Benjamin
BenjaminA.
A. Pierce
Pierce
•GENETICS Essentials
•Concepts and Connections
• SECOND EDITION
CHAPTER 4
Extensions and Modifications of Basic
Principles PART II
© 2013 W. H. Freeman and Company
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
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.
GENE INTERACTION THAT PRODUCES
NOVEL PHENOTYPE
• C. annuum comes in four colors
• Red, peach, yellow, cream
GENE INTERACTION THAT PRODUCES
NOVEL PHENOTYPE F2 GENERATION
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?
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
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
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.
DUPLICATE RECESSIVE EPISTASIS
• The case of the sweet pea flowers:
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
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. 4.19 & 4.20
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.
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
585000). 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 530000) 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.
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.
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
GENOMIC IMPRINTING
• Only the allele of one parent is expressed and
determines the phenotype.
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
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.
INHERITANCE OF CONTINUOUS
CHARACTERISTICS
• Characteristics with easily distinguishable
phenotypes- discontinuous characteristics
• Human height- continuous characteristic
(quantitative characteristics- can be describedmeasured)
• Polygenic characteristics- many loci influence the
characteristic (3n where n is number of loci)
• Pleiotropic characteristics- one gene incluences
many characteristics