2. Mendel's Laws of Inheritance

Probability and Genetics

Open Question
Labrador retrievers may be black, brown (chocolate), or golden (yellow) in color (see chapter-opening photo). While each color may breed true, many different outcomes are seen when numerous litters are examined from a variety of matings where the parents are not necessarily true breeding. Following are just some of the many possibilities.(a) black x brown → all black(b) black x brown → 1/2 black1/2 brown(c) black x brown → 3/4 black1/4 golden(d) black x golden → all black(e) black x golden → 4/8 golden3/8 black1/8 brown(f) black x golden → 2/4 golden1/4 black1/4 brown(g) brown x brown → 3/4 brown1/4 golden(h) black x black → 9/16 black4/16 golden3/16 brownPropose a mode of inheritance that is consistent with these data, and indicate the corresponding genotypes of the parents in each mating. Indicate as well the genotypes of dogs that breed true for each color.
Open Question
The wild-type allele of a gene has an A–T base pair at a particular location in its sequence, and a mutant allele of the same gene has a G–C base pair at the same location. Otherwise, the sequences of the two alleles are identical. Does this information tell you anything about the dominance relationship of the alleles? Explain why or why not.
Open Question
Galactosemia is an autosomal recessive disorder caused by the inability to metabolize galactose, a component of the lactose found in mammalian milk. Galactosemia can be partially managed by eliminating dietary intake of lactose and galactose. Amanda is healthy, as are her parents, but her brother Alonzo has galactosemia. Brice has a similar family history. He and his parents are healthy, but his sister Brianna has galactosemia. Amanda and Brice are planning a family and seek genetic counseling. Based on the information provided, complete the following activities and answer the questions.

If the first child has galactosemia, what is the probability that the second child will have galactosemia? Explain the reasoning for your answer.
Open Question
Galactosemia is an autosomal recessive disorder caused by the inability to metabolize galactose, a component of the lactose found in mammalian milk. Galactosemia can be partially managed by eliminating dietary intake of lactose and galactose. Amanda is healthy, as are her parents, but her brother Alonzo has galactosemia. Brice has a similar family history. He and his parents are healthy, but his sister Brianna has galactosemia. Amanda and Brice are planning a family and seek genetic counseling. Based on the information provided, complete the following activities and answer the questions.

What is the probability that the first child of Amanda and Brice will have galactosemia? Show your work.
Open Question
Galactosemia is an autosomal recessive disorder caused by the inability to metabolize galactose, a component of the lactose found in mammalian milk. Galactosemia can be partially managed by eliminating dietary intake of lactose and galactose. Amanda is healthy, as are her parents, but her brother Alonzo has galactosemia. Brice has a similar family history. He and his parents are healthy, but his sister Brianna has galactosemia. Amanda and Brice are planning a family and seek genetic counseling. Based on the information provided, complete the following activities and answer the questions.

