How many gametes we need, to get all the genotypes as Mendel received in his typical dihybrid cross?
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Some Genes Are Transmitted to Offspring in Groups via the Phenomenon of Gene Linkage
A schematic shows the dorsal side of a black-bodied fruit fly in silhouette with its wings outstretched. The wings are smaller than normal wings and have a jagged back edge.
Black fly with short wings
Although Mendel's principle of independent assortment states that alleles of different genes will segregate independently into gametes, in reality, this is not always the case. Sometimes, alleles of certain genes are inherited together, and they do not appear to undergo independent assortment at all.
Indeed, shortly after Mendel's discoveries about inheritance patterns became widely known, numerous researchers began to notice exceptions to his principles. For example, they realized that some crosses contradicted Mendel's principle of independent assortment, because these crosses produced organisms with certain phenotypes far more frequently than traditional Mendelian genetics predicted.
Based on these findings, these scientists hypothesized that certain alleles of one gene were somehow coupled with certain alleles of another gene; however, they were not sure how this could occur. This phenomenon is now known as genetic linkage, and it generally describes an inheritance pattern in which two genes located in close proximity to each other on the same chromosome have a biased association between their alleles. This, in turn, causes these alleles to be inherited together instead of assorting independently. Genetic linkage is a violation of the Mendelian principle of independent assortment.
Independent assortment in test crosses
To understand linkage, we must first compare it to an example of independent assortment of parental gametes. The best way to generate such an example is through a dihybrid test cross, which considers two different genes during a cross between two heterozygote parents. Mendel's principle of independent assortment predicts that the alleles of the two genes will be independently distributed into gametes.
Thus, according to Mendel's principles, a dihybrid cross between two heterozygous fruit flies with brown bodies and red eyes (BbEe X BbEe) should yield offspring with nine possible genotypes (BBEE, BBEe, BBee, BbEE, BbEe, Bbee, bbEE, bbEe, and bbee) and four possible phenotypes (brown body with red eyes, brown body with brown eyes, black body with red eyes, and black body with brown eyes) (Figure 1, left). In this case, the ratio of phenotypes observed among the offspring is 9 (brown body, red eyes): 3 (brown body, brown eyes): 3 (black body, red eyes): 1 (black body, brown eyes) (Figure 1, right). This 9:3:3:1 phenotypic ratio is the classic Mendelian ratio for a dihybrid cross in which the alleles of two different genes assort independently into gametes.
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