Chapter 14 - Mendel and the Gene Idea (Book)

What is the probability that each of the following pairs of parents will produce the indicated offspring? (a) AABBCC x aabbcc -> AaBbCc (b) AABbCc x AaBbCc -> AAbbCC (c) AaBbCc x AaBbCc -> AaBbCc (d) aaBbCC x AABbcc -> AaBbCc

(a) 1 (b) 1/32 (c) 1/8 (d) 1/2

The genotype of F1 individuals in a tetrahybrid cross is AaBbCcDd. Assuming independent assortment of these four genes, what are the probabilities that F2 offspring will have the following genotypes? (a) aabbccdd (b) AaBbCcDd (c)AABBCCDD (d)AaBBccDd (e) AaBBCCdd

(a) 1/256 (b) 1/16 (c) 1/256 (d) 1/64 (e) 1/128

Flower position, stem length, and seed shape are three characters that Mendel studied. Each is controlled by an independently assorting gene and has dominant and recessive expression. If a plant that is heterozygous for all three characters is allowed to self-fertilize, what proportion of the offspring would you expect to be as follows? (a) homozygous for the three dominant traits (b) homozygous for the three recessive traits (c) heterozygous for all three characters (d) homozygous for axial and tall, heterozygous for seed shape

(a) 1/64 (b) 1/64 (c) 1/8 (d) 1/32

Phenylketonuria (PKU) is an inherited disease caused by a recessive allele. If a woman and her husband, who are both carriers, have three children, what is the probability of each of the following? (a) All three children are of normal phenotype (b) One or more of the three children have the disease (c) All three children have the disease (d) At least one child is phenotypically normal

(a) 3/4 x 3/4 x 3/4 = 27/64 (b) 1 - 27/64 = 37/64 (c) 1/4 x 1/4 x 1/4 = 1/64 (d) 1 - 1/64 = 63/64

A man has six fingers on each hand and six toes on each foot. His wife and their daughter have the normal number of digits. Remember that extra digits is a dominant trait. What fraction of this couple's children would be expected to have extra digits?

1/2

For any gene with a dominant allele A and recessive allele a, what proportions of the offspring from an AA x Aa cross are expected to be homozygous dominant, homozygous recessive, and heterozygous?

1/2 homozygous dominant (AA), 0 homozygous recessive (aa), and 1/2 heterozygous (Aa)

Two organisms, with genotypes BbDD and BBDd are mated. Assuming independent assortment of the B/b and D/d genes, write the genotypes of all possible offspring from this cross and use the rules of probability to calculate the chance of each genotype occurring?

1/4 BBDD; 1/4 BbDD; 1/4 BBDd; 1/4 BbDd

Karen and Steve each have a sibling with sickle-cell disease. Neither Karen nor Steve nor any of their parents have the disease, and none of them have been tested to see if they have the sickle-cell trait. Based on this incomplete information, calculate the probability that if this couple has a child, the child will have sickle-cell disease.

1/9

Beth and Tom each have a sibling with cystic fibrosis, but neither Beth nor Tom nor any of their parents have the disease. Calculate the probability that if this couple has a child, the child will have cystic fibrosis. What would be the probability if a test revealed that Tom is a carrier but Beth is not? Explain your answers.

1/9 (Since cystic fibrosis is caused by a recessive allele, Beth and Tom's siblings who have CF must be homozygous recessive. Therefore, each parent must be a carrier of the recessive allele. Since neither Beth nor Tom has CF, this means they each have a 2/3 chance of being a carrier. If they are both carriers, there is a 1/4 chance that they will have a child with CF.); 0 (Both Beth and Tom would have to be carriers to produce a child with the disease.)

In tigers, a recessive allele of a particular gene causes both an absence of fur pigmentation (a white tiger) and a cross-eyed condition. If two phenotypically normal tigers that are heterozygous at this locus are mated, what percentage of their offspring will be cross-eyed? What percentage of cross-eyed tigers will be white?

