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Benjamin
BenjaminA.
A. Pierce
Pierce
•GENETICS Essentials
•Concepts and Connections
• SECOND EDITION
CHAPTER 4
Extensions and Modifications of Basic
Principles PART I
© 2013 W. H. Freeman and Company
CHAPTER 4 OUTLINE
• 4.1 Sex Is Determined by a Number of Different
Mechanisms, 74
• 4.2 Sex-Linked Characteristics Are Determined by
Genes on the Sex Chromosomes, 79
• 4.3 Additional Factors at a Single Locus Can Affect the
Results of Genetic Crosses, 87
• 4.4 Gene Interaction Takes Place When Genes at
Multiple Loci Determine a Single Phenotype, 91
• 4.5 Sex Influences the Inheritance and Expression of
Genes in a Variety of Ways, 97
• 4.6 The Expression of a Genotype May Be Influenced by
Environmental Effects
THE CASE OF YELLOW MICE
• All of the sudden Mendel’s rules do not work
as predicted
• What happened??
CHROMOSOMAL SEX-DETERMINATION SYSTEMS:
SEX CHROMOSOMES AND NON-SEX
CHROMOSOMES (AUTOSOMES)
• XX-XO system:
• XX – female (homogametic sex)
• XO – male (heterogametic sex)
• grasshoppers
• XX-XY system:
• XX – female (homogametic sex)
• XY – male (heterogametic sex)
• Mammals
• ZZ-ZW system:
• ZZ – male
• ZW – female
• Birds, snakes, butterflies, some amphibians, and fishes
• Autosomes- remaining of the chromosomes that are same in
both sexes
GENIC SEX-DETERMINING SYSTEM
• No sex chromosomes, only the sex-determining
genes at certain loci
• Plants, fungi, protists
ENVIRONMENTAL SEX DETERMINATION
• Environmental factors
• Temperature in turtles
SEX DETERMINATION IN DROSOPHILA
MELANOGASTER
• Genic balance system
• X:A ratio
• X, number of X chromosomes;
• A, number of haploid sets of
autosomes
SEX DETERMINATION IN HUMANS XX-XY
• SRY gene on the Y chromosome determines
maleness
1/1000
1/3000
1/1000
THE ROLE OF SEX CHROMOSOMES
• The X chromosome contains genetic information
essential for both sexes; at least one copy of an
X is required.
• The male-determining gene is located on the Y
chromosome. A single Y, even in the presence of
several X, still produces a male phenotype.
• The absence of Y results in a female phenotype.
THE MALE-DETERMINING GENE IN
HUMANS
• Sex-determining region Y (SRY) gene
• Androgen-insensitivity syndrome
• Caused by the defective androgen receptor
4.2 SEX-LINKED CHARACTERISTICS ARE
DETERMINED BY GENES ON THE SEX
CHROMOSOMES
• X-linked characteristics
• X-linked white eye in Drosophila
• X-linked color blindness in humans
Morgan’s Experiments
Drosophila eye color
• Growing Drosophila found white eyed male
• Cross of wild type red female with the white eyed male
• F1 all red eyed for that cross
Morgan’s Experiments
Drosophila eye color F1 cross
• The F1 generation offspring all had red eyes
• So far looks like Mendel’s dominant/recessive red over white
• Cross females and males of the F1 generation
Morgan’s Experiments
Drosophila eye color F2 generation
• As expected 3:1 red:white HOWEVER only males are
white eyed
• Embryonic lethal?
Morgan’s Experiments
Drosophila eye color the Reciprocal cross
• Cross of white eyed female with wild type male
• F1 ratio 1:1 red eyed females/white eyed males
Morgan’s Experiments
Drosophila eye color F1 cross
• The F1 generation offspring represents departure
from Mendel’s rules
• Cross females and males of the F1 generation
Morgan’s Experiments
Drosophila eye color F2 generation
• 1:1 red:white but now also white eyed females
COLOR BLINDNESS IN HUMANS
ALLELES AND CHROMOSOME LABELING
• There are rules for lableing of alleles in sex linked
traits:
•
•
•
•
X+ X+ is a female with both wild type alleles
X+Y is a male with wild type allele
X+ Xw is a female heterozygous for the white allele
Xw Y is a male hemizygous for the white allele and affected
• Since Y does not carry the homologous gene the
labeling can be expressed as X+/
X-LINKED CHARACTERISTICS:
DOSAGE COMPENSATION
• The amount of protein produced by X-linked
genes is equal in both sexes regardless to the
fact that females have two copies of those genes
• Solutions vary
• Fruit flies: dosage compensation is achieved by a doubling
of the activity of the genes on the X chromosome of the
male.
• The worm C. elegans: it is achieved by a halving of the
activity of genes on both of the X chromosomes in the
female
• Placental mammals: genes on one of the X chromosomes in
the female are inactivated creating Barr body
BARR BODY
X-INACTIVATION:
THE CATS
• X linked locus for fur color:
• X+, which produces nonorange (usually black) fur,
• Xo, which produces orange fur
• Tortoise cats
• Calico cats
Y-CHROMOSOME TRAITS
4.3 ADDITIONAL FACTORS AT A SINGLE
LOCUS CAN AFFECT THE RESULTS OF
GENETIC CROSSES
• Incomplete dominance
• Codominance
• Table 4.1
INCOMPLETE DOMINANCE
CODOMINANCE: BLOOD TYPES
4.3 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 character is
expressed.
PENETRANCE
• Penetrance: the percentage of individuals having a
particular genotype that express the expected
phenotype.
• The genotypes of the population have to be known
• If 83 individuals out of 100 with genotype that should produce a
certain phenotype express the phenotype the penetrance is 83%
• Polydactyly
EXPRESSIVITY
• Expressivity: The degree to which a character is
expressed.
• Polydactyly again:
• Some express extra finger and tows that are functional
• Others have small extra skin tags
PENETRANCE AND EXPRESSIVITY
Another example
• Neurofibromatosis
• Autosomal dominant
• 50-80% penetrance
• Individuals with the allele show a wide range of phenotypes:
• Mild few pigmented areas on the skin (café-au-lait spots).
• severe)
•
•
•
•
•
•
•
•
(1) Neurofibroma tumors of various sizes.
(2) High blood pressure.
(3) Speech impediments.
(4) Headaches.
(5) Large head.
(6) Short stature.
(7) Tumors of eye, brain, or spinal cord.
(8) Curvature of the spine
4.3 Additional Factors at a Single Locus
Can Affect the Results of Genetic Crosses
• A lethal allele: causes death at an early stage of
development, and so some genotypes may not appear
among the progeny.
• The example from the intro of this chapter the yellow coat and the
allele for it
• Yellow is dominant Y
• Viable in heterozygous Yy
• Lethal In homozygous YY