Learning Objectives

1. To become familiar with Mendel’s Laws of Inheritance.  

2. To provide an explanation behind the inheritance of two genes.  

3. To become familiar with how genetic expression can appear to alter Mendelian ratios.

4. To become familiar with the inheritance of complex genetic traits.

Key Concepts

1. Mendel's law of segregation states that gene variants (alleles) separate during meiosis as chromosomes are packaged into gametes. Patterns of single gene transmission and expression depend upon dominance relationships and whether a gene is on an autosome or a sex chromosome.

2. Mendel's law of independent assortment states that a gene transmitted on one chromosome does not influence transmission of a gene on a different chromosome. This law is used to predict the proportions of progeny classes when more than one trait is considered.

3. Sometimes Mendel's laws do not seem to be operating because expected ratios of progeny classes do not occur. Allele interactions and effects of other genes and the environment can alter phenotypes, but the laws still operate.

4. Many traits do not adhere to Mendel's laws because they are determined by more than one gene and also sometimes by the environment.

Chapter Concept 10.1: Tracing the Inheritance of One Gene

Mendel's law of segregation states that gene variants (alleles) separate during meiosis as chromosomes are packaged into gametes. Patterns of single gene transmission and expression depend upon dominance relationships and whether a gene is on an autosome or a sex chromosome.

Textbook Reading Assignment: Pages 178 - 186

Genetics, the study of inheritance, explains why offspring both resemble and differ from their parents. The study of genetics also allows biologists to explain how variation is maintained within populations over time. Transmission genetics, the topic of this chapter, explains how variation is passed down from one generation to the next.

Central to the understanding of transmission genetics are the concepts of phenotype and genotype. The phenotype of an organism encompasses its physical and behavioral characteristics. For example, eye color, height, skin color, etc. all constitute the phenotype of an individual. In other words, the phenotype refers to the physical characteristics of an individual. The genotype, on the other hand, consists of the genetic makeup of an organism. It represents the collection of all the genes found within an organism. A gene is a specific region of DNA that determines what types of proteins an individual will make, therefore partially determining the phenotype.

In reality, the phenotype of an individual is determined by both its genotype as well as the physical environment. One way to understand the difference between the phenotype and genotype is to analyze eye color. If your eyes are blue, you phenotype for eye color is blue and the genotype consists of the genes located within the DNA that construct proteins that make your eyes blue.

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Discussion Question 1

When you look at your classmates, are you directly observing their genotype or their phenotype?

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As discussed in Chapter 9 of the text, the significance of diploid cells is their ability to carry two copies of each chromosome. Since biologists have determined that genes reside on chromosomes, diploid cells must, therefore, carry two copies of each gene. Genes come in slightly different varieties termed alleles. For example, diploid cells in humans carry two genes for eye color, one on each of two homologous chromosomes. This does not, however, imply that both homologs necessarily carry the exact same form of the gene. In the case of eye color, one chromosome may carry an allele for brown eyes while its homolog may carry an allele for blue eyes. Both chromosomes are still said to be homologous to one another because they both carry eye color genes.

A good analogy that might help you understand the distinction between an allele and a gene involves a soda machine. If you were to visit the soda machine on campus, you would find a series of Pepsi products to choose from (Pepsi, Diet Pepsi, etc.). In this case, the soda machine might represent the “soda” gene and the specific types of soft drinks available would represent various “alleles” of the soda gene.

In diploid organisms, for any given gene, an individual carries two alleles of the gene. If both alleles are the same, the genotype for that individual is said to be homozygous. If the two alleles differ from one another, the genotype for that individual is said to be heterozygous. For example, an individual with two copies of the brown allele for eye color is homozygous, while an individual with one brown allele and one blue allele is heterozygous.

Figure 10.4 above demonstrates that alleles are located in specific locations on chromosomes. The term locus refers to the physical location of a gene/allele on a chromosome.

In 1900, the rediscovery of the work of the early geneticist, Gregor Mendel, led to a huge shift in our understanding of inheritance and the transfer of genetic information from one generation to the next. An Austrian monk by profession, Gregor Mendel was the first to accurately explain how traits are passed down from one generation to the next. Through the evaluation of many independent experiments performed on pea plants, Mendel found that individuals inherit two genes for each trait, one from each parent. In addition, if the alleles for these traits differ from one another, one of the alleles is expressed, while the other is not.

Figure 10.1
For example, as indicated in the text, Mendel noticed that height in garden peas was either tall or short. When he crossed peas homozygous for tall plant height with ones homozygous for short plant height, all of the plants in the next generation were tall. With this in mind, a dominant allele alone determines the phenotype of a heterozygote, while a recessive allele does not contribute to the resulting phenotype in a heterozygous individual.

