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Mendelian and Non-mendelian Genetics
Transcript of Mendelian and Non-mendelian Genetics
A gene with more than two alleles is said to have multiple alleles.
One of the best-known examples is human blood type. There are three alleles for blood type, A, B, and O, which combine to give us the familiar blood types of A, B, O, and AB. What are multiple alleles? How do you make a
Punnett square? When doing a genetic cross, the parent organisms are the P, or parental generation. How can you predict the outcomes of monohybrid crosses? The separation of alleles is called segregation. When gametes (sex cells) form, alleles segregate so that each gamete carries only one allele for each gene. The offspring gets a new combination of alleles: one from each parent.
The genetic makeup of an organism is called its genotype. The organism's physical traits are its phenotype. What is non-Mendelian inheritance? Some forms of inheritance are distinctly non-Mendelian inheritance because they do not follow the principles described by Gregor Mendel. Many traits are produced by the interaction of several genes. Traits controlled by two or more genes are said to be polygenic traits. What are polygenic traits? (contd.) A dihybrid cross is one in which there are two genes. How can you predict the outcomes of dihybrid crosses? In four o’clock plants, the alleles for red and white flowers show incomplete dominance. Heterozygous (RW) plants have pink flowers—a mix of red and white coloring. In incomplete dominance, alleles are neither dominant nor recessive.
Instead, the heterozygous phenotype lies somewhere between the two homozygous phenotypes. What are some other patterns
of inheritance? Traits are controlled by factors that pass from parent to offspring. These factors are called genes. The different forms of a gene are alleles. Mendel’s principle of dominance states that some alleles are dominant and others are recessive. The recessive allele is exhibited only when the dominant allele is not present. When both alleles are the same, the individual is homozygous for that trait. When the alleles are different, the individual is heterozygous for the trait. The offspring are called the F1, or first filial generation. A monohybrid cross is a cross in which there are two contrasting alleles for a single gene. A Punnett square is a diagram that helps predict combinations in genetic crosses. Punnett squares are easy to construct. The principle of independent assortment states that genes for different traits could segregate independently during the formation of gametes. To construct a Punnett square for a dihybrid cross, follow the same steps. The variety of skin color in humans comes about partly because more than four different genes probably control this trait. The environment can also affect the way in which an organism’s genes are expressed. For example, sometimes the color of a particular plant’s leaves depend only upon the color of tissue in the female parent, which produces the egg cells within its flowers. This is known as maternal inheritance. Human mitochondrial disorders also show a maternal pattern of inheritance.
In recent years, a new phenomenon has been added to the list of non-Mendelian patterns of inheritance. DNA bases in certain genes can be chemically modified so that they are not expressed in the next generation. This process is known as genetic imprinting. For example, in certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers. Heterozygous chickens have a color described as erminette, speckled with black and white feathers.