Video Transcript
The diagram provided shows a karyotype, which is used to view the chromosomes in an organism. How are chromosomes one to 22 ordered in a karyotype? (A) By increasing size and in matching pairs. (B) By decreasing size and in homologous pairs. (C) By decreasing size and in random pairs. Or (D) by the number of genes they contain and in matching pairs.
A karyotype allows for a visualization of chromosomes from an organism. This question asks about how the chromosomes are ordered in a karyotype. You may recall that humans contain 23 pairs of chromosomes. This makes 46 chromosomes in total for a normal individual. Each pair consists of one chromosome that was contributed from our biological mother and another that was contributed from our biological father. These pairs of chromosomes are known as homologous chromosomes, which means that the two chromosomes are approximately the same length and contain the same genes. These genes may be slightly different between the homologous chromosomes, and we call these different versions of genes alleles.
Once homologous chromosomes are paired, they are arranged from one to 22. The first chromosome, one, is the largest chromosome, while the 22nd chromosome is the smallest. Notice that the 23rd chromosome pair is not included in this count. This is because the 23rd pair refers to the sex chromosomes with sex-specific genes. An X chromosome contains genes for female development, while a Y chromosome contains genes for male development. Because the chromosomes in a karyotype are paired with their homologous chromosomes and those pairs are ordered by size from largest to smallest, we’re able to arrive at the correct answer to our question.
In response to how chromosomes one through 22 are ordered in a karyotype, the correct answer is (B), by decreasing size and in homologous pairs.
