BSC1005, Chapter 7, Smartwork
What could be a limitation of "personalized medicine" based on genetic testing?
All of the above
Rank these organisms based on the sizes (in megabases) of their genomes.
Lungfish, Red-spotted newt, White spruce, Human, Dog, Chicken Complexity is not always a reflection of the size of a genome.
Mendel discovered the basic laws of inheritance experimenting with peas. His basic rules can explain inheritance if there is only one gene involved. When there is more than one gene or the environment influences gene expression, the rules are more complicated. Determine whether the examples listed below are examples of Mendelian inheritance or complex inheritance.
Complex: tameness in silver foxes; skin color in humans; coat color in Labradors Mendelian: body size in dogs; flower color in peas Mendelian traits are flower color in peas and body size in dogs. These two traits are each controlled by one gene. Even though body size is an example of incomplete dominance, there is only one gene and one trait. Complex traits are tameness in silver foxes (pleiotropy), skin color in humans (polygenic), and coat color in Labradors (epistasis). Page 127
Carnations exhibit incomplete dominance in their flower color. The red (R) allele is incompletely dominant to the white allele (W). When red carnations are crossed with white carnations, all the F1 offspring are pink (RW). What would be the probable offspring in the F2 generation if the pink carnations were allowed to self-fertilize?
25% red, 25% white, 50% pink Crossing two heterozygotes produces a 25% chance of RR (red) carnations, a 25% chance of WW (white) carnations, and a 50% chance of RW (pink) carnations. Page 125
Two pea plants that are true-breeding for recessive white flowers are mated and their offspring analyzed. Which of the following statements about this situation is true?
The gametes produced by the offspring will carry only one allele for this gene. This cross involves two homozygous recessive parents. If you work out the cross (pp × pp) you will find that 100 percent of the offspring will have the same genotype (pp). All offspring, representing the F1 generation, will also be true-breeding for white flowers and will only produce recessive (p) alleles. A gamete only contains one allele for a given gene. Page 121
Study the infographic on genome size and then answer the questions below.
Chicken. This organism contains the least genetic material making it the easiest to study. The chicken has the smallest genome size, which makes it easiest to study genetically. Simply put, a smaller genome poses less material to sort through. Page 129
Sort the alleles below as either dominant or recessive.
Dominant: A, G, P Recessive: d, h, r In genetics, a capital letter is used to indicate a dominant allele or gene, and a lowercase letter indicates a recessive allele or gene. Page 120
The two dogs shown produce a litter of 12 puppies. Out of the 12 puppies, 11 are black and 1 is brown. Is the black dog shown in the figure homozygous dominant or heterozygous? Why?
Heterozygous; the brown offspring inherited a recessive allele from the black-haired parent. Although the actual phenotype ratio is not exactly what one would predict when crossing a homozygous recessive individual with a heterozygote, the one brown-haired offspring must have inherited a recessive allele from each parent. Therefore, the black-haired poodle must be a heterozygote (Bb). Page 120
A species of plant has a gene that controls its leaf color. Light green leaves are determined by the dominant allele G, and dark green leaves are determined by the recessive allele g. Below is a Punnett square showing the cross between a heterozygous plant that has light green leaves and a homozygous plant with light green leaves. The offspring are represented by the numbers 1, 2, 3, and 4. Sort whether the genotypes of the offspring are homozygous or heterozygous.
Homozygous: 1, 2 Heterozygous: 3, 4 Two offspring will be heterozygous, and two will be homozygous. Page 121
The color of peas is controlled by a gene called Y. The dominant allele Y produces yellow peas, and the recessive allele y produces green peas. How do you think it's possible that all the peas in the grocery store are green if the dominant allele is yellow?
If you breed green peas (yy) with other green peas (yy), you will always get green peas. All green peas have the genotype yy, and if bred with other green peas, they will always produce green peas: full of country goodness and green pea-ness. Page 121
What do researchers hope to achieve in terms of human health by studying the dog genome?
The goal is to find similar genes for diseases in the two different species, which might lead to treatment and cures. Since humans and dogs seem to share some of the same diseases, looking at dog genes may help us understand our own disease genes.