What is the probability that Amanda is a carrier of the allele for galactosemia? What is the probability that Brice is a carrier? Explain your reasoning for each answer.
Open Question
A true-breeding purple-leafed plant isolated from one side of El Yunque, the rain forest in Puerto Rico, was crossed to a true-breeding white variety found on the other side. The F₁ offspring were all purple. A large number of F₁ x F₁ crosses produced the following results:purple: 4219 white: 5781 (Total = 10,000)Propose an explanation for the inheritance of leaf color. As a geneticist, how might you go about testing your hypothesis? Describe the genetic experiments that you would conduct.
Open Question
In Dexter and Kerry cattle, animals may be polled (hornless) or horned. The Dexter animals have short legs, whereas the Kerry animals have long legs. When many offspring were obtained from matings between polled Kerrys and horned Dexters, half were found to be polled Dexters and half polled Kerrys. When these two types of F₁ cattle were mated to one another, the following F₂ data were obtained:3/8 polled Dexters3/8 polled Kerrys1/8 horned Dexters1/8 horned KerrysA geneticist was puzzled by these data and interviewed farmers who had bred these cattle for decades. She learned that Kerrys were true breeding. Dexters, on the other hand, were not true breeding and never produced as many offspring as Kerrys. Provide a genetic explanation for these observations.
Open Question
A geneticist from an alien planet that prohibits genetic research brought with him to Earth two pure-breeding lines of frogs. One line croaks by uttering 'rib-it rib-it' and has purple eyes. The other line croaks more softly by muttering 'knee-deep knee-deep' and has green eyes. With a newfound freedom of inquiry, the geneticist mated the two types of frogs, producing F₁ frogs that were all utterers and had blue eyes. A large F₂ generation then yielded the following ratios: 27/64 blue-eyed, 'rib-it' utterer 12/64 green-eyed, 'rib-it' utterer 9/64 blue-eyed, 'knee-deep' mutterer 9/64 purple-eyed, 'rib-it' utterer 4/64 green-eyed, 'knee-deep' mutterer 3/64 purple-eyed, 'knee-deep' muttererIn another experiment, the geneticist crossed two purple-eyed, 'rib-it' utterers together with the results shown here:9/16 purple-eyed, 'rib-it' utterer3/16 purple-eyed, 'knee-deep' mutterer3/16 green-eyed, 'rib-it' utterer1/16 green-eyed, 'knee-deep' muttererWhat were the genotypes of the two parents?
Open Question
A geneticist from an alien planet that prohibits genetic research brought with him to Earth two pure-breeding lines of frogs. One line croaks by uttering 'rib-it rib-it' and has purple eyes. The other line croaks more softly by muttering 'knee-deep knee-deep' and has green eyes. With a newfound freedom of inquiry, the geneticist mated the two types of frogs, producing F₁ frogs that were all utterers and had blue eyes. A large F₂ generation then yielded the following ratios: 27/64 blue-eyed, 'rib-it' utterer 12/64 green-eyed, 'rib-it' utterer 9/64 blue-eyed, 'knee-deep' mutterer 9/64 purple-eyed, 'rib-it' utterer 4/64 green-eyed, 'knee-deep' mutterer 3/64 purple-eyed, 'knee-deep' muttererOne set of crosses with his true-breeding lines initially caused the geneticist some confusion. When he crossed true-breeding purple-eyed, 'knee-deep' mutterers with true-breeding green-eyed, 'knee-deep' mutterers, he often got different results. In some matings, all offspring were blue-eyed, 'knee-deep' mutterers, but in other matings all offspring were purple-eyed, 'knee-deep' mutterers. In still a third mating, 1/2 blue-eyed, 'knee-deep' mutterers and 1/2 purple-eyed, 'knee-deep' mutterers were observed. Explain why the results differed.
Open Question
A geneticist from an alien planet that prohibits genetic research brought with him to Earth two pure-breeding lines of frogs. One line croaks by uttering 'rib-it rib-it' and has purple eyes. The other line croaks more softly by muttering 'knee-deep knee-deep' and has green eyes. With a newfound freedom of inquiry, the geneticist mated the two types of frogs, producing F₁ frogs that were all utterers and had blue eyes. A large F₂ generation then yielded the following ratios:27/64 blue-eyed, 'rib-it' utterer12/64 green-eyed, 'rib-it' utterer9/64 blue-eyed, 'knee-deep' mutterer9/64 purple-eyed, 'rib-it' utterer4/64 green-eyed, 'knee-deep' mutterer3/64 purple-eyed, 'knee-deep' muttererAfter years of experiments, the geneticist isolated pure-breeding strains of all six F₂ phenotypes. Indicate the F₁ and F₂ phenotypic ratios of the following cross using these pure-breeding strains: blue-eyed, 'knee-deep' 'rib-it' utterer.
Open Question
