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Genetics
Inheritance
Genetics Notes
Who is Gregor Mendel? “Father of Genetics”
Principle of Independent Assortment – Inheritance of one
trait has no effect on the inheritance of another trait
Traits
• Genetics – study of how traits are passed from parent
to offspring
• Traits are determined by the genes on the
chromosomes. A gene is a segment of DNA that
determines a trait.
• Chromosomes come in homologous pairs, thus genes
come in pairs.
Homologous pairs – matching genes – one from female
parent and one from male parent
• Example: Humans have 46 chromosomes or 23 pairs.
One set from dad – 23 in sperm
One set from mom – 23 in egg
• One pair of Homologous Chromosomes:
Gene for eye color
(blue eyes)
Homologous pair
of chromosomes
Gene for eye color
(brown eyes)
Alleles – different genes (possibilities) for the same trait –
ex: blue eyes or brown eyes
Dominant and Recessive Genes
• Gene that prevents the other gene from “showing” –
dominant
• Gene that does NOT “show” even though it is present –
recessive
• Symbol – Dominant gene – upper case letter – T
Recessive gene – lower case letter – t
Dominant
color
Recessive
color
Example: Straight thumb is dominant to hitchhiker thumb
T = straight thumb t = hitchhikers thumb
(Always use the same letter for the same alleles—
No S = straight, h = hitchhiker’s)
Straight thumb = TT
Straight thumb = Tt
Hitchhikers thumb = tt
* Must have 2 recessive alleles
for a recessive trait to “show”
• Both genes of a pair are the same –
homozygous or purebred
TT – homozygous dominant
tt – homozygous recessive
• One dominant and one recessive gene –
heterozygous or hybrid
Tt – heterozygous
BB – Black
Bb – Black w/
white gene
bb – White
Genotype and Phenotype
• Combination of genes an organism has (actual gene
makeup) – genotype
Ex: TT, Tt, tt
• Physical appearance resulting from gene make-up –
phenotype
Ex: hitchhiker’s thumb or straight thumb
Punnett Square and Probability
• Used to predict the possible gene makeup of offspring –
Punnett Square
• Example: Black fur (B) is dominant to white fur (b) in mice
1. Cross a heterozygous male with a homozygous recessive female.
Black fur (B)
Heterozygous
male
White fur (b)
White fur (b)
Homozygous
recessive female
White fur (b)
Male = Bb X Female = bb
b
Male gametes - N
(One gene in
sperm)
B
b
b
Bb
Bb
bb
bb
Female gametes – N
(One gene in egg)
Possible offspring – 2N
Write the ratios in the following orders:
Genotypic ratio = 2 Bb : 2 bb
50% Bb : 50% bb
Genotypic ratio
homozygous : heterozygous : homozygous
dominant
recessive
Phenotypic ratio = 2 black : 2 white
50% black : 50% white Phenotypic ratio
dominant : recessive
Sex Determination
• People – 46 chromosomes or 23 pairs
• 22 pairs are homologous (look alike) – called autosomes –
determine body traits
1 pair is the sex chromosomes – determines sex (male or female)
• Females – sex chromosomes are homologous (look alike) – label XX
Males – sex chromosomes are different – label XY
Crossing involving 2 traits – Dihybrid crosses
• Example: In rabbits black coat (B) is dominant over brown (b) and
straight hair (H) is dominant to curly (h). Cross 2 hybrid rabbits
and give the phenotypic ratio for the first generation of offspring.
Possible gametes:
BbHh X BbHh
BH
BH
Gametes
Bh
Bh
bH
bH
BH
bh
bh
BH
Bh
bH
bh
BBHH
BBHh
BbHH
BbHh
Phenotypes - 9:3:3:1
9 black and straight
Bh
BBHh
BBhh
BbHh
Bbhh
3 black and curly
3 brown and straight
1 brown and curly
bH
BbHH
BbHh
bbHH
bbHh
bh
BbHh
Bbhh
bbHh
bbhh
Independent Assortment and Segregation during Meiosis
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A
B
A
Aa
a
B
Bb
b
A
A
B
B
AB
A
B
a
a
a
b
b
b
ab
a
b
A
a
B
b
A
A
A
b
b
b
Ab
A
Parent cell has two
pairs of homologous
chromosomes.
