x BP

(b) Male karyotype

(a) Female karyotype

(b) Male karyotype figure 12-2

Human female and male karyotypes have in common 22 chromosome pairs, called autosomes. The karyotype of females (a) differs from that of males (b) in only the 23rd pair, which consists of the sex chromosomes. The 23rd chromosome pair has two X chromosomes in females and one X chromosome and one Y chromosome in males.

Word Roots and Origins autosome from the Greek autos, meaning "self," and soma, meaning "a body"

Sex Chromosomes and Autosomes

The sex chromosomes contain genes that determine the sex (gender) of an individual. The remaining chromosomes that are not directly involved in determining the sex of an individual are called autosomes. As in fruit flies, human males have an X and a Y chromosome, and human females have two X chromosomes. Figure 12-2a shows an example of the 23 pairs of human chromosomes from a female; Figure 12-2b shows an example of chromosomes from a male. In certain organisms, such as chickens and moths, males have two identical sex chromosomes, and females have two different sex chromosomes. Most plants and some fish lack sex chromosomes entirely.

Sex Determination

Like other homologous chromosomes, sex chromosomes pair during meiosis I. As meiosis proceeds, the paired chromosomes separate and move to different cells. As a result, a sperm cell has an equal chance of receiving an X chromosome or a Y chromosome. Each egg, however, receives only an X chromosome. This system of sex determination results in a one-to-one ratio of males to females. Each egg and sperm cell also receives a single copy of each autosome.

In mammals, when an egg that carries an X chromosome is fertilized by a sperm carrying a Y chromosome, the offspring has an XY pair and is male. Likewise, when an egg is fertilized by a sperm cell carrying an X chromosome, the offspring has an XX pair and is female. In a male mammal, the Y chromosome contains a gene called SRY for Sex-determining Region Y. This gene codes for a protein that causes the gonads of an embryo to develop as testes. Because female embryos do not have the SRY gene, the gonads develop as ovaries.

Sirens Sleep Solution

Sirens Sleep Solution

Discover How To Sleep In Peace And Harmony In A World Full Of Uncertainty And Dramatically Improve Your Quality Of Life Today! Finally You Can Fully Equip Yourself With These “Must Have” Tools For Achieving Peace And Calmness And Live A Life Of Comfort That You Deserve!

Get My Free Ebook

Post a comment