25% or 1/4 will be cross-eyed; all (100%) of the cross-eyed offspring will also be white

Pea plants heterozygous for flower position and stem length (AaTt) are allowed to self-pollinate, and 400 of the resulting seeds are planted. Draw a punnett square for this cross. How many offspring would be predicted to have terminal flowers and be dwarf?

According to the law of independent assortment, 25 plants are predicted to be aatt, or recessive for both characters. The actual result is likely to differ slightly from this value.

The first filial, hybrid (heterozygous) offspring arising from a parental (P generation) cross

F1 generation

The offspring resulting from interbreeding (or self-pollination) of the hybrid F1 generation.

F2 generation

If a man with type AB blood marries a woman with type O, what blood types would you expect of each type?

Half of the children would be expected to have type A blood and half type B blood.

Explain how the change of a single amino acid in hemoglobin to the aggregation of hemoglobin into long fibers?

In normal hemoglobin, the sixth amino acid is glutamic acid (Glu), which is acidic (has a negative charge on its side chain). In sickle-cell hemoglobin, Glu is replaced by valine (Val), which is a nonpolar amino acid, very different from Glu. The primary structure of a protein (its amino acid sequence) ultimately determines the shape of the protein and thus its function. The substitution of Val for Glu enables the hemoglobin molecules to interact with each other and form long fibers, leading to the protein's deficient function and the deformation of the red blood cell.

Incomplete dominance and epistasis are both terms that define genetic relationships. What is the most basic distinction between these terms?

Incomplete dominance describes the relationship between two alleles of a single gene, whereas epistasis relates to the genetic relationship between two genes (and the respective alleles of each).

Joan was born with six toes on each foot, a dominant trait called polydactyly. Two of her five siblings and her mother, but not her father, also have extra digits. What is Joan's genotype for the number-of digits character? Explain your answer. Use D and d to symbolize the alleles for this character.

Joan's genotype is Dd. Because the allele for polydactyly (D) is dominant to the allele for five digits per appendage (d), the trait is expressed in people with either the DD or Dd genotype. But because Joan's father does not have polydactyly, his genotype must be dd, which means that Joan inherited a d allele from him. Therefore Joan, who does have the trait, must be heterozygous.

A man with A blood marries a woman with type B blood. Their child has type O blood. What are the genotypes of these three individuals? What genotypes, and in what frequencies, would you expect in future offspring from this marriage?

Man IAi, Woman IBi, child ii; Genotypes for future children are predicted to be 1/4 IAIB, 1/4 IAi, 1/4 IBi, 1/4 ii

In 1981, a stray black cat with unusual rounded, curled-back ears was adopted by a family in California. Hundreds of descendants of the cat have since been born, and cat fanciers hope to develop the curl cat into a show breed. Suppose you owned the first curl cat and wanted to develop a true-breeding variety. How would you determine whether the curl allele is dominant or recessive? How would you obtain true-breeding curl cats? How could you be sure they are true-breeding?

Matings of the original mutant cat with true-breeding noncurl cats will produce both curl and noncurl F1 offspring if the curl allele is dominant, but only noncurl offspring if the curl allele is recessive. You would obtain some true-breeding offspring homozygous for the curl allele from matings between the F1 cats resulting from the original curl X noncurl crosses whether the curl trait is dominant or recessive. You know that cats are true-breeding when curl x curl matings produce only curl offspring. As it turns out, the allele that causes curled ears is dominant.

The true-breeding (homozygous) parent individuals from which F1 hybrid offspring are derived in studios of inheritance; P stands for "parental".

P generation

A diagram used in the study of inheritance to show the predicted genotypic results of random fertilization in genetic crosses between individuals of known genotype

Punnett Square

EXPERIMENT: Mendel crossed true-breeding purple-flowered plants and white-flowered plants (crosses are symbolized by X). The resulting F1 hybrids were allowed to self-pollinate or were cross-pollinated with other F1 hybrids . The F2 generation plants were then observed for flower color.

RESULTS: Both purple-flowered and white-flowered plants appeared in the F2 generation, in a ratio of approximately 3:1. CONCLUSION: The "heritable factor" for the recessive trait (white flowers) had not been destroyed, deleted, or "blended" in the F1 generation but was merely masked by the presence of the factor for purple flowers, which is the dominant trait.