An organism's _____ is an expression of its _____.
phenotype; genotype An organism's phenotype is the physical expression of its genotype (its genetic makeup). Page 119
Different alleles are the result of which?
mutations in DNA within a specific gene Mutations in DNA within a gene can result in different alleles. Page 119
Which statement accurately describes the outcome of independent assortment as shown in this animation?
Alleles associated with pea color and shape appear in a 9:3:3:1 ratio in the F2 offspring. In this animation, the two traits illustrated are pea color (yellow or green) and pea shape (smooth or wrinkled). Each individual has two alleles (a pair of alleles) for each trait, and the two pairs of alleles (one pair for each trait) segregate independently. Offspring in the F2 generation may have new combinations of these two traits that are not seen in the parental generation due to independent assortment. The ratio of traits in the F2 generation is 9:3:3:1. Page 124
Sort the examples of phenotypes in dogs to the correct type of inheritance.
Codominance: Gum color Dominance: Coat color Incomplete Dominance: Body Size Page 125
Which of the following statements regarding genes is true?
Genes are the basic unit of information affecting a genetic trait. Genes are segments of chromosomes, which are found within the nucleus of a cell. In humans and other sexually reproducing species, each cell (except for gametes) contains two copies of every gene, one copy inherited from each parent. Page 119
Rank the terms describing genetic material in descending order, from largest to smallest.
Genome, Homologous pair of chromosomes, A single DNA molecule, Gene for hair color The genome refers to all of the genes in an organism. These genes are found on chromosomes that are each made of long DNA molecules associated with proteins. A small, physical section of a DNA molecule that encodes a particular trait is a gene, and a gene can exist as one of several different variations or alleles. Page 120
Why was it important that Mendel used parental pea plants that bred true in his experiments?
It allowed him to know what traits his pea plants had before he started experimenting with them. In order to follow the traits through the generations, Mendel needed to know what traits his parent plants "truly" had. Page 121
In cats, the length of fur is determined by a pair of alleles. The dominant allele L codes for short fur, and the recessive allele l codes for long fur. If a cat had short fur, could you tell by looking at it what its genotype would be?
No, if it is showing the dominant phenotype, then its genotype could be LL or Ll. If a cat has short fur, it is displaying the dominant phenotype, and its genotype could be LL or Ll. Page 120
The figure shown indicates a homologous pair of chromosomes. Which of the following indicates the relationship between Gene A and the circled gene?
The circled gene must encode information for the same trait as Gene A. Remember that homologous pairs are inherited when each parent passes one chromosome on to offspring. Genes in the same location on homologous chromosomes carry information for the same traits. The alleles on each chromosome in the pair may be the same or different. Page 119
Dihybrid crosses are crosses involving
two genes with two alleles each. Dihybrid crosses are used to test the hypothesis that alleles of one gene segregate independently of any two alleles of another gene. Dihybrid crosses follow the inheritance of two separate traits. Page 124
A heterozygous pea plant and a white pea plant are crossed. What is the expected percentage of purple plants produced by this cross?
50% purple Crossing a plant that has the genotype Pp with one that has the genotype pp will produce 50% Pp and 50% pp. Page 121
The black fur allele (B) is dominant in dogs, and the brown fur allele is recessive (b). Label each dog below with the correct possible genotype (s) based on their phenotype.
Black: BB and Bb Brown: bb only The brown dog has two copies of the recessive allele bb. The black dog could be either BB or Bb since it exhibits the dominant phenotype. Page 119
Mendel wanted to trace the inheritance of two traits in pea plants. He decided to look at height, which is controlled by two alleles. The dominant allele T produces a tall plant, while the recessive allele t produces a short plant. Flower color is controlled by one gene with two alleles as well. The dominant allele P produces purple flowers, and the recessive p produces white flowers. If the female parent has the genotype TtPp, which allele combinations can be found in her gametes?
TP, Tp, tP, tp The female plant can make gametes with all combinations of the flower and height alleles. Page 124
The figure shown indicates the genotypes of two pea plants. Which of the following is true for a cross of these two plants?
