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Mendel’s “Updated” Postulates
• Unit factors in pairs – A pair of genes controls a trait
• Dominance / Recessiveness
For different alleles of a gene, one will be expressed over the other
• Segregation
Alleles are specifically dispersed into gametes
• Independent Assortment
Genes for different traits are assorted independently of each other
during gamete formation
Things don’t always work out…
• Other researchers observed ratios other than the typical “Mendelian” ratios.
• Mendel’s postulate of dominance vs. recessiveness is not always applicable.
– Incomplete Dominance
– Co-Dominance
– Lethal Alleles
Incomplete Dominance
Incomplete Dominance
• Heterozygote phenotype is intermediate between two different homozygous phenotypes
Incomplete Dominance
• Genotypic and
phenotypic ratios
are both 1:2:1
Fig. 4-1
Co-dominance
Codominance
• Joint expression of two alleles which produce two distinct products
– F2 ratios are similar to incomplete dominance
– 1:2:1 for both genotype and phenotype
ABO Blood Typing • A,B: antigens expressed on rbc surface
– O: no antigen is expressed
• Both alleles contribute to phenotype in AB
heterozygotes
Genotype Antigen Phenotype
IAIA A A
IA i A A
IBIB B B
IB i B B
IAIB A, B AB
i i - O
If you cross a yellow frog with a green frog and get
offspring which are green with yellow spots, the
skin color allele is inherited by which pattern?
• A) Incomplete dominance
• B) Co-dominance
Lethal Alleles
Lethal Alleles
• Many gene products essential for survival
– Mutations to these genes may result in nonfunctional gene products
• Typically behave as recessive lethal alleles
– Heterozygotes may survive
– Homozygote recessives will not
• May also result in a distinctive dominant phenotype
Lethal Alleles
• Mutant allele for coat color in mice
– Agouti (brown) = A
– Yellow = AY
Lethal Alleles
Fig. 4-3
Lethal Alleles
• Huntington Disease
– Lethal allele is dominant
• Rare
• Must reproduce before allele expressed otherwise
mutation cannot be maintained in population, thus
age of disease onset ~40yrs.
• Nervous degeneration death
Lethal alleles are usually…
• A) Dominant
• B) Recessive
• C) Codominant
Dihybrid Crosses with Mendelian Deviation
• Ex., inheritance of albinism and blood type
Fig. 4-4
Dihybrid Crosses with Mendelian Deviation
Fig. 4-4
Dihybrid Crosses with Mendelian Deviation
• Multiplication rule only works if genes are
not linked, and if genes do not interact with
each other
Fig. 4-4
Gene Interaction
• Phenotype may be affected by more than
one gene
• Epistasis
– The expression of a gene at one locus masks
or modifies the expression of another gene
Epistasis
• Ex. 1: A dominant allele for one gene
masks the genotype of another gene
• Squash color (white, yellow, green) is due to the
interaction of 2 genes (A, B)
A--- = white
aaB- = yellow
aabb = green
F2 ratio = 12:3:1
Epistasis
• Ex. 2: Homozygous recessive alleles for one gene masks expression of another gene
Bombay Phenotype (ABO blood typing)
Child type O even though one parent was AB
Due to incomplete H substance
Precursor to glycolipids forming A & B antigens
Product of another gene (H)
Result
Can exhibit O phenotype even though genotype is A , B or AB
Bombay Phenotype
Bombay Phenotype
Epistasis
• Ex. 3: Two gene pairs complement each other; one dominant allele is needed at each locus to express a phenotype
Bateson & Punnett
Epistasis
• Bateson & Punnett:
– Two genes for flower color: C, P
C-P- = purple
C-pp = white
ccP- = white
ccpp = white
True or False: A lethal allele cannot participate in
epistasis.
• A) True
• B) False
What is the scientific name for the fruit fly?
• A) drosophila melanogaster
• B) Drosophila melanogaster
• C) Drosophila melanogaster
• D) Drosophila melanogaster
Complementation
Complementation
• For two mutant strains with a similar
phenotype, how can you tell if the
similarity is due to two different mutations
in the same gene, or if two separate genes
are involved in the production of that
phenotype?
