12: Mendel's Experiments and Heredity

Question 1

What is the blending theory of inheritance?

Answer 1

A hypothetical inheritance pattern in which parental traits are blended together in the offspring to produce an intermediate physical appearance.

Question 2

What is continuous variation?

Answer 2

An inheritance pattern in which a character shows a range of trait values with small gradations rather than large gaps between them.

Question 3

What is discontinuous variation?

Answer 3

An inheritance pattern in which traits are distinct and are transmitted independently of one another.

Question 4

What is a dominant trait?

Answer 4

A trait which confers the same physical appearance whether an individual has two copies of the trait or one copy of the dominant trait and one copy of the recessive trait.

Question 5

What is F₁?

Answer 5

The first filial generation in a cross; the offspring of the parental generation.

Question 6

What is F₂?

Answer 6

The second filial generation produced when F1 individuals are self-crossed or fertilized with each other.

Question 7

What is hybridization?

Answer 7

The process of mating two individuals that differ with the goal of achieving a certain characteristic in their offspring.

Question 8

What is a model system?

Answer 8

A species or biological system used to study a specific biological phenomenon to be applied to other different species.

Question 9

What is P₀?

Answer 9

The parental generation in a cross.

Question 10

What is the product rule?

Answer 10

The probability of two independent events occurring simultaneously can be calculated by multiplying the individual probabilities of each event occurring alone.

Question 11

What is a recessive trait?

Answer 11

A trait that appears “latent” or non-expressed when the individual also carries a dominant trait for that same characteristic; when present as two identical copies, the recessive trait is expressed.

Question 12

What is a reciprocal cross?

Answer 12

A paired cross in which the respective traits of the male and female in one cross become the respective traits of the female and male in the other cross.

Question 13

What is the sum rule?

Answer 13

The probability of the occurrence of at least one of two mutually exclusive events is the sum of their individual probabilities.

Question 14

What is a trait?

Answer 14

A variation in the physical appearance of a heritable characteristic.

Question 15

Who was Mendel?

Answer 15

Johann Gregor Mendel (1822–1884) was a lifelong learner, teacher, scientist, and man of faith. As a young adult, he joined the Augustinian Abbey of St. Thomas in Brno in what is now the Czech Republic. Supported by the monastery, he taught physics, botany, and natural science courses at the secondary and university levels.

Question 16

When did Mendel discover the laws of classical genetics?

Answer 16

In 1856, he began a decade-long research pursuit involving inheritance patterns in honeybees and plants, ultimately settling on pea plants as his primary model system (a system with convenient characteristics used to study a specific biological phenomenon to be applied to other systems). In 1865, Mendel presented the results of his experiments with nearly 30,000 pea plants to the local Natural History Society. He demonstrated that traits are transmitted faithfully from parents to offspring independently of other traits and in dominant and recessive patterns. In 1866, he published his work, Experiments in Plant Hybridization, in the proceedings of the Natural History Society of Brünn.

Question 17

What were the scientific beliefs about inheritance when Mendel presented his results?

Answer 17

Mendel’s work went virtually unnoticed by the scientific community that believed, incorrectly, that the process of inheritance involved a blending of parental traits that produced an intermediate physical appearance in offspring; this hypothetical process appeared to be correct because of what we know now as continuous variation.

Question 18

How did Mendel avoid continuous variation?

Answer 18

Instead of continuous characteristics, Mendel worked with traits that were inherited in distinct classes (specifically, violet versus white flowers); this is referred to as discontinuous variation. Mendel’s choice of these kinds of traits allowed him to see experimentally that the traits were not blended in the offspring, nor were they absorbed, but rather that they kept their distinctness and could be passed on.

Question 19

What was the impact of Mendel’s discoveries?

Answer 19

In 1868, Mendel became abbot of the monastery and exchanged his scientific pursuits for his pastoral duties. He was not recognized for his extraordinary scientific contributions during his lifetime. In fact, it was not until 1900 that his work was rediscovered, reproduced, and revitalized by scientists on the brink of discovering the chromosomal basis of heredity.

Question 20

What model organism did Mendel use and why?

