Deck 21: Quantitative Genetics and Multifactorial Traits

Question 1

Provide a brief description of discontinuous inheritance and continuous inheritance.How are the two related? How are they different?

Answer 1

At the transmission level,one sees "stepwise" distributions in discontinuous inheritance,but "smoother" or more bell-shaped distributions in continuous inheritance as shown in the following figures:
Both patterns are formed from normal Mendelian principles of segregation and independent assortment.The differences are in the manner in which the gene products interact; continuous inheritance involves additive effects.

Question 2

Continuous inheritance is often related to the term quantitative inheritance.Why?

Answer 2

In continuous inheritance,each involved locus has a quantitative input on the production of a single characteristic of the phenotype.In addition,although it may not always be the case,each gene product would be considered to be qualitatively similar.

Question 3

Which formula can be used to determine the number of categories (phenotypes)possible in the F? results of a polygenic system?

Answer 3

2n + 1 (where n is the number of gene pairs)

Question 4

What is the term given to a random subset of individuals who are selected for measurement in a particular study?

Question 5

The 9:3:3:1 ratio is typical of a dihybrid cross in which complete dominance and independent assortment occur.What is the dihybrid ratio with independent assortment of polygenes?

Question 6

If the proportion of F? individuals resembling either of the two most extreme parental phenotypes can be determined,which formula can be applied to determine the number of gene pairs involved?

Question 7

How many gene pairs are involved in generating a typical 1:4:6:4:1 ratio?

Question 8

Assume that in the F? of a series of crosses,1/64 of the offspring resemble one of the parents (P).How many gene pairs are involved in producing these results?

Question 9

Given the following numbers,calculate the mean: 10,12,14,16,18.

Question 10

Environmental factors typically influence inheritance of ________.

A)multiple alleles
B)codominance
C)trihybrid crosses
D)polygenic traits
E)dominantly inherited traits

Question 11

Assume that four polygenic gene pairs are involved in determining phenotypes of F?.How many phenotypic classes are expected?

Question 12

Quantitative inheritance involves the interaction of a number of gene loci.The pattern of genetic transmission typical of quantitative inheritance is ________.

A)discontinuous distributions such as 3:1
B)typical of Mendelian inheritance
C)continuous variation of phenotypic expression
D)a 9:3:3:1 ratio
E)usually a pattern that clearly reflects dominance and recessiveness

Question 13

When a trait is polygenic but distinguished from continuous and meristic traits by having a small number of discrete classes,it is said to be a(n)________ trait.

Question 14

List at least three statistical values often used to represent a sample.

Question 15

In the early part of the twentieth century,Nilsson-Ehle and others described experiments showing that multiple loci may be involved in the inheritance of certain traits.Such patterns are often called ________.

Question 16

Assume that a cross is made between tall and dwarf tobacco plants.The F? generation showed intermediate height,whereas the F? generation showed a distribution of height ranging from tall to dwarf,like the original parents,and many heights between the extremes.These data are consistent with which one of the following modes of inheritance?

A)multiple-factor inheritance
B)alternation of generations
C)codominance
D)incomplete dominance
E)hemizygosity

Question 17

Assume that you are studying a trait determined by a number of polygenes.If there are seven phenotypic categories,how many genes are probably involved?

Question 18

Characteristics exhibited by continuously varying traits include ________.

A)sex-linked genes only
B)autosomal genes only
C)quantification by measuring,weighing,counting,etc.
D)a 9:3:3:1 ratio
E)3:1 and 1:1 ratios

Question 19

What are the name and formula for describing sample variability?

Question 20

Bell-shaped distributions produced by plotting results of F? and F? crosses are typical of which type of inheritance?

A)multiple-factor inheritance
B)alternation of generations
C)codominance
D)incomplete dominance
E)hemizygosity

Question 21

Individuals with the same genetic background and a high degree of homozygosity are said to be isogenic.Of what value are isogenic strains in genetic studies?

Question 22

Provide a formal equation for h² (narrow-sense heritability).

Question 23

Name two mammalian traits with relatively high heritability (h²)values and two with relatively low h² values.

Question 24

We often refer to the host of genetic factors in the genome that have an influence on a particular gene's expression as its genetic background.What influence does genetic background have over the phenotype of the organism?

Question 25

As used in twin studies in mammals,distinguish between the terms concordant and discordant.

Question 26

Name two human traits that would be expected to have high concordance.

Question 27

What is meant by the term heritability? Describe the components of heritability and provide a brief explanation of each.Of what interest is heritability to animal and plant breeders?

Question 28

What is the formal expression used to examine the relative importance of genetic versus environmental factors?

Question 29

What is the name of the process of selecting a specific group of organisms from an initially heterogeneous population for future breeding purposes?

Question 30

Name the three components of phenotypic variance.

Question 31

List at least two statistical terms commonly used in the analysis of quantitative traits.

Question 32

Provide a brief definition of the terms additive variance,dominance variance,and interactive variance.

Question 33

Interpret the meaning of an H² (broad-sense heritability)value that approaches 0.0.

Question 34

Huntington disease is inherited in humans as an autosomal dominant gene.Affected individuals show progressive brain deterioration from cell death in the cerebral cortex.Onset of the disease usually occurs between ages 30 and 50.Give two possible reasons for the observed variation in the age of onset of this disease.

Question 35

To estimate how much the means of a variety of like samples drawn from the same population might vary,which statistic is often used and what is its formula?

Question 36

Inbred strains with individuals of a relatively homogeneous or constant genetic background are often used to determine the influence of genes or the environment on phenotypic variation.Variation observed between different inbred strains reared in a constant or homogeneous environment would likely be caused by genetic factors.What would be the source of variation observed among members of the same inbred strain reared under varying environmental conditions?

Question 37

Interpret the meaning of an H² (broad-sense heritability)value that approaches 1.0.

Question 38

What is the relationship between the narrow-sense heritability (h²)index and the impact of selection?

Question 39

Describe the value of using twins in the study of questions relating to the relative impact of heredity versus environment.

Question 40

Define the term broad-sense heritability (H²?.What is implied by a relatively high value of H²? Express aspects of broad-sense heritability in equation form.

Question 41

A "marker" in a genetic sense usually represents a site along a chromosome where a specific nucleotide sequence exists.Which specific phrase is used when such markers are identified by restriction endonucleases and a particular set of DNA fragments is generated?

Question 42

Polygenes are involved in determining continuously varying or multiple-factor traits.

Question 43

How would the use of a large series of monozygotic and dizygotic twins enhance studies on the genetic basis of human behavior?

Question 44

Additive alleles are those that are epistatic over nonallelic genes that influence the same phenotypic characteristic.

Question 45

Heritability is a measure of the degree to which the phenotypic variation of a given trait is due to genetic factors.

Question 46

Traits such as height,general body structure,skin color,and some behavioral traits are probably caused primarily by genes that behave codominantly or epistatically.

Question 47

Concordance refers to the frequency with which members of a twin pair express a different trait.

Question 48

Which two relatively recent extensions to QTL mapping center on molecular definitions of phenotypes with quantitative traits?

Question 49

The multiple factor hypothesis suggests that many factors or genes contribute to the phenotype in a cumulative or quantitative manner.

Question 50

In the analysis of quantitative traits,positions on chromosomes called quantitative trait loci (QTLs)are often discussed.In the same context,restriction fragment length polymorphisms (RFLPs)are also discussed.What is the relationship between QTLs and RFLPs?

Question 51

What are QTLs and RFLPs?

Question 52

Typically,one thinks of identical (MZ)twins being genetically identical.However,genomic differences do exist between MZ twins by epigenetic processes.What are epigenetic processes?