GED Science: Mendelian vs. Non-Mendelian Inheritance

During Mendel’s study of pea plants in the 1800’s, nothing was known about DNA, chromosomes, genes, or meiosis.  Mendel’s conclusions were drawn from observation and modeling.  The discovery of these features of the cell and the cell cycle both confirmed Mendel’s findings, and helped to explain some inconsistencies with Mendelian genetics.  Some of those inconsistencies are described below:

Incomplete Dominance:  Mendel observed that there was no blending of alleles in inheritance (e.g., lavender flowers resulting from a cross between purple and white flowers).  However, some traits do blend together.  Another way of saying this is that heterozygous organisms express phenotypes with characteristics of both the dominant and the recessive allele.  Some flowers exhibit incomplete dominance.

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Co-dominance:  In some cases, neither allele is dominant over the other, but they also do not blend together.  In offspring expressing co-dominant traits, both traits are expressed equally.  Another way of saying this is to say that in co-dominance, there are three phenotypes:  two separate phenotypes and a third in which the other two phenotypes appear together.

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Another example of co-dominance that is not as visible is blood type.  The allele for A blood type and the allele for B blood type are co-dominant, resulting the AB blood types.

Incomplete dominance and co-dominance are examples of non-Mendelian inheritance patterns.  Remember that genes contain information to make proteins, as discussed in the central dogma section of the last lesson.  So it could be that incomplete dominance and co-dominance exist due to how the genes are expressed. The expression of a gene refers to when, how often, how quickly, and/or how effectively it gets converted into a protein.  Many things can affect gene expression, including availability of nutrients, poisons, and other environmental conditions during development.

Still other forms of non-Mendelian inheritance patterns are due more to how genetic information gets split up during meiosis.

Linked Traits:  Linked traits are ones that tend to be inherited together.  Remember that one of Mendel’s genetic laws was the Law of Independent Assortment.  In terms of his pea plants, that meant that flower color and seed coat were separated independently of each other as alleles got separated to pass on to offspring.  However, some traits do not separate independently, and are thus called linked traits.  Red hair and freckles are linked traits in humans.  Humans who have red hair often have freckles.

The reason for linked traits has to do with their proximity on a chromosome. Remember that during meiosis I, a process called crossing over occurs in which portions of one chromosome can swap places with the same portion on its paired chromosome.  The following picture shows a quick summary of meiosis and crossing over:

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In the above example, the genes represented by alleles A and B are likely to be linked, or inherited together, because they are so close together on the chromosome.  It is unlikely that crossing over would occur between A and B.

Sex-linked Traits: Another example of non-Mendelian inheritance pattern is sex linkage. Sex-linked traits are located on the sex chromosomes (in humans, these are the X and Y chromosome – females have are XX and males are XY). In humans, alleles that show up on the X chromosome only need one copy to appear phenotypically in men, regardless whether the allele is dominant or recessive. For women, who have two X chromosomes, they would have to have two copies of a recessive gene in order for it to show up.  Women who have just one copy of such a gene are said to carriers of the gene.  For this reason, most sex-linked traits – such as colorblindness – are far more commonly observed in males. Another interesting feature of sex-linked traits that show up in males is that the traits skip generations.  The gene for male-pattern baldness is a recessive, sex-linked gene.  Fathers who are bald are likely to produce daughters who produce grandsons who are bald.  In fact, men often say that if you want to know if you will go bald, look at your mother’s father.  The reason for this skipping of generations is shown in the diagram below.  Notice that because the father can only pass on a Y-chromosome to a son, there is no way for him to pass on a trait to a son that is on an X-chromosome; only the mother can give a son an X-chromosome:

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