Dominance and recessiveness were observed by Mendel and were crucial for him to conclude that plants had two copies of each element (gene).
However, some genes have more than two alleles and some do not mask the expression of others.
Incomplete dominance is where the hybrid has an intermediate phenotype: Red crossed with white yield pink flowered plants. See Figure 4.1.
Codominance is when both phenotypes are present.
An example would be the human blood typing system: MM, NN or MN. (Figure 4.2)
Each person must have 2 copies. The MN has both types of cell surface antigens present on their blood cells.
Multiple alleles are when there are more than two alternative alleles.
If more than two different forms of a gene are present in a population, the gene is classified a polymorphic gene.
See Chapter 3 notes and problem solving sections for additional coverage of the topic.
An allelic series is a hierarchical relationship can be established for alleles of polymorphic genes.
Make heterozygotes from all the possible combinations of true breeding types to determine which allele in each pair is dominant.
Recessive alleles are often called amorphic (without form).
An interesting exception is polled allele in cattle where the recessive form has horns and the dominant is hornless.
Blue eyes are the absence of brown pigment and therefore are amorphic.
White rabbits lack fur pigmentation. (See Figure 4.3 and 4.4)
Allelism: Just because a mutation has affected a trait such as coat color does not mean that it is an allele to another coat color gene.
There are many genes that affect the phenotype of fur color.
To test if the new form is allelic to another brown gene, do a cross.
The F1 cross should yield either a 3:1 or 1:2:1 ratio, if allelic.
Otherwise, expect some derivative of a 1:2:1:2:4:2:1:2:1 or a 9:3:3:1 ratio (or a combination of these two possible outcomes, i.e., a derivative of a 1:2:1 X 3:1 (3:6:3:1:2:1).
Another way, which works if the alleles are recessive, is that if you cross a pure bred strain with the other pure bred recessive strain, you might get neither recessive phenotypes but the wild type trait.
General Types of Mutants
Visible mutations because they generate an observable change in appearance;
sterile mutations, which are unable to sexually reproduce; and
lethal mutations, which cause death. See Figure 4.7.
Genes
Genes code for an RNA that is used to make polypeptides or functional RNAs. See Figure 4.8.
Remember that polypeptides are made of amino acids.
Mutations can either change a polypeptide's structure, make it so the polypeptide is not made at all, change the amount made or change when it is made. (Or change a functional RNA’s sequence…)
Why are some mutations dominant while others are recessive?
Recessive alleles usually code for non-functional proteins. It is more likely that a protein is changed to a non-functional form than to another form that works differently.
Gain of function is less frequent than loss of function and can be dominant or codominant in its expression mode.
Why is loss of function recessive (an incomplete dominant)? Because if a good copy of the gene, the dominant allele, exists, it is sufficient to code for the protein that can function within the organism's cells.
Why is gain of function dominant? Because they produce an effect that is otherwise not produced in the organism. Therefore, the presence of the gene means the presence of the new effect or phenotype. See Figure 4.9a and 4.9b.
Genes and the Environment
An organisms genetic makeup interacts with the environment.
The environment influences gene expression.
Different gene expression patterns can be due to different temperatures, nutritional sources, light exposures, humidity, and other variable conditions.
Even differences in the social environment can influence the phenotype of humans, e.g. cultures that eat high fat meals versus vegetarian cultures.
Penetrance and Expressivity
Penetrance is the percentage of individuals that show a particular phenotype among those capable of showing it.
Not all that have a genotype for a certain phenotype will express it.
An example is polydactyl (six fingers) (Figure 4.12)
Expressivity is the degree of expression of a trait controlled by a gene.
For example, if you have gout, how bad is it?
Epistasis
One trait can be influenced by more than one gene.
Epistasis is the interaction between products of nonallelic genes. The genes suppressed are said to be hypostatic.
Different genes are responsible for a final phenotype.
See Chapter 3 notes and problem solving sections for more information on this topic.
Pleiotropy — more than one trait can be affected by one gene (conceptionally contrasting with epistasis).
PKU is an example — it causes mental impairment and lightens hair.
Continuous Phenotypic Variation
It has been said that humans are pattern seekers.
It might also be said that they tend to observe things that contrast greatly versus those with gradations.
Many traits are not all or none.
Height, skin color, intelligence, strength, shape of the nose, feet and other features, in fact most features of the our phenotype, have shades or gradations.
Where as Mendel learned from studying dominant/recessive modes of inheritance the knowledge that two copies of each gene were possessed by peas. Many traits are each affected by many genes. This is called polygenic inheritance and will be covered in our last chapter.