Modern Genetics

1. Objectives and Reading Guide

Lesson Objectives

• Explain Mendel’s laws with our modern understanding of chromosomes.
• Explain how codominant traits are inherited.
• Distinguish between phenotype and genotype.
• Explain how polygenic traits are inherited.

2. Vocabulary Defined

Vocabulary

allele

An alternative form of a gene.

co-dominance

A pattern of inheritance where both alleles are equally expressed.

genotype

The genetic makeup of a cell or organism, defined by certain alleles for a particular trait.

homozygous

Having identical alleles for a particular trait.

heterozygous

Having two different alleles for a particular trait.

incomplete dominance

A pattern of inheritance where the offspring has a phenotype that is halfway between the two parents’ phenotypes.

phenotype

The physical appearance that is a result of the genotype.

polygenic inheritance

A pattern of inheritance where the trait is controlled by many genes and each dominant allele has an additive effect.

3. Introduction

Introduction

Although Mendel laid the foundation for modern genetics, there were still a lot of questions left unanswered. How is inheritance determined for traits that do not seem to follow a simple dominant-recessive pattern? What exactly are the hereditary factors that determine traits in organisms? And how do these factors work? One of the great achievements of this past century was the discovery of DNA as the genetic material. And it is the DNA that makes up the hereditary factors that Mendel identified. By applying our modern knowledge of DNA and chromosomes, we can explain Mendel’s findings and build on them.

4. Traits Genes and Alleles

Traits, Genes, and Alleles

Interpreting Mendel’s discoveries through the eye of modern genetics, we now know that Mendel’s hereditary factors are made up of DNA. Recall that our DNA is wound into chromosomes. Each of our chromosomes contains a long chain of DNA that encodes hundreds, if not thousands, of genes. Each of these genes can have slightly different versions from individual to individual. These variants of genes are called alleles. For example, remember that for the height gene in pea plants there are two possible alleles, the dominant allele for tallness (T) and the recessive allele for shortness (t).

5. Genotype and Phenotype

Genotype and Phenotype

Genotype refers to the combination of alleles that an individual has for a certain gene. For each gene, an organism has two alleles, one on each chromosome of a homologous pair of chromosomes. The genotype is often referred to with the letter combinations that were introduced in the previous lesson, such as TT, Tt, and tt. When an organism has two of the same alleles for a specific gene, it is homozygous for that gene. An organism can be either homozygous dominant (TT) or homozygous recessive (tt). If an organism has two different alleles (Tt) for a certain gene, it is known as heterozygous. Genes have a specific place on a specific chromosome, so in the heterozygous individual these alleles are in the same location on each homologous chromosome.

Phenotype refers to the visible traits or appearance of the organism, as determined by the genotype. For example, the phenotypes of Mendel’s pea plants were either tall or short, or were purple-flowered or white-flowered. Keep in mind that plants with different genotypes can have the same phenotype. For example, both a pea plant that is homozygous dominant for the tall trait (TT) and heterozygous plant (Tt) would have the phenotype of being tall plants. The recessive phenotype only occurs if the dominant allele is absent, which is when an individual is homozygous recessive (tt).

6. Incomplete Dominance and Codominance

Incomplete Dominance and Codominance

In all of Mendel’s experiments, he worked with traits where a single gene controlled the trait and where one allele was always dominant to the other. Although the rules that Mendel derived from his experiments explain many inheritance patterns, the rules do not explain them all. There are in fact exceptions to Mendel’s rules, and these exceptions usually have something to do with the dominant allele.

One exception to Mendel’s rules is that one allele is always completely dominant over a recessive allele. Sometimes an individual has an intermediate phenotype between the two parents, as there is no dominant allele. This pattern of inheritance is called incomplete dominance.

An example of incomplete dominance is the color of snapdragon flowers. 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 will have red flowers, while a plant that is homozygous for the white allele will have white flowers. On the other hand, the heterozygote will have pink flowers (Figure below). Neither the red nor the white allele is dominant, so the phenotype of the offspring is a blend of the two parents.

Another example of incomplete dominance is sickle cell anemia, a disease in which the hemoglobin protein is produced incorrectly and the red blood cells have a sickle shape. A person that is homozygous recessive for the sickle cell trait will have red blood cells that all have the incorrect hemoglobin. A person who is homozygous dominant will have normal red blood cells. And because this trait has an incomplete dominance pattern of expression, a person who is heterozygous for the sickle cell trait will have some misshapen cells and some normal cells (Figures below and below). These heterozygous individuals have a fitness advantage; they are resistant to severe malaria. Both the dominant and recessive alleles are expressed, so the result is a phenotype that is a combination of the recessive and dominant traits.

