Aa Aa Aa. In humans and many other animal species , sex is determined by specific chromosomes. The chromosomes that carry those genes related to sexual characteristics have a long and intriguing story of exploration. As often happens during such a long course of discovery, scientists noted the appearance of these chromosomes before knowing exactly what these structures were.
The X Element. The Accessory Chromosome. Counting Chromosomes. Figure 1: Male human chromosomes. Courtesy of Betsy Hirsch and Olga Shilko. All rights reserved. One primary question facing the scientists of the time was whether maleness was associated with the absence or the presence of the misshapen chromosome.
Yet again, E. Wilson set out to find the answer. Wilson took a deep interest in the work of his colleague Nettie Stevens, and he set out to combine her surveys of multiple beetle species with his. Stevens had observed that in the Tenebrio species of beetle, there was an unusual-looking pair of chromosomes that separated to form different sperm cells. Based on comparisons of chromosome appearance in other cells of male and female Tenebrio , she proposed that these accessory chromosomes were likely related to the inheritance of sex.
Wilson independently observed similar patterns in a variety of insects. Bolstered by Stevens' work, Wilson thus proposed that, among the two recognizable "sex chromosomes," it was possible that one was a "female-determinant" and the other was a "male-determinant" Wilson, However, in some insects, including wasps, maleness was associated with the absence of an extra chromosome. How, then, could the observations in beetles be reconciled with those in wasps?
His descriptions were as follows, and they are also summarized in Table 2: Type A species: In organisms of these species, male somatic cells have one unpaired chromosome, the accessory. So, while females have n chromosomes, males will always have n- 1 chromosomes. On the other hand, in sperm formation, the unpaired accessory chromosome passes into only half of the sperm cells, so half will have this chromosome, while the other half will be without it.
Type B species: In organisms of these species, all chromosome pairs are of equal size in female somatic cells, and each chromosome has a partner. In male somatic cells, all chromosome pairs are of equal size except for one pair—these are the "idiochromosomes. However, in sperm cells, the unequally sized chromosomes separate into two different types of male gametes, one with the larger idiochromosome X and one with the smaller, crumpled looking idiochromosome.
Wilson's proposal from his observations about chromosome number was that there were two types of systems by which these mismatched chromosomes separated into gametes. The Type A system resulted in an X or no-X distribution among sperm, while the Type B system resulted in an X or crumpled-looking X partner distribution among sperm. The relationship between the observations made in the gametes and the counts in the species' somatic cells confirmed that patterns seen in gametes directly corresponded with the sex of individual organisms.
Later research showed that males aren't always the sex with the deviation from identical chromosome pairs, however. Lessons Learned. References and Recommended Reading Brown, S. Entomological contributions to genetics: Studies on insect germ cells linked genes to chromosomes and chromosomes to Mendelian inheritance.
Anatomischer Anzeiger 20 , — link to article ———. Biological Bulletin 3 , 43—84 link to article Pierce, B. Genetics: A Conceptual Approach , 2nd ed. New York, W. Freeman, Stevens, N. Biological Bulletin 4 , 24—39 link to article Wilson, E. Science 22 , — link to article ———.
Article History Close. Share Cancel. Revoke Cancel. Each species of eukaryote has a characteristic number of chromosomes in the nuclei of its cells. Human body cells somatic cells have 46 chromosomes. A somatic cell contains two matched sets of chromosomes, a configuration known as diploid.
The letter n is used to represent a single set of chromosomes; therefore a diploid organism is designated 2 n. Human cells that contain one set of 23 chromosomes are called gametes , or sex cells; these eggs and sperm are designated n , or haploid. The matched pairs of chromosomes in a diploid organism are called homologous chromosomes.
Homologous chromosomes are the same length and have specific nucleotide segments called genes in exactly the same location, or locus. Genes, the functional units of chromosomes, determine specific characteristics by coding for specific proteins. However, he was unable to gather any direct evidence to support his hypothesis. More than a decade after Henking's work, Nettie Stevens surveyed multiple beetle species and examined the inheritance patterns of their chromosomes.
In , while studying the gametes of the beetle Tenebrio molitor Figure 2 , Stevens noted an unusual-looking pair of chromosomes that separated to form sperm cells in the male beetles. Based on her comparisons of chromosome appearance in cells from male and female beetles, Stevens proposed that these accessory chromosomes were related to the inheritance of sex.
Over time, other scientists studied the appearance of chromosomes in a wide variety of animal species, and it became clear that there was a relationship between the physical appearance and number of chromosomes in gametes and somatic cells from males and females of a given species.
Figure 6: Sex determination in birds. On the upper left, the female parent genotype is ZW. The W is labeled to the left of the top quadrant, while the Z is labeled outside the left quadrant. On the upper right, the male parent genotype is ZZ. The first Z is labeled to the right of the top quadrant, while the second Z is labeled outside the right quadrant.
Each of the four quadrants contains the two-letter genotype that results from combining one female parent allele with one male parent allele. In this case, two of the four quadrants contain the genotype ZZ, resulting in male birds; the remaining two quadrants contain the genotype ZW, resulting in female birds.
For example, the ZZ-ZW sex determination system used in birds, snakes, and some insects relies upon females to carry the mismatched chromosome pair ZW and males to carry the identical pair ZZ Figure 6.
If the three systems discussed above are compared in side-by-side Punnett squares Figure 7 , it is easy to see that sex determination is simply a matter of gamete assortment. Determinations of male and female character arise from a variety of different gamete combination patterns, all of which are the result of gender coding in sexually reproducing organisms. This page appears in the following eBook. Aa Aa Aa. In humans and many other animal species, sex is determined by specific chromosomes.
How did researchers discover these so-called sex chromosomes? The path from the initial discovery of sex chromosomes in to an understanding of their true function was paved by the diligent efforts of multiple scientists over the course of many years. As often happens during a lengthy course of discovery, scientists observed and described sex chromosomes long before they knew their function.
An idea inspired by the "X element". This is the 2 n or diploid state. Human gametes have 23 chromosomes or one complete set of chromosomes; a set of chromosomes is complete with either one of the sex chromosomes.
This is the n or haploid state. Genes are segments of DNA that code for a specific protein. Duplicated chromosomes are composed of two sister chromatids. Chromosomes are compacted using a variety of mechanisms during certain stages of the cell cycle.
Several classes of protein are involved in the organization and packing of the chromosomal DNA into a highly condensed structure. The condensing complex compacts chromosomes, and the resulting condensed structure is necessary for chromosomal segregation during mitosis. Eukaryotic chromosomes are thousands of times longer than a typical cell.
Explain how chromosomes can fit inside a eukaryotic nucleus. Human somatic cells have 46 chromosomes: 22 pairs and 2 sex chromosomes that may or may not form a pair. This is the 2 n or diploid condition. Human gametes have 23 chromosomes, one each of 23 unique chromosomes, one of which is a sex chromosome. This is the n or haploid condition. Skip to main content. Cell Division and Cell Cycle. Search for:. Cell Division Learning Objectives By the end of this section, you will be able to: Describe the structure of prokaryotic and eukaryotic genomes Distinguish between chromosomes, genes, and traits Describe the mechanisms of chromosome compaction.
Link to Learning This animation illustrates the different levels of chromosome packing:. Additional Self Check Questions 1.
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