2. Mendel's Laws of Inheritance

Probability and Genetics

Open Question
A 'wrongful birth' case was recently brought before a court in which a child with Smith–Lemli–Opitz syndrome was born to apparently healthy parents. This syndrome is characterized by a cluster of birth defects including cleft palate, and an array of problems with the reproductive and urinary organs. Originally considered by their physician as having a nongenetic basis, the parents decided to have another child, who was also born with Smith–Lemli–Opitz syndrome. In the role of a genetic counselor, instruct the court about what occurred, including the probability of the parents having two affected offspring, knowing that the disorder is inherited as a recessive trait.What is the probability that one of them is a carrier and the other is not? [Hint: The p values in (b), (c), and (d) should equal 1.]
Open Question
A 'wrongful birth' case was recently brought before a court in which a child with Smith–Lemli–Opitz syndrome was born to apparently healthy parents. This syndrome is characterized by a cluster of birth defects including cleft palate, and an array of problems with the reproductive and urinary organs. Originally considered by their physician as having a nongenetic basis, the parents decided to have another child, who was also born with Smith–Lemli–Opitz syndrome. In the role of a genetic counselor, instruct the court about what occurred, including the probability of the parents having two affected offspring, knowing that the disorder is inherited as a recessive trait.What is the probability that neither of them is a carrier?
Open Question
A 'wrongful birth' case was recently brought before a court in which a child with Smith–Lemli–Opitz syndrome was born to apparently healthy parents. This syndrome is characterized by a cluster of birth defects including cleft palate, and an array of problems with the reproductive and urinary organs. Originally considered by their physician as having a nongenetic basis, the parents decided to have another child, who was also born with Smith–Lemli–Opitz syndrome. In the role of a genetic counselor, instruct the court about what occurred, including the probability of the parents having two affected offspring, knowing that the disorder is inherited as a recessive trait.Calculate the probability that both the male and female are carriers for TSD.
Open Question
Datura stramonium (the Jimsonweed) expresses flower colors of purple and white and pod textures of smooth and spiny. The results of two crosses in which the parents were not necessarily true breeding are shown below.Assuming that true-breeding strains of all combinations of traits are available, what single cross could you execute and carry to an F₂ generation that will prove or disprove your hypothesis? Assuming your hypothesis is correct, what results of this cross will support it?
Open Question
Datura stramonium (the Jimsonweed) expresses flower colors of purple and white and pod textures of smooth and spiny. The results of two crosses in which the parents were not necessarily true breeding are shown below.Based on these results, put forward a hypothesis for the inheritance of the purple/white and smooth/spiny traits.
Open Question
In pea plants, plant height, seed shape, and seed color are governed by three independently assorting genes. The three genes have dominant and recessive alleles, with tall (T) dominant to short (t), round (R) dominant to wrinkled (r), and yellow (G) dominant to green (g).

What proportion of the that produce round, green seeds (regardless of the height of the plant) are expected to breed true?
Open Question
In pea plants, plant height, seed shape, and seed color are governed by three independently assorting genes. The three genes have dominant and recessive alleles, with tall (T) dominant to short (t), round (R) dominant to wrinkled (r), and yellow (G) dominant to green (g).

What proportion of the F₂ are expected to be tall, wrinkled, yellow? ttRRGg?
Open Question
In pea plants, plant height, seed shape, and seed color are governed by three independently assorting genes. The three genes have dominant and recessive alleles, with tall (T) dominant to short (t), round (R) dominant to wrinkled (r), and yellow (G) dominant to green (g).

If a true-breeding tall, wrinkled, yellow plant is crossed to a true-breeding short, round, green plant, what phenotypic ratios are expected in the F₁ and F₂?
Open Question
In an 1889 book titled Natural Inheritance (Macmillan, New York), Francis Galton, who investigated the inheritance of measurable (quantitative) traits, formulated a law of 'ancestral inheritance.' The law stated that individuals inherit approximately one-half of their genetic traits from each parent, about one-quarter of the traits from each grandparent, one-eighth from each great grandparent, and so on. In light of the chromosome theory of heredity, argue either in favor of Galton's law or against it.
Open Question
Albinism, caused by a mutational disruption in melanin (skin pigment) production, has been observed in many species, including humans. In 1991, and again recently in 2017, the only documented observations of an albino humpback whale (named 'Migaloo') were observed near New South Wales. Recently, Polanowski and coworkers (Polanowski, A., S. Robinson-Laverick, and D. Paton. (2012). Journal of Heredity 103:130–133) studied the genetics of humpback whales from the east coast of Australia, including Migaloo.What data would be helpful in determining the answer to part (a)?
Open Question
Albinism, caused by a mutational disruption in melanin (skin pigment) production, has been observed in many species, including humans. In 1991, and again recently in 2017, the only documented observations of an albino humpback whale (named 'Migaloo') were observed near New South Wales. Recently, Polanowski and coworkers (Polanowski, A., S. Robinson-Laverick, and D. Paton. (2012). Journal of Heredity 103:130–133) studied the genetics of humpback whales from the east coast of Australia, including Migaloo.Do you think that Migaloo's albinism is more likely caused by a dominant or recessive mutation? Explain your reasoning.
Open Question
A male and a female are each heterozygous for both cystic fibrosis (CF) and phenylketonuria (PKU). Both conditions are autosomal recessive, and they assort independently.

What proportion of the children of this couple will have neither condition?
Open Question
Albinism, caused by a mutational disruption in melanin (skin pigment) production, has been observed in many species, including humans. In 1991, and again recently in 2017, the only documented observations of an albino humpback whale (named 'Migaloo') were observed near New South Wales. Recently, Polanowski and coworkers (Polanowski, A., S. Robinson-Laverick, and D. Paton. (2012). Journal of Heredity 103:130–133) studied the genetics of humpback whales from the east coast of Australia, including Migaloo.Assuming that Migaloo's albinism is caused by a rare dominant gene, what would be the likelihood of the establishment of a natural robust subpopulation of albino white humpback whales in this population?
Open Question
Albinism, caused by a mutational disruption in melanin (skin pigment) production, has been observed in many species, including humans. In 1991, and again recently in 2017, the only documented observations of an albino humpback whale (named 'Migaloo') were observed near New South Wales. Recently, Polanowski and coworkers (Polanowski, A., S. Robinson-Laverick, and D. Paton. (2012). Journal of Heredity 103:130–133) studied the genetics of humpback whales from the east coast of Australia, including Migaloo.Assuming that Migaloo's albinism is caused by a rare recessive gene, what would be the likelihood of the establishment of a natural robust subpopulation of albino white humpback whales in this population?
Open Question
A woman expressing a dominant phenotype is heterozygous (Dd) for the gene.

What is the probability that two grandchildren of the woman who are first cousins to one another will each inherit the dominant allele?