All of the offspring will have the same phenotype as one of the parents. The gametes produced by the parent with the RRYY genotype can only carry the combination RY. The gametes produced by the parent with the rryy genotype can only carry the combination ry. When these gametes combine, the only genotype produced is RrYy. Individuals with the RrYy genotype will show the dominant phenotype, which is seen in the RRYY parent. Page 124
In peas, round (R) is dominant to wrinkled (r) and yellow (Y) is dominant to green (y). Which of these crosses will produce 100% round, yellow seeds?
RRyy × RrYY
In addition to pea flower color, Mendel also examined the inheritance of several other traits, including pea plant height. When he crossed true-breeding tall plants with true-breeding short plants, all of the F1 generation was tall. Which trait is determined by a dominant allele in plant height?
Tall is dominant because it appears in the F1 generation, but short does not. Tall is dominant because all the plants in the F1 generation are tall. Page 121
Study the infographic on genome size and then answer the questions below.
The lungfish cells must each contain more DNA. Genome size refers to the amount of DNA found in each cell of an organism's body. An organism's genome is all of the genetic material that an organism contains. Page 129
The presence of one gene in Labrador retrievers influences coat color by affecting expression of another gene. This is one example of
epistasis Complex traits whose patterns of inheritance cannot be predicted by Mendelian inheritance include pleiotropy, polygenic traits, epistasis, and traits affected by the environment. Epistasis involves one gene influencing the phenotypic effect of another, such as with coat color in Labrador retrievers.
Most traits in humans are controlled by the action of multiple genes. These traits are referred to as
polygenic Traits are often not simple Mendelian traits with a single-gene, single-phenotype pattern of inheritance and expression. More commonly, patterns of inheritance are polygenic, meaning they are controlled by the actions of multiple genes.
Mendel experimented with peas to determine the basic laws of genetics. Put the steps of his experiment in order.
1. Mendel crossed true-breeding peas with purple flowers with true-breeding peas with white flowers. 2. Mendel harvested seeds from an F1 generation. 3. Mendel planted F1 plants and let them self-fertilize. he then planted those seeds and let them grow. 4. Mendel observed and recorded the flower color of the plants in the F2 generation. 5. Mendel analyzed his data using statistics. 6. Mendel repeated the experiment several times for thousands of pea plants. Page 121
In Labrador retrievers, coat color is determined by two genes. The dominant B allele produces a black coat while two copies of the recessive b allele produce a brown coat. When two copies of the e allele of the second gene are present, it prevents the pigment from being produced, and the fur appears yellow. If the allele E is present, then the color black or brown is produced. Imagine if the two Labs in the image were bred with each other. If the brown Lab is heterozygous at the E gene and the yellow Lab is heterozygous at the B gene, what would be the possible phenotypes for the offspring?
1/4 brown Lab, 1/4 black Lab, 1/2 yellow Lab There would be a ratio of 1 brown Lab : 1 black Lab : 2 yellow Labs in the offspring. Page 127
In Portuguese water dogs, black fur is determined by the B allele and brown fur by the b allele. The texture of fur is controlled by the W allele, which results in wavy fur, whereas w results in curly fur. If a dog who is heterozygous for both black and wavy fur is mated with a brown, curly-furred dog, what would be the possible phenotypes of their offspring?
25% black and curly, 25% black and wavy, 25% brown and curly, 25% brown and wavy This mating can produce all four possible combinations of phenotypes, with a 25% chance of each resulting. Page 124
What are the genotypes for the pea plants in the P generation?
PP × pp The plants in the P generation are "true-breeding." This means that they are homozygous for the trait of interest. The purple-flowered plant is PP (homozygous dominant), and the white-flowered plant is pp (homozygous recessive). Page 121
What is indicated by the arrow in the figure shown?
The alleles in an offspring A Punnett square is used to indicate the separation of parental alleles into gametes and to show the possible combinations of these alleles in offspring. Each allele is represented by a letter. Page 121
How could the Dog Genome Project potentially help researchers understand human diseases?
