13
______
Observing patterns
in inherited traits
Chapter Outline
Observing Patterns in Inherited Traits 109
13.1 Menacing mucus
13.2 MENDEL, Pea plants, and iNHERITANCE PATTERNS
Mendel’s Experiments
Inheritance in Modern Terms
13.3 mendel’s LAW of Segregation
13.4 mendel’s LAW of Independent Assortment
The Contribution of Crossovers
13.5 beyond simple dominance
Codominance
Incomplete Dominance
Epistasis
Pleiotropy
13.6 nature and nurture
Some Environmental Effects
Alternative Phenotypes in Water Fleas
Seasonal Changes in Coat Color
Effects of Altitude on Yarrow
Psychiatric Disorders
13.7 complex variation in traits
Continuous Variation
MEnacing mucus (revisited)
SUMMARY
Self-Quiz
Genetics Problems
Data Analysis ACTIVITIES
Patterns of Inheritance 68
Patterns of Inheritance 68
Learning Objectives
13.1 Examine the negative consequences of the genetic disorder known as cystic fibrosis.
13.2 Examine how alleles contribute to traits.
13.3 Demonstrate Mendel’s law of segregation using a monohybrid cross.
13.4 Demonstrate Mendel’s law of independent assortment using a dihybrid cross.
13.5 Outline the different ways in which an allele can influence an inherited trait using Punnett squares.
13.6 Examine the influence of the environment on the phenotype of an organism using examples.
13.7 Examine the characteristics of continuous variation.
Key Terms
Observable Patterns of Inheritance 93
bell curve
codominance
continuous variation
dihybrid cross
dominant
epistasis
genotype
heterozygous
homozygous
hybrid
incomplete dominance
law of independent assortment
law of segregation
linkage group
locus
monohybrid cross
multiple allele system
phenotype
pleiotropy
Punnett-square
recessive
short tandem repeats
testcross
Observable Patterns of Inheritance 79
Lecture Outline
13.1 Menacing Mucus
A. Cystic fibrosis is the most common fatal disorder in the United States.
1. This disorder occurs when a mutation prevents the CFTR protein from reaching the cell surface.
2. The normal functioning CFTR protein also helps the body defend itself against bacteria.
a. In normal individuals the CFTR protein binds bacteria and initiates cellular defenses.
b. Cystic fibrosis patients do not have a CFTR protein on the cell surface; bacteria are not recognized and destroyed, thus chronic bacterial infections are a problem in these individuals.
B. About 1 in 25 people carry the CFTR mutation in one of their two copies of the gene; in about 1 in 3300 births, both copies of the CFTR are mutated, causing cystic fibrosis.
13.2 Mendel, Pea Plants, and Inheritance Patterns
A. Gregor Mendel used experiments in plant breeding and knowledge of mathematics to form his hypotheses.
B. Mendel’s Experimental Approach
1. Mendel used the garden pea in his experiments.
a. This plant can fertilize itself; true-breeding varieties were available to Mendel.
b. Peas can also be cross-fertilized by human manipulation of the pollen.
2. Mendel cross-fertilized true-breeding garden pea plants that had clearly contrasting traits (e.g., white versus purple flowers).
3. Mendel hypothesized that clearly observable differences might help him track the trait and identify inheritance patterns and heredity.
C. Terms Used in Modern Genetics
1. Genes are units of information about specific traits, each located at a particular locus on a chromosome.
2. Diploid cells have two genes (a gene pair) for each trait—each on a homologous chromosome.
3. Mutation alters a gene’s molecular structure; alleles are various molecular forms of a gene for the same trait.
4. True-breeding lineage occurs when offspring inherit identical alleles, generation after generation; non-identical alleles produce hybrid offspring.
5. When both alleles are the same, the condition is called the homozygous condition; if the alleles differ, it is the heterozygous condition.
6. When heterozygous, one allele is dominant (A), and the other is recessive (a).
7. Homozygous dominant = AA; homozygous recessive = aa; and heterozygous = Aa
8. Genotype is the particular alleles an individual carries; phenotype is how the genes are expressed physically (what you observe).
9. P = true-breeding parental generation; F1 = first-generation offspring; F2 = second-generation offspring of self-fertilized or intercrossed F1 individuals.
13.3 Mendel’s Law of Segregation
A. Alleles and Meiosis
1. Each member of a chromosome pair contains one allele of a gene; individuals have two copies of each chromosome and thus two copies of every gene.