Cross breeding involves breeding two genetically distinct organisms with one another. The offspring of such a cross are termed hybrids. For example, in the cross discussed above, crossing a pea plant homozygous for tall plant height to one homozygous for short plant height produces heterozygous tall offspring. The original parents in any one cross are the parental, or P, generation and the offspring represent the first filial, or F1, generation. Reproduction of the F1 individuals with one another subsequently result in the F2 generation.

Let’s use this information to explain the cross presented on pages 180 and 181 of the text. In pea plants, plant height is determined by a single gene with two alleles: tall and short. Since tall is dominant we let:

T = tall

t = short

These two traits simply represent alternative alleles of the gene for plant height. Recall from Chapter 9 that most cells are diploid and have two copies of each gene. Therefore, the homozygous tall plant has two T ’s and the homozygous short plant has has two t’s.

TALL

TT

SHORT

tt

Also recall from Chapter 9 that meiosis produces haploid gametes containing only one copy of each gene for each trait. As a result, in the case of the tall plant, all of the gametes will contain one T and in the short plant all of the gametes will contain one t.

PHENOTYPE TALL SHORT
GENOTYPE TT tt
GAMETES all T all t

Obviously, all of the F1 offspring were Tt and had a tall phenotype (since T is dominant to t). The F1 offspring are said to be monohybrids—they are heterozygous for only one set of alleles.

Figure 10.6
Notice from Figure 10.6 above that when the F1 hybrid offspring (Tt) were crossed with one another, the F2 generation consists of a 3:1 ratio of tall to short. In other words, ¾ of the offspring were tall and ¼ were short. Since each parent is heterozygous, ½ of their gametes will carry a T and the other ½ will carry a t. Notice that the gametes from each parent are placed on the sides of a Punnett square. This square serves to analyze all possible combinations of alleles from the parents. Remember that offspring only receive one gene from each parent (gametes are haploid).

Mendel explained results such as those demonstrated above in his principle of segregation, which suggests that each sexually reproducing organism has two genes for each characteristic; these two genes segregate from one another during gamete formation.

One way to test this principle is with the use of a testcross. A testcross involves crossing an individual with a dominant phenotype to one with the recessive phenotype. Such a cross allows one to determine the genotype of the dominant individual.

For example, the genotype of a tall plant could be either TT (homozygous dominant) or Tt (heterozygous). By crossing this plant with a short plant with the genotype tt (homozygous recessive) it is possible to determine the exact genotype of the tall plant. If all of the offspring from this cross are tall, the original plant is probably TT (homozygous dominant). However, if ½ of the offspring are short, the genotype of the tall parental plant must be Tt (heterozygous).

Ex:

TT x tt = All Tt (tall)

Tt x tt = 1/2 Tt (tall); 1/2 tt (short)

Genetic disorders are classified based upon their mode of inheritance. Most genetic traits and, therefore, disorders are carried on autosomes—non sex chromosomes (sex chromosomes (X and Y) are the subject of the next chapter).

Figure 10.10
Autosomal dominant disorders are caused by a dominant allele and affect individuals that are either AA or Aa. The pedigree chart above (Figure 10.10) demonstrates an autosomal dominant trait. In the chart, individuals in black are affected, while individuals in white are not. Notice from the chart (males are squares and females are circles) that anyone with at least one copy of the dominant allele is affected by the disease. Also notice that under such inheritance, two unaffected individuals cannot have affected children. Autosomal dominant disorders include neurofibromatosis and Huntington disease (please refer to Table 10.4 of the text).

Figure 10.9
In autosomal recessive disorders, only individuals who are homozygous recessive (aa) will actually have the disease. The pedigree chart located above (Figure 10.9) demonstrates the inheritance of an autosomal recessive disorder. Notice that heterozygous individuals (Aa) are not affected, but they can pass the disorder on to their children. These heterozygous individuals (Aa) are said to be carriers, since they appear normal but are capable of having a child with the disorder. Autosomal recessive disorders include Tay-Sachs Disease, Cystic Fibrosis, and Phenylketonuria (PKU). Please review these disorders on Table 10.4 of the text.

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Discussion Question 2

What would be the expected phenotypes of offspring in a cross between a pea plant homozygous for tall height with a heterozygous pea plant? Show your work in a Punnett square.

Discussion Question 3

Make the following crosses: (Don't let the use of the letter p bother you, work these just like the problem we worked above)

PP x pp

Pp x pp

Pp x Pp

In all cases, use a Punnett square to determine the genotypic and phenotypic ratios in the F1 generation.

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Chapter Concept 10.2: Tracing the Inheritance of Two Genes

Mendel's law of independent assortment states that a gene transmitted on one chromosome does not influence transmission of a gene on a different chromosome. This law is used to predict the proportions of progeny classes when more than one trait is considered.

