1. Extensions and Modifications of
Benjamin A. Pierce
GENETICS
A Conceptual Approach
SIXTH EDITION
CHAPTER 5
Extensions and Modifications of
Basic Principles
© 2017 W. H. Freeman and Company

2. The direction of shell coiling in snails is determined by genetic maternal effect.

3. 5.1 Additional Factors at a Single Locus Can Affect the Results of Genetic Crosses
Genes at the same locus—two versions of the same gene; each version of the same gene is defined as allele
Types of dominance
Complete dominance
Incomplete dominance
Codominance

4. 5.1 The type of dominance exhibited by a trait depends on how the phenotype of the heterozygote relates to the phenotypes of the homozygotes.

5. 5.2 Fruit color in eggplant is inherited as an incompletely dominant trait.

6. TABLE 5.1
Differences between complete dominance, incomplete dominance, and codominance
Type of Dominance
Definition
Complete dominance
Phenotype of the heterozygote is the same as the phenotype of one of the homozygotes.
Incomplete dominance
Phenotype of the heterozygote is intermediate (falls within the range) between the phenotypes of the two homozygotes.
Codominance
Phenotype of the heterozygote includes the phenotypes of both homozygotes.

7. Level of phenotype may affect dominance. Example: CFTR
5.1 Additional Factors at a Single Locus Can Affect the Results of Genetic Crosses
Level of phenotype may affect dominance.
Example: CFTR
Channel in the membrane that regulates chloride levels
Mutated in cystic fibrosis
In CF, mutated channel stays closed, so chloride ions build up in cell
In heterozygotes, enough functional CFTR is made to prevent cystic fibrosis

8. 5.1 Additional Factors at a Single Locus Can Affect the Results of Genetic Crosses
Penetrance: the percentage of individuals having a particular genotype that express the expected phenotype
Expressivity: the degree to which a trait is expressed

9. 5.3 Human polydactyly is an example of incomplete dominance and variable expressivity.

10. Concept Check 1
Assume that long fingers are inherited as a recessive trait with 80% penetrance.
Two people heterozygous for long fingers mate. What is the probability that their first child will have long fingers?

11. Concept Check 1
Assume that long fingers are inherited as a recessive trait with 80% penetrance.
Two people heterozygous for long fingers mate. What is the probability that their first child will have long fingers?
¼ x 80% = 20%

12. 5.1 Additional Factors at a Single Locus Can Affect the Results of Genetic Crosses
A lethal allele causes death at an early stage of development, so some genotypes may not appear among the progeny.
Affects the Mendelian genotypic and phenotypic ratios in progeny

13. 5.4 The 2 : 1 ratio produced by a cross between two yellow mice results from a lethal allele.

14. Concept Check 2
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 and the white progeny?

15. Concept Check 2
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 and the white progeny?
White genotype: GG
Green genotype: Gg
gg: lethal allele causing death in homozygous

16. 5.1 Additional Factors at a Single Locus Can Affect the Results of Genetic Crosses
Multiple alleles: For a given locus, more than two alleles are present within a group of individuals.
ABO blood group

17. 5.5 Mendel’s principle of segregation applies to crosses with multiple alleles. In this example, three alleles determine the type of plumage in mallard ducks: MR (restricted) > M (mallard) > md (dusky).

18. 5.6 ABO blood types and possible blood transfusions.

19. Concept Check 3
What blood types are possible among the children of a cross between a man who is blood-type A and a woman of blood-type B?

20. Concept Check 3
What blood types are possible among the children of a cross between a man who is blood-type A and a woman of blood-type B?
Could be A type with a genotype of IAIA and IAi
Could be B type with a genotype of IBIB or IBi
Could be AB type with a genotype of IAIB
Could also be a O type with a genotype of ii

21. 5.2 Gene Interaction Takes Place When Genes at Multiple Loci Determine a Single Phenotype
Gene interaction: Effects of genes at one locus depend on the presence of genes at other loci.
Gene interaction produces novel phenotypes.
Gene interaction with epistasis: One gene masks the effect of another gene.

22. 5.7 Gene interaction in which two loci determine a single characteristic, fruit color, in the pepper Capsicum annuum.

23. 5.2 Gene Interaction Takes Place When Genes at Multiple Loci Determine a Single Phenotype
Epistasis: One gene masks the effect of another gene.
Recessive epistasis: Fig. 5.8
Dominant epistasis: Fig. 5.9
Duplicate recessive epistasis: Fig. 5.10

24. 5. 8 Expression of the ABO antigens depends on alleles at the H locus
5.8 Expression of the ABO antigens depends on alleles at the H locus. The H locus encodes a precursor to the antigens called compound H. Alleles at the ABO locus determine which types of terminal sugars are added to compound H.

25. 5.9 Yellow pigment in summer squash is produced in a two-step pathway.

26. 5.10 Pigment is produced in a two-step pathway in snails.

27. Modified dihybrid phenotypic ratios due to gene interaction
TABLE 5.2
Modified dihybrid phenotypic ratios due to gene interaction
Ratio*
Genotype
Type of Interaction
Example Discussed in Chapter
A_ B_
A_ bb
aa B_
aa bb
9 : 3 : 3 : 1 9 3 1
None
Seed shape and seed color in peas
9 : 3 : 4 4
Recessive epistasis
Coat color in Labrador retrievers
12 : 3 : 1 12
Dominant epistasis
Color in squash
9 : 7 7
Duplicate recessive epistasis
Albinism in snails
9 : 6 : 1 6
Duplicate interaction —
15 : 1 15
Duplicate dominant epistasis
13 : 3 13
Dominant and recessive epistasis
*Each ratio is produced by a dihybrid cross (Aa Bb × Aa Bb). Shaded bars represent combinations of genotypes that give the same phenotype.

