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“Nature vs. Nurture”
Environment and Gene Expression
Gene expression can be affected by many factors,
including:


the activity of other genes
climate conditions – Ex.: Siamese cat &
Himalayan rabbit fur, flower color in hydrangeas,
etc.
“Nature vs. Nurture”
Environment and Gene Expression
Gene expression can be affected by many factors,
including:

social environment of organism – Ex.: Japanese
goby fish changes sex depending on the gender
of other fish present
“Nature vs. Nurture”
Environment and Gene Expression
Gene expression can be affected by many factors,
including:


internal environment of organism – diet
(presence of nutrients) affects height; shyness in
children is affected by socialization;
External environment of organism – Ex.:
Mutagens/carcinogens – cancer development
Refining Mendel’s Laws
Linked Genes
•Genes located on the same chromosome travel together into
the same gamete;
•the two traits are inherited together, instead of sorting
independently
•this is an exception to Mendel’s law of independent
assortment.
Intermediate Inheritance:
Polygenic Traits
Most traits are polygenic – affected by more than one gene.
Intermediate Inheritance:
Incomplete Dominance – “no win”
Result of incomplete dominance: blending of traits in heterozygote
Intermediate Inheritance:
Codominance – “win-win”
Codominance - both alleles in
heterozygote express themselves fully
Ex.: blood types are determined by
carbohydrates (sugars) on red blood
cells; two alleles  two types of
carbohydrate molecules on RBCs.
Human blood types also have
multiple alleles (more than two):
IA, IB and i (neither)
Rh factor (a different gene)
(+, dominant or -, recessive) also
determines blood type; ex.: A+, B-, etc.
Blood transfusions - People reject any
blood factor they do NOT have.
Phenotype
(blood type)
Antibody in
Plasma
Can Donate
to Blood
Types
A AA or
AO
B BB or
BO
Anti-B
A and
AB
Anti-A
B and
AB
AB
None
AB
OO
Anti-A All Blood
Anti-B Types
AB
O
Possible
genotype(s)
Antigen on
RBC
Sex-Linked Traits
&
Sex-Influenced Traits
* Add the missing examples
to your notes. *
Which sex has more chromosomes?
Which sex has more genes?
It starts at a young age and men just get worse
But wait! Things are not as bad as they seem… 
X Inactivation
( A.K.A. Revenge for Boys!  )

Early in embryonic development in females, one of the two X
chromosomes is randomly and permanently inactivated
in cells other than egg cells.

This ensures that females, like males, have one functional
copy of the X chromosome in each body cell.

Some genes on the X chromosome escape X-inactivation.
Many of these genes are located at the ends of each arm of
the X chromosome in areas known as the pseudoautosomal
regions.

Although many genes are unique to the X chromosome,
genes in the pseudoautosomal regions are present on both
sex chromosomes, and many are essential for normal
development.
Tortoiseshell /
Calico Cat
Primary gene for coat color on the
X chromosome has two codominant alleles,
Orange (XO) & Black (XB)
X inactivation (random) in different
embryonic cells (melanocytes)
produces the pattern
Delayed melanocyte migration
during development (another gene
does this) creates white in the
three-color cats (cells w/o
pigment)
Male or female?
Females,
except for rare XXY males
Two cell populations
in adult
Early embryo
Cell division
and random
X chromosome
inactivation
X chromosomes
Active X
Orange
fur
Inactive X
Inactive X
Allele for
orange fur
Active X
Black fur
Allele for
black fur
Figure 11.5
Sex-Linked Traits
Colorblindness
Hemophilia
Linked to X
chromosome,
are recessive
Sex-Linked Traits
Sex-linked traits are encoded by genes on the X or Y
chromosomes.
Ex.: Hemophilia, a defect in blood clotting, is a recessive,
X-linked trait (Xh) found mostly in boys.
Dad
Mom
(carrier)
XH
Y
XH
XH XH
girl
XH Y
boy
Xh
XH Xh
girl
XhY
Boy*
* Since the Y
chromosome does not
have the gene for the
blood clotting factor,
this boy has no good
copy of the gene and
has the recessive
phenotype.
Hairy Pinna
Linked to
Y chromosome
Sex Influenced Trait
Gene is on an Autosome (#1 - #22), but the
Phenotype is INFLUENCED by the SEX HORMONES
which are controlled by the SEX CHROMOSOMES
Sex Influenced Traits
Male Patterned Baldness
Sex Influenced Trait
Male
Female
Phenotype
Phenotype
Hair
BB
Hair
Patterned
Baldness
B’B
Hair
B’B’
Patterned
Baldness
Patterned
Baldness