Practice problems for these concepts can be found at: Heredity Review Questions for AP Biology
Terms Important in Studying Heredity
The following is a list of terms that will help in your understanding of heredity:
Allele: a variant of a gene for a particular character. For example, the two alleles for eye color discussed later in the chapter are B (dominant) and b (recessive).
F1: the first generation of offspring, or the first "filial" generation in a genetic cross.
F2: the second generation of offspring, or the second "filial" generation in a genetic cross.
Genotype: an organism's genetic makeup for a given trait. A simple example of this could involve eye color where B represents the allele for brown and b represents the allele for blue. The possible genotypes include homozygous brown (BB), heterozygous brown (Bb), and homozygous blue (bb).
Heterozygous (hybrid): an individual is heterozygous (or a hybrid) for a gene if the two alleles are different (Bb).
Homozygous (pure): an individual is homozygous for a gene if both of the given alleles are the same (BB or bb).
Karyotype: a chart that organizes chromosomes in relation to number, size, and type.
Nondisjunction: the improper separation of chromosomes during meiosis, which leads to an abnormal number of chromosomes in offspring. A few classic examples of nondisjunction- related syndromes are Down, Turner, and Klinefelter syndromes.
P1: the parent generation in a genetic cross.
Phenotype: the physical expression of the trait associated with a particular genotype. Some examples of the phenotypes for Mendel's peas were round or wrinkled, green or yellow, purple flower or white flower.
Mendel and His Peas
The person whose name is most often associated with heredity is Gregor Mendel. Mendel spent many years working with peas. It was a very strange hobby, indeed, but it proved quite useful to the world of science. He mated peas to produce offspring and recorded the phenotype results in order to determine how certain characters are inherited. A character is a genetically inherited characteristic that differs from person to person.
Before he began his work in the 1850s, the accepted theory of inheritance was the "blending" hypothesis, which stated that the genes contributed by two parents mix as colors do. For example, a blue flower mixed with a yellow flower would produce a green flower. The exact genetic makeup of each parent could never be recovered; the genes would be as inseparable as the blended colors. Mendel used plant experiments to test this hypothesis and developed his two fundamental theories: the law of segregation and the law of independent assortment.
When Mendel was observing a single character during a mating, he was doing something called a monohybrid cross—a cross that involves a single character in which both parents are heterozygous (Bb ???????Bb). A monohybrid cross between heterozygous gametes gives a 3 : 1 phenotype ratio in the offspring (Figure 10.1). As you can see in Figure 10.1, an offspring is three times more likely to express the dominant B trait than the recessive b trait.
Mendel also experimented with multiple characters simultaneously. The crossing of two different hybrid characters is termed a dihybrid cross (BbRr × BbRr.) A dihybrid cross between heterozygous gametes gives a 9 : 3 : 3 : 1 phenotype ratio in the offspring (Figure 10.2).
From his experiments, Mendel developed two major hereditary laws: the law of segregation and the law of independent assortment.
The law of segregation. Every organism carries pairs of factors, called alleles, for each trait, and the members of the pair segregate (separate) during the formation of gametes. For example, if an individual is Bb for eye color, during gamete formation, one gamete would receive a B, and the other made from that cell would receive a b.
The law of independent assortment. Members of each pair of factors are distributed independently when the gametes are formed. Quite simply, inheritance of one trait or characteristic does not interfere with inheritance of another trait. For example, if an individual is BbRr for two genes, gametes formed during meiosis could contain BR, Br, bR, or br. The B and b alleles assort independently of the R and r alleles.
The law of dominance. Also based on Mendel's work, this states that when two opposite pure-breeding varieties (homozygous dominant vs. homozygous recessive) of an organism are crossed, all the offspring resemble one parent. This is referred to as the dominant trait.
The variety that is hidden is referred to as the recessive trait. It is time for you to answer a question for me (of course, I have no way of knowing whether or how you will answer this question): Can the phenotype of an organism be determined from simple observation? Yes—just look at the organism and determine whether it is tall or short, has blue eyes or brown eyes, and so on. However, the genotype of an organism cannot always be determined from simple observation. In the case of a recessive trait, the genotype is known. If a person has blue eyes (recessive to brown), the genotype is bb. But if that person has brown eyes, you cannot be sure if the genotype is Bb or BB—the individual can be either homozygous dominant or heterozygous dominant. To determine the exact genotype, you must run an experiment called a test cross. Geneticists breed the organism whose genotype is unknown with an organism that is homozygous recessive for the trait. This results in offspring with observable phenotypes. If the unknown genotype is heterozygous, probability indicates one-half of the offspring should express the recessive phenotype. If the unknown genotype is homozygous dominant, all the organism's offspring should express the dominant trait. Of course, such experiments are not done on humans.
Intermediate Inheritance
The inheritance of traits is not always as simple as Mendel's pea experiments seem to indicate. Traits are not always dominant or recessive, and phenotype ratios are not always 9 : 3 : 3 : 1 or 3 : 1. Mendel's experiments did not account for something called intermediate inheritance, in which an individual heterozygous for a trait (Yy) shows characteristics not exactly like either parent. The phenotype is a "mixture" of both of the parents' genetic input. There are two major types of intermediate inheritance:
- Incomplete dominance or "blending inheritance."
- Codominance.
Incomplete Dominance ("Blending Inheritance")
In incomplete dominance ("blending inheritance") the heterozygous genotype produces an "intermediate" phenotype rather than the dominant phenotype; neither allele dominates the other. A classic example of incomplete dominance is flower color in snapdragons— crossing a snapdragon plant that has red flowers with one that has white flowers yields offspring with pink flowers.
