Genetics Dynamic Study Module Ch.4 Extensions Of Mendelian Genetics

A trait is known to exhibit codominance. Two heterozygous organisms are crossed (Aa X Aa). What is the phenotypic ratio?

A: 1:2:1 Explanation: In codominance, both the phenotypic and genotypic ratio is 1:2:1 (for a heterozygous self cross). The Punnett square produces 1 organism with AA genotype & phenotype, 2 organisms with the Aa genotype & phenotype, 1 organism with the aa genotype & phenotype. If the characteristic exhibits classical Mendelian inheritance, a 3:1 ratio would be produced. The 9:3:3:1 ratio is the ratio expected from a dihybrid cross. 9:3:4 is the modified ratio produced by a dominant epistasis relationship between two genes.

Which type of hemoglobin molecules are produced by an individual who is a heterozygous carrier for Sickle Cell Disease?

A: 50% of the hemoglobin has the standard shape and 50% of the hemoglobin has the altered shape. Explanation: The two alleles for production of adult hemoglobin interact in a codominant manner. Since they are codominant, heterozygous individuals are capable of producing normal hemoglobin and the altered shape hemoglobin. 50% of the hemoglobin has the standard shape and 50% of the hemoglobin has the altered shape. A heterozygous individual does not produce the normal hemoglobin exclusively. Only individuals who are homozygous for the non-mutated allele produce exclusively normal hemoglobin. A heterozygous individual does not produce the abnormal hemoglobin exclusively. Only individuals who are homozygous for the mutated allele produce exclusively abnormal hemoglobin. A ratio of 3:1 describes a group of organisms produced by a genetic cross (a monohybrid F1 self cross, for example).

A homozygous dominant genotype for gene A prevents the expression of gene B. Which gene is epistatic?

A: A Explanation: When a specific genotype for one gene prevents the expression of a separate gene, the one that masks another is said to be epistatic. The gene whose expression is suppressed is called the hypostatic gene. In this case, where a homozygous dominant genotype for gene A prevents the expression of gene B, gene A is epistatic. Gene B is being suppressed in the situation described, therefore it is the hypostatic gene. In an epistatic interaction between two separate genes, only one is suppressive/epistatic. In an epistatic interaction between two separate genes, one is suppressive/epistatic while the other is hypostatic/ suppressed.

If a white-eyed female Drosophila is crossed with a red-eyed male, which offspring are expected?

A: All female offspring have red eyes; all male offspring have white eyes. Explanation: An X-linked gene influences eye color in Drosophila. Red is a dominant allele and white is a recessive allele. A white-eyed female is homozygous recessive (Xw Xw) and a red-eyed male is hemizygous dominant (XR Y). When these two flies are crossed, all male offspring receive the Xw from their mother, and will have white eyes. Similarly, all female offspring receive an XR from their father and express red eyes. The cross described cannot produce females with white eyes because the male parent gives an XR to every daughter. Further, it cannot produce males with red eyes because the female parent gives the white allele (Xw) to all of her sons. The cross described cannot produce all red-eyed offspring because the female parent gives the white allele (Xw) to all of her sons. The cross described cannot produce all white-eyed offspring because the male parent gives an XR to every daughter.

What term is used to indicate the alternative forms of genes?

A: Alleles Explanation: The alternative forms of genes are called alleles. Diploid organisms receive one copy of each gene (one allele) from each of their parents. A locus is the location/physical position a gene occupies along the length of a chromosome. A chromosome is a piece of DNA that is condensed and organized into a linear structure in eukaryotic cells. There are alleles/loci along each chromosome. The centromere is a constricted region on a chromosome, used to attach the chromosome to the cell division spindle.

An individual whose blood type is AB marries someone whose blood type is O. Which phenotypes/ blood types would you expect among their offspring?

A: Blood type A and blood type B. Explanation:50% of the offspring from this cross are expected to have blood type A, and 50% of the offspring are expected to have blood type B. The first parent produces A gametes and B gametes; the second parent produces all i gametes. A standard Punnett square yields equal numbers of genotypes IAi and IBi. Neither AB nor O blood types would be produced from this cross. The parental gene pair separates during sexual reproduction, therefore the A allele and the B allele would not be present in any one offspring. Since the pair of alleles separate during gamete production, it is not possible for an offspring to include both the A and B alleles present in parent one. Blood type inheritance is well understood; therefore, a prediction can be made concerning this ratio.