Answer 20

Mendel’s seminal work was accomplished using the garden pea, Pisum sativum, to study inheritance. This species naturally self-fertilizes, such that pollen encounters ova within individual flowers. The flower petals remain sealed tightly until after pollination, preventing pollination from other plants. The result is highly inbred, or “true-breeding,” pea plants. These are plants that always produce offspring that look like the parent. By experimenting with true-breeding pea plants, Mendel avoided the appearance of unexpected traits in offspring that might occur if the plants were not true breeding. The garden pea also grows to maturity within one season, meaning that several generations could be evaluated over a relatively short time. Finally, large quantities of garden peas could be cultivated simultaneously, allowing Mendel to conclude that his results did not come about simply by chance.

Question 21

How did Mendel perform hybridizations?

Answer 21

Mendel performed hybridizations, which involve mating two true-breeding individuals that have different traits. In the pea, which is naturally self-pollinating, this is done by manually transferring pollen from the anther of a mature pea plant of one variety to the stigma of a separate mature pea plant of the second variety. To prevent the pea plant that was receiving pollen from self-fertilizing and confounding his results, Mendel painstakingly removed all of the anthers from the plant’s flowers before they had a chance to mature.

Question 22

Which generations were important in Mendel’s crosses?

Answer 22

Mendel collected the seeds belonging to the P₀ plants that resulted from each cross and grew them the following season. Once Mendel examined the characteristics in the F₁ generation of plants, he allowed them to self-fertilize naturally. He then collected and grew the seeds from the F₁ plants to produce the F₂ generation. Mendel’s experiments extended beyond the F₂ generation to the F₃ and F₄ generations, and so on, but it was the ratio of characteristics in the P₀−F₁−F₂ generations that were the most intriguing and became the basis for Mendel’s postulates.

Question 23

Which garden pea characteristics did Mendel study?

Answer 23

In his 1865 publication, Mendel reported the results of his crosses involving seven different characteristics, each with two contrasting traits. The characteristics included plant height, seed texture, seed color, flower color, pea pod size, pea pod color, and flower position. For the characteristic of flower color, for example, the two contrasting traits were white versus violet.

Question 24

How many plants did Mendel use in his research?

Answer 24

To fully examine each characteristic, Mendel generated large numbers of F₁ and F₂ plants, reporting results from 19,959 F₂ plants alone. His findings were consistent.

Question 25

What were Mendel’s results for self-crossed reproduction on flower color?

Answer 25

First, Mendel confirmed that he had plants that bred true for white or violet flower color. Regardless of how many generations Mendel examined, all self-crossed offspring of parents with white flowers had white flowers, and all self-crossed offspring of parents with violet flowers had violet flowers. In addition, Mendel confirmed that, other than flower color, the pea plants were physically identical.

Question 26

What were Mendel’s results from crossing plants with different flower colors?

Answer 26

Mendel would apply the pollen from a plant with violet flowers to the stigma of a plant with white flowers. After gathering and sowing the seeds that resulted from this cross, Mendel found that 100 percent of the F₁ hybrid generation had violet flowers. Conventional wisdom at that time would have predicted the hybrid flowers to be pale violet or for hybrid plants to have equal numbers of white and violet flowers. In other words, the contrasting parental traits were expected to blend in the offspring. Instead, Mendel’s results demonstrated that the white flower trait in the F₁ generation had completely disappeared.

Question 27

What were Mendel’s results from self-crossing the F₁ plants?

Answer 27

He would allow the F₁ plants to self-fertilize and found that, of F₂-generation plants, 705 had violet flowers and 224 had white flowers. This was a ratio of 3.15 violet flowers per one white flower, or approximately 3:1. When Mendel transferred pollen from a plant with violet flowers to the stigma of a plant with white flowers and vice versa, he obtained about the same ratio regardless of which parent, male or female, contributed which trait.

Question 28

What were Mendel’s results for garden pea characteristics other than flower color?

Answer 28

For the other six characteristics Mendel examined, the F₁ and F₂ generations behaved in the same way as they had for flower color. One of the two traits would disappear completely from the F₁ generation only to reappear in the F₂ generation at a ratio of approximately 3:1.

Question 29

What were Mendel’s conclusions?

Answer 29

Upon compiling his results for many thousands of plants, Mendel concluded that the characteristics could be divided into expressed and latent traits. He called these, respectively, dominant and recessive traits. Mendel also proposed that plants possessed two copies of the trait for the flower-color characteristic, and that each parent transmitted one of its two copies to its offspring, where they came together. Moreover, the physical observation of a dominant trait could mean that the genetic composition of the organism included two dominant versions of the characteristic or that it included one dominant and one recessive version. Conversely, the observation of a recessive trait meant that the organism lacked any dominant versions of this characteristic.

