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Modern Genetics
What was Mendel missing?
Incomplete Dominance
• Incomplete Dominance- Neither allele is
completely dominant over the other. Results
in a “blending” of traits.
– Every genotype has its
own phenotype
RR = Red
Rr = Pink
rr = White
Incomplete Dominance
Incomplete Dominance
• (a.k.a. partial dominance or blending)
• occurs when both alleles contribute to a trait that is
not like either parent.
• this condition is only seen in the heterozygous
individual
– Snap Dragons or Four O’Clocks or Morning
Glory or Primrose (Flowers)
– Curly (homozygous) or straight (homozygous),
but if you are heterozygous you have wavy hair.
Type of
Dominance?
Incomplete
Dominance!
Codominance
•
Both alleles contribute to the phenotype, neither is dominant over the
other.
– Example: In some chickens
Black Chicken x White  Speckled Chicken
May see alleles written differently. (I will explain)
Codominance
Both dominant and recessive alleles are expressed
in heterozygous individuals.
In codominance, neither allele are dominant; both
are expressed. A cross between organisms with
two different phenotypes produces offspring
which has both phenotypes of the parental traits
shown.
Identify as co-dominance or
incomplete dominance.
1. Birds can be blue, white, or white with blue-tipped
feathers.
2. Flowers can be white, pink, or red.
3. A Sneech can be tall, medium, or short.
4. A Bleexo can be spotted, black, or white.
5. A mouse can be black, white, or gray
Codominance
• BOTH alleles are expressed equally in
heterozygous individuals
• Neither allele is dominant over the other
– Example: blood type
IB
IB
IA
IAIB
IAIB
i
IBi
IBi
IA
IB
i
IAi
IBi
i
IAi
IBi
Codominance
• Examples:
• male Type B (IBIB) x female Type A (IAi)
1/2 = IAIB
1/2 = IBi
•
male Type O (ii) x female type AB (IAIB)
1/2 = IAi
1/2 = IBi
Codominance
• Question:
If a boy has a blood type O and his sister has
blood type AB, what are the genotypes and
phenotypes of their parents?
boy - type O (ii) X girl - type AB (IAIB)
Codominance
• Answer:
IA
IB
i
i
IAIB
ii
Parents:
genotypes = IAi and IBi
phenotypes = A and B
Polygenetic Traits
• Polygenetic trait- controlled by two or more
genes
– Each dominant Allele “adds” to the expression of
the next dominant allele.
– Height
– Skin Color
– Eye Color
– Blood Type
Polygenic Inheritance of Skin Color
Gametes
ABC
ABC
ABc
AABBCC
AABBCc
AbC
Abc
aBC
AABbCc
AaBBCC
aBc
ABc
AbC
Abc
aBC
AAbbcc
aaBBCc
aBc
abC
abc
aaBbcc
abC
abc
Sex-linked Traits
• Traits (genes) located on the sex
chromosomes
• Sex chromosomes are X and Y
– XX genotype for females
– XY genotype for males
• Many sex-linked traits carried on X
chromosome
Sex – Linked Traits
• Example: Colorblindness
– If the mother carries the colorblindness gene on
her X chromosome, her son could get it.
– As long as one X chromosome
X
Y
is ok, a female will not
cX
cY
X
X
C
express the trait
X
X
XX
XY
Sex-linked Traits
Example: Colorblindness
Sex Chromosomes
Colorblindness
XX chromosome - female
Xy chromosome - male
Genetic Disorders
• Autosomal Recessive Disorders –
– Expressed only when an individual has two copies of the
mutant allele. When just one copy of the mutant allele is
present, an individual is a carrier of the mutation, but does
not develop the condition.
• Autosomal recessive disorders are typically not
seen in every generation of an affected family.
– Cystic Fibrosis: buildup of thick, sticky mucus
that can damage many of the body's organs.
– Tay-Sachs: disorder that progressively destroys
nerve cells (neurons) in the brain and spinal cord.
Genetic Disorders
• Autosomal Dominant Disorders
– One mutated copy of the gene in each cell is sufficient for
a person to be affected by an autosomal dominant
disorder.
• Autosomal dominant disorders tend to occur
in every generation of an affected family.
– Huntington’s Disease: progressive brain
disorder that causes uncontrolled movements,
emotional problems, and loss of thinking ability
Chromosomal Disorders
• Extra Chromosomes
– Down syndrome- mentally disabled
and physical deformities.
• Baby receives an extra chromosome from a parent.
– 3 copies of chromosome 21 (Trisomy 21)
– Klinefelter syndrome- difficulty learning, elongated limbs,
undeveloped sex organs
• male receives an extra X chromosome
Chromosomal Disorders
• Damaged Chromosomes
– Cat’s cry syndrome- misshapen facial features, infants cry
resembles a cat’s cry.
• tiny portion of chromosome 5 is missing
Genetic Advances
•
•
•
•
Biotechnology- genetic engineering
Genetic modification
Cloning- creating exact replica of an organism.
Gene therapy- insertion of genes into a
persons cells to cure a genetic disorder.
• In Agriculture(transgenic crops)- placement of
genes in a crop for a beneficial trait.
Human Genome Project
• Human Genome Project- an effort to
sequence all 3 billion bases that make up our
DNA and to identify within this code more
than 20,000 human genes. 1990-2003
– Chromosome map
– Now have test for genetic disorders.
– Knowing genetic risk for specific diseases.
– People may eventually be able to sequence own
genome.