Multiple
Alleles The source of new alleles was recognized as a gene change or a mutation. Alleles are located in corresponding parts of homologous chromosomes, only one member of a pair can be present in a given chromosome and only two are present in a cell of a diploid. Alleles are genes that are members of the same gene pair, each kind of allele affecting a
trait differently than the other. A diploid organism has, by its definition, only two alleles at one time, yet exceptions to the rules do appear. Many examples were found where more than two alternative alleles, also called multiple alleles, are present. In these cases two or more different mutations must have taken place at the same locus but indifferent individuals or at different times. Multiple alleles are alternatives tates at the same locus. The different alleles of a series are usually represented by the same symbol. Subscripts and superscripts are used to identify different members of a series of alleles. Most alleles produce variations of the same trait, but some produce very different phenotypes. The most famous example of multiple alleles was discovered in rabbits. It was known that
Albino rabbits were produced on occasion in variously colored rabbit populations. After conducting a monohybrid cross between a colored and Albino rabbit, it was discovered that the members of a pair of alternative genes, either c or c+, must be responsible for colored or albino rabbits. A cross of homozygous colored (c+ c+) and albino (c c) rabbits was made and the F1
generation were all colored, while the F2 generation had three colored and one albino. This showed that one pair of alleles was involved, the wild c+ and the mutant allele c. It was determined that c+ was dominant over c. Basic Thomas Hunt Morgan and Sex-Linked Traits.In 1910 Morgan studied the Drosophila fly and found amutant
male fly, which expressed the trait of
white eyes in stead of the normalred eyes. This trait was very unusual in that species and Morgan wanted to seeif the trait would be passed on to its offspring. He experimented to find ifthis strange trait would be in herited according to Mendel's research. First he crossed the mutant male fly with a normal female with
red eyes, to observe whether the white or red eyes were dominant. The F1 generation all had redeyes, which made Morgan conclude that red eyes were dominant over white. Hecontinued the steps of Mendel's experiment by crossing two flies from the F1 generation with each other. Out of 4252 flies in his F2 generation, 782 had white eyes but surprisingly all the flies with white eyes were also male. This strange observation puzzled Morgan to wonder why there weren't any females withwhite eyes. He then crossed flies from the F1 generation with the original malefly with white eyes. This cross resulted in white-eyed and red-eyed males and females, making a 1:1:1:1 ratio.In Drosophila the sex is determined by the number of copies of the X chromosome. An individual that has two X chromosomes is female and an individual with one X chromosome, which then joins with the Y chromosome, is male. During fertilization, if the egg joins with an X sperm, the zygote is XX, which becomes female. If the Y sperm is involved in fertilization, there is a XY zygote, which develops into a male. The reasoning for Morgan's results is dueto the fact that the gene for while eyes in Drosophila is located on the X chromosome and not the Y chromosome. Genes on the X chromosome that determine a trait are called sex linked. After one understands how the white-eye trait is recessive to the red-eye trait, one can easily notice that Morgan's results follow Mendel's assortment of chromosomes. Morgan's experiment has been calle done of the most important events in genetics. His work with Drosophila proved Sutton's theory that Mendel's "traits" are found on chromosomes.
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