28. 5.2 Gene Interaction Takes Place When Genes at Multiple Loci Determine a Single Phenotype
Complementation: determine whether mutations are at the same locus or at different loci.
The complex genetics of coat color in dogs:
Agouti (A) locus
Black (B) locus
Extension (E) locus
Spotting (S) locus

29. 5.11 Coat color in dogs is determined by interactions between genes at a number of loci.

30. Common genotypes in different breeds of dogs
TABLE 5.3
Common genotypes in different breeds of dogs
Breed
Usual Homozygous Genotypes*
Other Alleles Present Within the Breed
Basset hound
BB EE
ay,at S, sP, si
Beagle
asas BB sPsP
E, e
English bulldog BB
As, ay, at Em, E, ebr S, si, sP, sw
Chihuahua
As, ay, as, at B, b Em, E, ebr, e S, si, sP, sw
Collie
ay, at si, sw
Dalmatian
AsAs EE swsw
B, b
Doberman
atat EE SS
German shepherd
BB SS
ay, a, as, at Em, E, e
Golden retriever
AsAs BB SS
Greyhound
As, ay E, ebr, e S, sP, sw, si
Irish setter
BB ee SS
As, at
Labrador retriever
AsAs SS
B, b E, e
Poodle SS
As, at B, b E, e
Rottweiler
atat BB EE SS
St. Bernard
ayay BB
Em, E si, sp, sw
*Most dogs in the breed are homozygous for these genes; a few individual dogs may possess other alleles at these loci. Source: Data from M. B. Willis, Genetics of the Dog (London: Witherby, 1989).

31. Sex-influenced and sex-limited characteristics
5.3 Sex Influences the Inheritance and Expression of Genes in a Variety of Ways
Sex-influenced and sex-limited characteristics
Sex-influenced characteristics (Fig. 5.12)
Sex-limited characteristics (Fig. 5.13)
Cytoplasmic inheritance (Figs and 5.16)

32. 5.12 Genes that encode sex-influenced traits are inherited according to Mendel’s principles but are expressed differently in males and females.

33. 5.13 A sex-limited characteristic is encoded by autosomal genes that are expressed in only one sex.

34. Sex-limited characteristics are inherited according to Mendel’s principles. Precocious puberty is an autosomal dominant trait that is limited to males.

35. 5.15 Cytoplasmically inherited characteristics frequently exhibit extensive phenotypic variation because cells and individual offspring contain various proportions of cytoplasmic genes. Mitochondria that have wild-type mtDNA are shown in red; those having mutant mtDNA are shown in blue.

36. Characteristics of cytoplasmically inherited traits
TABLE 5.4
Characteristics of cytoplasmically inherited traits
1. Present in males and females.
2. Usually inherited from one parent, typically the maternal parent.
3. Reciprocal crosses give different results.
4. Exhibit extensive phenotypic variation, even within a single family.

37. 5.16 Crosses for leaf type in four-o’clocks illustrate cytoplasmic inheritance.

38. 5.3 Sex Influences the Inheritance and Expression of Genes in a Variety of Ways
Sex-influenced and sex-limited characteristics
Genetic maternal effect (Fig. 5.17)
Genomic imprinting: differential expression of genetic material depending on whether it is inherited from the male or female parent (Fig. 5.18)
Epigenetics: phenomena due to alterations to DNA that do not include changes in the base sequence; often affect the way in which the DNA sequences are expressed

39. 5.17 In genetic maternal effect, the genotype of the maternal parent determines the phenotype of the offspring. The shell coiling of a snail is a trait that exhibits genetic maternal effect.

40. 5.18 Genomic imprinting of the Igf2 gene in mice and humans affects fetal growth.

41. Influences of sex on heredity
TABLE 5.5
Influences of sex on heredity
Genetic Phenomenon
Phenotype Determined by
Sex-linked characteristic
Genes located on the sex chromosomes
Sex-influenced characteristic
Autosomal genes that are more readily expressed in one sex
Sex-limited characteristic
Autosomal genes whose expression is limited to one sex
Genetic maternal effect
Nuclear genotype of the maternal parent
Cytoplasmic inheritance
Cytoplasmic genes, which are usually inherited from only one parent
Genomic imprinting
Genes whose expression is affected by the sex of the transmitting parent

42. 5.4 Anticipation Is the Stronger or Earlier Expression of Traits in Succeeding Generations
Anticipation: A genetic trait becomes more strongly expressed or is expressed at an earlier stage as it is passed from generation to generation.
Anticipation occurs due to expansion of an unstable region of DNA from generation to generation.

43. 5.5 The Expression of a Genotype May Be Influenced by Environmental Effects
Temperature-sensitive allele is an allele whose product is functional only at certain temperature.
5.19 The expression of the vestigial mutation in Drosophila is temperature dependent. When reared at temperatures below 29°C, flies homozygous for vestigial have greatly reduced wings; at temperatures above 31°C, the flies develop normal wings. [Data from M. H. Harnly, Journal of Experimental Zoology 56:363–379, 1936.]

44. 5. 20 The expression of the himalayan allele is temperature dependent
5.20 The expression of the himalayan allele is temperature dependent. This rabbit was reared below 25°C. Its pigment is restricted to the extremities where the body temperature falls below 25°C and the enzyme that produces pigment is functional. [petra Wegner/alamy.]

45. The Inheritance of Continuous Characteristics
Discontinuous characteristics: relatively few phenotypes
Continuous characteristics: continuous distribution of phenotypes; occurs when genes at many loci interact
Polygenic characteristics: characteristics encoded by genes at many loci
Pleiotropy: one gene affects multiple characteristics