One genetic condition in humans that exhibits incomplete dominance is hypercholesterolemia—a recessive disorder (hh) that causes cholesterol levels to be many times higher than normal and can lead to heart attacks in children as young as 2 years old. Those who are HH tend to have normal cholesterol levels, and those who are Hh have cholesterol levels somewhere in between the two extremes. As with many conditions, the environment plays a major role in how genetic conditions express themselves. Thus, people who are HH do not necessarily have normal cholesterol levels if, for example, they have poor diet or exercise habits.
One important side note—try not to confuse the terms blending "hypothesis" and blending "inheritance." The latter is another name for incomplete dominance, whereas the former was the theory on heredity before Mendel worked his magic. The blending "hypothesis" says that the HH and hh extremes can never be retrieved. In reality, and according to blending inheritance, if you were to cross two Hh individuals, the offspring could still be HH or hh, which the blending "hypothesis" says cannot happen once the blending has occurred.
Codominance
Codominance is the situation in which both alleles express themselves fully in a heterozygous organism. A good example of codominance involves the human blood groups: M, N, and MN. Individuals with group M blood have M molecule on the surface of the blood cell, individuals with group N blood have N molecules on the blood cell, and those with group MN blood have both. This is not incomplete dominance because both alleles are fully expressed in the phenotype—they are codominant.
Other Forms of Inheritance
Polygenic Traits
Another interesting form of inheritance involves polygenic traits, or traits that are affected by more than one gene. Eye color is an example of a polygenic trait. The tone (color), amount (blue eyes have less than brown eyes), and position (how evenly distributed the pigment is) of pigment all play a role in determining eye color. Each of these characteristics is determined by separate genes. Another example of this phenomenon is skin color, which is determined by at least three different genes working together to produce a wide range of possible skin tones.
Multiple Alleles
Many monogenic traits (traits expressed via a single gene) correspond to two alleles, one dominant and one recessive. Other traits, however, involve more than two alleles. A classic example of such a trait is the human blood type. On the most simplistic level, there are four major blood types: A, B, AB, and O. They are named based on the presence or absence of certain carbohydrates on the surface of the red blood cells. The gene for blood type has three possible alleles (multiple alleles): IA, which causes carbohydrate A to be produced on the surface of the red blood cell; IB, which causes carbohydrate B to be produced; and i, which causes no carbohydrate to be produced. The following are the possible genotypes for human blood type: IAi (type A), IAIA (type A), IBi (type B), IBIB (type B), IAIB (type AB), ii (type O). Type AB blood displays the codominance of blood type. As we saw in MN blood groups, both the A and the B alleles succeed in their mission—their carbohydrate appears on the surface of the red blood cell (Figure 10.3). Analyzing blood type can be really complex because human blood types involve not only multiple alleles (IA, IB, and i) and codominance (type AB blood) but classic dominance of IA and IB over i as well.
If you have ever watched an episode of ER on television, you have heard one of the doctors frantically scream, "We need to type her and bring some O blood down here stat!" Why is it important for the physician to determine what type of blood the patient has, and why is it okay to give the patient O blood in the meantime? People with type A blood produce anti-B antibodies because the B carbohydrate that is present on type B and type AB blood is a foreign molecule to someone with type A blood. This is simply the body's defense mechanism doing its job. Following the same logic, those with type B blood make anti-A antibodies, and those with type O blood make anti-A and anti-B antibodies. People who are type AB make none, and are therefore the universal acceptor of blood. It is important to find out what kind of blood a person has because if you give type B blood to a person with type A blood, the recipient will have an immune response to the transfused blood. Why is O blood given while they wait to see what blood type the patient is? This is because type O blood has neither carbohydrate on the surface of red blood cells. People with type O blood are universal donors because few people will have an adverse reaction to type O blood. Immune reactions are discussed in further detail in Chapter 15, Human Physiology.
Epistasis
In epistasis the expression of one gene affects the expression of another gene. A classic example of epistasis involves the coat color of mice. Black is dominant over brown, and brown fur has the genotype bb. There is also another gene locus independent of the coat color gene that controls the deposition of pigment in the fur. If a mouse has a dominant allele of this pigment gene (Cc or CC), it leads to pigment deposition and the coloring of the fur according to the coat color gene's instructions. If a mouse is double recessive for this trait (cc), it will have white fur no matter what the coat-color gene wants because it will not put any pigment into the fur. It is almost as if the pigment gene were overruling the coat color gene. If you mate two black mice that are BbCc, the ratio of phenotypes in the offspring would not be the 9 : 3 : 3 : 1 ratio that Mendel predicts, but rather 9 : 4 : 3 black : white : brown because the epistatic gene alters the phenotype.
Pleiotropy
In pleiotropy a single gene has multiple effects on an organism. A good example of pleiotropy is the mutation that causes sickle cell anemia. This single gene mutation "sickles" the blood cells, leading to systemic symptoms such as heart, lung, and kidney damage; muscle pain; weakness; and generalized fatigue. The problems do not stop there; these symptoms can lead to disastrous side effects such as kidney failure. The mutation of a single gene wreaks havoc on the system as a whole.
Practice problems for these concepts can be found at: Heredity Review Questions for AP Biology
View Full Article
From 5 Steps to a 5 AP Biology. Copyright © 2010 by The McGraw-Hill Companies. All Rights Reserved.
Celebrate Memorial Day! Worksheets and Activities About American History 
5 Outdoor Games to Play in Under 5 Minutes 
Spring Fever! 6 Ways to Settle Kids Down 
Add your own comment