Exam 4: Extensions and Modifications of Basic Principles

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?

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Question 1

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Cross a bulldog homozygous for brindle with a Chihuahua homozygous for brindle. If the two brindle genes are allelic, all the offspring will be brindle: bb × bb → all bb (brindle). If, on the other hand, brindle in the two breeds is due to recessive genes at different loci, then none of the offspring will be brindle: a⁺a⁺ bb × aa b⁺b⁺ → a⁺a b⁺b.

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?

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Question 2

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Polygeny refers to the influence of multiple genes on the expression of a single characteristic. Pleiotropy refers to the effect of a single gene on the expression of multiple characteristics.

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?

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Question 3

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Gene interaction is interaction between genes at different loci. Dominance is interaction between alleles at a single locus.

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?

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Question 4

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?

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Question 5

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?

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Question 6

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?

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Question 7

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?

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Question 8

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?

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Question 9

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?

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Question 10

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?

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Question 11

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?

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Question 12

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?

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Question 13

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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?

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?

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?

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?

Introduction to Genetics, Chromosomes and Cellular Reproduction

Basic Principles of Heredity

Sex Determination and Sex-Linked Characteristics

Pedigree Analysis, Applications, and Genetic Testing

Linkage, Recombination, and Eukaryotic Gene Mapping

Bacterial and Viral Genetic Systems

Chromosome Variation

DNA: The Chemical Nature of the Gene

Chromosome Structure and Transposable Elements

DNA Replication and Recombination

Transcription

RNA Molecules and RNA Processing

The Genetic Code and Translation

Control of Gene Expression in Prokaryotes

Control of Gene Expression in Eukaryotes

Gene Mutations and DNA Repair

Molecular Genetic Analysis and Biotechnology

Genomics and Proteomics

Organelle DNA

Developmental Genetics and Immunogenetics

Cancer Genetics

Quantitative Genetics

Population Genetics

Evolutionary Genetics

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