Question 30

How did Mendel use probability in his research?

Answer 30

In his experiment, Mendel demonstrated that the probability of the event “round seed” occurring was 100 percent in the F₁ offspring of true-breeding parents, one of which has round seeds and one of which has wrinkled seeds. When the F₁ plants were subsequently self-crossed, the probability of any given F₂ offspring having round seeds was now 75 percent. Using large numbers of crosses, Mendel was able to calculate probabilities and use these to predict the outcomes of other crosses.

Question 31

What relationships between traits did Mendel discover in his research?

Answer 31

Mendel demonstrated that the pea-plant characteristics he studied were transmitted as discrete units from parent to offspring. Mendel also determined that different characteristics, like seed color and seed texture, were transmitted independently of one another and could be considered in separate probability analyses. The characteristics of color and texture did not influence each other.

Question 32

How can rolling a die and flipping a coin be used to illustrate the product rule?

Answer 32

Imagine that you are rolling a six-sided die (D) and flipping a penny (P) at the same time. The die may roll any number from 1–6 (D#), whereas the penny may turn up heads (PH) or tails (PT). The outcome of rolling the die has no effect on the outcome of flipping the penny and vice versa. There are 12 possible outcomes of this action, and each event is expected to occur with equal probability.

Of the 12 possible outcomes, the die has a 2/12 (or 1/6) probability of rolling a two, and the penny has a 6/12 (or 1/2) probability of coming up heads. By the product rule, the probability that you will obtain the combined outcome 2 and heads is: (D2) x (PH) = (1/6) x (1/2) or 1/12 (table above). Notice the word “and” in the description of the probability. The “and” is a signal to apply the product rule.

Question 33

How can flipping coins be used to illustrate the sum rule?

Answer 33

Imagine you are flipping a penny (P) and a quarter (Q). What is the probability of one coin coming up heads and one coin coming up tails? This outcome can be achieved by two cases: the penny may be heads (PH) and the quarter may be tails (QT), or the quarter may be heads (QH) and the penny may be tails (PT). Either case fulfills the outcome. By the sum rule, the probability of obtaining one head and one tail can be calculated as

[(PH)×(QT)] + [(QH)×(PT)] = [(1/2)×(1/2)] + [(1/2)×(1/2)] = 1/2

Question 34

What is an allele?

Answer 34

A gene variations that arise by mutation and exist at the same relative locations on homologous chromosomes.

Question 35

What are autosomes?

Answer 35

Any of the non-sex chromosomes.

Question 36

What is codominance?

Answer 36

In a heterozygote, complete and simultaneous expression of both alleles for the same characteristic.

Question 37

What is a dominant lethal inheritance pattern?

Answer 37

Inheritance pattern in which an allele is lethal both in the homozygote and the heterozygote; this allele can only be transmitted if the lethality phenotype occurs after reproductive age.

Question 38

What is a genotype?

Answer 38

The underlying genetic makeup, consisting of both physically visible and non-expressed alleles, of an organism.

Question 39

What does it mean to be hemizygous?

Answer 39

Presence of only one allele for a characteristic, as in X-linkage; hemizygosity makes descriptions of dominance and recessiveness irrelevant.

Question 40

What does it mean to be heterozygous?

Answer 40

Having two different alleles for a given gene on the homologous chromosome.

Question 41

What does it mean to be homozygous?

Answer 41

Having two identical alleles for a given gene on the homologous chromosome.

Question 42

What is incomplete dominance?

Answer 42

In a heterozygote, expression of two contrasting alleles such that the individual displays an intermediate phenotype.

Question 43

What is a monohybrid?

Answer 43

The result of a cross between two true-breeding parents that express different traits for only one characteristic.

Question 44

What is a phenotype?

Answer 44

Observable traits expressed by an organism.

Question 45

What is a Punnett square?

Answer 45

A visual representation of a cross between two individuals in which the gametes of each individual are denoted along the top and side of a grid, respectively, and the possible zygotic genotypes are recombined at each box in the grid.

Question 46

What is a recessive lethal inheritance pattern?

Answer 46

An inheritance pattern in which an allele is only lethal in the homozygous form; the heterozygote may be normal or have some altered, non-lethal phenotype.

Question 47

What does it mean to be sex-linked?

Answer 47

Any gene on a sex chromosome.

Question 48

What is a test cross?