The genes in dogs that cause diseases that are similar to diseases in humans may be caused by a similar gene. Dogs and humans share at least 300 similar genetic conditions, including cancer, heart disease, epilepsy, and Addison's disease. Knowing the genetic basis in dogs and treatments that work could lead to the discovery of similar genes and treatments in humans. Page 120
The round trait in peas is controlled by the R allele, and the r allele controls the wrinkled trait. If you were to perform a cross between one parent that was Rr and another that was rr, what is the chance that there would be wrinkled peas in the F1 generation?
There is a 2 out of 4, or 50%, chance. The cross between these two parents has the probability of producing two offspring with the genotype Rr and two offspring with the genotype rr. page 121
Horses can have a coat color that is called roan. Horses with this coat color have some pieces of hair that are all red and some pieces of hair that are all white. Red hair and white hair are coded by two different alleles, and roan horses are heterozygous for these two alleles of the gene. What kind of inheritance is this an example of?
codominance Both traits (white hairs and red hairs) are fully expressed at the same time. Page 119
Mendel's breeding experiments with pea plants indicate that alleles of a gene separate during
meiosis. The event responsible for the law of segregation occurs in meiosis, when homologous chromosomes separate into different daughter cells. The law of independent assortment, which states that two alleles segregate independently of any two alleles of another gene, is also based on events that occur during meiosis. Page 121
Use this figure to answer the questions below. Seed shape is controlled by one gene with the alleles R (round) and r (wrinkled). Seed color is controlled by a separate gene with the alleles Y (yellow) and y (green). What new phenotype combinations were found in the F2 generation? What is the total number of genotypes in the F2 generation?
round, green wrinkled, yellow Neither the P generation nor the F1 generation contained seeds that were round+green or wrinkled+yellow. These are the new combinations. 16 The genotype is the combination of alleles. Page 124
Chromosomes (not individual genes) are the paired structures that are shuffled, recombined, and then separated into gametes. Which explains the relationship of chromosomes to Mendel's laws?
the chromosome theory of inheritance
Mendel's primary contribution to our understanding of genetic inheritance was
providing a mechanism that explains patterns of inheritance. Mendel gave us a viable mechanism for explaining inheritance. All of the other contributions were made by scientists who did their work after Mendel introduced the laws of segregation and independent assortment. Page 121
Consider a gene with two alleles that show complete dominance. When a homozygous recessive individual (qq) is crossed with a heterozygous individual (Qq), they have a 50 percent chance of producing a homozygous recessive offspring. The next time these two individuals breed, what are the chances that they will once again have a homozygous recessive progeny?
50 percent Each reproduction event is the result of the fusion of a single gamete from each of the two parents, and therefore they are independent events. Thus, the probabilities remain the same each time. Page 121
A pea plant that is heterozygous for the flower color gene makes gametes. What is the probability that a specific gamete contains the recessive allele for flower color?
50 percent Heterozygotes can make two kinds of gametes, each with equal probability of occurrence. Homozygotes can only make one kind of gamete. Page 121
Rank the amount of genetic information contained at each level, from smallest to largest.
Gene, chromosome, cell, organism
When geneticists are looking for genes linked to particular disorders, such as cardiomyopathy, why is it important that they look at healthy individuals in addition to individuals affected by the disorder?
Healthy individuals provide a baseline for comparison. It is important to have both healthy and affected individuals to allow geneticists to know what the normal, healthy form of the gene looks like for comparison. When geneticists find an alternative form of the same gene in all individuals affected, they are able to locate the gene responsible for causing the disorder. Page 125
In 1959, a Russian geneticist began breeding silver foxes. He chose the tamest individuals from each generation for breeding. In addition to becoming tamer, there were observable changes in the appearance of the foxes. The foxes began to look more puppy-like with wider faces and floppier ears. Could this pattern of inheritance be predicted by Mendel's laws of inheritance?
No; this is not a single-gene, single-phenotype pattern and cannot be predicted by Mendel's laws of inheritance. The changes in behavioral and physical traits in the silver fox are an excellent example of a pleiotropy in which a single gene controls multiple traits. Page 127
Traits that can be inherited and can be detected are called genetic traits. They can be divided into three categories: physical, biochemical, and behavioral. Below are some genetic traits of dogs. Determine whether these traits are physical traits, biochemical traits, or behavioral traits.
