Download SY Sy sY

Patterns of Inheritance
By observing how traits are passed to
the next generation, how can the
inheritance patterns be used to
understand the principles of heredity?
Use of Garden Pea for
Genetics Experiments
Carpel
(female)
produces
eggs
Intact pea flower
Stamens
(male)
produce
pollen
Flower dissected to show
reproductive structures
Mendel’s Experiment With
Peas Differing in a Single Trait
Parental:
Smooth seed x Wrinkled seed
F1:
All smooth seed coats
F1 smooth plants x F1 smooth plants
F2: 5474 smooth: 1850 wrinkled
(3/4 smooth to 1/4 wrinkled)
Patterns of Inheritance
Mendel needed to explain
1. Why one trait seemed to disappear
in the first generation.
2. Why the same trait reappeared in
the second generation in one-fourth
of the offspring.
Mendel’s Proposal
1. Each trait is governed by two
factors – now called genes.
2. Genes are found in alternative
forms called alleles.
3. Some alleles are dominant and
mask alleles that are recessive.
Mendel’s Experiment With Peas
Differing in a Single Trait
Parental:
Smooth seed x Wrinkled seed
SS
ss
Homozygous
Homozygous
Dominant
Recessive
F1:
All smooth seed coats
Ss Heterozygous
F1 smooth plants x F1 smooth plants
Ss
Ss
Heterozygous
Heterozygous
F2
Homozygous parents can only pass
one form of an allele to their offspring.
S
S
S
S
Heterozygous parents can pass either of
two forms of an allele to their offspring.
S
s
S
s
Locus: Area on the chromosome where a gene is located.
For a heterozygote, homologous chromosomes will have
different alleles at the same locus.
Additional Genetic Terms
Term
Genotype
Definition
Alleles carried by an
individual
Example
SS, Ss, ss
Phenotype Physical characteristic smooth or
or appearance of an
wrinkled
individual
Mendel’s Principle of
Genetic Segregation
In the formation of gametes, the
members of a pair of alleles separate
(or segregate) cleanly from each other
so that only one member is included in
each gamete.
Each gamete has an equal probability of
containing either member of the allele
pair.
Genetic Segregation
Parentals:
SS x ss
S
S
s
s
F1 x F1:
Ss x Ss
s
S
s
S
Ss
Ss
S
Ss
Ss
S
s
s
S
S
SS
Ss
Ss
ss
s
s
100% Smooth seeds
75% Smooth seeds
25% Wrinkled seeds
Traits Studied by Mendel
Seed shape
Seed color
Pod shape
Pod color
Flower color
Flower location
Plant size
Mendel’s Experiment With Peas Differing in Two Traits
Parental: Smooth Yellow x Wrinkled Green
F1: All smooth yellow seed coats
F1 plants x F1 plants
F2
315 smooth, yellow
9/16
108 smooth, green
3/16
101 wrinkled, yellow
3/16
32 wrinkled, green
1/16
Patterns of Inheritance
Mendel needed to explain
1. Why non-parental combinations
appeared in the F2 offspring.
2. Why the ratio of phenotypes in the
F2 generation was 9:3:3:1.
Mendel’s Principle of
Independent Assortment
When gametes are formed, the
alleles of one gene segregate
independently of the alleles of
another gene producing equal
proportions of all possible gamete
types.
Genetic Segregation + Independent Assortment
Parentals: SSYY
x
SY SY SY SY
sy sy
sy
F1:
SY
ssyy
SsYy
100% smooth, yellow
sy sy
Genetic Segregation + Independent Assortment
F1 x F1 : S s Y y
SY Sy sY sy
x
SsYy
SY Sy sY sy
Four different types of gametes
are formed in equal proportions.
