Transcript Document 7803314

Principles of Heredity
What patterns of inheritance
can be observed when traits
are passed to the next generation?
Mendel’s Experiment With
Peas Differing in a Single Trait
Parental:
Round seed x
F1:
Wrinkled seed
All round seed coats
F1 round plants
x
F1 round plants
F2: 5474 round: 1850 wrinkled
(3/4 round to 1/4 wrinkled)
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:
Round seed x Wrinkled seed
RR
rr
Homozygous
Homozygous
Dominant
Recessive
F1:
All round seed coats
Rr Heterozygous
F1 round plants
x
Rr
F1 round plants
Rr
Heterozygous
Heterozygous
F2
Homozygous parents can only pass
one form of an allele to their offspring.
R
R
R
R
Heterozygous parents can pass either of
two forms of an allele to their offspring.
R
r
R
r
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
RR, Rr, rr
Phenotype Physical characteristic round 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:
RR x rr
R
R
r
r
F1 x F1:
Rr x Rr
r
R
r
R
Rr
Rr
R
Rr
Rr
R
r
r
R
R
RR
Rr
Rr
rr
r
r
100% Round seeds
75% Round seeds
25% Wrinkled seeds
Mendel’s Experiment With Peas Differing in Two Traits
Parental: Round Yellow x Wrinkled Green
F1: All round yellow seed coats
F1 plants x F1 plants
F2
315 round, yellow
9/16
108 round, green
3/16
101 wrinkled, yellow
3/16
32 wrinkled, green
1/16
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:
RRYY
x
RY RY RY RY
rryy
ry ry
ry
F1:
RY
RrYy
100% round, yellow
ry ry
Genetic Segregation + Independent Assortment
F1 x F1 : R r Y y
RY Ry rY ry
x
RrYy
RY Ry rY ry
Four different types of gametes
are formed in equal proportions.
F1 x F1
RrYy X RrYy
1
4 RY
1
4 Ry
Pollen
1
4 rY
1
4
ry
1
16
1
16
1
16
1
16
1
4
Eggs
1
1
4 Ry
4 rY
RY
RRYY
1
16
RRYy
1
16
RrYY
1
16
RrYy
1
16
RRYy
1
16
RRyy
1
16
RrYy
1
16
Rryy
1
16
1
4
ry
RrYY
1
16
RrYy
RrYy
1
16
Rryy
rrYY
1
16
rrYy
rrYy
1
16
rryy
F2 Genotypes and Phenotypes
Phenotypes
Round
Yellow
Genotypes
Round
Green
Wrinkled
Yellow
Wrinkled
Green
1/16 RRyy+ 2/16 Rryy
Total = 3/16 R_yy
1/16 RRYY + 2/16 RRYy +
2/16 RrYY + 4/16 RrYy
Total = 9/16 R_Y_
1/16 rrYY+ 2/16 rrYy
Total = 3/16 rrY_
1/16 rryy
Gene Products
• A gene is a segment of DNA that directs the
synthesis of a specific protein.
• Alleles of the same gene direct the
synthesis of different forms of the same
protein.
Dominant Allele
Codes for a functional protein
Recessive Allele
Codes for a non-functional
protein or prevents any protein
product from forming
Solving Genetics Problems
1. Convert parental phenotypes to
genotypes
2. Use Punnett Square to determine
genotypes of offspring
3. Convert offspring genotypes to
phenotypes
Additional Genetic
Patterns
Mendel’s peas
Other Patterns
Complete
Dominance
Lethal Alleles
Codominance
Incomplete Dominance
Two alleles per gene Multiple Alleles
One gene affects
one trait
One gene affects many
traits (pleiotropic effects)
One gene affects
one trait
Many genes affect one
trait (polygenic inheritance)
Lethal Alleles
Example: Manx cat
ML = tailless, lethal in homozygote
M = tail
Tailless male x Tailless female
MLM
x
MLM
ML
M
2/3 tailless + 1/3 tails
ML
MLML
X
dies
MLM
tailless
M
MLM
tailless
MM
tail
Multiple Alleles
• Multiple Alleles: three or more
alleles exist for one trait
(Note: A diploid individual can only carry two
alleles at once.)
Blood Type
Allele
Type A
IA
Type B
IB
Type O
i
Codominance
• Codominance: Neither allele masks the
other so that effects of both alleles are
observed in heterozygote without
blending
IA = IB > i
IA and IB are codominant.
IA and IB are completely dominant over i.
Codominance
Type A
Phenotype Genotype Gene
Product
Type A
IAIA or IAi Antigen A
Type B
IBIB or IBi Antigen B
Type O
ii
none
Type B
Type O
Type AB
IAIB
Antigen A
and
Antigen B
Effects of both
alleles observed
in phenotype
Type AB
Inheritance of Rh Factor
(another gene with multiple alleles)
Phenotype
Rh Positive
Genotype* Gene
Product
RR or Rr Rhesus Protein
Rh Negative rr
None
*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 are inherited independently.
Example of Multiple Alleles and Codominance
Mom
iirr
Dad
iirr
Type A, Rh positive x Type B, Rh negative
(mother is Type O, Rh-)
(father is Type O, Rh-)
ir
ir
IAiRr
IAR
IBirr
x
IBr IAIBRr
IAr
IAIBrr
iR
IBiRr
ir
IBirr
ir
IAirr
iiRr
iirr
IAiRr
Child with Type AB, Rh negative blood
IAIBrr = 1/8
Incomplete Dominance
Neither allele masks the other and both are
observed as a blending in the heterozygote
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
Epistasis
• An allele of one gene masks the expression
of alleles of another gene and expresses its
own phenotype instead.
• Example of Epistasis
H = enzyme that attaches antigen H
to protein on red blood cells
h= no enzyme to attach antigen H
• Antigens A and B of ABO blood typing
(from alleles IA and IB) are attached to
antigen H. Someone with the genotype
hh will have Type O blood irrespective
of their genotype for the I allele
A
A
H
H
A H
A
H
A
H
A
H
H A
Type A
H
A
H
A
H
A
H
A
HA
IA__H__
Type O
ii__ __ or
IA__hh or IB__hh
Example of Epistasis
IAiHh
IBH
IBh
x IBiHh
iH
ih
IAH IAIBHH
IAIBHh
IAiHH
IAiHh
Type A = 3/16
IAh IAIBHh
IAIBhh*
IAiHh
IAihh*
Type B = 3/16
iH
Type AB = 3/16
IBiHH
IBiHh
iiHH*
iiHh*
*Type O = 7/16
ih
IBiHh
IBihh*
iiHh*
iihh*
Pleiotropic Effects
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
Polygenic Inheritance: Many genes affect one trait
Example: Skin color
Number of Skin Color*
Dominant (Phenotype)
Alleles
0
White
Genotypes
% Pigmentation*
aabb
Aabb or aaBb
0-11%
1
Light Black
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)
ab
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