Physical: tail length; coat texture Biochemical: susceptibility to a type of cancer; inability to produce an enzyme Behavioral: love of swimming; nervousness around strangers Page 117
Drag each example to the type of complex inheritance the example displays.
Pleiotropy: A single gene controls a dog's head shape and limb shape. Epistasis: Labrodor coat color depends of a pigment color gene and an expression gene. Polygenic Inheritance: Eye color in humans is controlled by more than one gene. Dog head and limb shape is an example of pleiotropy. Labrador coat color is an example of epistasis. Human eye color is a polygenic trait. Page 127
Which best describes a gene?
a segment of DNA that codes for at least one trait Page 119
Use this figure to answer the questions below. What is the genotype of the F1 generation? Which of the following appropriately describes the phenotype ratio in the F2 generation?
heterozygous The F1 generation is made up of heterozygous individuals. The prefix hetero comes from the Greek word meaning "different." Heterozygous individuals exhibit the dominant phenotype (purple flowers, in this case) but contain one dominant and one recessive allele. 3/4 purple:1/4 white 3 purple:1 white 75% purple:25% white Since these all refer to the correct numbers of colors seen, they properly express the phenotype ratio in the generation. Page 119
In dogs, body size is controlled by a gene called IGF1 which has two alleles I and B. A dog with the genotype II is large, while a dog with the genotype BB is small. Dogs with the genotype IB are a medium size. This is an example of ______ and a cross between two medium sized dogs would have a ____ chance of producing another medium sized dog.
incomplete dominance; 50% This is an example of incomplete dominance. There is a 50% chance the offspring will be medium sized (lB) like its parents. Page 125
The chance of a parent passing on a given allele to a child ___________ each time he or she has another child.
is the same The probability of a parent passing on a given allele to an individual offspring is an independent event. The outcome of one event does not affect the outcome of the event occurring in the future. Page 121
Cystic fibrosis is a genetic disorder that is caused by a mutation in a gene called CFTR. Due to this mutation, almost all the body systems of individuals with cystic fibrosis are affected. This is an example of
pleiotropy In cystic fibrosis, one gene influences many traits. Page 125
The observation that individuals who are deaf also have heart problems is an example of
pleiotropy Recall that pleiotropy refers to situations in which a single gene has influence over multiple, different traits. As we saw in the text, a pleiotropic gene is one that can influence two or more different traits simultaneously, such as in the example of head shape and limb shape.
Human hair color shows an incredible range of phenotypes, from white blonde to light brown to jet black. This is an example of _____. It is a type of complex inheritance because _____.
polygenic traits; more than one gene controls the trait Hair color is an example of polygenic inheritance. More than gene controlling a trait will result in a large range of phenotypes. Page 127
A genome is
the complete set of genetic information encoded within a type of organism. Page 125
If a person has two different alleles for a given gene, the person is ________ for that gene.
Heterozygous Individuals with two different alleles are heterozygous. Individuals with two copies of the same allele are homozygous. Page 120
Study the infographic on genome size and then answer the questions below.
Much of the DNA in a larger genome is not necessary for life. There are certain genes that all living organisms contain. These genes must be found in all genomes, whether large or small. Therefore, the extra genetic material is probably not material required for life (although it could be important for the organism that contains it).
In mice, the allele for black fur (B) is dominant to the recessive allele for gray fur (b). Also, mice may have long tails, which are dominant (L allele), or short tails, which are recessive (l allele). A male mouse with black fur and a long tail has babies with a female mouse with gray fur and a short tail. All 15 of their babies have black fur and long tails. What is the most probable genotype of the father?
BBLL The genotype of the father is most likely BBLL because all of the children received a dominant allele in each category. Page 119
In 1981, a couple found a stray kitten whose unusual ears curled up and back from her head. They decided to breed her with their male cat who is homozygous for the allele for normal ears. The first litter of kittens produced two kittens with normal ears and two kittens with curled ears. Subsequent litters with the same parents showed the same ratio of curled ears to normal ears. When curled-ear offspring were mated with other curled-ear offspring, three-fourths of the kittens had curled ears and one-fourth had normal ears. This new trait was determined to be the result of a new and unique mutation in the ear gene of cats, and cats with this trait were named American curl cats. In American curl cats, the allele that produces the ear-curling trait is which? The allele that produces normal ears is which?