F1 x F1
SsYy X SsYy
1
4 SY
1
4 Sy
Pollen
1
4 sY
1
4
sy
1
16
1
16
1
16
1
16
1
4
Eggs
1
1
4 Sy
4 sY
SY
SSYY
1
16
SSYy
1
16
SsYY
1
16
SsYy
1
16
SSYy
1
16
SSyy
1
16
SsYy
1
16
Ssyy
1
16
1
4
sy
SsYY
1
16
SsYy
SsYy
1
16
Ssyy
ssYY
1
16
ssYy
ssYy
1
16
ssyy
F2 Genotypes and Phenotypes
Phenotypes
Smooth
Yellow
Genotypes
Smooth
Green
Wrinkled
Yellow
Wrinkled
Green
1/16 SSyy+ 2/16 Ssyy
Total = 3/16 S_yy
1/16 SSYY + 2/16 SSYy +
2/16 SsYY + 4/16 SsYy
Total = 9/16 S_Y_
1/16 ssYY+ 2/16 ssYy
Total = 3/16 ssY_
1/16 ssyy
Meiotic Segregation Explains Independent Assortment
Two possible
orientations
Additional Genetic
Patterns
Mendel’s peas
Alternative Pattern
Complete Dominance Incomplete Dominance
Incomplete dominance: neither allele
masks the other and both are observed as
a blending in the heterozygote
Incomplete Dominance
Red
RR
x White
R’R’
Four o’clock flowers
R = red, R’ = white
Pink
RR’
Incomplete Dominance
F1 x F1
Pink x Pink
RR’ x RR’
½R
½ R’
½R
½ R’
¼ RR ¼ RR’
¼ RR’ ¼ R’R’
Genotypic Ratio: ¼ RR + ½ RR’ + ¼ R’R’
Phenotypic Ratio: ¼ red + ½ pink + ¼ white
Additional Genetic Patterns
Mendel’s peas
Alternative Patterns
Complete Dominance
Codominance
Two alleles per gene
Multiple Alleles
Codominance: Neither allele masks the other
so that effects of both alleles are observed in
heterozygotes without blending
Multiple Alleles: Three or more alleles exist for
one trait
Note: A diploid individual can only carry any two
of these alleles at once.
Multiple Alleles and Codominance
ABO Blood Type in Humans
Blood Type
Allele
Type A
A
Type B
B
Type O
o
A= B > o
A and B are codominant.
A and B are completely dominant over o.
Human ABO Blood Types
•Type
•Genotype
•Antigen on RBCs
•Antibodies
•Receives
•Donates
•Freq
•A
•AA or Ao
Type A
•B
•A or O
•A or AB
•40%
•B
•BB or Bo
Type B
•A
•B or O
•B or AB
•10%
•Neither
•AB, A,
B, O
•AB
•AB
•AB
A and B
(universal)
•O
•oo
Neither
•Both
•O
(universal)
•O,AB,
A,B
(universal)
Codominance is observed for Type AB Blood since the
products of both the A and B alleles are found on the cells.
•4%
•46%
Inheritance of Rh Factor
Phenotype
Genotype* Gene
Product
Rh Positive RR or Rr
Rh Negative rr
Antibodies
Present
Rhesus Protein None
None
None
unless
exposed
*Although there are multiple R alleles, R1, R2, R3, etc. all are
completely dominant over all of the r alleles, r1, r2, r3, etc.
ABO Blood Type and Rh Factor are controlled by
separate genes. They show independent assortment.
Multiple Alleles and Codominance
Type A, Rh positive x Type B, Rh negative
AoRr
AR
x
Borr
Br ABRr
Ar
ABrr
oR
BoRr
or
Borr
or
Aorr
ooRr
oorr
AoRr
Phenotypic Ratio of Offspring
1/8 Type AB positive
1/8 Type AB negative
1/8 Type B positive
1/8 Type B negative
1/8 Type A positive
1/8 Type A negative
1/8 Type O positive
1/8 Type O negative
Additional Genetic Patterns
Mendel’s peas
Alternative Patterns
One gene affects
one trait
Polygenic Inheritance
Polygenic Inheritance: Many genes affect
one trait
Example of Polygenic Inheritance
Two genes affecting one trait
Number of Skin Color*
Dominant (Phenotype)
Alleles
0
White
Genotypes
% Pigmentation*
aabb
0-11%
1
Light Black
Aabb or aaBb
12-25%
2
26-40%
3
Medium Black AAbb or AaBb or
aaBB
Dark Black
AABb or AaBB
4
Darkest Black AABB
56-78%
41-55%
*Based on a study conducted in Jamaica.
Example of Polygenic Inheritance
Grandma
aabb
Medium Black Woman X Darkest Black Man
(her mother is white)
AABB
AaBb
AB
AB
Ab
AABB
AABb
Darkest
Black
Dark
Black
aB
ab
AaBB
AaBb
Dark
Black
Medium
Black
¼ Darkest Black + ½ Dark Black + ¼ Medium Black
Additional Genetic Patterns
Mendel’s peas
Alternative Patterns
One gene affects
one trait
Pleiotropy
Pleiotropy: One gene affects many traits
Sickle-Cell Anemia
One gene affects many
phenotypic characteristics
Gene
Product
Cell Shape
Disease
Conditions
SS
Hemoglobin A
Spherical, slightly
concave
No anemia
SS’
Hemoglobin A
Hemoglobin S
Some sickling under
extreme conditions
Sickle Cell Trait
Resistance to
Malaria
S’S’
Hemoglobin S
Sickled under low O2
tension
Sickle Cell
Anemia