The simple rules of Mendelian genetics do not always apply. There are exceptions to every rule in life.
Here are some examples:
1. Polygenic Traits - Poly = many and genic = genes
Traits are determined by the combined effect of more than one pair of genes. These result in continuous traits such as tallness in humans. Human stature is the combined size of all of the body parts from head to foot. Each of these body part sizes is determined by numerous genes. (Remember the video about homeobox genes) Human skin, hair and eye color are also influenced by more than one allele at different loci... (spots on the chromosome).
2. Intermediate Expression also known as Partial or Incomplete Dominance - This is defined as blending in the phenotype of a heterozygous trait. Flowers can exhibit pink because the dominant allele is not completely dominant and can't produce enough red pigment to make dark red. The pitch of male voices is also apparently determined by an allele - homozygous AA and aa are the lowest (bass) and highest (tenor) pitches and Aa are intermediate range baritones.
3. Codominance - For some traits, two alleles can be codominant. An example is AB blood type. When tested, these people have the characteristics of both type A and type B blood - it's not a mix. Another example is sickle cell anemia.
To answer Jake's question, see Genetic Dominance: Genotype-Phenotype Relationships , by Ilona Miko, Phd, 2008.
4. Multiple Allele Series - ABO blood type series is an example of three alleles (there are three allelic possibilities, A,B and O) but each individual only inherits 2 of them, one from each parent.
Type A - Anti B antibodies - can only receive Type O or A
Type B - Anti A antibodies - can only receive Type O or B
Type AB - Anti A and B antigens - universal receivers but can't be donors
Type O - no Anti ABO antigens - universal donors but can only receive Type O
5. Modifying Genes - Alter how other genes are expressed in the phenotype. For example, there is a dominant cataract gene that is expressed if the companion allele for the modifying gene is present. (Cataracts can also be affected by diabetes or other environmental factors)
6. Regulator genes can either block or initiate the expression of other genes. They are like switches that turn on the production of proteins or a sequence of developmental in our bodies. They also control the aging process. Foxo gene movie.
7. Sex related Genetic effects -
Sex limited Genes are inherited by both sexes, but only expressed by one sex. An example is facial hair.
8. Environment - Environment can play a role in an individual's phenotypic expression. Poor nutrition in the mother during pregnancy, accidents, and hormone levels can all contribute to altering an individual's phenotype.
There are other examples, but they are not necessary to understand the point --- Which is????
Although researchers have found at least 18,000 human traits that follow basic Mendelian rules, there are at least 5000 genetically inherited diseases and abnormalities that don't follow the rules. More exceptions to the rule leads to more diversity.
Homework : Read The Social Impact of Genetics on Reproduction handout and answer the two questions at the end of the article. We'll discuss your answers when you're finished.
Sources:
Miko, I. (2008) Genetic dominance: genotype-phenotype relationships. Nature Education 1(1)
O'Neil, D. (2011). Anthropology Tutorials [website]. Retrieved from http://anthro.palomar.edu/tutorials/biological.htm
Here are some examples:
1. Polygenic Traits - Poly = many and genic = genes
Traits are determined by the combined effect of more than one pair of genes. These result in continuous traits such as tallness in humans. Human stature is the combined size of all of the body parts from head to foot. Each of these body part sizes is determined by numerous genes. (Remember the video about homeobox genes) Human skin, hair and eye color are also influenced by more than one allele at different loci... (spots on the chromosome).
2. Intermediate Expression also known as Partial or Incomplete Dominance - This is defined as blending in the phenotype of a heterozygous trait. Flowers can exhibit pink because the dominant allele is not completely dominant and can't produce enough red pigment to make dark red. The pitch of male voices is also apparently determined by an allele - homozygous AA and aa are the lowest (bass) and highest (tenor) pitches and Aa are intermediate range baritones.
3. Codominance - For some traits, two alleles can be codominant. An example is AB blood type. When tested, these people have the characteristics of both type A and type B blood - it's not a mix. Another example is sickle cell anemia.
To answer Jake's question, see Genetic Dominance: Genotype-Phenotype Relationships , by Ilona Miko, Phd, 2008.
4. Multiple Allele Series - ABO blood type series is an example of three alleles (there are three allelic possibilities, A,B and O) but each individual only inherits 2 of them, one from each parent.
Type A - Anti B antibodies - can only receive Type O or A
Type B - Anti A antibodies - can only receive Type O or B
Type AB - Anti A and B antigens - universal receivers but can't be donors
Type O - no Anti ABO antigens - universal donors but can only receive Type O
5. Modifying Genes - Alter how other genes are expressed in the phenotype. For example, there is a dominant cataract gene that is expressed if the companion allele for the modifying gene is present. (Cataracts can also be affected by diabetes or other environmental factors)
Homework: Genetic definition page
6. Regulator genes can either block or initiate the expression of other genes. They are like switches that turn on the production of proteins or a sequence of developmental in our bodies. They also control the aging process. Foxo gene movie.
7. Sex related Genetic effects -
Sex limited Genes are inherited by both sexes, but only expressed by one sex. An example is facial hair.
8. Environment - Environment can play a role in an individual's phenotypic expression. Poor nutrition in the mother during pregnancy, accidents, and hormone levels can all contribute to altering an individual's phenotype.
There are other examples, but they are not necessary to understand the point --- Which is????
Although researchers have found at least 18,000 human traits that follow basic Mendelian rules, there are at least 5000 genetically inherited diseases and abnormalities that don't follow the rules. More exceptions to the rule leads to more diversity.
Sources:
Miko, I. (2008) Genetic dominance: genotype-phenotype relationships. Nature Education 1(1)
O'Neil, D. (2011). Anthropology Tutorials [website]. Retrieved from http://anthro.palomar.edu/tutorials/biological.htm
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