Dominant Recessive Page 120
Mendel's law of independent assortment says which?
During meiosis the two alleles of a gene separate from each other independently of the alleles for any other genes. During meiosis, the two alleles of a gene separate from each other independently of the alleles for any other genes. Page 124
Sort whether the following alleles as either "homozygous" or "heterozygous."
Homozygous: aa, ff, GG Heterozygous: Aa, Ee, Rr When two alleles for a trait are the same, the organism is said to be homozygous for that gene. When two alleles for a trait are different, the organism is said to be heterozygous for that gene. Page 120
In poodles, the coat color can be black due to the B allele, which is dominant; or it can be brown due to the recessive b allele. If a heterozygous black poodle mated with a brown poodle, what colors could their puppies be?
50% black, 50% brown This is what you'd see when a heterozygote poodle mates with a homozygous recessive brown poodle. Page 121
In Labrador retrievers, coat color is controlled by two different genes. The B gene has two alleles. The dominant allele B produces a black coat while the recessive allele b produces a brown coat. The second gene E can block the expression of the B allele. If two copies of the recessive allele e are present, the coat of the dog will be yellow. If at least one copy of the E allele is present, the coat will be whatever color is determined by the B/b alleles. Imagine a cross between the two Labrador retrievers pictured. If the black dog is heterozygous for both genes and the brown dog is heterozygous for the E gene, what is the chance they will have a yellow puppy?
There will be 1/4 chance. If the parents are BbEe and bbEe, there is a 1/4 chance they will have a yellow puppy with the ee genotype. Page 127
Which of the following best represents a "genetic cross"?
A genetic cross is a controlled mating experiment used to gather information about the inheritance pattern of a given trait. Watch the animation again, and pay attention during the first 30 seconds to see how a genetic cross is used with pea plants. Page 121
Which of the following is an example of incomplete dominance?
A heterozygote cat has a medium-sized body instead of being either large or small. Incomplete dominance occurs when neither allele is fully expressed. Heterozygotes will show an intermediate phenotype. Page 125
In Labrador retrievers, coat color is affected by the interaction of two genes. One gene has two alleles that result in black fur (dominant allele B), and another allele that results in brown fur (recessive allele b). Another gene can inhibit the production of the pigment melanin in fur. If a dog has two recessive copies of this allele (ee), it completely blocks the production of melanin, and the fur appears yellow. If the dominant allele E is present, then the fur color is a result of the B/b alleles.
Black: BBEE, BbEe Brown: bbEe, bbEE Yellow: BBee, Bbee, bbee The black lab could have the genotypes BBEE or BbEe. The brown lab could have the genotypes bbEe or bbEE. The yellow lab could have the genotypes BBee, Bbee, or bbee. Page 127
Some genetic traits are invariant in a species, which means they don't change from individual to individual. Other genetic traits in a species are highly variable from individual to individual. Think about what you know about dogs, and determine whether the traits below are variable or invariable.
Invariable: having two eyes; having four legs Variable: coat color; ear shape; fur texture All dogs have four legs and two eyes. There is a great variety in dogs in coat color, ear shape, and texture of fur. Page 117
Which of the following would NOT be an example of the environment influencing the expression of a gene?
The color of Labrador retrievers is controlled by the interaction of two genes. The presence of homozygous recessive alleles in the second gene turns off the expression of the first gene, which changes the color of the fur. Fur color in Labradors is not an example of the environment influencing genes but is an example of epistasis. 127
How did the ratio of F2 offspring support Mendel's first law, the law of segregation?
