Biology - Chapter 14

Dihybrid Cross

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

Monohybrid Cross

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

Pedigree

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

Punnett Square

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.

What is the difference between a character and a trait? Explain using an example.

A heritable feature that varies among individuals, such as flower color, is called a character. Each variant for a character, such as purple or white color for flowers, is called a trait. For example, the varying color of the flowers on pea plants is a character, and the specific variations, white and purple, are traits.

quantitative characters

A heritable feature that varies continuously over a range rather than in an either-or fashion, ex: skin color and height.

Huntington's disease

A human genetic disease caused by a dominant allele; characterized by uncontrollable body movements and degeneration of the nervous system; usually fatal 10 to 20 years after the onset of symptoms.

Tay-Sachs disease

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.

cystic fibrosis

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.

sickle cell disease

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

Epistasis

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

What is a true monohybrid cross and what mendelian law does it represent?

Aa x Aa = 3: 1 phenotypic ratio = 1:2:1 genotypic ratio Law of segregation

What is a true dihybrid cross and what mendelian law does it represent?

AaBb x AaBb = 9:3:3:1 ratio Law of independent assortment

polygenic inheritance

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

Dominant Alleles

An allele that is fully expressed in the phenotype of a heterozygote.

Recessive Alleles

An allele whose phenotypic effect is not observed in a heterozygote.

An African violet grower observes that genetically identical African violet plants growing near the walls of the greenhouse have white flowers, that plants growing farther away from the walls have pale blue flowers, and that plants growing nearest the center of the greenhouse have dark blue flowers. Which of the following best explains the differences in flower color of the African violets in the greenhouse?

An enzyme responsible for flower color does not fold correctly in cooler temperatures, and the greenhouse is warmest in the center.

Character

An observable heritable feature that may vary among individuals.

Monohybrids

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.

Dihybrids

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.

Alleles

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

Butterflies of the genus Colias live in the Rocky Mountains, where they experience a wide range of temperatures. Different variants of a particular glycolytic enzyme in the flight muscles are optimally active at different temperatures. Within the same population, some individual butterflies fly most effectively at 29°C , while others fly most effectively at 40°C. Still others can be equally active at both temperatures. Which of the following claims is most consistent with the observed butterfly behavior?

Butterflies that express two variants of the enzyme are active over a greater range of temperature.

In sexually reproducing organisms, why are there exactly two chromosomes in each homologous pair?

Each somatic cell in a diploid organism has two sets of chromosomes, one set inherited from each parent.

In the spring and summer, the fur of an arctic fox contains a pigment called melanin that gives the fox's fur a dark color. In the fall and winter, the fur of the arctic fox is white.

Environmental factors cause changes in gene expression, resulting in seasonal variations in pigment production.

Explain epistasis.

Epistasis is a type of gene interaction in which the phenotypic expression of one gene alters that of another independently inherited gene.

Mendel's model consists of four concepts. Describe each concept.

First concept: Alternative versions of genes account for variations in inherited characters. Second concept For each character, an organism inherits two copies of a gene, one from each parent. Third concept: If the two alleles at a locus differ, then one, the dominant allele, determines the organism's appearance; the other, the recessive allele, has no noticeable effect on the organism's appearance. Fourth concept (law of segregation): The two alleles for a heritable character segregate (separate from each other) during gamete formation and end up in different gametes.

Compare and contrast codominance with incomplete dominance.

In codominance, the phenotypes of both alleles are exhibited in the heterozygote because both alleles affect the phenotype in separate, distinguishable ways, such as in the human MN blood group, determined by the codominant alleles for two specific molecules located on the surface of red blood cells, the M and N molecules. In incomplete dominance, the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele; neither allele is completely dominant, and the F1 hybrids have a phenotype somewhere between those of the two parental varieties.

Give examples of incomplete dominance, codominance, sex - linked, epistasis, pleiotropy, and polygenic.

In corresponding order, pink flower color, blood type, hemophilia/colorblindness, coat color, dwarfism/gigantism, and skin color/height.

Carriers

In genetics, an individual who is heterozygous at a given genetic locus for a recessively inherited disorder. The heterozygote is generally phenotypically normal for the disorder but can pass on the recessive allele to offspring.

Hybridization

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

Alleles can show different degrees of dominance. Explain how incomplete dominance is different from complete dominance and give an example of incomplete dominance.

Incomplete dominance is the situation in which the phenotype of heterozygotes is intermediate between the phenotypes of individuals homozygous for either allele. Complete dominance is the situation in which the phenotypes of the heterozygote and dominant homozygote are indistinguishable. An example of incomplete dominance is the crossing of red snapdragons with white snapdragons to produce F1 hybrids with pink flowers.

Law of Segregation

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

law of independent assortment

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.

Dominant alleles are not necessarily more common than recessive alleles in the gene pool. Explain why this is true.

Natural selection determines how common an allele is in the gene pool. For example, having six fingers (polydactyly) is dominant to five fingers, but the presence of six fingers is not common in the human gene pool.

Trait

One of two or more detectable variants in a genetic character.

A blue-flowered African violet of unknown ancestry self-pollinated and produced 50 seeds. These seeds germinate and grow into flowering plants. Of these plants, 36 produce blue flowers and 14 produce pink flowers. What is the best explanation for the pink-flowered offspring?

