Chapter 14- Lecture Outline

Question 1

♣ Chapter 14
♣ Drawing from the Deck of Genes
♣ What principles account for the passing of traits from parents to offspring?
♣ The “blending” hypothesis is the idea that genetic material from the two parents blends together
(like blue and yellow paint blend to make green)

Answer 1

The “particulate” hypothesis is the idea that parents pass on discrete heritable units (genes)
♣ Mendel documented a particulate mechanism through his experiments with garden peas
♣ Figure 14.1a
♣ Concept 14.1: Mendel used the scientific approach to identify two laws of inheritance
♣ Mendel discovered the basic principles of heredity by breeding garden peas in carefully planned experiment

Question 2

♣ Mendel’s Experimental, Quantitative Approach
♣ Mendel’s approach allowed him to deduce principles that had remained elusive to others
♣ A heritable feature that varies among individuals (such as flower color) is called a character
♣ Each variant for a character, such as purple or white color for flowers, is called a trait
♣ Peas were available to Mendel in many different varieties

Answer 2

♣	Other advantages of using peas
♣	Short generation time
♣	Large numbers of offspring
♣	Mating could be controlled; plants could be allowed to self-pollinate or could be cross pollinated
♣	Figure 14.2

Question 3

♣ Mendel chose to track only those characters that occurred in two distinct alternative forms
♣ He also used varieties that were true-breeding (plants that produce offspring of the same variety when they self-pollinate)

Answer 3

♣ In a typical experiment, Mendel mated two contrasting, true-breeding varieties, a process called hybridization
♣ The true-breeding parents are the P generation
♣ The hybrid offspring of the P generation are called the F1 generation
♣ When F1 individuals self-pollinate or cross- pollinate with other F1 hybrids, the F2 generation is produced

Question 4

♣ The Law of Segregation
♣ When Mendel crossed contrasting, true-breeding white- and purple-flowered pea plants, all of the F1 hybrids were purple
♣ When Mendel crossed the F1 hybrids, many of the F2 plants had purple flowers, but some had white
♣ Mendel discovered a ratio of about three to one, purple to white flowers, in the F2 generation
♣ Figure 14.3-1
♣ Figure 14.3-2
♣ Figure 14.3-3

Answer 4

Mendel reasoned that only the purple flower factor was affecting flower color in the F1 hybrids
♣ Mendel called the purple flower color a dominant trait and the white flower color a recessive trait
♣ The factor for white flowers was not diluted or destroyed because it reappeared in the F2 generation
♣
♣ Mendel observed the same pattern of inheritance in six other pea plant characters, each represented by two traits
♣ What Mendel called a “heritable factor” is what we now call a gene
♣ Table 14.1
♣ Mendel’s Model
♣ Mendel developed a hypothesis to explain the 3:1 inheritance pattern he observed in F2 offspring
♣ Four related concepts make up this model
♣ These concepts can be related to what we now know about genes and chromosomes

Question 5

♣ First: alternative versions of genes account for variations in inherited characters
♣ For example, the gene for flower color in pea plants exists in two versions, one for purple flowers and the other for white flowers
♣ These alternative versions of a gene are called alleles
♣ Each gene resides at a specific locus on a specific chromosome
♣ Figure 14.4

Answer 5

♣ Third: if the two alleles at a locus differ, then one (the dominant allele) determines the organism’s appearance, and the other (the recessive allele) has no noticeable effect on appearance
♣ In the flower-color example, the F1 plants had purple flowers because the allele for that trait
is dominant
Second: for each character, an organism inherits two alleles, one from each parent
♣ Mendel made this deduction without knowing about chromosomes
♣ The two alleles at a particular locus may be identical, as in the true-breeding plants of Mendel’s P generation
♣ Alternatively, the two alleles at a locus may differ, as in the F1 hybrids

Question 6

♣ Fourth (the law of segregation): the two alleles for a heritable character separate (segregate) during gamete formation and end up in different gametes
♣ Thus, an egg or a sperm gets only one of the two alleles that are present in the organism
♣ This segregation of alleles corresponds to the distribution of homologous chromosomes to different gametes in meiosis

Answer 6

♣ The model accounts for the 3:1 ratio observed in the F2 generation of Mendel’s crosses
♣ Possible combinations of sperm and egg can be shown using a Punnett square
♣ A capital letter represents a dominant allele, and a lowercase letter represents a recessive allele

Question 7

♣ An organism with two identical alleles for a character is homozygous for the gene controlling that character

Answer 7

♣ An organism that has two different alleles for a gene is heterozygous for the gene controlling that character

Question 8

♣ Because of the different effects of dominant and recessive alleles, an organism’s traits do not always reveal its genetic composition

