5.7 Exceptions to Mendel’s Rules

Learning Objectives

• Describe how some patterns of inheritance violate Mendel's rules.

Introduction

Are all people either short or tall?

Unlike Mendel's peas, people do not all fall into two categories: short or tall. Most people, in fact, are somewhere in between. This is true for many human traits. In some cases, Mendel's rules are too simple to explain the inheritance of traits. Each characteristic Mendel investigated was controlled by one gene that had only two possible alleles, one of which was completely dominant over the other. We now know that inheritance is often more complicated than this. In blood types, for example, there are actually three alleles instead of two.

Guided Learning

Exceptions to Mendel's Rules

In all of Mendel’s experiments, he worked with traits where a single gene controlled the trait. Each also had one allele that was always dominant over the recessive allele. But this is not always true.

There are exceptions to Mendel’s rules, and these exceptions usually have something to do with the dominant allele. If you cross a homozygous red flower with a homozygous white flower, according to Mendel's laws, what color flower should result from the cross? Either a completely red or completely white flower, depending on which allele is dominant. But since Mendel's time, scientists have discovered this is not always the case.

Incomplete Dominance

One allele is not always completely dominant over another allele. Sometimes an individual has a phenotype between the two parents because one allele is not dominant over another. This pattern of inheritance is called incomplete dominance. For example, snapdragon flowers show incomplete dominance. One of the genes for flower color in snapdragons has two alleles, one for red flowers and one for white flowers.

A plant that is homozygous for the red allele (RR) will have red flowers, while a plant that is homozygous for the white allele will have white flowers (WW). But the heterozygote will have pink flowers (RW) (Figure below). Neither the red nor the white allele is dominant, so the phenotype of the offspring is a blend of the two parents.

^[1]

Pink snapdragons are an example of incomplete dominance.

Codominance and Multiple Alleles

Another exception to Mendel's laws is a phenomenon called codominance. For example, our blood type shows codominance. Do you know what your blood type is? Are you A? B? O? AB? Those letters actually represent alleles. Unlike other traits, your blood type has three alleles (A, B, and O), instead of two!

The A allele is dominant, as is the B allele. They are both dominant over the allele that codes for type O blood, but when they are present together (AB), they are both expressed. This is called codominance because the prefix co- means both or together. This pattern of inheritance is significantly different than Mendel’s rules for inheritance, because both alleles are expressed completely, and one does not mask the other.

Polygenic Traits

Another exception to Mendel’s rules is polygenic inheritance, which occurs when a trait is controlled by more than one gene. This means that each dominant allele "adds" to the expression of the next dominant allele.

Usually, traits are polygenic when there is wide variation in the trait. For example, humans can be many different sizes. Height is a polygenic trait, controlled by at least three genes with six alleles. If you are dominant for all of the alleles for height, then you will be very tall. There is also a wide range of skin color across people. Skin color is also a polygenic trait.

Sex-Linked Traits

One special pattern of inheritance that doesn’t fit Mendel’s rules is sex-linked inheritance, referring to the inheritance of traits that are located on genes on the sex chromosomes. Since males and females do not have the same sex chromosomes, there will be differences between genders in how these sex-linked traits—traits linked to genes located on the sex chromosomes—are expressed.

One example of a sex-linked trait is red-green colorblindness. People with this type of color blindness cannot tell the difference between red and green. They often see these colors as shades of brown (Figure below). Boys are much more likely to be colorblind than girls. This is because colorblindness is a sex-linked, recessive trait, and girls would need to inherit two copies of the gene whereas boys only need to inherit one.

Boys only have one X chromosome, so if that chromosome carries the gene for color blindness, they will be colorblind. As girls have two X chromosomes, a girl can have one X chromosome with the colorblind gene and one X chromosome with a normal gene for color vision. Since colorblindness is recessive, the dominant normal gene will mask the recessive color blind gene. Females with one color blindness allele and one normal allele are referred to as carriers. They carry the allele but do not express it.

How would a female become color-blind? She would have to inherit two genes for color blindness, which is very unlikely. Many sex-linked traits are inherited in a recessive manner.

Asexual Reproduction

Mendel's rules also assume that there are two parents contributing to the genotype of the offspring. In asexual reproduction, however, there is only one parent, and an exact copy of the parent's DNA is passed onto the offspring. As a result, the offspring are clones of the parent. A clone is an organism that has identical DNA to another individual.

Review

• In polygenic inheritance, a trait is controlled by more than one gene.
• Examples of polygenic inheritance include height or skin color.
• Incomplete dominance, as seen in snapdragon flower color, is a form of inheritance in which one allele is only partly dominant over the other allele, resulting in an intermediate phenotype.
• Codominance, as in human blood type, is a form of inheritance in which both alleles are expressed equally in the phenotype of the heterozygote.
• Organisms that reproduce asexually pass on all of their DNA, and therefore all of their traits, onto their offspring.
• Each individual has two sex chromosomes; females have two X chromosomes (XX), while males have one X chromosome and one Y chromosome (XY).
• Sex-linked traits are located on genes on the sex chromosomes.

Vocabulary

Asexual Reproduction

Reproduction in which there is only one parent and all genetic material is passed on to offspring.

Carrier

Person who carries the recessive allele for a trait but does not express the trait.

Clone

An exact copy of another organism; contains identical DNA.

Codominance

Form of inheritance in which two alleles are expressed equally in the phenotype of the heterozygote.

Incomplete Dominance

Form of inheritance in which one allele for a specific trait is not completely dominant over the other allele, resulting in an intermediate phenotype.

Polygenic Trait

Trait that is controlled by more than one gene.

Sex-Linked Trait

Trait linked to genes located on the sex chromosomes.