A geneticist from an alien planet that prohibits genetic research brought with him to Earth two pure-breeding lines of frogs. One line croaks by uttering 'rib-it rib-it' and has purple eyes. The other line croaks more softly by muttering 'knee-deep knee-deep' and has green eyes. With a newfound freedom of inquiry, the geneticist mated the two types of frogs, producing F₁ frogs that were all utterers and had blue eyes. A large F₂ generation then yielded the following ratios:27/64 blue-eyed, 'rib-it' utterer12/64 green-eyed, 'rib-it' utterer9/64 blue-eyed, 'knee-deep' mutterer9/64 purple-eyed, 'rib-it' utterer4/64 green-eyed, 'knee-deep' mutterer3/64 purple-eyed, 'knee-deep' muttererIndicate the genotypes of the six F₂ phenotypes.
Open Question
A geneticist from an alien planet that prohibits genetic research brought with him to Earth two pure-breeding lines of frogs. One line croaks by uttering 'rib-it rib-it' and has purple eyes. The other line croaks more softly by muttering 'knee-deep knee-deep' and has green eyes. With a newfound freedom of inquiry, the geneticist mated the two types of frogs, producing F₁ frogs that were all utterers and had blue eyes. A large F₂ generation then yielded the following ratios:27/64 blue-eyed, 'rib-it' utterer12/64 green-eyed, 'rib-it' utterer9/64 blue-eyed, 'knee-deep' mutterer9/64 purple-eyed, 'rib-it' utterer4/64 green-eyed, 'knee-deep' mutterer3/64 purple-eyed, 'knee-deep' muttererAssign gene symbols for all phenotypes and indicate the genotypes of the P₁ and F₁ frogs.
Open Question
A geneticist from an alien planet that prohibits genetic research brought with him to Earth two pure-breeding A geneticist from an alien planet that prohibits genetic research brought with him to Earth two pure-breeding lines of frogs. One line croaks by uttering 'rib-it rib-it' and has purple eyes. The other line croaks more softly by muttering 'knee-deep knee-deep' and has green eyes. With a newfound freedom of inquiry, the geneticist mated the two types of frogs, producing F₁ frogs that were all utterers and had blue eyes. A large F₂ generation then yielded the following ratios: 27/64 blue-eyed, 'rib-it' utterer 12/64 green-eyed, 'rib-it' utterer 9/64 blue-eyed, 'knee-deep' mutterer 9/64 purple-eyed, 'rib-it' utterer 4/64 green-eyed, 'knee-deep' mutterer 3/64 purple-eyed, 'knee-deep' muttererOf these, how many are controlling eye color? How can you tell? How many are controlling croaking?
Open Question
A geneticist from an alien planet that prohibits genetic research brought with him to Earth two pure-breeding lines of frogs. One line croaks by uttering 'rib-it rib-it' and has purple eyes. The other line croaks more softly by muttering 'knee-deep knee-deep' and has green eyes. With a newfound freedom of inquiry, the geneticist mated the two types of frogs, producing F₁ frogs that were all utterers and had blue eyes. A large F₂ generation then yielded the following ratios:27/64 blue-eyed, 'rib-it' utterer12/64 green-eyed, 'rib-it' utterer9/64 blue-eyed, 'knee-deep' mutterer9/64 purple-eyed, 'rib-it' utterer4/64 green-eyed, 'knee-deep' mutterer3/64 purple-eyed, 'knee-deep' muttererHow many total gene pairs are involved in the inheritance of both traits? Support your answer.
Open Question
Alkaptonuria is an infrequent autosomal recessive condition. It is first noticed in newborns when the urine in their diapers turns black upon exposure to air. The condition is caused by the defective transport of the amino acid phenylalanine through the intestinal walls during digestion. About 4 people per 1000 are carriers of alkaptonuria.
Sara and James had never heard of alkaptonuria and were shocked to discover that their first child had the condition. Sara's sister Mary and her husband, Frank, are planning to have a family and are concerned about the possibility of alkaptonuria in one of their children.
The four adults (Sara, James, Mary, and Frank) seek information from a neighbor who is a retired physician. After discussing their family histories, the neighbor says, 'I never took genetics, but I know from my many years in practice that Sara and James are both carriers of this recessive condition. Since their first child had the condition, there is a very low chance that the next child will also have it, because the odds of having two children with a recessive condition are very low. Mary and Frank have no chance of having a child with alkaptonuria because Frank has no family history of the condition.' The two couples each have babies and both babies have alkaptonuria.

The couples are worried that one of their grandchildren will inherit alkaptonuria. How would you assess the risk that one of the offspring of a child with alkaptonuria will inherit the condition?