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Mendelian InheritancePart 1
BIO 2215
Oklahoma City Community College
Dennis Anderson
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Gregor Mendel
• Father of Genetics• 1823-1884• Monk in Austria• Experimented with
garden peas
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Short
Tall
Tall
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Mendel’s Hypotheses
• Each parent has two factors (alleles)
• Each parent gives one of those factors to the offspring
• Tall has TT
• Short has tt
• Tall is dominant
• Short is recessive
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TT tt
T t
Tt
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TT TT
T T
TT
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tt tt
t ttt
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Law of Segregation
• Alleles separate during gamete production
• Gametes have one allele for each trait
• During fertilization gametes combine at random to form individuals of the next generation
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Discovery of Chromosomes in 1900 Confirmed Law of Segregation
• Chromosomes are in pairs
• Each chromosome has one of the allele pair
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Chromosomes line up in a double row.
Meiosis Metaphase
Assume a T allele on each red chromatid and a t allele on each blue chromatid
T T t t
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Chromosomes separate
Each each daughter cell gets doubled chromosomes
T T t t
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Doubled Chromosomes Separate in Second Meiotic Division
T T t t
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Each gamete will have a T allele or a t allele
T T t t
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Allele
• Member of a paired gene– One allele comes from each parent
• Represented by a single letter
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Dominant & Recessive Alleles
• Dominant alleles are expressed
• Recessive alleles are not expressed in the presence of a dominant allele– Recessive alleles are only expressed if both
recessive alleles are present
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Gene
• A unit of heredity that controls the development of one trait
• Made of DNA
• Most genes are composed of two alleles
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Homozygous
• Both alleles alike
• AA or aa
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Heterozygous
• Alleles are different
• Aa
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Genotype
• Genetic make up
• Represented by alleles
• TT & Tt are genotypes for tall pea plants
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Phenotype
• A trait
• Genotype determines the phenotype
• Tall is a phenotype
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Homologous Chromosomes
• Chromosomes of the same pair
• Each homologue will have one allele for a paired gene
• Homologous chromosomes pair up during meiosis
• Only one of each homologue will be in each gamete
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Sickle Cell Anemia
• RBCs sickle shaped• Anemia• Pain• Stroke• Leg ulcers• Jaundice• Gall stones• Spleen, kidney & lungs
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Sickle Cell Anemia
• Recessive allele s, codes for hemoglobin S – Long rod-like molecules– Stretches RBC into sickle shape
• Homozygous recessive ss, have sickle cell anemia
• Heterozygous Ss, are carriers
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Albinism
• Lack of pigment– Skin
– Hair
– Eyes
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Amino Acids Melanin PigmentEnzyme
A a
AA = Normal pigmentation
Aa = Normal pigmentation
aa = Albino
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PKU Disease
• Phenylalanine excess• Mental retardation if
untreated
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Phenylalanine TyrosineEnzyme
P p
PP = Normal
Pp = Normal
pp = PKU
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A man & woman are both carriers (heterozygous) for albinism. What is the chance their children will inherit albinism?
Monohybrid Cross or One Trait
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AA = Normal pigmentation
Aa = Normal pigmentation (carrier)
aa = Albino
Man = Aa Woman = Aa
A
a a
A
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A
a
a
A AA
Aa
Aa
aa
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AA
Aa
Aa
aa
Genotypes1 AA, 2Aa, 1aa
Phenotypes
3 Normal
1 Abino
Probability
25% for albinism
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A man & woman are both carriers (heterozygous) for PKU disease. What is the chance their children will inherit PKU disease?
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p
p
P PP
Pp
Pp
pp
P
PP = Normal
Pp = Normal (carrier)
pp = PKU disease
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PP
Pp
Pp
pp
Genotypes1 PP, 2Pp, 1pp
Phenotypes
3 Normal
1 PKU disease
Probability
25% for PKU disease
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A man with sickle cell anemia marries a woman who is a carrier. What is the chance their children will inherit sickle cell anemia?
