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Exam 4: Extensions and Modifications of Basic Principles

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Question 1

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Epistasis is the masking of the expression of one gene by another gene at a different locus. The epistatic gene does the masking; the hypostatic gene is masked. Epistatic genes can be dominant or recessive. -A number of all-white cats are crossed and they produce the following types of progeny: 12/16 all-white, 3/16 black, and 1/16 gray. What is the genotype of the black progeny?

A) Aa
B) Aa Bb
C) A_ B_
D) A_ bb

E) A) and B)
F) A) and C)

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Question 2

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Incomplete dominance is exhibited when the heterozygote has a phenotype intermediate between the phenotypes of the two homozygotes. When a trait exhibits incomplete dominance, a cross between two heterozygotes produces a 1 : 2 : 1 phenotypic ratio in the progeny -If an F₁ eggplant in Figure above is used in a testcross, what proportion of the progeny from this cross will be white?

A) All the progeny
B) 1/2
C) 1/4
D) 0

E) A) and B)
F) A) and C)

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Question 3

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Characteristics exhibiting cytoplasmic inheritance are encoded by genes in the cytoplasm and are usually inherited from one parent, most commonly the mother. In genetic maternal effect, the genotype of the mother determines the phenotype of the offspring. -How might you determine whether a particular trait is due to cytoplasmic inheritance or to genetic maternal effect?

Penetrance is the percentage of individuals having a particular genotype that express the associated phenotype. Expressivity is the degree to which a trait is expressed. Incomplete penetrance and variable expressivity result from the influence of other genes and environmental factors on the phenotype. -How does incomplete dominance differ from incomplete penetrance?

Sex-influenced characteristics are encoded by autosomal genes that are more readily expressed in one sex. Sex-limited characteristics are encoded by autosomal genes whose expression is limited to one sex. -How do sex-influenced and sex-limited traits differ from sex-linked traits?

A lethal allele causes death, frequently at an early developmental stage, and so one or more genotypes are missing from the progeny of a cross. Lethal alleles modify the ratio of progeny resulting from a cross. -A cross between two green corn plants yields 2/3 progeny that are green and 1/3 progeny that are white. What is the genotype of the green progeny?

The expression of many genes is modified by the environment. A phenocopy is a trait produced by environmental effects that mimics the phenotype produced by the genotype. -How can you determine whether a phenotype such as reduced eyes in fruit flies is due to a recessive mutation or is a phenocopy?

Discontinuous characteristics exhibit a few distinct phenotypes; continuous characteristics exhibit a range of phenotypes. A continuous characteristic is frequently produced when genes at many loci and environmental factors combine to determine a phenotype. -What is the difference between polygeny and pleiotropy?

In gene interaction, genes at different loci contribute to the determination of a single phenotypic characteristic. -How does gene interaction differ from dominance between alleles?

In genomic imprinting, the expression of a gene is influenced by the sex of the parent transmitting the gene to the offspring. Epigenetic marks are reversible changes in DNA that do not alter the base sequence but may affect how a gene is expressed. -What type of epigenetic mark is responsible for genomic imprinting?

Dominance entails interactions between genes at the same locus (allelic genes) and is an aspect of the phenotype; dominance does not affect the way in which genes are inherited. The type of dominance exhibited by a characteristic frequently depends on the level of the phenotype examined. -How do complete dominance, incomplete dominance, and codominance differ?

More than two alleles (multiple alleles) may be present within a group of individual organisms, although each individual diploid organism still has only two alleles at that locus. -How many genotypes are present at a locus with five alleles?

A complementation test is used to determine whether two mutations occur at the same locus (are allelic) or occur at different loci. -Brindle (tiger-striped appearance) is a recessive trait in bulldogs and in Chihuahuas. What types of crosses would you carry out to determine whether the brindle genes in bulldogs and in Chihuahuas are at the same locus?