1. Concepts and Connections
Benjamin A. Pierce
Benjamin A. Pierce
GENETICS Essentials
Concepts and Connections
SECOND EDITION
CHAPTER 4
Extensions and Modifications of Basic
Principles PART II
© 2013 W. H. Freeman and Company

2. 4.3 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 population.
ABO blood group
Fig. 4.16

3. Figure 4.16 ABO blood types and possible blood transfusions.

4. 4.4 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 that produces novel phenotypes.
Fig. 4.17
Gene interaction with epistasis
Epistasis: one gene masks the effect of another gene.

5. Gene Interaction that produces novel Phenotype
C. annuum comes in four colors
Red, peach, yellow, cream

6. Gene Interaction that produces novel Phenotype F2 generation

7. Gene Interaction that produces novel Phenotype
Calculate probabilities to see gene interactions:
Testcross reveals different ratios
Do the math of such testcross:
Y+y C+ cross with yy cc
What are the ratios?

8. 4.4 Gene Interaction Takes Place When Genes at Multiple Loci Determine a Single Phenotype
Gene interaction with epistasis
Epistasis: one gene masks the effect of another gene.
Recessive epistasis
Dominant epistasis: Fig. 4.18
Table 5.2

9. Recessive epistasis
Coat color determination in labrador retriever dogs
i. Gene B/_ makes black pigment, while b/b makes brown.
ii. Another gene, E/_, allows expression of the B gene, while e/e does not. The E locus encodes the melanocortin 1 receptor, a regulator of hair and skin color.
Iii. Genotypes and their corresponding phenotypes:
(1) B/_ E/_ is black.
(2) b/b E/_ is brown (chocolate).
(3) _/_ e/e produces yellow with nose and lips either dark (B/_ e/e) or pale (b/b e/e).
Ratio 9black:3brown:4yellow

10. Dominant epistasis
In dominant epistasis A/_ B/_ and A/_ b/b have the same phenotype, producing an F2 ratio of 12:3:1. Examples:
a. Summer squash fruit have three common colors: white, yellow, and green.
i. Yellow is recessive to white but dominant to green.
ii. Gene pairs are W/w and Y/y.
(1)W/_ are white no matter the genotype of the other locus.
(2) w/w are yellow in Y/_ and green in y/y.

11. Duplicate recessive epistasis
The case of the sweet pea flowers:

13. 4.4 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.
Single locus

14. 4.5 Sex Influences the Inheritance and Expression of Genes in a Variety of Ways
Sex-influenced and sex-limited characteristics
Sex-influenced characteristics
Sex-limited characteristics
Cytoplasmic inheritance
Figs & 4.20

15. SEX-INFLUENCED AND SEX-LIMITED
Sex-influenced: somatic gene with Mendelian inheritance that is differently expressed in males and females
Sex-limited: somatic gene that is exclusively expressed in only one sex; zero penetrance in the other.

16. Cytoplasmic inheritance
Genes that are not on nuclear chromosomes;
Mitochondria and chloroplasts
DNA inherited from one parent, usually female egg
Reciprocal cross gives different outcomes
Extensive phenotypic variations
Leber’s hereditary optic neuropathy (LHON, OMIM ). Optic nerve degeneration results in complete or partial blindness in midlife adults.
LHON is caused by mutations in mtDNA genes for electron transport chain proteins.
Kearns-Sayre syndrome (OMIM ) produces three types of neuromuscular defects:
i. Progressive paralysis of certain eye muscles.
ii. Abnormal pigment accumulation on the retina, and degeneration of the retina.
iii. Heart disease.

17. 4.5 Sex Influences the Inheritance and Expression of Genes in a Variety of Ways
Sex-influenced and sex-limited characteristics
Genetic maternal effect
Fig. 4.21
Genomic imprinting: differential expression of genetic material depending on whether it is inherited from the male or female parent.
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.

18. Genetic maternal effect
Phenotype of the offspring is determined by the genotype of the mother. (nuclear genes)
Mostly from the genes expresses in the eggs (oocytes)
Example snail Limnea peregra shell coiling

19. Genomic imprinting
Only the allele of one parent is expressed and determines the phenotype.

20. epigenetics Modification of nucleotides; methylation, acetylation
Changes gene expression and can be different in gametes from each parent
Epigenetic modifications change through life and is influenced by
diet (bee queen and workers are females yet different)
Cell differentiation and signaling
Chemicals

21. 4.6 The Expression of a Genotype May Be Influenced by Environmental Effects
Temperature-sensitive allele:
An allele whose product is functional only at a certain temperature.

22. Inheritance of continuous characteristics
Characteristics with easily distinguishable phenotypes- discontinuous characteristics
Human height- continuous characteristic (quantitative characteristics- can be described-measured)
Polygenic characteristics- many loci influence the characteristic (3n where n is number of loci)
Pleiotropic characteristics- one gene incluences many characteristics