2. During meiosis, the chromosomes pair and separate such that each gamete contains one copy of each chromosome, and thus one copy of every gene (law of segregation).
B. Testcrosses
1. Individuals that have a dominant trait, which genotypically could be homozygous or heterozygous, are crossed to individuals that have the recessive trait, which are known to be homozygous.
2. If the progeny segregate 1:1 for the dominant and recessive trait, then the parent with the dominant trait was a heterozygote.
3. If the progeny all have the dominant trait, the parent with the dominant trait was a homozygote.
C. Monohybrid Experiment Predictions
1. Mendel’s first experiments were monohybrid crosses.
a. Monohybrid crosses have two parents that are true-breeding for contrasting forms of a trait (white versus purple flowers).
b. One form of the trait (white) disappears in the first generation offspring (F1), only to show up in the second generation (F2).
c. We now know that all members of the F1 offspring are heterozygous (Aa) because one parent could produce only an A gamete and the other could produce only an a gamete.
2. In the F2 generation, the white flowers reappeared.
a. The numerical ratios of crosses suggested that genes do not blend.
b. For example, the F2 offspring showed a 3:1 phenotypic ratio of purple to white.
c. Mendel assumed that each sperm has an equal probability of fertilizing an egg. This can be seen most easily by using the Punnett square.
d. Thus, each new plant has three chances in four of having at least one dominant allele.
13.4 Mendel’s Law of Independent Assortment
A. Dihybrids are the offspring of parents that breed true for different versions of two traits.
1. Mendel also performed experiments involving two traits—a dihybrid cross.
2. A dihybrid cross is started by first crossing two true-breeding parents, each exhibiting two forms of two different traits. The resulting F1 hybrids were allowed to self-fertilize, giving rise to offspring of 16 different combinations.
a. Mendel correctly predicted that all F1 plants would show both of the dominant alleles (e.g., all purple flowers and all tall plants).
b. Mendel wondered if the genes for flower color and plant height would travel together when two F1 plants were crossed.
3. The F2 results showed nine-sixteenths were tall and purple-flowered (both dominant forms of the trait), and one-sixteenth were dwarf and white-flowered (both recessive forms of the trait)—as were the original parents; however, there were three-sixteenths each of two new combinations: dwarf purple-flowered and tall white-flowered.
B. Mendel’s law of independent assortment states that as meiosis ends genes on different pairs of homologous chromosomes are sorted into gametes independently of each other; the same is true for genes located far apart on the same chromosome.
C. Genes located close together on the same chromosome do not independently assort; instead they are frequently inherited together (linked).
13.5 Beyond Simple Dominance
A. Codominance in ABO Blood Types
1. Codominance is when a pair of nonidentical alleles affecting two phenotypes are both expressed at the same time.
2. In codominance, both alleles are expressed in heterozygotes.
3. Blood type is determined by markers produced by three genes.
4. The occurrence of more than three alleles for a single gene locus available to individuals in the population is called a multiple allele system.
a. IA and IB are each dominant to i, but are codominant to each other.
b. Therefore, some persons can express both genes and have AB blood.
B. Incomplete Dominance
1. In incomplete dominance, a dominant allele cannot completely mask the expression of another.
2. Phenotype will be an intermediate between the two traits.
3. For example, a true-breeding red-flowered snapdragon crossed with a white-flowered snapdragon will produce pink flowers because there is not enough red pigment (produced by the dominant allele) to completely mask the effects of the white allele.
C. Epistasis
1. Many phenotypes are due to the action of multiple gene products.
2. In Labrador retrievers, one gene pair codes for the quantity of melanin produced while another codes for melanin deposition.
3. Still another gene locus determines whether melanin will be produced at all—absence of melanin produces an albino (recessive).
4. Two interacting gene pairs can give rise to a phenotype in which neither produces by itself.
5. Another example: there are four comb shapes in chickens; these four different shapes are due to the interaction between two genes (R and P); the phenotypes are walnut comb (R-P-), rose comb (R-pp), pea comb (rrP-), and single comb (rrpp).
D. Pleiotropy
1. Sometimes the expression of alleles at one location can have effects on two or more traits; this is termed pleiotropy.
2. Marfan syndrome is caused by an autosomal-dominant mutation of the gene for fibrillin, a protein found throughout the body in connective tissue.
