CONCEPTS IN BIOLOGY

PART III. MOLECULAR BIOLOGY, CELL DIVISION, AND GENETICS

10. Patterns of Inheritance

10.2. The Fundamentals of Genetics

Three questions represent the biological principles behind understanding the genetics problems presented in this chapter:

1. What alleles do the parents have?

2. What alleles are present in the gametes that the parents produce?

3. What is the likelihood that gametes with specific combinations of alleles will be fertilized?

To solve genetics problems and understand biological inheritance, it is necessary to understand how to answer each of these questions and to understand how the answer to one of these questions can affect the others.

Phenotype and Genotype

The interaction of alleles determines the appearance of the organism. The genotype of an organism is the combination of alleles that are present in the organism’s cells. The phenotype of an organism is how it appears outwardly and is a result of the organism’s genotype.

Reconsider the example of earlobe type to explore the ideas of phenotype and genotype. Earlobes can be attached or free. If a person’s earlobes are attached, the person’s phenotype is “attached earlobes.” Likewise, if a person’s earlobes are free, his or her phenotype is “free earlobes.” Each person has 2 alleles for earlobe type. However, the 2 alleles do not need to be identical.

To make understanding genotype easier, we can use a shorthand notation that is commonly used in genetics. The capital letter E can be used to represent the allele that codes for free earlobe development. A lowercase e can be used to refer to the allele that codes for attached earlobe development. Because each person has 2 alleles, a person can have one of these combinations of alleles:

• (EE)—2 alleles for free earlobes

• (ee)—2 alleles for attached earlobes

• (Ee)—1 allele for free earlobes and 1 allele for attached earlobes

The 2 alleles will interact with each other when they are in the same cell and their proteins are synthesized as described in chapter 9. Consider what happens in a cell when the allele combination is EE, ee, or even Ee. When the cell has EE, it is only capable of producing proteins associated with free earlobes. The organism will have free earlobes. When both alleles code for attached earlobe development (ee) then the person will develop attached earlobes. Continue reading to understand what happens when the cells are Ee.

Dominant and Recessive Alleles

What does the organism look like if it has 1 allele that codes for free earlobes and 1 allele that codes for attached earlobes—(Ee)? In this particular situation, the organism develops free earlobes. The E allele produces proteins for free earlobes that “outperforms” the e allele. Therefore, E is able to dominate the appearance of the organism. A dominant allele is one that masks another allele (called the recessive allele) in the phenotype of an organism. A recessive allele is one that is masked by another, the dominant allele. In the previous example, the free earlobes allele (E) is dominant and the attached earlobes allele (e) is recessive, because in an (Ee) individual the phenotype that develops is free earlobes. Geneticists use the capital letter to denote that an allele is dominant. The lowercase letter denotes the recessive allele.

Take a closer look at the genotypes for free and attached earlobes. Notice that organisms with attached earlobes always have 2 e alleles (ee), whereas organisms with free earlobes might have 2 E alleles—(EE)—or both an E and an e allele—(Ee). A dominant allele may hide a recessive allele.

The term recessive has nothing to do with the significance or value of the allele—it simply describes how it is expressed when inherited with a dominant allele. The term recessive also has nothing to do with how frequently the allele is passed on to offspring.

In individuals with 2 different alleles, each allele has an equal chance of being passed on. The Gene Key that immediately follows this text organizes the information about how earlobe shape is inherited. This format will also be used later in this chapter to summarize information about other genes.

Gene Key

Gene or Condition: earlobe shape

Summary: Geneticists describe an organism by its genotype and its phenotype. One rule that describes how the genotype of an organism influences its phenotype involves the principle of dominant and recessive interaction.

Application: Use the dominant and recessive principle to infer information that is not provided. Example: If a person has attached earlobes, you can infer that his or her genotype is ee. If a person has free earlobes, you can infer that he or she has at least 1 E allele. The second allele is uncertain without additional information.