Answer 8

♣ Therefore, we distinguish between an organism’s phenotype, or physical appearance, and its genotype, or genetic makeup
♣ In the example of flower color in pea plants, PP and Pp plants have the same phenotype (purple) but different genotypes

Question 9

♣ An individual with the dominant phenotype could be either homozygous dominant or heterozygous

Answer 9

♣ To determine the genotype we can carry out a testcross: breeding the mystery individual with a homozygous recessive individual
If any offspring display the recessive phenotype, the mystery parent must be heterozygous

Question 10

♣ The F1 offspring produced in this cross were monohybrids, heterozygous for one character

Answer 10

♣ A cross between such heterozygotes is called

a monohybrid cross

Question 11

♣ Crossing two true-breeding parents differing in two characters produces dihybrids in the F1 generation, heterozygous for both characters

Answer 11

♣ A dihybrid cross, a cross between F1 dihybrids, can determine whether two characters are transmitted to offspring as a package or independently

Question 12

♣ Using a dihybrid cross, Mendel developed the law of independent assortment
♣ It states that each pair of alleles segregates independently of each other pair of alleles during gamete formation
♣ This law applies only to genes on different, nonhomologous chromosomes or those far apart on the same chromosome
♣ Genes located near each other on the same chromosome tend to be inherited together

Answer 12

♣ Concept 14.2: Probability laws govern Mendelian inheritance
♣ Mendel’s laws of segregation and independent assortment reflect the rules of probability
♣ When tossing a coin, the outcome of one toss has no impact on the outcome of the next toss
♣ In the same way, the alleles of one gene segregate into gametes independently of another gene’s alleles
The Multiplication and Addition Rules Applied to Monohybrid Crosses

Question 13

♣ The multiplication rule states that the probability that two or more independent events will occur together is the product of their individual probabilities
♣ Probability in an F1 monohybrid cross can be determined using the multiplication rule
♣ Segregation in a heterozygous plant is like flipping a coin: Each gamete has a ½ chance of carrying the dominant allele and a ½ chance of carrying the recessive allele
♣ Figure 14.9

Answer 13

♣ The addition rule states that the probability that any one of two or more exclusive events will occur is calculated by adding together their individual probabilities
♣ The rule of addition can be used to figure out the probability that an F2 plant from a monohybrid cross will be heterozygous rather than homozygous
♣ Solving Complex Genetics Problems with the Rules of Probability

Question 14

♣ Complete dominance occurs when phenotypes of the heterozygote and dominant homozygote are identical

Answer 14

♣ In incomplete dominance, the phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
♣ In codominance, two dominant alleles affect the phenotype in separate, distinguishable ways

Question 15

The Relation Between Dominance and Phenotype

♣ A dominant allele does not subdue a recessive allele; alleles don’t interact that way

Answer 15

♣ Alleles are simply variations in a gene’s nucleotide sequence
♣ For any character, dominance/recessiveness relationships of alleles depend on the level at
which we examine the phenotype

Question 16

♣ Tay-Sachs disease is fatal; a dysfunctional enzyme causes an accumulation of lipids in
the brain

Answer 16

At the organismal level, the allele is recessive
♣ At the biochemical level, the phenotype (i.e., the enzyme activity level) is incompletely dominant
♣ At the molecular level, the alleles are codominant

Question 17

♣ Dominant alleles are not necessarily more common in populations than recessive alleles
♣ For example, one baby out of 400 in the United States is born with extra fingers or toes
♣
♣ The allele for this unusual trait is dominant to the allele for the more common trait of five digits per appendage
♣ In this example, the recessive allele is far more prevalent than the population’s dominant allele
♣ Multiple Alleles

Answer 17

♣ Most genes exist in populations in more than two allelic forms
♣ For example, the four phenotypes of the ABO blood group in humans are determined by three alleles for the enzyme (I) that attaches A or B carbohydrates to red blood cells: IA, IB, and i.
♣ The enzyme encoded by the IA allele adds the A carbohydrate, whereas the enzyme encoded by the IB allele adds the B carbohydrate; the enzyme encoded by the i allele adds neither

Question 18

♣ Pleiotropy
♣ Most genes have multiple phenotypic effects, a property called pleiotropy
♣ For example, pleiotropic alleles are responsible for the multiple symptoms of certain hereditary diseases, such as cystic fibrosis and sickle-cell disease
♣ Extending Mendelian Genetics for Two or More Genes
♣ Some traits may be determined by two or more genes
♣ Epistasis
♣ In epistasis, a gene at one locus alters the phenotypic expression of a gene at a second locus