An example of a codominant trait is ABO blood types (Figure below), named for the carbohydrate attachment on the outside of the blood cell. In this case, two alleles are dominant and completely expressed (designated I^A and I^B), while one allele is recessive (i). The I^A allele encodes for red blood cells with the A antigen, while the I^B allele encodes for red blood cells with the B antigen. The recessive allele (i) doesn’t encode for any antigens. An antigen is a substance that provokes an immune response, your body’s defenses against disease, which will be discussed further in the Diseases and the Body's Defenses chapter. Therefore a person with two recessive alleles (ii) has type O blood. As no dominant (I^A and I^B) allele is present, the person cannot have type A or type B blood.

There are two possible genotypes for type A blood, homozygous (I^AI^A) and heterozygous (I^Ai), and two possible genotypes for type B blood (I^Bi and I^BI^B). If a person is heterozygous for both the I^A and I^B alleles, they will express both and have type AB blood with both antigens on each red blood cell. 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.

7. Polygenic Traits and Environmental Influences

Polygenic Traits and Environmental Influences

Another exception to Mendel’s rules is polygenic inheritance, which is when a trait is controlled by more than one gene. Often these traits are in fact controlled by many genes on many chromosomes. Each dominant allele has an additive effect, so the resulting offspring can have a variety of genotypes, from no dominant alleles to several dominant alleles. In humans, some examples of polygenic traits are height and skin color. People are neither short nor tall, as was seen with the pea plants studied by Mendel, which has only one gene that encodes for height. Instead, people have a range of heights determined by many genes. Similarly, people have a wide range of skin colors. Polygenic inheritance often results in a bell shaped curve when you analyze the population (Figure below). That means that most people are intermediate in the phenotype, such as average height, while very few people are at the extremes, such as very tall or very short.

Most polygenetic traits are partially influenced by the environment. For example, height is partially influenced by nutrition in childhood. If a child is genetically programmed to be average height but does not get a proper diet, he or she may be below average in size.

Other examples of environmentally influenced traits are mental illnesses like schizophrenia and depression. A person may be genetically predisposed to have depression, so when that person's environment contributes major stresses like losing a job or losing a close relative, the person is more likely to become depressed.

8. Linkage

Linkage

Linkage refers to particular genetic position or loci, of alleles inherited together, suggesting that they are physically on the same chromosome, and located close together on that chromosome. A crossing-over event during prophase I of meiosis is rare between linked loci. Alleles for genes on different chromosomes are not linked; they sort independently (independent assortment) of each other during meiosis.

A gene is also said to be linked to a chromosome if it is physically located on that chromosome. For example, a gene (or loci) is said to be linked to the X-chromosome if it is physically located on the X-chromosome.

Linkage Maps

The frequency of recombination refers to the rate of crossing-over (recombination) events between two loci. This frequency can be used to estimate genetic distances between the two loci, and create a linkage map. In other words, the frequency can be used to estimate how close or how far apart the two loci are on the chromosome.

In the early 20^th century, Thomas Hunt Morgan demonstrated that the amount of crossing over between linked genes differs. This led to the idea that the frequency of crossover events would indicate the distance separating genes on a chromosome. Morgan's student, Alfred Sturtevant, developed the first genetic map, also called a linkage map.

Sturtevant proposed that the farther apart linked genes were on a chromosome, the greater the chance that non-sister chromatids would cross over in the region between the genes during meiosis. By determining the number of recombinants - offspring in which a cross-over event has occurred - it is possible to determine the approximate distance between the genes. This distance is called a genetic map unit (m.u.), or a centimorgan, and is defined as the distance between genes for which one product of meiosis in 100 products is a recombinant. So, a recombinant frequency of 1% (1 out of 100) is equivalent to 1 m.u. Loci with a recombinant frequency of 10% would be separated by 10 m.u. The recombination frequency will be 50% when two genes are widely separated on the same chromosome or are located on different chromosomes. This is the natural result of independent assortment. Linked genes have recombination frequencies less than 50%.

Determining recombination frequencies between genes located on the same chromosome allows a linkage map to be developed. Linkage mapping is critical for identifying the location of genes that cause genetic diseases.

9. Lesson Summary

Lesson Summary

• Variants of genes are called alleles.
• Genotype is the combination of alleles that an individual has for a certain gene, while phenotype is the appearance caused by the expression of the genotype.
• Incomplete dominance and codominance do not fit Mendel’s rules because one allele does not entirely mask the other.
• In polygenic inheritance, many genes control a trait with each dominant allele having an additive effect.