What you’ll learn to do: Explain complications to the phenotypic expression of genotype, including mutations
Mendel’s experiments with pea plants suggested the following:
- Two “units” or alleles exist for every gene.
- Alleles maintain their integrity in each generation (no blending).
- In the presence of the dominant allele, the recessive allele is hidden and makes no contribution to the phenotype.
Therefore, recessive alleles can be “carried” and not expressed by individuals. Such heterozygous individuals are sometimes referred to as “carriers.” Further genetic studies in other plants and animals have shown that much more complexity exists, but that the fundamental principles of Mendelian genetics still hold true. In the sections to follow, we consider some of the extensions of Mendelism. If Mendel had chosen an experimental system that exhibited these genetic complexities, it’s possible that he would not have understood what his results meant.
- Explain how a trait with incomplete dominance will appear in a population
- Explain how a trait with codominant inheritance will appear in a population
- Explain how a trait with sex-linkage will appear in a population
- Explain how mutli-allele inheritance will impact a trait within in a population
Mendel’s results, that traits are inherited as dominant and recessive pairs, contradicted the view at that time that offspring exhibited a blend of their parents’ traits. However, the heterozygote phenotype occasionally does appear to be intermediate between the two parents.
For example, in the snapdragon, Antirrhinum majus (Figure 1), a cross between a homozygous parent with white flowers (CWCW) and a homozygous parent with red flowers (CRCR) will produce offspring with pink flowers (CRCW). (Note that different genotypic abbreviations are used for Mendelian extensions to distinguish these patterns from simple dominance and recessiveness.)
This pattern of inheritance is described as incomplete dominance, denoting the expression of two contrasting alleles such that the individual displays an intermediate phenotype. The allele for red flowers is incompletely dominant over the allele for white flowers. However, the results of a heterozygote self-cross can still be predicted, just as with Mendelian dominant and recessive crosses. In this case, the genotypic ratio would be 1 CRCR:2 CRCW:1 CWCW, and the phenotypic ratio would be 1:2:1 for red:pink:white.
A variation on incomplete dominance is codominance, in which both alleles for the same characteristic are simultaneously expressed in the heterozygote.
Codominance can also be seen in human blood types: the AB blood type is a result of both the IA allele and the IB allele being codominant. (An A blood type would only have the IA allele, and a B blood type would only have IB allele.)
The roan coat color in horses is also an example of codominance. A “red” roan results from the mating of a chestnut parent and a white parent (Figure 2). We know this is codominance because individual hairs are either chestnut or they are white, leading to the red roan overall appearance.
So what’s the difference between incomplete dominance and codominant inheritance? While they are very similar, the key difference is this: in incomplete dominance, the two traits are blended together, whereas in codominance, both traits are expressed.
We’ve already discussed incomplete dominance in flowers (Figure 1). What do you think a flower would look like if the red and white phenotypes were codominant instead?
In humans, as well as in many other animals and some plants, the sex of the individual is determined by sex chromosomes. The sex chromosomes are one pair of non-homologous chromosomes. Until now, we have only considered inheritance patterns among non-sex chromosomes, or autosomes. In addition to 22 homologous pairs of autosomes, human females have a homologous pair of X chromosomes, whereas human males have an XY chromosome pair. Although the Y chromosome contains a small region of similarity to the X chromosome so that they can pair during meiosis, the Y chromosome is much shorter and contains many fewer genes. When a gene being examined is present on the X chromosome, but not on the Y chromosome, it is said to be X-linked. And if the gene being investigated is only on the Y chromosome, it is said to by Y-linked.
Eye color in Drosophila was one of the first X-linked traits to be identified. Thomas Hunt Morgan mapped this trait to the X chromosome in 1910. Like humans, Drosophila males have an XY chromosome pair, and females are XX. In flies, the wild-type eye color is red (XW) and it is dominant to white eye color (Xw) (Figure 3). Because of the location of the eye-color gene, reciprocal crosses do not produce the same offspring ratios. Males are said to be hemizygous, because they have only one allele for any X-linked characteristic. Hemizygosity makes the descriptions of dominance and recessiveness irrelevant for XY males. Drosophila males lack a second allele copy on the Y chromosome; that is, their genotype can only be XWY or XwY. In contrast, females have two allele copies of this gene and can be XWXW, XWXw, or XwXw.
What ratio of offspring would result from a cross between a white-eyed male and a female that is heterozygous for red eye color?
Discoveries in fruit fly genetics can be applied to human genetics. When a female parent is homozygous for a recessive X-linked trait, she will pass the trait on to 100 percent of her offspring. Her male offspring are, therefore, destined to express the trait, as they will inherit their father’s Y chromosome. In humans, the alleles for certain conditions (some forms of color blindness, hemophilia, and muscular dystrophy) are X-linked. Females who are heterozygous for these diseases are said to be carriers and may not exhibit any phenotypic effects. These females will pass the disease to half of their sons and will pass carrier status to half of their daughters; therefore, recessive X-linked traits appear more frequently in males than females.
In some groups of organisms with sex chromosomes, the gender with the non-homologous sex chromosomes is the female rather than the male. This is the case for all birds. In this case, sex-linked traits will be more likely to appear in the female, in which they are hemizygous.
Mendel implied that only two alleles, one dominant and one recessive, could exist for a given gene. We now know that this is an oversimplification. Although individual humans (and all diploid organisms) can only have two alleles for a given gene, multiple alleles may exist at the population level such that many combinations of two alleles are observed. Note that when many alleles exist for the same gene, the convention is to denote the most common phenotype or genotype among wild animals as the wild type (often abbreviated “+”); this is considered the standard or norm. All other phenotypes or genotypes are considered variants of this standard, meaning that they deviate from the wild type. The variant may be recessive or dominant to the wild-type allele.
Multiple Alleles (ABO Blood Types) and Punnett Squares
An example of multiple alleles is the ABO blood-type system in humans. In this case, there are three alleles circulating in the population. The IA allele codes for A molecules on the red blood cells, the IB allele codes for B molecules on the surface of red blood cells, and the i allele codes for no molecules on the red blood cells. In this case, the IA and IB alleles are codominant with each other and are both dominant over the i allele. Although there are three alleles present in a population, each individual only gets two of the alleles from their parents. This produces the genotypes and phenotypes shown in the figure below. Notice that instead of three genotypes, there are six different genotypes when there are three alleles. The number of possible phenotypes depends on the dominance relationships between the three alleles. We will analyze how to do this in more detail in a section about non-Mendelian Punnett squares.
Non-Mendelian Punnett Squares
This practice activity will help you remember the difference between types of non-Mendelian inheritance and remember just how they work.
Watch this video for a summary of the three “special” cases of non-Mendelian inheritance you just practiced.
Check Your Understanding
Answer the question(s) below to see how well you understand the topics covered in the previous section. This short quiz does not count toward your grade in the class, and you can retake it an unlimited number of times.
Use this quiz to check your understanding and decide whether to (1) study the previous section further or (2) move on to the next section.