A true-breeding Black rodent is crossed with a true breeding White rodent. All of the offspring are speckled Black and White. Which inheritance pattern is represented?

A: Codominance Explanation: In codominance heterozygous individuals express both alleles at the same time. Per the example described in this question, both Black pigment and White pigment are produced in the heterozygous F1 individuals. Partial dominance refers to the allelic relationship in which heterozygotes express neither the standard dominant phenotype nor the standard recessive phenotype. Instead, they appear to be an intermediate of the two alleles/phenotypes. Incomplete dominance is the same as partial dominance, as described above. X linkage refers to alleles that are located on the X chromosome. The distribution of phenotypes in males and in females is affected by X linkage.

Purple is the wild type color in a certain flower. Two white mutants are crossed, and all resulting offspring are purple. Which phenomenon is being exhibited?

A: Complementation Explanation: When two recessive mutants are crossed and their offspring exhibit the wild type phenotype, this is complementation. The two mutants are phenotypically the same, but the gene responsible for the mutant phenotype is different in the two mutants. Since the genes in the mutants are different, they can complete each other's deficiencies and produce wild type offspring. Codominance is an inheritance pattern in which heterozygous organisms exhibit a phenotype that includes both alleles. Partial dominance is an inheritance pattern in which heterozygous organisms exhibit an intermediate phenotype. Epigenesis is an increasingly complex series of interactions during embryonic development.

Which inheritance pattern is characterized by heterozygous organisms who exhibit an intermediate phenotype?

A: Incomplete dominance Explanation: In incomplete dominance neither of the alleles is able to dominate the other. As a result, heterozygous organisms show a blended or intermediate phenotype. For example, if one allele produces a black pigment and the other allele produces a white pigment, all heterozygous organisms would be gray. In codominance both alleles are expressed in a heterozygous individual (black & white spots). A null mutation is a change in DNA composition that completely prevents the function of the protein the gene codes for. A neutral mutation is a change in DNA composition that does not alter the function of the protein the gene codes for.

Polydactyly is the presence of extra digits (extra toes or fingers). Many individuals have the genotype for polydactyly, but they do not develop extra digits. Which inheritance principle is this?

A: Incomplete penetrance Explanation: Polydactyly exhibits incomplete penetrance. In any situation where a genotype is not expressed, it can be said that the expected phenotype did not penetrate. There are many gene interactions and environmental influences that can be the ultimate source of incomplete penetrance. Variable expressivity is a condition in which the degree of expression varies greatly among individuals with the same genotype. For example, there are many different degrees of cleft palate, even among individuals with the same genotype. Sex-influenced inheritance is a sex-based form of variable expressivity. It is not an example of incomplete penetrance because it does not lead to complete absence of a trait in one sex. Partial dominance is an allelic interaction in which heterozygous organisms exhibit an intermediate phenotype.

How are new alleles formed?

A: Mutation Explanation: Randomly generated mutations lead to the production of modified proteins. These new proteins have slightly different functions; as variations of the original, they are new alleles. Some alleles function in a dominant fashion in heterozygous organisms. Being dominant does not generate new alleles. Sexual reproduction produces organisms with diverse combinations of alleles. Sexual reproduction does not cause new alleles to develop. New alleles can contribute to evolutionary fitness by creating protein variants that provide a survival advantage.

A dihybrid cross produced a 9:3:4 ratio. What does this ratio suggest?

A: Recessive epistasis Explanation: Recessive epistasis yields a 9:3:4 phenotypic ratio, as a modification to the expected 9:3:3:1 ratio. The modified ratio is the result of the homozygous recessive genotype for one of the genes masking the expression of the other gene. The organisms in whom the masking occurs are represented by the 4 in the modified ratio (3 organisms with the masked phenotype, plus 1 organism actually being homozygous recessive for both genes). Dominant epistasis changes expected dihybrid ratios. The ratio presented with dominant epistasis is 12:3:1. The group of 12 includes 9 individuals who are dominant for both traits, plus 3 individuals in whom the masking occurred. In incomplete dominance, phenotypic ratios are altered, yielding a 1:2:1 ratio. Lethal alleles can alter phenotypic ratios. The modifications made by lethal alleles are modifications to a 3:1 ratio.

