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                 The complete set of chromosomes possessed by an organism is called its karyotype. An organism’s karyotype is usually presented as a picture of metaphase chromosomes lined up in descending order of their size. Karyotypes are prepared from actively dividing cells, such as white blood cells, bone marrow cells, or cells from meristematic tissues of plants. After treatment with a chemical (such as colchicine) that prevents them from entering anaphase, the cells are chemically preserved. They are then burst open to release the chromosomes onto a microscope slide, and the chromosomes are stained and photographed. The photograph is then enlarged, and the individual chromosomes are cut out and arranged in a karyotype. For human chromosomes, karyotypes are routinely prepared by automated machines, which scan a slide using a video camera attached to a microscope, looking for a chromosome spread (a group of chromosomes that are well separated). When a spread has been located, the camera takes a picture of the chromosomes, the image is digitized, and the chromosomes are sorted and arranged electronically by a computer.

In 1866, John Langdon Down, physician and medical superintendent of the Earlswood Asylum in Surrey, England, noticed a remarkable resemblance among a number of his intellectually disabled patients: all of them possessed a broad, flat face, a small nose, and oval-shaped eyes. Their features were so similar, in fact, that he felt that they might easily be mistaken for children from the same family. Down did not understand the cause of their intellectual disability, but his original description faithfully records the physical characteristics of people with this genetic form of intellectual disability. In his honor, the disorder is today known as Down syndrome.

Aneuploidy usually alters the phenotype drastically. In most animals and many plants, aneuploidies are lethal. Because aneuploidy affects the number of gene copies, but not their nucleotide sequences, the effects of aneuploidy are most likely due to abnormal gene dosage. Aneuploidy alters the dosage for some, but not all, genes, disrupting the relative concentrations of gene products and often interfering with normal development.  A major exception to the relation between gene number and gene dosage pertains to genes on the mammalian X chromosome. In mammals, X-chromosome inactivation ensures that males (who have a single X chromosome) and females (who have two X chromosomes) receive the same functional dosage for X-linked genes for further discussion of X-chromosome inactivation).

One of the first aneuploids to be recognized was a fruit fly with a single X chromosome and no Y chromosome, discovered by Calvin Bridges in 1913. Another early study of aneuploidy focused on mutants in the Jimson weed (Datura stramonium). A. Francis Blakeslee began breeding this plant in 1913, and he observed that crosses with several Jimson-weed mutants produced unusual ratios of progeny. For example, the globe mutation (which produces a globe-shaped seedcase) was dominant, but was inherited primarily from the female parent. When globe mutants self-fertilized, only 25% of the progeny had the globe phenotype. If the globe mutation were strictly dominant, Blakeslee should have seen 75% of the progeny with the trait, so the 25% that he observed was unsual. Blakeslee isolated 12 different mutants that exhibited peculiar patterns of inheritance.

          Aneuploidy can arise in several ways. First, a chromosome may be lost in the course of mitosis or meiosis if, for example, its centromere is deleted. Loss of the centromere prevents the spindle microtubules from attaching, so the chromosome fails to move to the spindle pole and does not become incorporated into a nucleus after cell division. Second, the small chromosome generated by a Robertsonian translocation may be lost in mitosis or meiosis. Third, aneuploidy may arise through nondisjunction, the failure of homologous chromosomes or sister chromatids to separate in meiosis or mitosis. Nondisjunction leads to some gametes or cells that contain an extra chromosome and other gametes or cells that are missing a chromosome

Natural selection favors individual birds that achieve the greatest lifetime reproductive success. The investments of males and females in small sperm and large eggs, respectively, drive different options, including their mating opportunities and how best to invest in quality offspring. Most birds pair with a single male and both then raise the offspring together. Both parents are needed to provide adequate care for their young. Females strive to protect their investments in large, expensive eggs. Males must balance the options of mating with extra females against caring for their own young. Conversely, females can improve the quality of their offspring through extra-pair copulations with high-quality males. 

Most birds are monogamous: it is when they form simple pairs of one male and one female on one territory. Sexual selection is manifest in the initial stages of competition among males for a breeding territory and then in the decisions by females to reside with particular males on their property. Where they can control high-quality territories, some species such as the Red-winged Blackbird of North America are polygynous, often pairing with two or more females. They also exhibit striking sexual dimorphism (the differences in appearance between males and females of the same species, such as in color, shape, size, and structure, that are caused by the inheritance of one or the other sexual pattern in the genetic material) and large variation in their sexual success.

Mutual assessment of prospective partners is a vital aspect of the early stages of courtship and pair formation. The ornaments and displays favored and maintained by sexual selection are those that reliably reflect the superior condition of certain males, enabling females to select the best possible mates.​ For example, House Finch females prefer brightly colored males, which have better survival rates and are better family providers.

Male and female Blue Tits looks almost the same to the human eye but not to each other. The pulmage of both sexes includes strong ultraviolet, reflectance, which makes the birds more conpicuous to each other agianst the background colors of the woods in which they live. Males also have a brillant purple crown patch that we cannot see. The males display ultraviolet crown prominently during courtship, especially in the early morning light. Consistent with the process of sexual selection, females prefer males with brightest crown patches. They also pair assortatively: those with the brightest UV reflectance in their own plumage pair with the most brightly colored males.

Striking sexual differences in plumage (and size) are typical of many birds. Darwin concluded that exaggerated sexual differences such as the tail of a peacock or displays of the Wild Turkey evolve as a result of what he called sexual selection namely, contests among males and females for mates and females preferences for particular males. As potential male reproduction success increases so does the value of the characteristics. Large size, fancy plumage , intricate songs, and striking displays that are responsible for the success.