Predicting Gametes from Meiosis

To predict the types of offspring that parents may produce, it is important to predict the kinds of alleles that may be in the sex cells produced by each parent. Remember that during meiosis the 2 alleles will end up in different sex cells. If an organism contains two copies of the same allele, such as in EE or ee, it can produce sex cells with only one type of allele. EE individuals can produce sex cells with only the E allele, likewise ee individuals can produce sex cells with only the e allele. The Ee individual can produce two different types of sex cells. Half of the sex cells carry the E allele. The other half carry the e allele. If an organism has 2 identical alleles for a characteristic and can produce sex cells with only one type of allele, the genotype of the organism is homozygous (homo = same or like). If an organism has 2 different alleles for a characteristic and can produce two kinds of sex cells with different alleles, the organism is heterozygous (hetero = different). This is summarized in the Gene Key at the bottom of this page.

Notice that the 2 alleles separate into different sex cells. This is true whether the cell is homozygous or heterozygous. The Law of Segregation states that in a diploid organism the alleles exist as two separate pieces of genetic information, and that these two different pieces of genetic information are on different chromosomes and are separated into different cells during meiosis.

Gene Key

Gene or Condition: earlobe type

Summary: When sex cells form, they receive only 1 allele for each characteristic. Homozygous organisms can produce only one kind of sex cell. In heterozygous organisms, meiosis produces two genetically different sex cells. The 2 different alleles are represented equally in the sex cells that are produced. Half the cells contain 1 of the alleles and half the cells contain the other.

Application: Make two predictions using the Law of Segregation. The first prediction describes the genetic information a sex cell can carry. The second prediction describes the expected ratios of these sex cells. If the organism is homozygous, then all sex cells will be the same. If the organism is heterozygous, half of the sex cells will carry one allele (one out of two). The other allele will be in the other half of the sex cells.

Fertilization

Recall from chapter 9 that fertilization is the process of two haploid (n) sex cells joining to form a zygote (2n). The zygote divides by mitosis to produce additional diploid cells as the new organism grows. The diploid genotype of all the cells of that organism is determined by the alleles carried by the two sex cells that joined to form the zygote.

A genetic cross is a planned breeding or mating between two organisms. Although the cross is planned, the exact sperm and egg that join when fertilization occurs are not entirely predictable, because the process of fertilization is random. Any one of the many different sperm produced by meiosis may fertilize a given egg. Despite this element of randomness, generalizations can be made about possible results from two parents. These generalizations can be seen by drawing a diagram called a Punnett square. A Punnett square shows the possible offspring of a particular genetic cross.

Genetic crosses can be designed to investigate one or more characteristics. A single-factor cross is designed to look at how one genetically determined characteristic is inherited. A unique single factor cross is a monohybrid cross. A monohybrid cross is a cross between two organisms that are both heterozygous for the one observed gene. A double-factor cross is a genetic study in which two different genetically determined characteristics are followed from the parental generation to the offspring at the same time. Because double-factor crosses involve two genes, their outcomes are more complex than singlefactor crosses. Let’s look at the following single-factor cross where we observe earlobe attachment.

The cross shown is between two heterozygous (Ee) individuals. The individuals in this cross can each produce two types of sex cells, E and e. The colors (red and blue) used in this monohybrid cross and Punnett square allow us to trace what happens to the sex cells from each parent. The top row lists the sex cells that can be produced by one parent, and the left-most column of the Punnett square lists the sex cells that can be produced by the other parent. The letter combinations within the four boxes represent the possible genotypes of the offspring. Each combination of letters is simply the combination of the alleles listed at the top of each column and the left of each row.

Let’s look at the type of offspring that can be produced by this cross. The Punnett square contains three genotypes: EE, Ee, and ee. Additionally, by counting how many times each genotype is shown in the Punnett square, we can predict how frequently we expect to observe each genotype in the offspring of these parents. Here, we expect to see Ee twice for every time we see EE or ee. Remember that a Punnet square only generalizes. If many (at least 30 or more) offspring are produced from the cross, we might expect to see nearly 1/4 EE, 2/4 Ee, and 1/4 ee. Geneticists may abbreviate this ratio as 1:2:1. These ratios can be written in a different manner but still mean the same thing:

1/4 EE, 1/2 Ee, 1/4 ee

Summary: The outcome of a genetic cross cannot be exactly determined. The outcome can only be described by general trends.

Application: The Punnett square can be used to predict the types and ratios of offspring.

10.2. CONCEPT REVIEW

4. Distinguish between phenotype and genotype.

5. What types of symbols are typically used to express genotypes?

6. How many kinds of games are possible with the genotype Aa?

7. What is the difference between a single-factor cross and double-factor cross?