Question 12 of 25 Which best describes a gene?
O A. A piece of a chromosome
B. A sister chromatid
C. A tetrad
D. A chromosome

a piece of chromosome

Explanation:

Each species has its own characteristic number of chromosomes. Humans, for instance, have 46 chromosomes in a typical body cell (somatic cell), while dogs have 78^1  

1

start superscript, 1, end superscript. Like many species of animals and plants, humans are diploid (2n), meaning that most of their chromosomes come in matched sets known as homologous pairs. The 46 chromosomes of a human cell are organized into 23 pairs, and the two members of each pair are said to be homologues of one another (with the slight exception of the X and Y chromosomes; see below).

Human sperm and eggs, which have only one homologous chromosome from each pair, are said to be haploid (1n). When a sperm and egg fuse, their genetic material combines to form one complete, diploid set of chromosomes. So, for each homologous pair of chromosomes in your genome, one of the homologues comes from your mom and the other from your dad.

Image of the karyotype of a human male, with chromosomes from the mother and father false-colored purple and green, respectively.

Image of the karyotype of a human male, with chromosomes from the mother and father false-colored purple and green, respectively.

Image modified from "Karyotype," by the National Institutes of Health (public domain).

The two chromosomes in a homologous pair are very similar to one another and have the same size and shape. Most importantly, they carry the same type of genetic information: that is, they have the same genes in the same locations. However, they don't necessarily have the same versions of genes. That's because you may have inherited two different gene versions from your mom and your dad.

As a real example, let's consider a gene on chromosome 9 that determines blood type (A, B, AB, or O)^2  

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squared. It's possible for a person to have two identical copies of this gene, one on each homologous chromosome—for example, you may have a double dose of the gene version for type A. On the other hand, you may have two different gene versions on your two homologous chromosomes, such as one for type A and one for type B (giving AB blood).

The sex chromosomes, X and Y, determine a person's biological sex: XX specifies female and XY specifies male. These chromosomes are not true homologues and are an exception to the rule of the same genes in the same places. Aside from small regions of similarity needed during meiosis, or sex cell production, the X and Y chromosomes are different and carry different genes. The 44 non-sex chromosomes in humans are called autosomes.

they fuse to form they have half as many chromosomes as a somatic cell of the same individual. b