Chromosome

http://www.britannica.com/EBchecked/topic/116055/chromosome

http://en.wikipedia.org/wiki/Chromosome

Chromosome, the microscopic threadlike part of the cell that carries hereditary information in the form of genes. A defining feature of any chromosome is its compactness. For instance, the 46 chromosomes found in human cells have a combined length of 200 nm (1 nm = 10^{− 9} metre); 

A chromosome is a structure of DNA, protein, and RNA found in cells. It is a single piece of coiled DNA containing manygenes, regulatory elements and other nucleotide sequences. Chromosomes also contain DNA-bound proteins, which serve to package the DNA and control its functions. Chromosomal DNA encodes most or all of an organism's genetic information; some species also contain plasmids or other extrachromosomal genetic elements.

Every eukaryotic species has a characteristic number of chromosomes (chromosome number). In species that reproduce asexually, the chromosome number is the same in all the cells of the organism. Among sexually reproducing organisms, the number of chromosomes in the body (somatic) cells is diploid (2n; a pair of each chromosome), twice the haploid (1n) number found in the sex cells, or gametes. The haploid number is produced during meiosis. During fertilization, two gametes combine to produce a zygote, a single cell with a diploid set of chromosomes. 

The DNA molecule may be circular or linear, and can be composed of 100,000 to over 3,750,000,000 nucleotides in a long chain. Typically, eukaryotic cells (cells with nuclei) have large linear chromosomes and prokaryotic cells (cells without defined nuclei) have smaller circular chromosomes.

Chromosomes in humans can be divided into two types: autosomes and sex chromosomes. Certain genetic traits are linked to a person's sex and are passed on through the sex chromosomes. The autosomes contain the rest of the genetic hereditary information. All act in the same way during cell division. Human cells have 23 pairs of chromosomes (22 pairs of autosomes and one pair of sex chromosomes), giving a total of 46 per cell. In addition to these, human cells have many hundreds of copies of the mitochondrial genome.

Sexually reproducing species have somatic cells (body cells), which are diploid [2n] having two sets of chromosomes (23 pairs in humans with one set of 23 chromosomes from each parent), one set from the mother and one from the father. Gametes, reproductive cells, are haploid [n]: They have one set of chromosomes. Gametes are produced by meiosis of a diploid germ linecell. During meiosis, the matching chromosomes of father and mother can exchange small parts of themselves (crossover), and thus create new chromosomes that are not inherited solely from either parent. When a male and a female gamete merge (fertilization), a new diploid organism is formed.

Some animal and plant species are polyploid [Xn]: They have more than two sets of homologous chromosomes. Plants important in agriculture such as tobacco or wheat are often polyploid, compared to their ancestral species. Wheat has a haploid number of seven chromosomes, still seen in some cultivars as well as the wild progenitors. The more-common pasta and bread wheats are polyploid, having 28 (tetraploid) and 42 (hexaploid) chromosomes, compared to the 14 (diploid) chromosomes in the wild wheat.

Prokaryotes

Prokaryote species generally have one copy of each major chromosome, but most cells can easily survive with multiple copies.For example, Buchnera, a symbiont of aphids has multiple copies of its chromosome, ranging from 10–400 copies per cell.However, in some large bacteria, such as Epulopiscium fishelsoni up to 100,000 copies of the chromosome can be present.

Diagram of a replicated and condensedmetaphase eukaryotic chromosome. (1)Chromatid – one of the two identical parts of the chromosome after S phase. (2)Centromere – the point where the two chromatids touch, and where the microtubules attach. (3) Short arm. (4) Long arm.

       Organization of DNA in a eukaryotic cell.

                                                                      Human chromosomes during metaphase

                                                                            Karyogram of a human male

Human Genome

http://en.wikipedia.org/wiki/Human_genome

The human genome is the complete set of genetic information for humans (Homo sapiens). This information is encoded as DNA sequences within the 23 chromosome pairs in cell nuclei and in a small DNA molecule found within individual mitochondria. Human genomes include both protein-coding DNA genes and noncoding DNA. Haploid human genomes (contained in egg and sperm cells) consist of three billion DNA base pairs,( As compared to 7 Traits in the Pea Plant) while diploid genomes (found in somatic cells) have twice the DNA content. 

The haploid human genome contains approximately 20,000 protein-coding genes, significantly fewer than had been anticipated. Protein-coding sequences account for only a very small fraction of the genome (approximately 1.5%), and the rest is associated with non-coding RNA molecules, regulatory DNA sequences, LINEs, SINEs, introns, and sequences for which as yet no function has been elucidated.

Graphical representation of the idealized human diploid karyotype, showing the organization of the genome into chromosomes.This drawing shows both the female (XX) and male (XY) versions of the 23rd chromosome pair. Chromosomes are shown aligned at their centromeres. The mitochondrial DNA is not shown.

Chromosomes appear in Pairs (Diploid, Two Alleles for each Trait, Inherited one each from each parent) The Last pair of chromosomes are different XX ( Female) and Male ( XY). The male and female genomes differ in only these two last pair of chromosomes ( Allele Pair in Pea Plant) .All the other 22 Pair of chromosomes are same in Human Males and Females. So the last pair are the sex determining pairs or the sex chromosomes.

In The Pea Plant the Alleles for Pea Colour are Shown as (YY) ( Diploid, Two Alleles for each Trait, Inherited one each from each parent) for Yellow Pea colour and (GG) ( Diploid, Two Alleles for each Trait, Inherited one each from each parent)  for Green Pea Colour.