Answer 48

A cross between a dominant expressing individual with an unknown genotype and a homozygous recessive individual; the offspring phenotypes indicate whether the unknown parent is heterozygous or homozygous for the dominant trait.

Question 49

What does it mean to be X-linked?

Answer 49

Gene present on the X, but not the Y, chromosome.

Question 50

How did Mendel’s experiments demonstrate the difference between phenotype and genotype?

Answer 50

When true-breeding plants in which one parent had yellow pods and one had green pods were cross-fertilized, all of the F₁ hybrid offspring had yellow pods. That is, the hybrid offspring were phenotypically identical to the true-breeding parent with yellow pods. However, the allele donated by the parent with green pods was not simply lost because it reappeared in some of the F₂ offspring. Therefore, the F₁ plants must have been genotypically different from the parent with yellow pods.

Question 51

Why did Mendel use homozygous parent plants?

Answer 51

The P₁ plants that Mendel used in his experiments were each homozygous for the trait he was studying. Mendel’s parental pea plants always bred true because both of the gametes produced carried the same trait. When P₁ plants with contrasting traits were cross-fertilized, all of the offspring were heterozygous for the contrasting trait, meaning that their genotype reflected that they had different alleles for the gene being examined.

Question 52

How are dominant and recessive alleles expressed?

Answer 52

For a gene that is expressed in a dominant and recessive pattern, homozygous dominant and heterozygous organisms will look identical (that is, they will have different genotypes but the same phenotype). The recessive allele will only be observed in homozygous recessive individuals.

Question 53

Who created the Punnett square?

Answer 53

A Punnett square was devised by the British geneticist Reginald Punnett.

Question 54

Why are Punnett squares useful?

Answer 54

If each possibility is equally likely, genotypic ratios can be determined from a Punnett square. If the pattern of inheritance (dominant or recessive) is known, the phenotypic ratios can be inferred as well.

Question 55

What is used as an alternative to test crosses for identifying recessive genetic disorders in humans?

Answer 55

Geneticists use pedigree analysis to study the inheritance pattern of human genetic diseases.

Question 56

What were the fundamental principles that Mendel discovered?

Answer 56

Mendel’s experiments with pea plants suggested that: (1) two “units” or alleles exist for every gene; (2) alleles maintain their integrity in each generation (no blending); and (3) in the presence of the dominant allele, the recessive allele is hidden and makes no contribution to the phenotype. Therefore, recessive alleles can be “carried” and not expressed by individuals. Such heterozygous individuals are sometimes referred to as “carriers.”

Question 57

What is an example of incomplete dominance?

Answer 57

In the snapdragon, Antirrhinum majus, a cross between a homozygous parent with white flowers (C^WC^W) and a homozygous parent with red flowers (C^RC^R) will produce offspring with pink flowers (C^RC^W).

Question 58

What is an example of codominance?

Answer 58

An example of codominance is the MN blood groups of humans. The M and N alleles are expressed in the form of an M or N antigen present on the surface of red blood cells. Homozygotes (L^ML^M and L^NL^N) express either the M or the N allele, and heterozygotes (L^ML^N) express both alleles equally.

Question 59

How do multiple alleles interact within a population?

Answer 59

Although individual humans (and all diploid organisms) can only have two alleles for a given gene, multiple alleles may exist at the population level such that many combinations of two alleles are observed. When many alleles exist for the same gene, the convention is to denote the most common phenotype or genotype among wild animals as the wild type (often abbreviated “+”). All other phenotypes or genotypes are considered variants of this standard. The variant may be recessive or dominant to the wild-type allele.

Question 60

What is an example of multiple alleles?

Answer 60

An example of multiple alleles is coat color in rabbits. Here, four alleles exist for the c gene. The wild-type version, C⁺C⁺, is expressed as brown fur. The chinchilla phenotype, c^chc^ch, is expressed as black-tipped white fur. The Himalayan phenotype, c^hc^h, has black fur on the extremities and white fur elsewhere. Finally, the albino, or “colorless” phenotype, cc, is expressed as white fur. In cases of multiple alleles, dominance hierarchies can exist. In this case, the wild-type allele is dominant over all the others, chinchilla is incompletely dominant over Himalayan and albino, and Himalayan is dominant over albino. This hierarchy, or allelic series, was revealed by observing the phenotypes of each possible heterozygote offspring.

Question 61

Why do wild types prevail over other phenotypes?

