Download Teacher - Application Genetics Notes Pre AP 13-14

Pre AP - Applications of Genetics Notes
Incomplete dominance and Codominance
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When one allele is NOT completely dominant over another (they blend) – incomplete dominance
Example: In carnations the color red (R) is incompletely dominant over white (r). The hybrid color is
pink. Give the genotypic and phenotypic ratio from a cross between 2 pink flowers.
RR = red
Rr X Rr
R
R
RR
r

Rr
rr = white
Rr = pink
r
Rr
rr
Genotypic =
1 RR : 2 Rr : 1 rr
Phenotypic = 1 red : 2 pink : 1 white
When both alleles are expressed – Codominance
Example: In certain species of chickens black feathers (FB) are codominant with white feathers (FW).
Heterozygous chickens have black and white speckled feathers. Show the F1 from crossing
2 hybrid chickens. Give the genotypic and phenotypic ratio.
FBFB = black
FWFW = white
FBFW = speckled
FBFW X FBFW
FB
FW
FB
FBFB
FBFW
FW
FBFW
FWFW
Genotypic =
1 FBFB : 2 FBFW : 1 FWFW
Phenotypic = 1 black : 2 speckled : 1 white
Sex – linked Traits

Genes for these traits are located only on the X chromosome (NOT on the Y chromosome)

X linked alleles always show up in males whether dominant or recessive because males have only one X
chromosome

Examples of recessive sex-linked disorders:
1. colorblindness – inability to distinguish between certain colors (most common red/green)
2. hemophilia – blood won’t clot (blood clotting factor VIII defective)
3. Duchenne muscular dystrophy – wasting away of skeletal muscles
Carriers – individual that has inherited the trait, but doesn’t display the trait due to being heterozygous
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
Example: A female that has normal vision but is a carrier for colorblindness marries a male with normal
vision. Give the expected phenotypes of their children.
N = normal vision
n = colorblindness
XN Xn X XN Y
XN
N
X
Xn
N N
X X
N n
Phenotype: 1 normal vision female
X X
1 normal vision female (carrier)
1 normal vision male
Y
XNY
XnY
1 colorblind male
Pedigrees


Graphic representation of how a trait is passed from parents to offspring
Tips for making a pedigree
1. Circles are for females
2. Squares are for males
3. Horizontal lines connecting a male and a female represent a marriage
4. Vertical line and brackets connect parent to offspring
5. A shaded circle or square indicates a person has the trait
6. A circle or square NOT shaded represents an individual who does NOT have the trait
7. Partial shade indicates a carrier – someone who is heterozygous for the trait

Example: Make a pedigree chart for the following couple. Dana is color blind; her husband Jeff is not.
They have two boys and two girls. HINT: colorblindness is a recessive sex-linked trait
XnXn
Has trait
XNY
Can pass trait to
offspring
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Multiple Alleles


3 or more alleles of the same gene that code for a single trait
In humans, blood type is determined by 3 alleles – IA, IB and iO
BUT genes come in pairs, so each human can only inherit 2 alleles
1. Codominant – IA, IB
Recessive – iO
2. Blood type – A = IAIA or IA iO
B = IBIB or IBiO
AB = IAIB
O = iO iO

Example: A woman homozygous for type B blood marries a man who is heterozygous type A. What will
be the possible genotypes and phenotypes of their children?
IBIB X IA iO
IA
iO

IB
IB
IAIB
IAIB
IB iO
IB iO
Genotypic =
2 IAIB : 2 IB iO
Phenotypic = 2 AB : 2B
Blood types used for paternity and crime scene investigations before DNA evidence
Polygenic Inheritance

Effect of 2 of more genes on a single phenotypic characteristic

Examples: eye color, skin color, height, body mass

Skin color: dark is dominant to light, genes are not linked
Ex: aabbcc = very light
AABBCC = very dark
AaBbCc = medium color – 3 dominant, 3 recessive
AABbcc = medium color – 3 dominant, 3 recessive

Disorders that are polygenic: autism, diabetes, cancer
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Mutations


Mutation – sudden genetic change (change in base pair sequence of DNA)
Can be :
Harmful mutations – organism less able to survive: genetic disorders, cancer, death
5-8 genes in humans results in death – lethal mutation
Beneficial mutations – allows organism to better survive: provides genetic variation
Neutral mutations – neither harmful nor helpful to organism


Mutations can occur in 2 ways: chromosomal mutation or gene/point mutation
Only mutations in sex cells are passed from parent to offspring
Chromosomal mutation:





less common than a gene mutation
more drastic – affects entire chromosome, so affects many genes rather than just one
caused by failure of the homologous chromosomes to separate normally during meiosis
chromosome pairs no longer look the same – too few or too many genes, different shape
Examples:
Down’s syndrome – (Trisomy 21) 47 chromosomes, extra chromosome at pair #21
Turner’s syndrome – only 45 chromosomes, missing a sex chromosome (X)
Klinefelter’s syndrome – 47 chromosomes, extra X chromosomes (XXY)
Gene or Point Mutation


most common and least drastic
usually only one gene is altered

Examples:
Recessive gene mutations:
Sickle cell anemia – red blood cells are sickle shaped instead of round and get stuck in the blood vessels
– can cut off blood supply to organs – heterozygous condition protects people from
malaria
Cystic fibrosis – mucus clogs lungs, liver and pancreas
Tay-Sachs Disease – deterioration of the nervous system – early death
Phenylketonuria (PKU) – an amino acid common in milk cannot be broken down and as it builds up it
causes mental retardation – newborns are tested for this
Dominant gene mutations:
Huntington’s disease – gradual deterioration of brain tissue, shows up in middle age and is fatal
Dwarfism – variety of skeletal abnormalities
Text p. 263-271
A. Who is Gregor Mendel? Father of genetics
B. What is the Principle of Dominance? When individuals with contrasting traits are crossed, the
offspring will express the dominant trait only.
C. What is the Principle of Independent Assortment? Genes can be inherited separately (or
independently) from each other
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