Answer 18

♣ For example, in Labrador retrievers and many
other mammals, coat color depends on two genes
♣ One gene determines the pigment color (with alleles B for black and b for brown)
♣ The other gene (with alleles E for color and e for
no color) determines whether the pigment will be deposited in the hair
♣ Figure 14.12
♣ Polygenic Inheritance
♣ Quantitative characters are those that vary in the population along a continuum
♣ Quantitative variation usually indicates polygenic inheritance, an additive effect of two or more genes on a single phenotype
♣ Skin color in humans is an example of polygenic inheritance

Question 19

♣ Nature and Nurture: The Environmental Impact on Phenotype
♣ Another departure from Mendelian genetics arises when the phenotype for a character depends on environment as well as genotype

Answer 19

♣ The phenotypic range is broadest for polygenic characters

♣ Traits that depend on multiple genes combined with environmental influences are called multifactorial

Question 20

♣ A Mendelian View of Heredity and Variation
♣ An organism’s phenotype includes its physical appearance, internal anatomy, physiology, and behavior
♣ An organism’s phenotype reflects its overall genotype and unique environmental history
♣ Concept 14.4: Many human traits follow Mendelian patterns of inheritance

Answer 20

♣ Humans are not good subjects for genetic research
♣ Generation time is too long
♣ Parents produce relatively few offspring
♣ Breeding experiments are unacceptable
♣ However, basic Mendelian genetics endures as the foundation of human genetics
♣ Pedigree Analysis
♣ A pedigree is a family tree that describes the interrelationships of parents and children across generations
♣ Inheritance patterns of particular traits can be traced and described using pedigrees

Question 21

♣ Pedigrees can also be used to make predictions about future offspring
♣ We can use the multiplication and addition rules to predict the probability of specific phenotypes
♣ Recessively Inherited Disorders
♣ Many genetic disorders are inherited in a recessive manner
♣ These range from relatively mild to life-threatening
♣ The Behavior of Recessive Alleles
♣ Recessively inherited disorders show up only in individuals homozygous for the allele
♣ Carriers are heterozygous individuals who carry the recessive allele but are phenotypically normal; most individuals with recessive disorders are born to carrier parents
♣ Albinism is a recessive condition characterized by a lack of pigmentation in skin and hair

Answer 21

♣ If a recessive allele that causes a disease is rare, then the chance of two carriers meeting and mating is low
♣ Consanguineous matings (i.e., matings between close relatives) increase the chance of mating between two carriers of the same rare allele
♣ Most societies and cultures have laws or taboos against marriages between close relatives
♣ Cystic Fibrosis
♣ Cystic fibrosis is the most common lethal genetic disease in the United States, striking one out of every 2,500 people of European descent

Question 22

♣ The cystic fibrosis allele results in defective or absent chloride transport channels in plasma membranes leading to a buildup of chloride ions outside the cell
♣ Symptoms include mucus buildup in some internal organs and abnormal absorption of nutrients in the small intestine
♣ Dominantly Inherited Disorders
♣ Some human disorders are caused by dominant alleles

Answer 22

♣ Dominant alleles that cause a lethal disease are rare and arise by mutation
♣ Achondroplasia is a form of dwarfism caused by a rare dominant allele
♣ Figure 14.18
♣
♣ The timing of onset of a disease significantly affects its inheritance
♣ Huntington’s disease is a degenerative disease of the nervous system
♣ The disease has no obvious phenotypic effects until the individual is about 35 to 40 years of age
♣ Once the deterioration of the nervous system begins the condition is irreversible and fatal

Question 23

♣ Multifactorial Disorders
♣ Many diseases, such as heart disease, diabetes, alcoholism, mental illnesses, and cancer have both genetic and environmental components
♣ No matter what our genotype, our lifestyle has a tremendous effect on phenotype
♣ Genetic Testing and Counseling
♣ Genetic counselors can provide information to prospective parents concerned about a family history for a specific disease

♣ In amniocentesis, the liquid that bathes the fetus is removed and tested
♣ In chorionic villus sampling (CVS), a sample of the placenta is removed and tested
♣ Newborn Screening

Answer 23

♣ Counseling Based on Mendelian Genetics and Probability Rules
♣ Using family histories, genetic counselors help couples determine the odds that their children
will have genetic disorders
♣ It is important to remember that each child represents an independent event in the sense that its genotype is unaffected by the genotypes of older siblings
♣ Tests for Identifying Carriers
♣ For a growing number of diseases, tests are available that identify carriers and help define the odds more accurately
♣ The tests enable people to make more informed decisions about having children
♣ However, they raise other issues, such as whether affected individuals fully understand their genetic test results
♣ Fetal Testing

Question 24

♣ Some genetic disorders can be detected at birth by simple tests that are now routinely performed in most hospitals in the United States

Answer 24

♣ One common test is for phenylketonuria (PKU), a recessively inherited disorder that occurs in one of every 10,000–15,000 births in the United States