sex chromosomes

Examples of sex chromosomes in the following topics:

• Sex-Linked Traits

  • A gene present on one of the sex chromosomes (X or Y in mammals) is a sex-linked trait because its expression depends on the sex of the individual.
  • In humans, as well as in many other animals and some plants, the sex of the individual is determined by sex chromosomes.
  • The sex chromosomes are one pair of non-homologous chromosomes.
  • Until now, we have only considered inheritance patterns among non-sex chromosomes, or autosomes.
  • Insects also follow an XY sex-determination pattern and like humans, Drosophila males have an XY chromosome pair and females are XX.

• X-Inactivation

  • The presence of extra X chromosomes in a cell is compensated for by X-inactivation in which all but one X chromosome are silenced.
  • Several errors in sex chromosome number have been characterized.
  • Turner syndrome, characterized as an X0 genotype (i.e., only a single sex chromosome), corresponds to a phenotypically female individual with short stature, webbed skin in the neck region, hearing and cardiac impairments, and sterility.
  • In addition to the loss or gain of an entire chromosome, a chromosomal segment may be duplicated or lost.
  • Duplicated chromosomal segments may fuse to existing chromosomes or may be free in the nucleus.

• Sex Determination

  • Mammalian sex is determined genetically by the presence of X and Y chromosomes .
  • Avian sex determination is dependent on the presence of Z and W chromosomes.
  • The W appears to be essential in determining the sex of the individual, similar to the Y chromosome in mammals.
  • The presence of X and Y chromosomes are one of the factors responsible for sex determination in mammals, with males being the heterozygous sex.
  • In birds, Z and W chromosomes determine sex, with females being the heterozygous sex.

• Identification of Chromosomes and Karyotypes

  • A karyotype is the number and appearance of chromosomes.
  • In a human karyotype, autosomes or "body chromosomes" (all of the non–sex chromosomes) are generally organized in approximate order of size from largest (chromosome 1) to smallest (chromosome 22).
  • However, chromosome 21 is actually shorter than chromosome 22.
  • The X and Y chromosomes are not autosomes and are referred to as the sex chromosomes.
  • Finally, the karyotype can pinpoint translocations, which occur when a segment of genetic material breaks from one chromosome and reattaches to another chromosome or to a different part of the same chromosome.

• Chromosomal Theory of Inheritance

  • The Chromosomal Theory of Inheritance identified chromosomes as the genetic material responsible for Mendelian inheritance.
  • During meiosis, homologous chromosome pairs migrate as discrete structures that are independent of other chromosome pairs.
  • The gametic chromosomes combine during fertilization to produce offspring with the same chromosome number as their parents.
  • Despite compelling correlations between the behavior of chromosomes during meiosis and Mendel's abstract laws, the Chromosomal Theory of Inheritance was proposed long before there was any direct evidence that traits were carried on chromosomes.
  • They have three pair of autosomes and a pair of sex chromosomes.

• Genomic DNA and Chromosomes

  • Human body cells have 46 chromosomes, while human gametes (sperm or eggs) have 23 chromosomes each.
  • Human cells that contain one set of chromosomes are called gametes, or sex cells; these are eggs and sperm, and are designated 1n, or haploid.
  • Matched pairs of chromosomes in a diploid organism are called homologous ("same knowledge") chromosomes.
  • The sex chromosomes, X and Y, are the single exception to the rule of homologous chromosome uniformity.
  • In this image, the chromosomes were exposed to fluorescent stains for differentiation of the different chromosomes.

• Genes as the Unit of Heredity

  • A gene is made up of short sections of DNA that are contained on a chromosome within the nucleus of a cell.
  • You inherited 23 chromosomes from your mother and 23 from your father .
  • Together they form 22 pairs of autosomal chromosomes and a pair of sex chromosomes (either XX if you are female, or XY if you are male).
  • Homologous chromosomes have the same genes in the same positions, but may have different alleles (varieties) of those genes.

• The Evolution of Reproduction

  • The evolution of sex contains two related, yet distinct, themes: its origin and its maintenance.
  • These cells develop through meiosis, an adaption of mitosis, which reduced the number of chromosomes in each reproductive cell by half, while increasing the number of cells through cell division .
  • Further evolution of reproductive systems resulted in the development of reproductive systems that are sex specific.
  • The development of sperm and eggs occurs via the process of meiosis which reduces the number of chromosomes in each cell by half.
  • The evolution of sex-specific gonads led to the development of sperm (male) and eggs (female).

• Introduction to Mendelian Inheritance

  • Gregor Johann Mendel set the framework for genetics long before chromosomes or genes had been identified, at a time when meiosis was not well understood.
  • He began studying heredity using mice, but his bishop did not like one of his friars studying animal sex, so he switched to plants.
  • We now know that genes, carried on chromosomes, are the basic functional units of heredity with the capability to be replicated, expressed, or mutated.
  • We can now view a human karyotype of all of the chromosomes in an individual to visualize chromosomal abnormalities in offspring, even before birth.
  • Shortly after Mendel proposed that traits were determined by what are now known as genes, other researchers observed that different traits were often inherited together, and thereby deduced that the genes were physically linked by being located on the same chromosome.

• Disorders in Chromosome Number

  • Aneuploidy, an abnormal number of chromosomes in a cell, is caused by nondisjunction, or the failure of chromosomes to separate at meiosis.
  • Of all of the chromosomal disorders, abnormalities in chromosome number are the most obviously identifiable from a karyotype and are referred to as aneuploidy.
  • If homologous chromosomes fail to separate during meiosis I, the result is two gametes that lack that particular chromosome and two gametes with two copies of the chromosome.
  • If sister chromatids fail to separate during meiosis II, the result is one gamete that lacks that chromosome, two normal gametes with one copy of the chromosome, and one gamete with two copies of the chromosome .
  • Nondisjunction occurs when homologous chromosomes or sister chromatids fail to separate during meiosis, resulting in an abnormal chromosome number.