Video Transcript
Fill in the blank. According to the chromosomal theory of inheritance, the sorting of chromosomes into gametes is blank. (A) Organized, (B) random, (C) predetermined, or (D) consistent.
In order to accurately answer this question, let’s first quickly review the chromosomal theory of inheritance proposed by Boveri and Sutton. This provides some observations about what happens to chromosomes during the process of meiosis in which gametes, or sperm and egg, are created.
Let’s first remember that during meiosis, an original diploid cell with two copies of each chromosome divides into two. During this division, the homologous pairs of chromosomes, or the two chromosomes in each set, separate so that each of the resulting cells will have only one replicated chromosome from each set. In this illustration, we’re showing the separation of just one pair of replicated homologous chromosomes, although typically cells will have many more pairs. Then, in the second meiotic division, the replicated sides of each chromosome separate so that in the resulting gametes, there is only one copy of each chromosome and therefore of each gene.
Now let’s look a bit more closely at what happens to homologous pairs of chromosomes during the first meiotic division. For this example, we’ll use a drawing of a cell that has two homologous pairs of replicated chromosomes, although again typically cells will have many more pairs than this.
In the first meiotic division, an original diploid cell with two replicated copies of each chromosome divides in two. During this process, the homologous chromosomes line up in pairs, somewhat like what you see in this picture. Then, these homologous pairs will separate, with one replicated chromosome from each pair going to each of the daughter cells. So this is one possibility of the chromosomes that might end up in the daughter cells.
However, the homologous pairs won’t necessarily line up this way every time. For example, the homologous pairs might instead line up like this, which would result in a different combination of chromosomes in each daughter cell. Or the homologous pairs might line up like this, resulting in yet another different combination of chromosomes in the daughter cells.
Essentially, the way that one homologous pair of chromosomes lines up does not affect the way that the other homologous pairs of chromosomes line up, which leads us to the first idea of the chromosomal theory of inheritance, which is that during meiosis, each homologous chromosome pair moves independently of other homologous chromosome pairs. And the sorting of chromosomes into gametes, that is, which chromosome from each pair ends up in which gamete, is a random process.
We also saw in our previous diagram of meiosis that the gametes a parent produces contain half the genetic material of their body cells. The last idea of the chromosomal theory of inheritance is that when two gametes combine in fertilization, they produce offspring with the same number of chromosomes as the parent’s, that is, two chromosomes of each set rather than just the one that each gamete had. So, since we have here a quick and concise summary of the chromosomal theory of inheritance, our answer should be found in this list.
So let’s look back at our question. We’re asked to describe the sorting of chromosomes into gametes. The chromosomal theory of inheritance states that the sorting of chromosomes into gametes is a random process. So therefore our answer must be (B). According to the chromosomal theory of inheritance, the sorting of chromosomes into gametes is random.