Textbook Reading Assignment: Pages 186 - 188

In addition to simple single trait crosses, Mendel constructed crosses in which he observed the inheritance of two traits at the same time. He was interested in determining if the genes for these two traits were inherited together or independently from one another. Mendel’s resulting principle of independent assortment suggests that each pair of genes is distributed independently from every other pair during gamete formation. Today, we recognize that this principle is explained by the fact that most traits are located on different chromosomes that assort independently from one another during meiosis.

Figure 10.12
Mendel’s concepts of segregation and independent assortment can both be visualized in Figure 10.12 above. Segregation occurs due to the fact that homologous chromosomes separate from one another during Meiosis I. Independent assortment occurs at the same time. For example, notice from the figure above that the homologous chromosomes line up randomly at the metaphase plate. As a result, homologous chromosomes and the alleles they carry segregate independently during gamete formation. All possible combinations of chromosomes and alleles occur in the gametes.

In order to better comprehend this idea, let’s use the example of the two-trait cross provided for us on page 187 of the text. In this cross, we are dealing with two genes—seed shape (either round or wrinkled) and seed color (either yellow or green). The legend for the cross is presented below:

R = round

r = wrinkled

Y = yellow

y = green

Since plants are diploid organisms, any one plant will have two copies of each gene. In other words, a single individual will have two R’s and two Y’s (remember, one came from each parent). Notice that in this cross one parent is RRYY (round, yellow) and the other is rryy (wrinkled, green).

Notice from Figure 10.11 on page 187 that the F1 genertion receive one gene for each trait from each parent (one parent gives the offspring RY and the other parent gives the offspring ry). As a result, the genotype of the F1 is RrYy. This genotype is termed a dihybrid because the individual is heterozygous for both traits: seed shape and seed color.

Figure 10.12

Each F1 individual with the genotype RrYy generates four separate types of gametes.

Why is this the case?

Refer again to Figure 10.11. Notice that the F2 generation receives one gene for seed shape and one gene for seed color from each F1 individual. If you take a look at the genotype of the F1 you will find it to be RrYy. Since this individual will pass one gene for each trait on to the offspring, you must look at all possible combinations of alleles when constructing the gametes.

For example, in the case of RrYy, each gamete will have one gene for seed shape and one gene for seed color(remember, gametes are haploid). With this in mind, what are all possible combinations?

RY, Ry, rY and ry

In other words, each F1 parent can make 4 different types of gametes. This is what independent assortment means—just because the R goes into one gamete, that does not mean that the Ymust follow. In other words, the R may travel into the gamete with either the Y or the y.

Since each parent can generate four different gametes, the Punnett square is a 4 x 4 box. Once all the squares are filled in one finds the following ratios in the F2:

9/16 round, yellow

3/16 round, green

3/16 wrinkled, yellow

1/16 wrinkled, green

Your book discusses the use of the product rule in solving dihybrid problems: this is a shortcut and should only be used once you completely understand how to solve the problem the "long way". Since we will never deal with the inheritance of 3 or more genes, I'd suggest skipping the section on the use of the product rule.

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Discussion Question 4

Use the following information to examine a genetic cross involving two traits:

P = purple flowers, p = white flowers, T = tall plant, t = short plant

Use a Punnett square and determine the phenotypic ratios of the following crosses:

PPTt x PpTt

PpTt x PpTt

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Chapter Concept 10.3: How Gene Can Appear to Alter Mendelian Ratios

Sometimes Mendel's laws do not seem to be operating because expected ratios of progeny classes do not occur. Allele interactions and effects of other genes and the environment can alter phenotypes, but the laws still operate.

Textbook Reading Assignment: Pages 188 - 194

O.K--Let's simplify a bit. This section gets a bit too detailed for our purposes in this course. Don't worry about the following:

Penetrance and Expressivity

Pleiotropy

Epistasis

Environmental Influences (phenocopies)

Genetic Heterogeneity

What you do need to know about are:

1) lethal alleles

2) multiple alleles

3) codominance

4) incomplete dominance

Let's spend some time briefly reviewing these 4 concepts:

1. Lethal alleles- Lethal alleles cause death early in development when in the homozygous condition. Since death occurs so rapidly, homozygous offspring are never seen in the next generation.

For example, in cross 1 below (Figure 10.14), homozygous individuals die early in development. As a result, the ratios seen in the F1 generation are not 3:1, but 2/3:1/3. Whenever you see results in a 2:1 ratio such as this it indicates the presence of lethal alleles.

Figure 10.14

2. Multiple alleles and codominace can be covered together. In addition to the above departure from Mendelian Genetics, differences in the number of alleles associated with a particular gene exist as well. In the classic dominant-recessive interaction, there are only two alleles which can occupy a given locus within a chromosome. However, in the case of the ABO human blood group, more than 2 standard alleles exist in the determination of blood type within the population. In other words, any single individual can only have two alleles for blood type (since we are diploid), however, three alleles exist within the human population.

