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Genes and Medical Genetics
Chapter 2
Genotype and Phenotype
Genotype = all the
genes that an
individual has
Phenotype = physical
appearance of the
individual
Pictures of
Jennifer Aniston
Genotype (con’t)
– Alleles can be dominant (capital letter) or
recessive (lower case letter).
– Alternate forms of a gene having the same
position (locus) on a pair of matching
chromosomes that control the same trait are
called alleles
– An individual has two alleles for each trait
because a chromosome pair carries alleles for
the same traits
– How many alleles for each trait will be in the
Example – in
humans
unattached
earlobe is
dominant over
attached
earlobes
• Could use capital E
to show dominant
allele and lower
case e to show
recessive (nondominant) allele
Note: it is customary
to designate alleles
by the same letter,
with uppercase to
Fig. 02-01
• When the two alleles
are both dominant it is
called a homozygous
dominant genotype
– The phenotype will be
unattached earlobes
• When the two alleles
are both recessive it is
called homozygous
recessive
– The phenotype will be
attached earlobes
• When the two alleles
are different (one
dominant, one
recessive) it is called a
heterozygous genotype
– The phenotype will be
________________.
Fig. 02-01
Forming gametes (gametogenesis)
• Recall: gametes have ½
normal # of chromosomes
• Happens when
chromosome pair
separates during meiosis
• Since alleles are on
chromosomes, they also
separate during meiosis,
thus gametes only carry
one allele for each trait.
– Ex. If individual was Ee, and
produced four gametes, ½
would be E and ½ would be
e.
(Chromosomes
already doubled)
Fig. 02-02 Gametogenesis
Genotypes
WW or Ww
ww
EE or Ee
ee
SS or Ss
ss
Fig. 02-03
Common
Inherited
Character
-istics in
Humans
Phenotypes
Genetic Crosses
(One-trait Crosses)
• If know genotype of parents, can predict
chances of having a child with certain
genotypes (and thus certain phenotypes).
– Ex. If one parent is homozygous dominant
(EE) the chance of having a child with
unattached earlobes is 100% because parent
only has dominant allele (E) to pass on to
baby.
– But, if both parents are homozygous
recessive (ee), the chance of having a child
with attached earlobes is 100% because
One-trait
Crosses (con’t)
• If both parents are
heterozygous (Dad is Ee,
and Mom is Ee), what are
the chances that baby
will have unattached or
attached earlobes?
• Determine using a
Punnett Square
• Each child will have a
75% chance of dominant
phenotype (unattached),
and 25% chance of
having recessive
phenotype (attached).
Fig. 02-04
Heterozygous-byHeterozygous cross
One-trait
crosses
(con’t)
• What if one
parent is Ee and
the other is ee?
• Each child will
have a 50%
chance of having
unattached
earlobes
Fig. 02-05 Heterozygous
By Homozygous cross
Genetic Crosses
(Two-trait Crosses)
• What if want to look at two traits at the
same time?
– Ex. Widow’s peak (W) and short fingers (S)
(both dominant traits).
• If one parent is homozygous for widow’s
peak and short fingers (WWSS) and other
is homozygous for straight hairline and
long fingers (wwss), what will
– Children look like?
– Grandchildren look like?
Fig. 02-06
Dihybrid cross
using Punnett
Square
• Expected phenotypic
ratio for a dyhibrid
cross is always 9:3:3:1
• Can use this expected
ratio to predict
chances of each child
receiving a certain
phenotype
– Ex. Chance of getting
two dominant
phenotypes together is
9 out of 16
– Chance of getting two
recessive phenotypes is
1 out of 16
Autosomal Dominant Disorders
• Genetic disorders are caused by mutations
• Mutations – permanent changes in genes (DNA)
• If disorder is autosomal dominant  mutation is
a single allele, and heterozygotes will exhibit the
disorder
• If know genotype of parents, can determine
chances of children having the disorder (see
Table 2.2, p. 26)
• Can provide genetic counseling to parents who
can make decision about best courses of action
(Chapter 4).
Autosomal Dominant Disorders
• Neurofibromatosis
– Most common genetic disorders (1 in 3,500 newborns,
all races and ethnicities)
– Symptoms
• Large tan spots on skin that get darker with age
• Small, benign (?) tumors (neurofibromas) often occur in nerves
• If severe case
– Skeletal deformities (including a large head)
– May develop eye and ear tumors  become blind and deaf
• Children with non-severe symptoms may still have learning
disabilities and be hyperactive
– Gene for neurofibromatosis has been located on
chromosome 17
– Found that gene controls production of protein that
usually blocks cell growth
– If gene not working  certain cells grow out of control
=_______________ .
Autosomal Dominant Disorders
• Huntington Disease
– Neurological (nervous system) disorder that causes
progressive degeneration of brain cells.
– Symptoms
–
–
–
–
• Most patients appear normal until middle age (may already
have children)
• Severe muscle spasms
• Personality disorders
Treatment – none (patients die in 10 – 15 years after symptoms
appear)
Gene (mutated) located on chromosome 4  causes abnormal
protein that clumps inside neurons (nerve cells)
Test developed for presence but most people do not want to know
Two minute paper: “Would you want to be tested for a fatal genetic
disease?
Why or why not?”
Autosomal Recessive Disorders
• Cystic Fibrosis (CF)
– Most common lethal genetic disease among
Caucasians in the US
– 1 in 20 are a carrier, 1 in 2,500 newborns has
it
– Caused by defect in plasma membrane (gene
on chromosome 7)
– Symptoms
• Thickened mucous in bronchial tubes (problems
breathing) and pancreatic duct (problems
digesting)
– Treatment – mucous in lungs manually
loosened and other treatments have raised
Fig. 02-08
Treatment
for CF
Autosomal Recessive Disorders
• Phenylketonuria (PKU)
– Not so common as CF
– Affects nervous system development
• Caused by missing enzyme that normally allows
metabolism of the amino acid phenylalanine
• Causes abnormal breakdown product
(phenylketone) in urine
– Symptoms – severe mental retardation, black
urine
• PKU allele located on chromosome 12  prenatal
DNA test can detect it (and elevated phenylalanine
in blood)
• If detect  place newborn on diet low in
phenylalanine until at least age 7
Autosomal Recessive Disorders
• Tay-Sachs Disease
– Usually occurs in Jewish people
– Symptoms
• Development slows at age 4 to 8 months
• Neurological and Psychomotor impairment
• Child gradually becomes blind and helpless,
seizures, paralyzed, death by age 3 – 4 years old
– Caused by gene on chromosome 15 
caused buildup of nonfunctional lysosomes in
neurons
Autosomal Recessive Disorders
• Albinism
– Person unable to produce
pigment melanin  colors skin,
hair, eyes
– Symptoms
• White hair, pink eyes, superwhite skin
• Vision problems
– Example of epistasis  one gene
affects the expression of other
genes
• In albino  any gene for
coloring cannot be expressed
because mutated gene
prevents them from producing
melanin
• Picture of Johnny
Winter