Question

n this chapter we learned not only about the importance of Mendel's Laws (segregation and independent assortment) to heredity, but also that there are a number of exceptions to Mendel's Laws. Inheritance follows some basic ideas, but can certainly appear to be a tricky thing if you don't fully understand the basis of how the traits of living things come about. This discussion, therefore, will focus on the exceptions to Mendel's Laws so that it will help you better understand how traits come about. You should look up more information on an exception and provide us with a better understanding of it in your postings. Different examples, importance, effects on traits including diseases, and other things can certainly be addressed.I need an outside source with this with in paragraph citation as well as a reference citing at the end..

Answer 1

Some of the exceptions to Mendel's law of basic inheritence are given as follows:

• Polygenic Inheritence- Many human traits such as height, hair color, eye color, skin color etc. are determined by the additive effect of multiple gene expression. Other than that, these traits are significantly influenced by the environment the concerned perosn is residing in. Due to involvement of multiple alleles of multiple genes at multiple loci, the usual laws of dominance, segregation and independent assortment are not followed in polygenic inheritances.
• Co-dominance and Incomplete Dominance - One of the best studied examples of co-dominance is the inheritence of human blood types, where the the allele for blood types A and B are codominant to each other. This violates Mendel's law of dominance that out of the alleles involved in controlling the trait, one allele will be dominant while the other will be recessive. Another similar exception to this law is 'Incomplete Dominane', where a blending may occurin phenotypes such as in snapdragons, where a cross between a plant with red flowers and a plant with whote flowers will produce a plant with pink flowers in F1 generation. The other two laws of inheritence are followed in this example as after selfing F1 plants, red and white flowered plants are observed, hence we camn conclude the the alleles were not mixed permanently, nor was it a result of some mutation.
• Sex related Genetic effects- There are three categories of genes that may have different effects depending on an individual's gender. These are referred to as:
  • Sex limited genes: Genes that are present in both male and females, but expressed only in only one gender. Example- men's facial hair.
  • Sex controlled genes : are expressed in both sexes but differently. An example of this is gout, a disease that causes painfully inflamed joints. If the gene is present, men are nearly eight times more likely than women to have severe symptoms.
  • Genomic Imprinting : A central assumption of Mendel's laws of inheritance is that genes originating from maternal and paternal genomes are equally expressed in the offspring. In some disorders such as Parader-Willi syndrome or Anglemann's disease the sex of the transmitting parent plays a role in the expression of the phenotype in his or her affected children. This led to the discovery that for some genes, only the allele inherited from a particular parent is expressed in the offspring. A gene that is expressed only from the paternally inherited chromosome is maternally imprinted (the maternal allele is inactivated); a gene that is expressed only from the maternally inherited chromosome is paternally imprinted (the paternal allele is inactivated).

These were just some of the general exceptions to the Mendel's principles of basic inheritence as commonly observed. Other exceptions to this are Pleiotropic inheritence or environmental effect, maternal inheritence etc. In short, the exceptions to Mendel's principles are far and widespread as depicted in some of the examples above.