomes will be present in a haploid gamete produced by a bird of this species? Remember, the diploid cells are represented as 2n as they have twice the number of chromosomes of a haploid cell. Therefore, a bird that has 80 chromosomes in a typical diploid cell would have 80 divided by two chromosomes in a typical haploid cell. So each of the pigeons’ gametes would have 40 chromosomes. So far, we’ve confirmed that a diploid body cell will contain twice the number of chromosomes of a haploid gamete. A cell cannot contain a half chromosome. So whether they’re haploid or diploid, they will always contain a whole number of chromosomes.
Any whole number, whether it’s odd or even, when multiplied by two will give us an even number. So whether the haploid gamete contains an odd number of chromosomes like in humans or an even number of chromosomes like in pigeons, when this number is doubled to form a diploid cell, the number of chromosomes in this diploid cell will always be even. Now, let’s review our answer choices to arrive at the correct response.
We’ve learned that skin cells, muscle cells, nerve cells, and kidney cells will all contain an even number of chromosomes because they are all diploid cells, regardless of the organism they come from. So none of the answer options that include these could contain an odd number of chromosomes in the nucleus. The only answer choice that could contain an odd number of chromosomes would be a haploid gamete as these cells could contain either an odd number of chromosomes or an even number of chromosomes depending on the species they come from. As we know that an egg cell or ovum is one such haploid gamete, a plant’s ovum or egg cell is the only possible cell that might have an odd number of chromosomes.