Answer 61

The complete dominance of a wild-type phenotype over all other mutants often occurs as an effect of “dosage” of a specific gene product, such that the wild-type allele supplies the correct amount of gene product whereas the mutant alleles cannot.

Question 62

Why is the wild type allele in rabbit coats dominant?

Answer 62

For the allelic series in rabbits, the wild-type allele may supply a given dosage of fur pigment, whereas the mutants supply a lesser dosage or none at all. Interestingly, the Himalayan phenotype is the result of an allele that produces a temperature-sensitive gene product that only produces pigment in the cooler extremities of the rabbit’s body.

Question 63

Under what circumstances might a mutant allele be dominant over the wild type?

Answer 63

Alternatively, one mutant allele can be dominant over all other phenotypes, including the wild type. This may occur when the mutant allele somehow interferes with the genetic message so that even a heterozygote with one wild-type allele copy expresses the mutant phenotype. One way in which the mutant allele can interfere is by enhancing the function of the wild-type gene product or changing its distribution in the body.

Question 64

What is an example of a dominant mutant allele?

Answer 64

One example of this is the Antennapedia mutation in Drosophila. In this case, the mutant allele expands the distribution of the gene product, and as a result, the Antennapedia heterozygote develops legs on its head where its antennae should be.

Question 65

What is malaria?

Answer 65

Malaria is a parasitic disease in humans that is transmitted by infected female mosquitoes, including Anopheles gambiae, and is characterized by cyclic high fevers, chills, flu-like symptoms, and severe anemia.

Question 66

What are the most common causes of malaria?

Answer 66

Plasmodium falciparum and P. vivax are the most common causative agents of malaria, and P. falciparum is the most deadly.

Question 67

How effective is the treatment of malaria?

Answer 67

When promptly and correctly treated, P. falciparum malaria has a mortality rate of 0.1 percent. However, in some parts of the world, the parasite has evolved resistance to commonly used malaria treatments, so the most effective malarial treatments can vary by geographic region.

Question 68

Which drugs has P. falciparum developed resistance to?

Answer 68

In Southeast Asia, Africa, and South America, P. falciparum has developed resistance to the anti-malarial drugs chloroquine, mefloquine, and sulfadoxine-pyrimethamine.

Question 69

Which gene is responsible for drug resistance in P. falciparum?

Answer 69

P. falciparum, which is haploid during the life stage in which it is infectious to humans, has evolved multiple drug-resistant mutant alleles of the dhps gene. Varying degrees of sulfadoxine resistance are associated with each of these alleles. Being haploid, P. falciparum needs only one drug-resistant allele to express this trait.

Question 70

What is the impact of sulfadoxine-resistance in P. falciparum?

Answer 70

In Southeast Asia, different sulfadoxine-resistant alleles of the dhps gene are localized to different geographic regions. This is a common evolutionary phenomenon that occurs because drug-resistant mutants arise in a population and interbreed with other P. falciparum isolates in close proximity. Sulfadoxine-resistant parasites cause considerable human hardship in regions where this drug is widely used as an over-the-counter malaria remedy.

Question 71

How can anti-malarial drug resistance be overcome?

Answer 71

As is common with pathogens that multiply to large numbers within an infection cycle, P. falciparum evolves relatively rapidly (over a decade or so) in response to the selective pressure of commonly used anti-malarial drugs. For this reason, scientists must constantly work to develop new drugs or drug combinations to combat the worldwide malaria burden.

Question 72

What are the differences between the sex chromosomes in humans?

Answer 72

In addition to 22 homologous pairs of autosomes, human females have a homologous pair of X chromosomes, whereas human males have an XY chromosome pair. Although the Y chromosome contains a small region of similarity to the X chromosome so that they can pair during meiosis, the Y chromosome is much shorter and contains many fewer genes.

Question 73

What is an example of an X-linked trait?

Answer 73

Eye color in Drosophila was one of the first X-linked traits to be identified. Thomas Hunt Morgan mapped this trait to the X chromosome in 1910. Like humans, Drosophila males have an XY chromosome pair, and females are XX. In flies, the wild-type eye color is red (X^W) and it is dominant to white eye color (X^w). Because of the location of the eye-color gene, reciprocal crosses do not produce the same offspring ratios. Males are said to be hemizygous. Drosophila males lack a second allele copy on the Y chromosome; that is, their genotype can only be X^WY or X^wY. In contrast, females have two allele copies of this gene and can be X^WX^W, X^WX^w, or X^wX^w.