A
Aa
a
b
bB
B
b
a
a
B
B
a
B
aB
a
B
All orientations of homologous chromosomes
are possible at metaphase I in keeping with
the law of independent
assortment.
15
At metaphase II, each
daughter cell has only
one member of each
homologous pair in
keeping with the law of
segregation.
All possible combinations of chromosomes
and alleles occur in
the gametes as
suggested by Mendel's
two laws.
Exceptions To Mendel’s Original Principles
•
•
•
•
•
•
•
•
Incomplete dominance
Codominance
Multiple alleles
Polygenic traits
Epistasis
pleiotropy
Sex-linked traits
Gene-linkage
Incomplete
dominance
• Neither allele is dominant and
heterozygous individuals have an
intermediate phenotype that
has a blended appearance
P Generation
White
CW CW

Red
CRCR
CR
Gametes
CW
Pink
CRCW
F1 Generation
• For example, in Japanese “Four
o’clock”, plants with one red
allele and one white allele have
pink flowers:
Gametes
Eggs
F2 Generation
1⁄
2
1⁄
2
1⁄
2
CR
1⁄
2
CR
1⁄
2
CR
1⁄
2
CR
CR
Cw
CR CR
CR CW
CR CW
CW CW
Sperm
Incomplete Dominance
Gametes
CR
CW
CRCR
CRCW
CRCW
CWCW
CRCR
CR
Gametes
CW
F1 generation
All CRCW
F2 generation
CWCW
1:2:1
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Codominance
• Neither allele is dominant and both alleles are expressed
in heterozygous individuals
• Example ABO blood types
Polygenic Traits
• Most traits are not
controlled by a single gene
locus, but by the combined
interaction of many gene
loci. These are called
polygenic traits.
• Polygenic traits often show
continuous variation,
rather then a few discrete
forms:
Polygenic Traits
Gene 2
Trait
Gene 3
Polygenic traits traits controlled by more than one
gene or allele pair; gives perception of varying
degrees of pigment
Ex. skin color (3- 7 genes)
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 – A, B, and O
BUT each human can only inherit 2 alleles
1. Dominant – A and B (codominance)
Recessive – O
2. Blood type – A = AA or AO
B = BB or BO
AB = AB
O = OO
Multiple alleles – determined by a single gene that has at
least four different alleles. Ex: rabbit fur, blood types
The 4 different allele possibilities are: C = dark gray
c= no color or white
cch= peppered coloring
ch = color on ears, nose, tips of feet
Epistasis
• Type of polygenic inheritance where the alleles at one gene locus can hide or
prevent the expression of alleles at a second gene locus.
• Labrador retrievers one gene locus affects coat color by controlling how densely
the pigment eumelanin is deposited in the fur.
• A dominant allele (B) produces a black coat while the recessive allele (b) produces
a brown coat
• However, a second gene locus controls whether any eumelanin at all is deposited
in the fur. Dogs that are homozygous recessive at this locus (ee) will have yellow
fur no matter which alleles are at the first locus:
Epistasis
ee
No dark pigment in fur
eebb
Yellow fur
eeB_
Yellow fur
E_
Dark pigment in fur
E_bb
Brown fur
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E_B_
Black fur
Pleiotropy
• This is when a single gene locus affects more than one trait.
• For example, in Labrador retrievers the gene locus that controls how
dark the pigment in the hair will be also affects the color of the nose,
lips, and eye rims.
Pleiotropy – one gene controls many sometimes
unrelated traits; gene codes for substance widely
used throughout the body so lack of it has effect in
many areas
Ex. PKU (phenylketonuria) inability to process
phenylalinine results in build up of amino acid
causing mental retardation, reduction of pigment in
hair and skin
Environmental Effects on Gene Expression
• The phenotype of an organism depends not only on which genes it has (genotype), but
also on the environment under which it develops.
