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Exploring Mendelian Genetics 11-3 Independent Assortment When Mendel was looking at his principal of segregation he ended up with another question Do genes for traits segregate independently from one another? Independent Assortment In order to find this out Mendel needed to look at two genes at the same time. This process is called a two-factor cross Independent Assortment In Mendel’s first twofactor cross he crossed a plant that was true breeding for round, yellow seeds and crossed it with a plant that was true breeding for wrinkled, green seeds. Two factor cross between F1 hybrids Because the true breeding cross does not show us the answer we are looking for we will create an F2 generation from our F1 hybrids just like in Mendel’s original experiments Two factor cross between F1 hybrids When Mendel did this experiment he came created 556 seeds from the two original hybrid F1 seeds His results were very similar to what we see in our phenotypic probability ratio of 9:3:3:1 A summary of Mendel’s Principles The inheritance of biological characteristics is determined by individual units known as genes, which are passed from parents to offspring. In cases in which two or more forms of the gene for a single trait exist, some forms of the gene may be dominant and others may be recessive. A summary of Mendel’s Principles In most sexually reproducing organisms, each adult has two copies of a gene -- one from each parent. These genes are segregated from each other when gametes are formed. The alleles for different genes usually segregate independently of one another. Beyond Dominant and Recessive Some alleles are neither dominant nor recessive, and many traits are controlled by multiple alleles or multiple genes This can easily be seen in many complex organisms such as animals Incomplete Dominance Cases in which one allele is not dominant over the other is called incomplete dominance An example is the four o’clock flower Neither red or white is recessive, so when crossed, it produces pink flowers Codominance Codominance is similar to incomplete dominance. Both alleles will contribute to the phenotype of the offspring This creates a mixture effect in the offspring Multiple Alleles Many genes have more than two alleles and therefore are said to have multiple alleles This does not mean that they can be given more than two, just that there are more than two types known to exist Multiple Alleles Multiple alleles is commonly seen in the fur color of rabbits We can see here our pattern of dominance as well as the phenotypes and genotypes of the organism Polygenic Traits Many traits are produced by the interaction of several genes These are said to be polygenic, meaning “many genes” Polygenic Traits Examples of polygenic traits can be seen when looking at the skin color of humans Many genes come together to create the possible phenotypes that are seen