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7.1 Chromosomes and Phenotype
KEY CONCEPT
The chromosomes on which genes are located can
affect the expression of traits.
7.1 Chromosomes and Phenotype
• Sexually reproducing
organisms have two of each
chromosome.
•One from mother and one
from father
•Have the same genes, but
may have different alleles
• About 99.9% of all human
DNA is identical
7.1 Chromosomes and Phenotype
• Autosomal genetic disorders: disorders that have no
relation to the sex of an individual
– Autosomal recessive disorder: Any offspring with two
recessive genes (cc) will have the disease/disorder
– A carrier is heterozygous (Cc) for a recessive disorder
C
C
c
CC
c
Cc
*carrier
Cc
cc
*carrier
Disorder
Ex: Cystic
Fibrosis
7.1 Chromosomes and Phenotype
– Albinism is a autosomal recessive disorder
characterized by a lack of pigment I skin, hair and
eyes.
7.1 Chromosomes and Phenotype
7.1 Chromosomes and Phenotype
7.1 Chromosomes and Phenotype
A
a
A
A
a
A
a
a
Fill out these punnett sqs. What is the probability that the
offspring will have albinism?
7.1 Chromosomes and Phenotype
A
a
A
AA
Aa
A
AA
Aa
• O% will have albinism
• 50% carriers
A
a
a
Aa
aa
a
Aa
aa
• 50% will have albinism
• 50% carriers
Both Mom and Dad have to either have the disorder or
be carriers for offspring to be affected with an
autosomal recessive disorder!
7.1 Chromosomes and Phenotype
• Autosomal Dominant Disorders: any offspring showing a
dominant allele will have the disorder
– Dominant disorders are uncommon
– Ex: Huntington’s Disease
H
h
Hh
h
hh
Disorder
h
Hh
Disorder
hh
*anywhere
there is an
H the child
will have
the disease
7.1 Chromosomes and Phenotype
• Genes on sex chromosomes are called sex-linked genes.
– Y chromosome genes in mammals are responsible for
male characteristics.
– X chromosome genes in mammals affect many traits.
7.1 Chromosomes and Phenotype
• Male mammals have an XY genotype.
– All of a male’s sexlinked genes are
expressed
because Males
have no second
copies of sexlinked genes.
» One copy X, one
copy Y
7.1 Chromosomes and Phenotype
• Female mammals have an XX genotype.
– Expression of sex-linked genes is similar to autosomal
genes in females.
– X chromosome inactivation randomly “turns off” one X
chromosome
– ensures that females, like males, have one functional copy
of the X chromosome in each body cell
X
7.1 Chromosomes and Phenotype
Males only have one
X chromosome so
one color will be
present (recessive
OR dominant)
Female cats have
two X
chromosomes. Each
patch of fur can be
different depending
on which X
chromosome “turns
off”