In an X-linked cross, the genotypes of F₁ and F₂ offspring depend on whether the recessive trait was expressed by the male or the female in the P₁ generation. With regard to Drosophila eye color, when the P₁ male expresses the white-eye phenotype and the female is homozygous red-eyed, all members of the F₁ generation exhibit red eyes. The F₁ females are heterozygous (X^WX^w), and the males are all X^WY, having received their X chromosome from the homozygous dominant P₁ female and their Y chromosome from the P₁ male. A subsequent cross between the X^WX^w female and the X^WY male would produce only red-eyed females (with X^WX^W or X^WX^w genotypes) and both red- and white-eyed males (with X^WY or X^wY genotypes). Now, consider a cross between a homozygous white-eyed female and a male with red eyes. The F₁ generation would exhibit only heterozygous red-eyed females (X^WX^w) and only white-eyed males (X^wY). Half of the F₂ females would be red-eyed (X^WX^w) and half would be white-eyed (X^wX^w). Similarly, half of the F₂ males would be red-eyed (X^WY) and half would be white-eyed (X^wY).

Question 74

Why are recessive X-linked disorders disproportionately represented in human males?

Answer 74

Because human males need to inherit only one recessive mutant X allele to be affected, X-linked disorders are disproportionately observed in males. Females must inherit recessive X-linked alleles from both of their parents in order to express the trait. When they inherit one recessive X-linked mutant allele and one dominant X-linked wild-type allele, they are carriers of the trait and are typically unaffected. Carrier females can manifest mild forms of the trait due to the inactivation of the dominant allele located on one of the X chromosomes. However, female carriers can contribute the trait to their sons, resulting in the son exhibiting the trait, or they can contribute the recessive allele to their daughters, resulting in the daughters being carriers of the trait.

Question 75

Are the non-homologous sex chromosomes always found in males?

Answer 75

In some groups of organisms with sex chromosomes, the gender with the non-homologous sex chromosomes is the female rather than the male. This is the case for all birds. In this case, sex-linked traits will be more likely to appear in the female, in which they are hemizygous.

Question 76

What are some examples of X-linked disorders in humans?

Answer 76

Red-green color blindness, types A and B hemophilia, muscular dystrophy

Question 77

How prevalent are Y-linked disorders?

Answer 77

Although some Y-linked recessive disorders exist, typically they are associated with infertility in males and are therefore not transmitted to subsequent generations.

Question 78

How are recessive lethal inheritance patterns expressed in a population?

Answer 78

A large proportion of genes in an individual’s genome are essential for survival. Occasionally, a nonfunctional allele for an essential gene can arise by mutation and be transmitted in a population as long as individuals with this allele also have a wild-type, functional copy. The wild-type allele functions at a capacity sufficient to sustain life and is therefore considered to be dominant over the nonfunctional allele. However, consider two heterozygous parents that have a genotype of wild-type/nonfunctional mutant for a hypothetical essential gene. In one quarter of their offspring, we would expect to observe individuals that are homozygous recessive for the nonfunctional allele. Because the gene is essential, these individuals might fail to develop past fertilization, die in utero, or die later in life, depending on what life stage requires this gene.

Question 79

Are recessive lethal alleles observable in the phenotype?

Answer 79

For crosses between heterozygous individuals with a recessive lethal allele that causes death before birth when homozygous, only wild-type homozygotes and heterozygotes would be observed. The genotypic ratio would therefore be 2:1. In other instances, the recessive lethal allele might also exhibit a dominant (but not lethal) phenotype in the heterozygote.

Question 80

What is an example of a recessive lethal allele that can be observed in the phenotype?

Answer 80

The recessive lethal Curly allele in Drosophila affects wing shape in the heterozygote form but is lethal in the homozygote.

Question 81

How are dominant lethal inheritance patterns expressed in a population?

Answer 81

Individuals with mutations that result in dominant lethal alleles fail to survive even in the heterozygote form. Dominant lethal alleles are very rare because, as you might expect, the allele only lasts one generation and is not transmitted. However, just as the recessive lethal allele might not immediately manifest the phenotype of death, dominant lethal alleles also might not be expressed until adulthood. Once the individual reaches reproductive age, the allele may be unknowingly passed on, resulting in a delayed death in both generations.

Question 82

What is an example of a dominant lethal allele?