The flower colors produced followed a pattern predicted when two alleles separate during meiosis. The flower color patterns seen in Mendel's experiment followed a predictable pattern based on alleles of a given gene moving into separate gametes during meiosis. Page 121
Siamese cats have pale, cream-colored bodies with dark brown fur on their extremities (their ears, face, legs, and tail). Siamese cats that live outside in warm climates tend to have lighter fur, and those that live outside in cooler climates tend to have darker fur. What is the best explanation for this pattern?
The gene that controls fur color in Siamese cats is affected by the environment. Fur color in Siamese cats is controlled by temperature-sensitive alleles. When the temperature is higher, the fur is lighter in color. When the temperature is lower, the fur is darker in color. Figure 7.10 in your text has more questions about this example to further test your knowledge. Page 127
How is the genotype of an organism related to its phenotype?
The phenotype is an expression of the genotype. An organism's genotype determines its phenotype. Page 119
In the 1950s a Russian geneticist started breeding silver foxes for tameness. After many generations, it was noticed that some of the physical traits of the foxes began to change as well, with results in floppy ears, shorter tails, and wider faces, although only one gene was involved. This was due to _____, which is the result of _____.
pleiotropy; one gene controlling many traits The change in the physical and behavioral traits in association indicates that this is a result of pleiotropy. Page 127
In a particular plant, two genes control height and flower color. Tall height (T) is dominant to short height (t). Yellow flowers (Y) are dominant to white flowers (y). A tall, yellow-flowered plant is mated with a short, white-flowered plant. What are the genotypes of the plants involved in this cross?
T_Y_ × ttyy Without knowing whether the tall, yellow-flowered plant is true-breeding for the two traits, you cannot know whether that individual is homozygous dominant for those two traits or whether that individual is heterozygous. The underscore ("_") indicates an allele that is unknown. The genotype of the short, white-flowered plant is known because those traits are recessive and are only seen in individuals who are homozygous recessive. Page 124
Elaine Ostrander and Gordon Lark collaborated on a project to analyze the genetic basis of body size in dogs. They used both genotype and phenotype information to identify the gene IGF1 that controls growth. The IGF1 gene has two alleles, I and B. Large dogs have the genotype II, and small dogs have the genotype BB. If heterozygous dogs are medium-sized dogs, what does this indicate about the pattern of inheritance?
The I and B alleles exhibit incomplete dominance. Incomplete dominance is characterized by alleles that do not exert their full effect when found together in a heterozygote. In this case, heterozygotes are displaying an intermediate phenotype by being medium-sized. Page 125
In the animation, one P generation parent produces round, yellow seeds, and the other parent produces wrinkled, green seeds. In the F1 generation, all individuals produce round, yellow seeds. What mechanism causes approximately 60 percent of the F2 generation plants to produce seeds that are different from both the P generation and F1 generation plants?
The independent assortment of the alleles controls seed color and wrinkling of seeds. The two alleles for seed color and seed shape sort independently during the formation of gametes. This law of independent assortment is significant because it leads to new combinations of alleles and, therefore, new phenotypes in offspring that are not seen in either parent. Page 124
Being able to use a Punnett square to track the pattern of inheritance in a two-characteristic cross (for example, pea seed color and seed shape) demonstrates Mendel's idea of
independent assortment. The law of independent assortment states that the alleles for different genes segregate (assort) independently of one another during gamete formation. This is true because those genes are likely on separate chromosomes, which do not influence each other when they separate during meiosis. Page 124
A biology student decides to copy Mendel's experiment with pea plants. Unfortunately, some of the pea plants from the P generation, the "true-breeding" plants, get mixed in with some of the F1 generation plants. The student attempts to sort out the plants by crossing two purple-flowered plants that are believed to be heterozygous F1 plants. The results of this cross produce 100 offspring of which 72 have purple flowers and 28 have white flowers. Which of the following statements are true?
The information in this question gives phenotype frequencies that match the expected phenotype frequencies from an F1 cross (3 purple:1 white). Therefore, the parents in this cross are both heterozygotes, the same as the F1 generation plants seen in the animation beginning at 2:15. The purple-flowered offspring are a mix of homozygous dominant ("true-breeding") plants and heterozygous plants. The white-flowered plants are the result of the recessive alleles from each parent, p, combining to form a homozygous recessive offspring, pp. Page 121