APPLICATION: An organism that exhibits a dominant trait, such as purple flowers in pea plants, can be either homozygous for the dominant allele or heterozygous. To determine the organism's genotype, genetics can perform a testcross. TECHNIQUE: In a testcross, the individual with the unknown genotype is crossed with a homozygous individual expressing the recessive trait (white flowers in this example), and Punnett squares are used to predict the possible outcomes.

RESULTS: Matching the results to either prediction identifies the unknown parental genotype (either PP or Pp in this example). In this test-cross, we transferred pollen from a white-flowered plant to the carpels of a purple-flowered plant; the opposite (reciprocal) cross would have led to the same results.

EXPERIMENT: To follow the characters of seed color and seed shape through the F2 generation, Mendel crossed a true-breeding plant with yellow-rounded seeds with a true-breeding plant with green wrinkled seeds, producing dihybrid F1 plants. Self-pollination of the F1 dihybrids produced the F2 generation. The two hypotheses (dependent and independent assortment) predict different phenotypic ratios.

RESULTS: Phenotypic ratio appoximately 9:3:3:1 CONCLUSION: Only the hypothesis of independent assortment predicts two of the observed phenotypes: green-round seeds and yellow-wrinkled seeds. The alleles for each gene sequence independently of those of the other, and the two genes are said to assort independently.

APPLICATION: By crossing (mating) two true-breeding varieties of an organism, scientists can study patterns of inheritance. In this example, Mendel crossed pea plants that varied in flower color.

RESULTS: When pollen from a white flower was transferred to a purple flower, the first-generation hybrids all had purple flowers. The result was the same for the reciprocal cross, which involved the transfer of pollen from purple flowers to white flowers.

In some pea plant crosses, the plants are self-pollinated. Explain whether self-pollination is considered asexual or sexual reproduction.

Self-pollination is sexual reproduction because meiosis is involved in forming gametes, which unite during fertilization. As a result, the offspring in self-pollination are genetically different from the parent.

A human genetic disease caused by a recessive allele for a dysfunctional enzyme, leading to accumulation of certain lipids in the brain. Seizures, blindness, and degeneration of motor and mental performance usually become manifest a few months after birth, followed by death within a few years

Tay-Sachs disease

A rooster with gray feathers and a hen of the same phenotype produce 15 gray, 6 black, and 8 white chicks. What is the simplest explanation for the inheritance of these colors in chickens? What phenotypes would you expect in the offspring of a cross between a gray rooster and a black hen?

The black and white alleles are incompletely dominant, with heterozygotes being gray in color. A cross between a gray rooster and a black hen should yield approximately equal numbers of gray and black offspring.

In maize (corn) plants, a dominant allele I inhibits kernel color, while the recessive allele i permits color when homozygous. At a different locus, the dominant allele P causes purple kernel color, while the homozygous recessive genotype pp causes red kernels. If plants heterozygous at both loci are crossed, what will be the phenotypic ratio of the offspring?

The dominant allele I is epistatic to the P/p locus, and thus the genotypic ratio for the F1 generation will be 9 I-P- (colorless): 3 I-pp (colorless): 3 iiP- (purple): 1 iipp (red). Overall, the phenotypic ratio is 12 colorless: 3 purple: 1 red.

Three characters (flower color, seed color, and pod shape) are considered in a cross between two pea plants: PpYyIi x ppYyii. What fraction of offspring are predicted to be homozygous recessive for at least two of the three characters?

The genotypes that fulfill this condition are ppyyIi, ppYyii, Ppyyii, and ppyyii.

List all gametes that could be made by a pea plant heterozygous for seed color, seed shape, and pod shape. How large a punnett square would you need to draw to predict the offspring of a self-pollination of this "trihybrid"?

The plant could make eight different gametes (YRI, YRi, Yri, yRI, yRi, yrI, and yri). To fit all the possible gametes in a self-pollination, a Punnett square would need 8 rows and 8 columns. It would have spaces for the 64 possible unions of gametes in the offspring.