Answer 82

An example of this in humans is Huntington’s disease, in which the nervous system gradually wastes away. People who are heterozygous for the dominant Huntington allele (Hh) will inevitably develop the fatal disease. However, the onset of Huntington’s disease may not occur until age 40, at which point the afflicted persons may have already passed the allele to 50 percent of their offspring.

Question 83

What is a dihybrid?

Answer 83

The result of a cross between two true-breeding parents that express different traits for two characteristics.

Question 84

What is epistasis?

Answer 84

An antagonistic interaction between genes such that one gene masks or interferes with the expression of another.

Question 85

What is the law of dominance?

Answer 85

In a heterozygote, one trait will conceal the presence of another trait for the same characteristic.

Question 86

What is the law of independent assortment?

Answer 86

Genes do not influence each other with regard to sorting of alleles into gametes; every possible combination of alleles is equally likely to occur.

Question 87

What is the law of segregation?

Answer 87

Paired unit factors (i.e. genes) segregate equally into gametes such that offspring have an equal likelihood of inheriting any combination of factors.

Question 88

What is linkage?

Answer 88

A phenomenon in which alleles that are located in close proximity to each other on the same chromosome are more likely to be inherited together.

Question 89

What is the forked-line method?

Answer 89

A diagrammatic alternative to Punnett Squares that allows the calculation of the probabilities of the outcomes of multi-hybrid crosses that is more concise. It consists of a tree for each allele of a depth to the number of genes under consideration. The ratio for each allele in isolation is multiplied to achieve the ratios of the combinations of genes.

Question 90

What is an example in humans of multiple genes influencing a trait?

Answer 90

At least eight genes contribute to eye color in humans.

Question 91

How can multiple genes affect a phenotype?

Answer 91

In some cases, several genes can contribute to aspects of a common phenotype without their gene products ever directly interacting. In the case of organ development, for instance, genes may be expressed sequentially, with each gene adding to the complexity and specificity of the organ. Genes may function in complementary or synergistic fashions, such that two or more genes need to be expressed simultaneously to affect a phenotype. Genes may also oppose each other, with one gene modifying the expression of another.

Question 92

What does “epistasis” mean?

Answer 92

“Epistasis” is a word composed of Greek roots that mean “standing upon.”

Question 93

What is the term for an allele that is masked during epistasis?

Answer 93

The alleles that are being masked or silenced are said to be hypostatic to the epistatic alleles that are doing the masking.

Question 94

What is the cause of epistasis?

Answer 94

Often the biochemical basis of epistasis is a gene pathway in which the expression of one gene is dependent on the function of a gene that precedes or follows it in the pathway.

Question 95

What is an example of epistasis?

Answer 95

An example of epistasis is pigmentation in mice. The wild-type coat color, agouti (AA), is dominant to solid-colored fur (aa). However, a separate gene (C) is necessary for pigment production. A mouse with a recessive c allele at this locus is unable to produce pigment and is albino regardless of the allele present at locus A. Therefore, the genotypes AAcc, Aacc, and aacc all produce the same albino phenotype. A cross between heterozygotes for both genes (AaCc x AaCc) would generate offspring with a phenotypic ratio of 9 agouti:3 solid color:4 albino. In this case, the C gene is epistatic to the A gene.

Question 96

What is an example of epistasis occurring by a dominant allele that masks expression at a separate gene?

Answer 96

Fruit color in summer squash is expressed in this way. Homozygous recessive expression of the W gene (ww) coupled with homozygous dominant or heterozygous expression of the Y gene (YY or Yy) generates yellow fruit, and the wwyy genotype produces green fruit. However, if a dominant copy of the W gene is present in the homozygous or heterozygous form, the summer squash will produce white fruit regardless of the Y alleles. A cross between white heterozygotes for both genes (WwYy × WwYy) would produce offspring with a phenotypic ratio of 12 white:3 yellow:1 green.

Question 97

What is an example of epistasis occurring when either of two separate genes are dominant?

Answer 97

In the shepherd’s purse plant (Capsella bursa-pastoris), the characteristic of seed shape is controlled by two genes in a dominant epistatic relationship. When the genes A and B are both homozygous recessive (aabb), the seeds are ovoid. If the dominant allele for either of these genes is present, the result is triangular seeds. That is, every possible genotype other than aabb results in triangular seeds, and a cross between heterozygotes for both genes (AaBb x AaBb) would yield offspring with a phenotypic ratio of 15 triangular:1 ovoid.