31
Genetic Complementation Analysis
• Background:
– A “mutant screen” is an experiment which generates
mutations which affect specific phenotypes
– Mutant screens are used to discover new mutations
that may be due to previously unidentified genes
Complementation Test
Fig. 4-9
33
Genetic Complementation
• Principle of Complementation:
– two recessive allelic mutations yield mutant phenotype
– two non-allelic recessive mutations yield wt phenotype
• The complementation test groups mutants into
allelic classes called complementation groups
If you cross two different mutant strains of
Drosophila which both have white eye color, and
the offspring all have white eyes, this indicates that
the two mutations are in…
• A) the same gene
• B) different genes along the same
biochemical pathway
• C) different genes which are completely
unrelated to each other
X-linked traits
X-linkage
• Occurs in many animal & some plant
species
– Involves “unlike” pair of chromosomes
involved in sex determination
– Ex., XY in humans
• Female = XX
• Male = XY
Fig. 4-10
X-linkage in Drosophila
Fig. 4-10
X-linkage in Drosophila
X-linkage in Humans
• Red-green colorblindness
• Hemophilia types A & B
• Lesch-Nyhan syndrome
– Abnormal nucleic acid metabolism
– Mental retardation, palsy, self mutilation
– Onset: 6-8 months of age
• Duchenne muscular dystrophy
– Progressive muscular wasting
– Onset: 3-5 years of age
If a man with red-green colorblindness and a
woman whose father was also colorblind married
and had children, what would the likelihood be that
their daughter would be colorblind? (Assume the
woman’s mother was homozygous normal.)
• A) 1/1 (i.e., 100%
• B) ½ (50%)
• C) ¼ (25%)
• D) 2/3 (67%)
Sex-limited & Sex-influenced
Traits
• Autosomal
• An allele is dominant in one sex but recessive
in the other
• Hormone production influences gene
expression
• Sex-limited = phenotype limited to one sex
• Sex-influenced = sex influences expression
but not limited to one sex only
Pattern Baldness
Phenotype
Genotype Females Males
BB bald bald
Bb not bald bald
bb not bald not bald
If a trait is sex-influenced, what will the Mendelian
pattern of inheritance be when two heterzygotes
are crossed?
• A) 100% expressed in male, 0% in female
• B) 100% expressed in female, 0% in male
• C) we can’t tell based on this information
Extranuclear Inheritance
• Organelle heredity: mitochondria & chloroplasts
– Contain own DNA
– Self-replicating
– Inherited with maternal cytoplasm
• Maternal effect: Ephestia kuehniella pigment
– Phenotype is determined by mother’s genotype
• Phenotype under control of gene products already
present in the egg prior to fertilization
Environmental Influences on
Phenotypic Expression
• Temperature effects
(conditional mutations)
– Ex., Himalayan rabbit &
Siamese cats
• Enzymes responsible for
pigment production inactive
or less active at higher
temperatures
Environmental Influences on
Phenotypic Expression
Adiponectin mRNA in Microswine Offspring Exposed to
Either Low Protein or Normal Protein Diets in Utero.
Intra-Abdominal Adipose Tissue
Male Female0
1
2
3
*
*AB
D-A
T
Ad
ipo
necti
n /
18s r
RN
A
Subcutaneous Adipose Tissue
Male Female0
1
2
3
****
NPO
LPO
SC
-AT
Ad
ipo
necti
n /
18s r
RN
A
DuPriest, et al. J DOHAD 2011
Environmental Influences on
Phenotypic Expression • “Effects of Pro12Ala mutation in PPAR-
gamma interact with body size at birth”
J. Eriksson, D. Barker, et al.
Clinical Genetics, 2003
True or False: An individual who has an allele for a
particular trait will always express that trait.
• A) True
• B) False
49
Biochemical Genetics
• Variable expressivity refers to genes that are
expressed to different degrees in different
individuals, e.g.: severity of an inherited disease
• Incomplete penetrance means that the
phenotype predicted from a specific genotype is
not always expressed, e.g.: individual inherits
mutant gene but shows no effect
– Penetrance is a percentage of individuals with a
phenotype that express that phenotype
50
Biochemical Genetics
• Many genes code for enzymes which carry out
specific steps in biochemical pathways
• Mutations which alter the structure of genes
block enzyme production if both alleles of the
gene are defective
• Disorders were termed “inborn errors of
metabolism” by Garrod
51
Biochemical Genetics
• Recessive genes often
contain mutations which
block the formation of
gene product (ww)
• Heterozygotes (Ww) produce ½ the amount of
protein as the homozygous dominant (WW)
52
Biochemical Genetics
• Heterozygotes (Ww) may still produce sufficient
gene product to display dominant phenotype
E.g., round seed of Ww genotype = carrier
• For some genes reduction of gene product by ½
in the heterozygote may be physiologically
significant, especially for structural proteins =
dominant disorders
A patient with Marfan Syndrome may express the
disease even though they are heterozygous (a
carrier) because…
• A) the allele is dominant
• B) the allele is recessive
• C) the allele displays incomplete
dominance
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