The expression of some autosomal genes can be influenced by the sex of the organism. In particular, some genes are expressed more strongly (but not exclusively) in one sex. What is this called?

A: Sex-influenced Explanation: A sex-influenced gene is an autosomal gene, but the intensity of expression is modified by whether or not the individual is male or female. For example, pattern baldness is autosomally determined, therefore both males and females can inherit this characteristic. The degree of hair loss is greater in males than in females. Sex linked refers to a gene that is located on either the X or the Y chromosome. Pleiotropy is the ability of one gene to control the expression of more than one phenotype. Sex-limited genes are autosomal genes that cannot be expressed in both sexes.

What does the term multiple alleles mean?

A: There are more than two possible alleles for one gene in a population. Explanation: Each individual possesses a pair of alleles for each trait; this is represented as their genotype. While individuals possess one exact pair for each, when the allelic variants for an entire population are surveyed it is possible for more than two possible variants to exist in the whole population. While the sum of all allelic variants in a population can be three or more, each individual possesses precisely two alleles for each characteristic. One allele for each trait is donated by the individuals' two parents. There are more than two possible genotypes for all genes in diploid organisms; this is not related to the number of alleles in the population. It is not possible for one individual to express two different phenotypes for one gene.

Compared to standard Mendelian inheritance patterns, what do codominance, partial dominance, lethal Alleles, and epistasis have in common?

A: They alter phenotypic ratios. Explanation: All of these inheritance patterns (codominance, partial dominance, epistasis, lethal alleles) alter phenotypic ratios compared to those expected in Mendelian Inheritance. The alterations are different in each case, but ratios outside the scope of those Mendel observed will occur for each. Only lethal alleles will directly lead to the death of an organism. In codominance and in partial dominance, a unique phenotype is presented in heterozygous organisms. Epistasis leads to a masked phenotypic expression in separate genes.

What term describes the phenotype most commonly found in a population?

A: Wild-Type Explanation: If a random sample of organisms is collected in nature (in "the wild"), whichever phenotypic variation is observed in the majority of those organisms is designated as the "wild-type." In a population of organisms, any phenotypic variant that is different from the wild type is considered a "mutant" because it is a deviation from the "normal" type/ wild type. The term dominant describes an allele's affect within an individual gene. A dominant allele is observed in heterozygous organisms. The term recessive describes an allele's impact in a genotype. A recessive allele is not expressed in heterozygous organisms.

Which inheritance pattern is not a component in the inheritance of blood types in humans?

A: X linkage Explanation: The inheritance of blood types in humans is not determined by genes on the X chromosome. An individual's biological sex does not affect the manner in which his/her blood type is inherited. There are three alleles for gene "I." These include the dominant A version, the dominant B version, and the recessive i version. Any number of alleles greater than two is classified as multiple alleles. The A allele for gene "I" and the B allele for gene "I" are considered codominant. Individuals who have the heterozygous genotype IAIB produce both forms of blood type antigen. The function of the I gene in producing blood type is connected to the function of another gene "H." A homozygous recessive genotype hh prevents the expression of the I gene. As such, hh is epistatic to the I genotype.

Ebony body color in the fruit fly is considered a recessive mutant phenotype. What is the associated genotype of ebony fruit flies?

A: e / e Explanation: When using designations for mutant and wild-type, genotypes are written to include a letter symbolizing the locus and a "+" for the known wild-type allele. Sine Ebony eye color is recessive, the organism would have two alleles, neither of which includes the + designation, and thus would be e / e. The genotype e+ / e is a heterozygous genotype with one wild-type allele and one recessive allele. The genotype e+ / e + is a homozygous genotype with two wild-type alleles (two dominant alleles). The designation e+ includes only one allele, therefore it is not a genotype. Genotypes include two alleles.

The phenomenon in which a single gene influences the development of many phenotypes is called _____.

A: pleiotropy Explanation: The word pleiotropy means many (pleio) effects (tropy). It is a description of genes that produce a protein product that is used in many body tissues. Since the protein product is used in many body tissues, the gene affects many phenotypes. Multiple alleles is different from pleiotropy because only one phenotype is affected. The word "multiple" refers to alleles, not to affects. In epistasis more than one gene interacts in the production of a single phenotype. When an allele is dominant, it has preferential expression in heterozygous organisms. Dominance does not necessarily allow the gene to affect multiple phenotypes.