• Although scientists agree that phenotype depends on a complex interaction between genotype and environment, there is a lot of
debate and controversy about the relative importance of these 2 factors, particularly for complex human traits.
Patterns of Inheritance
Disorders Inherited as Recessive Traits –Autosomal Recessive
Over a thousand human genetic disorders are known to have Mendelian inheritance
patterns. Each of these disorders is inherited as a dominant or recessive trait
controlled by a single gene. Most human genetic disorders are recessive.
A particular form of deafness is
inherited as a recessive trait.
Many human disorders follow
Mendelian patterns of inheritance
Cystic fibrosis, which strikes one out of
every 2,500 whites of European
descent but is much rarer in other
groups. One out of 25 whites (4% ) is a
carrier.
The normal allele for this gene codes
for a membrane protein that functions
in chloride ion transport between
certain cells and the extracellular fluid.
These chloride channels are defective
or absent.
The result is an abnormally high
concentration of extracellular chloride,
which causes the mucus that coats
certain cells to become thicker and
stickier than normal.
Tay-Sachs disease is caused by a dysfunctional enzyme that fails to break down
brain lipids of a certain class. Is proportionately high incidence of Tay-Sachs
disease among Ashkenazic Jews, Jewish people whose ancestors lived in central
Europe
Sickle-cell disease, which affects one out of 400 African Americans. Sickle-cell
disease is caused by the substitution of a single amino acid in the hemoglobin
protein of red blood cells
Dominantly Inherited Disorders – Autosomal Dominant
Achondroplasia, a form of dwarfism with an incidence of one case among every
10,000 people. Heterozygous individuals have the dwarf phenotype.
Huntington’s disease, a degenerative disease of the nervous system, is caused by
a lethal dominant allele that has no obvious phenotypic effect until the individual
is about 35 to 45 years old.
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
You should see 58
(upper left), 18
(upper right), E
(lower left) and 17
(lower right).
Color blindness is the inability to distinguish the differences between certain colors. The most
common type is red-green color blindness, where red and green are seen as the same color.
2. hemophilia – blood won’t clot
3. Sex-Linked Disorders in Humans
Duchenne muscular dystrophy, affects about one out of every 3,500 males born in
the United States. People with Duchenne muscular dystrophy rarely live past their
early 20s. The disease is characterized by a progressive weakening of the muscles
and loss of coordination. Researchers have traced the disorder to the absence of a
key muscle protein called dystrophin and have tracked the gene for this protein to a
specific locus on the X chromosome.
Posture changes during
progression of Duchenne
muscular dystrophy.
Hemophilia is a sex-linked recessive trait defined by the absence of one or more of
the proteins required for blood clotting.
Pattern Baldness In Humans: A Sex Influenced Trait
Baldness is an autosomal trait and is apparently influenced by sex hormones after
people reach 30 years of age or older.
In men the gene is dominant, while in women it is recessive. A man needs only one
allele (B) for the baldness trait to be expressed, while a bald woman must be
homozygous for the trait (BB).
What are the probabilities for the children for a bald man and woman with no history of
baldness in the family?
Genetic Counseling geneticists that provide parents with the probabilities
of passing genetic disorders to their offspring
Tools Used for Genetic Counseling
Genetic karyotype - picture of an individual’s chromosomes
blood testing for enzymes
amniocentesis - removal of a small amount of amniotic
fluid during pregnancy to analyze cells from
fetus and make karyotype
fetoscopy – doctor inserts tiny camera through incision
in uterus to observe fetal development
ultrasound – uses sound waves to observe fetal development
chorion villi sampling – doctor takes cell sample of chorion
which has same genetic makeup of fetus
Pedigrees geneticists interview family members to develop a chart of
information through many generations concerning a particular disorder so
that it may be analyzed to predict future generations
Detecting Genetic Disorders
• picture of an individual’s chromosomes – karyotype
• amniotic fluid surrounding the embryo is removed for
analysis – amniocentesis
Female with Down’s syndrome