A rule of probability stating that the probability of any one of two or more mutually exclusive events occurring can be determined by adding their individual probabilities

addition rule

Any of the alternative versions of a gene that may produce distinguishable phenotypic effects

alleles

A technique associated with prenatal diagnosis in which amniotic fluid is obtained by aspiration from a needle inserted into the uterus. The fluid and the fetal cells it contains are analyzed to detect certain genetic and congenital defects in the fetus.

amniocentesis

An observer heritable feature that may vary among individuals

character

A technique associated with prenatal diagnosis in which a small sample of the fetal portion of the placenta is removed for analysis to detect certain genetic and congenital defects to the fetus

chrionic villus sampling (CVS)

The situation in which the phenotypes of both alleles are exhibited in the heterozygote because both alleles affect the phenotype in separate, distinguishable ways.

codominance

The situation in which the phenotypes of the heterozygous and dominant homozygote are indistinguishable.

complete dominance

A human genetic disorder caused by a recessive allele for a chloride channel protein; characterized by an excessive secretion of mucus and consequent vulnerability to infection; fatal if untreated

cystic fibrosis

A cross between two organisms that are each heterozygous for both of the characters being followed (or the self-pollination of plant that is heterozygous for both characters)

dihybrid cross

An organism that is heterozygous with respect to two genes of interest. All the offspring from a cross between parents doubly homozygous for different alleles are dihybrids. For example, parents of genotypes AABB and aabb produce a dihybrid of genotype AaBb.

dihybrids

An allele that is fully expressed in the phenotypic of a heterozygote

dominant allele

A type of gene interaction in which the phenotypic expression of one gene alters that of another independently inherited gene

epistasis

The genetic makeup, or set of alleles, of an organism

genotype

Having two different alleles for a given gene

heterozygous

Having two identical alleles for a given gene

homozygous

In genetics, the mating, or crossing, of two true-breeding varieties

hybridization

The situation in which the phenotype of heterozygous is intermediate between the phenotypes of individuals homozygous for either allele

incomplete dominance

Mendel's second law, stating that each pair of alleles segregates, or assorts, independently of each other pair during gamete formation; applies when genes for two characters are located on different pairs of homologous chromosomes or when they are far enough apart on the same chromosome to behave as though they are on different chromosomes.

law of independent assortment

Mendel's first law, stating that two alleles in a pair segregate (seperate from each other) into different gametes during gamete formation

law of segregation

A cross between two organisms that are heterozygous for the character being followed (or the self pollination of a heterozygous plant)

monohybrid cross

An organism that is heterozygous with respect to a single gene of interest. All the offspring from a cross between parents homozygous for different alleles are monohybrids. For example, parents of genotypes AA and aa produce a monohybrid of genotype Aa.

monohybrids

Referring to a phenotypic character that is influenced by multiple genes and environmental factors.

multifactorial

A rule of probability stating that the probability of two or more independent events occurring together can be determined by multiplying their individual probabilities.

multiplication rule

A diagram of a family tree with with conventional symbols, showing the occurrence of heritable characters in parents and offspring over multiple generations.

pedigree

The observable physical and physiological traits of an organism, which are determined by its genetic makeup.

phenotype

The ability of a single gene to have multiple effects

pleiotropy

An additive effect of two or more genes on a single phenotypic character

polygenic inheritance

A heritable feature that varies continuously over a range rather than in an either-or fashion

quantitative characters

An allele whose phenotypic effect is not observed in a heterozygote

recessive allele

A recessively inherited human blood disorder in which a single nucleotide change in the beta-globin gene causes hemoglobin to aggregate, changing red blood cell shape and causing multiple symptoms in afflicted individuals.

sickle-cell disease

Breeding an organism of unknown genotype with a homozygous recessive individual to determine the unknown genotype. The ratio of phenotypes in the offspring reveals the unknown genotype.

testcross

One of two or more detectable variants in a genetic character

trait

Referring to organisms that produce offspring of the same variety over many generation of self-pollination

true-breeding