The Same Diploid Alleles are also present in Human Beings also. Refer to the second picture all the Chromosomes appear in Pairs (Diploid, Two Alleles for each Trait, Inherited one each from each parent)

Mendel’s principles of inheritance

http://www.biotechlearn.org.nz/themes/mendel_and_inheritance/mendel_s_principles_of_inheritance

Mendel developed 3 principles of inheritance based on his experiments with pea plants.

Our understanding of how inherited traits are passed between generations comes from principles first proposed by Gregor Mendel in 1866.

Mendel followed the inheritance of 7 traits in pea plants (Pisum sativum). He chose traits that had 2 forms:

·         Pea shape (round or wrinkled)

·         Pea colour (yellow or green)

·         Flower colour (purple or white)

·         Flower position (terminal or axial)

·         Plant height (tall or short)

·         Pod shape (inflated or constricted)

·         Pod colour (yellow or green).

Mendel began with pure-breeding pea plants because they always produced progeny with the same characteristics as the parent plant.

1.  Fundamental theory of heredity

Inheritance involves the passing of discrete units of inheritance, or genes, from parents to offspring.

Mendel found that paired pea traits were either dominant or recessive. When pure-bred parent plants were cross-bred, dominant traits were always seen in the progeny, whereas recessive traits were hidden until the first-generation (F1) hybrid plants were left to self-pollinate. Mendel counted the number of second-generation (F2) progeny with dominant or recessive traits and found a 3:1 ratio of dominant to recessive traits

Mendel didn’t know about genes or discover genes, but he did speculate that there were 2 factors for each basic trait and that 1 factor was inherited from each parent.

We now know that Mendel’s inheritance factors are genes, or more specifically alleles – different variants of the same gene.

2.  Principle of segregation

During reproduction, the inherited factors (now called alleles) that determine traits are separated into reproductive cells by a process called meiosis and randomly reunite during fertilisation.

 Separation occurs during meiosis when the alleles of each gene segregate into individual reproductive cells (eggs and sperm in animals, or pollen and ova in plants).

3.  Principle of independent assortment

Genes located on different chromosomes will be inherited independently of each other.

Mendel observed that, when peas with more than one trait were crossed, the progeny did not always match the parents. This is because different traits are inherited independently – this is the principle of independent assortment. For example, he cross-bred pea plants with round, yellow seeds and plants with wrinkled, green seeds. Only the dominant traits (yellow and round) appeared in the F1 progeny, but all combinations of trait were seen in the self-pollinated F2 progeny. The traits were present in a 9:3:3:1 ratio (round, yellow: round, green: wrinkled, yellow: wrinkled, green).

Exceptions to Mendel’s rules

There are some exceptions to Mendel’s principles, which have been discovered as our knowledge of genes and inheritance has increased. The principle of independent assortment doesn’t apply if the genes are close together (or linked) on a chromosome. Also, alleles do not always interact in a standard dominant/recessive way, particularly if they are codominantor have differences in expressivity or penetrance.

The Gregor Mendel Experiments

http://en.wikipedia.org/wiki/Gregor_Mendel

http://anthro.palomar.edu/mendel/mendel_1.htm

http://www.nature.com/scitable/topicpage/gregor-mendel-and-the-principles-of-inheritance-593#

Ever wonder why you are the only one in your family with your grandfather's nose? The way in which traits are passed from one generation to the next-and sometimes skip generations-was first explained by Gregor Mendel. By experimenting with pea plant breeding, Mendel developed threeprinciples of inheritance that described the transmission of genetic traits, before anyone knew genes existed. Mendel's insight greatly expanded the understanding of genetic inheritance, and led to the development of new experimental methods.

Understanding Dominant Traits

Before Mendel's experiments, most people believed that traits in offspring resulted from a blending of the traits of each parent. However, when Mendel cross-pollinated one variety of purebred plant with another, these crosses would yield offspring that looked like either one of the parent plants, not a blend of the two. For example, when Mendel cross-fertilized plants with wrinkled seeds to those with smooth seeds, he did not get progeny with semi-wrinkly seeds. Instead, the progeny from this cross had only smooth seeds. In general, if the progeny of crosses between purebred plants looked like only one of the parents with regard to a specific trait, Mendel called the expressed parental trait the dominant trait. From this simple observation, Mendel proposed his first principle, the principle of uniformity; this principle states that all the progeny of a cross like this (where the parents differ by only one trait) will appear identical. Exceptions to the principle of uniformity include the phenomena of penetrance, expressivity, and sex-linkage, which were discovered after Mendel's time.

Mendel worked with seven characteristics of pea plants: plant height, pod shape and color, seed shape and color, and flower position and color. With seed color, he showed that when a yellow pea and a green pea were bred together their offspring plant was always yellow. However, in the next generation of plants, the green peas reappeared at a ratio of 1:3. To explain this phenomenon, Mendel coined the terms “recessive” and “dominant” in reference to certain traits. (In the preceding example, green peas are recessive and yellow peas are dominant.)

He came to three important conclusions from these experimental results:

1.	that the inheritance of each trait is determined by "units" or "factors" that are passed on to descendents unchanged      (these units are now called genes )
2.	that an individual inherits one such unit from each parent for each trait
3.	that a trait may not show up in an individual but can still be passed on to the next generation. The Experiment with Yellow and Green Seeds. The Experiment with Yellow and Green Seeds. The Experiment with two Traits. The Experiment with two Traits. The Seven Traits.