Pink flower color is a trait recessive to blue flower color.

What is pleiotropy? Explain how this is significant in cystic fibrosis and sickle-cell disease.

Pleiotropy is the ability of a single gene to have multiple effects. In humans, pleiotropic alleles are responsible for multiple symptoms associated with certain hereditary diseases, such as cystic fibrosis and sickle-cell disease.

True Breeding

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

Quantitative variation usually indicates polygenic inheritance. What is a good example of this?

Skin color is a good example of polygenic inheritance.

The offspring of purple and white-flowering plants all produce purple flowers, but still are able to form gametes for white-flowering offspring. What principle does this demonstrate?

The Law of Segregation

In fruit flies of the genus Drosophila, the allele for vestigial wings is recessive to the allele for round wings, and the allele for brown eye color is recessive to the allele for red eye color. A scientist crossed flies that are heterozygous for both traits and determined the number of offspring with each combination of phenotypes. The scientist performed a chi-square analysis to determine if the data are consistent with the expectations for independent assortment. The chi-square calculated value for the experiment was 6.03 Based on the chi-square calculated value, which of the following statements is most accurate?

The chi-square calculated value is less than the chi-square critical value; therefore the null hypothesis should not be rejected. Degrees of freedom equals n−1, where n is the number of different categories, or in this example, phenotypes, into which the data are divided. Degrees of freedom is important because a greater number of categories means a greater likelihood of deviation from the expected value. The chi-square critical value for three degrees of freedom = 3 and p>0.05 is 7.81. Because the calculated value 6.03 is less than 7.81, the observed frequencies do not significantly deviate from the expectations for independent assortment, and the null hypothesis is not rejected.

In pea plants, flower color and the length of the flower's pollen grains are genetically determined. Researchers studying pea plants crossed homozygous dominant pea plants with homozygous recessive pea plants. The F1 plants were then crossed, and the number of offspring with each phenotype was recorded. The researchers' observed data, however, differed from the expected data. The researchers did a chi-square analysis and calculated the chi-square value to be 5.5 Based on their calculation, the researchers would most likely conclude which of the following?

The differences between observed data and expected data are due to chance. The calculated chi-square value is less than the critical value for three degrees of freedom (four possible phenotype combinations −1), indicating the researchers will fail to reject the null hypothesis.

F1 generation

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

Genotype

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

A researcher observes that when two heterozygous plants with red flowers are crossed, the resulting offspring include plants with red, white, or pink flowers. The researcher proposes the null hypothesis that flower color is the result of independent assortment and incomplete dominance. The researcher calculates a chi-square value of 7.37.3. Assuming two degrees of freedom, which of the following is the correct interpretation of the chi-square analysis, using a pp-value of 0.050.05 ?

The null hypothesis should be rejected because the critical value is less than the calculated value.

Phenotype

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

F2 Generation

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

incomplete dominance

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

Codominance

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

complete dominance

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

P Generation

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

Explain why the dihybrid cross, detailed in Figure 14.12 in your text, has four yellow Labrador retrievers instead of the three that would have been predicted by Mendel's work.

This dihybrid cross results in four yellow Labrador retrievers rather than three because the dominant allele, symbolized by E, results in the deposition of either black or brown pigment. If the Lab is homozygous recessive for the second locus (ee), then the coat is yellow, regardless of the genotype at the black/brown locus. The E/e gene is epistatic to the B/b gene.

Describe what you think is medically important to know about the behavior of recessive alleles.

Thousands of genetic disorders are known to be inherited as simple recessive traits. These disorders range in severity from relatively mild, such as albinism (lack of pigmentation, which results in susceptibility to skin cancers and vision problems) to life-threatening, such as cystic fibrosis.

Why are test crosses used?

To determine genotype of individuals showing dominant phenotype. It is unknown (A__) x aa (homozygous recessive).

In peas the trait for tall plants is dominant (T) and the trait for short plants is recessive (t). The trait for yellow seed color is dominant (Y) and the trait for green seed color is recessive (y). A cross between two plants results in 296 tall yellow plants and 104 tall green plants. Which of the following are most likely to be the genotypes of the parents?

TtYy x TTYy

For the following human inherited conditions, describe the condition and give the pattern of inheritance.

albinism: A lack of skin pigmentation caused by inheriting two recessive alleles. Tay-Sachs disease: A human genetic disease caused by the inheritance of two recessive alleles 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. cystic fibrosis: 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. sickle-cell disease: A recessively inherited human blood disorder in which a single nucleotide change in theβ-globin gene causes hemoglobin to aggregate, changing red blood cell shape and causing multiple symptoms in afflicted individuals. achondroplasia: A form of dwarfism, caused by a dominant allele, that occurs in 1 of every 25,000 people. Heterozygous individuals have the dwarf phenotype. Like the presence of extra fingers or toes, achondroplasia is a trait for which the recessive allele is much more prevalent than the corresponding dominant allele. Huntington's disease: A human genetic disease caused by a dominant allele; characterized by uncontrollable body movements and degeneration of the nervous system; usually fatal 10 to 20 years after the onset of symptoms.