Figure 10.19
Two of these alleles are codominant to each other (A and B) and one (o) is recessive to both A and B. Codominant alleles are both expressed in the phenotype. Note the table above (Figure 10.19) comparing genotypes to phenotypes. Note that the only way an individual can have type AB blood is to have both an A allele as well as a B allele. However, a person with type A blood can have one of two genotypes: AA or AO. In this case, since O is recessive, it is not expressed in the heterozygous condition (AO). Likewise, the type B person can have one of two genotypes: BB or BO. Again, since O is recessive, it is not expressed in the heterozygous condition. Also note from the same figure that the only way to have type O blood is to have two O alleles (OO). Even though type O blood is determined by two recessive alleles, it is the most common blood type in the world (expressed in almost 50% of the world’s population).

3) The final variation on classical Mendelian Genetics is incomplete dominance. Under such a dominance condition, one allele does not completely mask the expression of another.

Figure 10.18

For example, flower color in Snapdragons is determined by alleles that are incompletely dominant to one another (refer to Figure 10.18 above). Notice that the symbols used for the alleles are R and R’ and not R and r. Using R and r would be misleading since it would imply that R is dominant to r. So, whenever you work a genetic cross dealing with incompletely dominant alleles, use capital letters and a ‘ to distinguish between the two alleles.

Notice from the figure above (Figure 10.18) that three phenotypes are possible: red, pink and white. In other words, incompletely dominant alleles generate intermediate phenotypes (in this case the intermediate phenotype of pink flower color is caused by the interaction of a red gene and a white gene).

Chapter Concept 10.4: Polygenic and Multifactorial Traits

Many traits do not adhere to Mendel's laws because they are determined by more than one gene and also sometimes by the environment.

Textbook Reading Assignment: Pages 194 - 197

You are in luck! We are going to skip almost this entire section (way tooooooooo detailed). The only portion of this section you are responsible for is the concept of polygenic inheritance.

Most traits are polygenic, indicating that they are influenced by more than one gene. For example, fur color in mice, skin color in humans and human height depend upon the interaction of many independent genes.

Figure 10.22
The result of many genes acting upon the phenotype of an individual is a continuous variation of phenotypes. In other words, if a single gene determined height, individuals would either be tall (T) or short (t) such as in Mendel’s pea plants. However, since many genes independently affect human height, a variety of phenotypes are apparent. This phenotypic distribution resembles the bell-shaped curve demonstrated above (Figure 10.22).

Self-Test Questions

1. The ___________ of an individual encompasses both its physical and behavioral characteristics.

a) phenotype

b) genotype

c) haplotype

d) gene

2. _________ cells contain two copies of each chromosome.

a) haploid

b) diploid

c) aneuploid

d) homologous

3. A ___________ individual has two different alleles of a particular gene.

a) heterozygous

b) homozygous

c) hemizygous

d) perizygous

4. Mendel’s ____________________ suggests that each sexually reproducing organism has two genes for each characteristic which separate from one another during gamete formation.

a) principle of inheritance

b) principle of independent assortment

c) principle of genetics

d) principle of segregation

5. Which of the following is an example of an autosomal dominant disorder?

a. PKU

b. Huntingtons Disease

c. Tay-Sachs Disease

d. Cystic Fibrosis

6. How many traits (genes) are involved in a monohybrid cross?

a. one

b. two

c. three

d. none

7. A heterozygous individual has

a. two dominant alleles

b. one dominant and one recessive allele

c. two recessive alleles

d. no phenotype

8. How many copies of a particular gene is/are found in a gamete?

a. one

b. two

c. three

d. four

9. A _______________ occurs when an individual with the dominant phenotype is crossed with an individual having the recessive phenotype.

a. dihybrid cross

b. independent assortment

c. testcross

d. lateral cross

10. ____________ inheritance occurs when one trait is governed by the interaction of two or more sets of alleles.

a. Monohybrid

b. Dihybrid

c. Polygenic

d. Sex-linked

Answers to Self-Test Questions

1. a

2. b

3. a

4. d

5. b

6. a

7. b

8. a

9. c

10. c

Answers/Insight to Discussion Questions

1.      You are directly observing their phenotype.

2.      All offspring will be tall.. Half of the offspring will be TT and the other half Tt.

3.      a: All Pp

b: ½ Pp, ½ pp

c: ¼ PP, ½ Pp, ¼ pp

4.      a: 6/8 Purple Tall; 2/8 Purple Short

b: 9/16 Purple Tall; 3/16 Purple Short; 3/16 White Tall; 1/16 White Short