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s
s
s Ss
Ss
ss
ss
S
SS = Normal
Ss = Normal (carrier)
ss = Sickle Cell
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Ss
Ss
ss
ss
Genotypes2 Ss, 2ss
Phenotypes
2 Normal (carriers)
2 Sickle cell
Probability
50% for Sickle cell
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Dwarfism = D
Normal height = d
DD = Dwarfism
Dd = Dwarfism
dd = Normal height
Dwarfism
Dwarf Band
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A man with heterozygous dwarfism marries a woman who has normal height. What is the chance their children will inherit dwarfism? Dwarfism is dominant.
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d
d
D Dd
dd
Dd
dd
d
DD = Dwarf
Dd = Dwarf
dd = Normal
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Dd
dd
Dd
dd
Genotypes2 Dd, 2dd
Phenotypes
2 Normal
2 Dwarfs
Probability
50% for Dwarfism
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Law of Independent Assortment
• The inheritance of one gene does not influence the inheritance of another gene if they are on separate chromosomes.
• The gene for albinism does not affect the gene for dwarfism
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Dihybrid Cross or Two Traits
• A heterozygous tall plant that is also heterozygous for yellow seeds is crossed with another plant with the same genotype
• Tall and yellow seeds are dominant to short and green seeds.
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Tall Yellow
TtYy
TY
What gametes can each parent produce?
TytYty
TtYy
TYTytYty
Tall Yellow
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9 Tall-Yellow
Match gametes on a Punnent Square
TY
Ty
tY
ty
TY Ty tY ty
TtYy
TtYy
TTYY TTYy TtYY
TTYy TTyy Ttyy
TtYY TtYy ttYY ttYy
TtYy Ttyy ttYy ttyy
3 Tall-Green
3 Short-Yellow
1 Short-Green
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A man with blue eyes and normal height marries a woman with heterozygous brown eyes and heterozygous dwarfism. What are the possible phenotypes of their children? Dwarfism & brown eyes are dominant.
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Normal height-Blue
ddbb
db
What gametes can each parent produce?
DdBb
DBDbdBdb
Dwarf-Brown
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DB
Db
dB
db
Match gametes on Punnent Square
db
DdBb
Ddbb
ddBb
ddbb
Dwarf-Brown eyes
Dwarf-blue eyes
Normal height-Brown eyes
Normal height-Blue eyes
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X-linked Traits
• Alleles on the X chromosome
• Women have two alleles
• Men have one allele
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Hemophilia
Blood clotting impaired
Recessive allele, h
carried on X cms
X-linked recessive trait
More common in males
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XH XH = Normal Female
XH Xh = Normal Female (Carrier)
Xh Xh = Hemophiliac Female
XHy = Normal Male
Xhy = Hemophiliac Male
Alleles must be written on X chromosome
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A man with hemophilia marries a normal woman who is not a carrier. What is the chance their children will inherit hemophilia? Hemophilia is X-linked recessive.
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y
XH
Xh XH Xh
XH
XH XH = Normal Female
XH Xh = Normal Female (Carrier)
Xh Xh = Hemophiliac Female
XHy = Normal Male
Xhy = Hemophiliac Male
XH Xh
XHy XHy
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Genotypes
2 XH Xh, 2XHy
Phenotypes
2 Carrier Females
2 Normal Males
Probability
O% for Hemophilia
y
XH
Xh XH Xh
XH
XH Xh
XHy XHy
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A normal man marries a normal woman who is a carrier for hemophilia. What is the chance their children will inherit hemophilia?
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y
Xh
XH
XH
XH XH = Normal Female
XH Xh = Normal Female (Carrier)
Xh Xh = Hemophiliac Female
XHy = Normal Male
Xhy = Hemophiliac Male
XH XH XH Xh
XHy Xhy
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Genotypes
1XH XH, 1 XH Xh, 1XHy, 1 XhyPhenotypes
2 Normal Females
1 Normal Males
1 Male Hemophiliac
Probability50% for Male Hemophiliac
0% for Female Hemophiliac
y
Xh
XH XH XH
XH
XH Xh
XHy Xhy
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Hypertrichosis
• X-linked dominant• Similar gene in apes
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The End