3. Marfan syndrome is characterized by these effects: lanky skeleton, leaky heart valves, weakened blood vessels, deformed air sacs in lungs, pain, and lens displacement in the eyes.
13.6 Nature and Nurture
A. The environment contributes to variations in gene expression among individuals in a population; the inheritance of chromatin modifications likely plays a role.
1. The fur of the snowshoe hare varies by season: brown in the summer, white in the winter.
2. Cuttings from yarrow plant clones grew differently at three different altitudes.
3. Water fleas have different phenotypes depending on the presence or absence of predators.
4. Some people tolerate stress better than others, perhaps due to differences in the gene for a serotonin-transporting protein. Serotonin compromises response to stress.
B. Environmentally driven changes in gene expression can be inherited due to epigenetic changes in gene expression .
13.7 Complex Variations in Traits
A. Continuous Variation
1. A given phenotype can vary, by different degrees, from one individual to the next in a population as a result of interactions with other genes and environmental influences.
a. This feature is known as polygenic inheritance, where many genes along a particular chromosome affect one trait.
b. In humans, eye color and height are examples.
2. Most traits are not qualitative, but show continuous variation and have an additive effect on phenotype.
B. Regarding the Unexpected Phenotype
1. Tracking even a single gene through several generations may produce results that are different than expected.
2. Camptodactyly (immobile, bent fingers) can express itself on one hand only, both hands, or neither due to the possibility that a gene product is missing in one of the several steps along the metabolic pathway.
Suggestions for Presenting the Material
• Students are usually naturally curious and interested in genetics. Start first with the simple examples of Mendel’s monohybrid and dihybrid crosses before fielding questions on human traits such as height or eye color. Emphasize the remarkable nature of Mendel’s work; remind the students that he knew nothing of chromosomes and their behavior, and that the term gene did not exist until several years after his death.
• Use Mendel’s experiments and his conclusions as real-life examples of the scientific method at work. Ask questions to make sure students understand monohybrid and dihybrid crosses and testcrosses.
• Emphasize genetic terms and the figures that make use of these terms. Use Fig. 13.5 to ensure that students can visualize homologous chromosomes, gene loci, alleles, and gene pairs.
• Students should be able to relate the events of meiosis to the concepts of segregation and independent assortment; if their understanding of meiosis is weak, they will have trouble doing this.
• Many students come into college biology classes with the misconception that dominant alleles are “better” or more common than the recessive alleles. This misconception also makes it difficult for these students to understand non-Mendelian patterns of inheritance discussed at the end of Chapter 13. Stress to your students that dominant does not mean the most abundant or most advantageous trait. Give some examples of complete dominance with alleles that are harmful, undesirable, or lethal, such as polydactyly (extra digits), achondroplasia (dwarfism), and Huntington’s disease (lethal degenerative neurological disorder).
• Beginning with this chapter, students will be quick to ask questions about human traits, many of which are governed by mechanisms more complex than those postulated by Mendel. Answer questions in this area during (or after) the discussion of variations on Mendel’s themes, presented in the second half of this chapter.
· Remind students that the statistical probabilities obtained from Punnett squares are not absolute. There will be some variance, but large sample sizes reduce the likelihood.
Classroom and Laboratory Enrichment
• Ask groups of students to conduct coin tosses. Demonstrate the importance of large sample size by having the students vary the number of tosses before calculating variation from expected ratios.
• Distribute PTC tasting paper to your students, and calculate the number of tasters and nontasters in the classroom. You can also use different physical traits such as tongue rolling or earlobe attachment.
• Expand the biographical sketch of Mendel, including his education and practice as a clergyman. Enliven your presentation with as many slides of photos as you can find.
• Hand out a partially completed pedigree, and show students how to assign squares and circles for their family. Then ask them to select a trait and complete the pedigree after surveying the family members for presence/absence of the trait.
• Select a portion of the class to reenact the photo in Figure 13.17A. If the quantity of students chosen does not provide a bell-shaped curve, use this as an illustration of how the greater number of trials/subjects/experiments tends to increase probability.
• Show this TEDEd video on genetics http://ed.ted.com/lessons/how-mendel-s-pea-plants-helped-us-understand-genetics-hortensia-jimenez-diaz
· Have students do a couple of quick practice Punnett squares (Tt X tt). Students should try to explain the given probabilities and describe possible phenotypes.
· There are many interesting websites that give possible phenotypes of offspring from parents that have different eye color; this shows students the complexity of polygenic inheritance.