Transcript Slide 1

Mendelian Genetics
The laws of probability govern
Mendelian inheritance
• Mendel’s laws of segregation and independent
assortment reflect the rules of probability
• The multiplication rule states that the
probability that two or more independent
events will occur together is the product of
their individual probabilities
• Probability in a monohybrid cross can be
determined using this rule

Rr
Segregation of
alleles into eggs
Rr
Segregation of
alleles into sperm
Sperm
1/
R
2
2
Eggs
4
r
2
r
R
R
1/
1/
r
2
R
R
1/
1/
1/
4
r
r
R
r
1/
4
1/
4
Punnett Square
Monohybrid Cross
The rule of addition
• States that the probability that any one of two
or more exclusive events will occur is
calculated by adding together their individual
probabilities
Each box in this
dihybrid cross has
a 1/16 chance of
occurring.
Add them up
for chances
of any phenotype
Dihybrid cross - The traits are:
long tail (s), short tail (S), brown hair (B) and white hair (b)
Solving Complex Genetics Problems with
the Rules of Probability
• We can apply the rules of probability to predict the
outcome of crosses involving multiple characters
• A dihybrid or other multicharacter cross is equivalent
to two or more independent monohybrid crosses
occurring simultaneously
• In calculating the chances for various genotypes from
such crosses each character first is considered
separately and then the individual probabilities are
multiplied together
Trihybrid Cross of
PpYyRr x Ppyyrr
ppyyRr
ppYyrr
Ppyyrr
PPyyrr
ppyyrr
1/ (yy)  1/ (Rr)
(probability
of
pp)

4
2
2
1/  1/  1/
4
2
2
1/  1/  1/
2
2
2
1/  1/  1/
4
2
2
1/  1/  1/
4
2
2
1/
Chance of at least two recessive traits
 1/16
 1/16
 2/16
 1/16
 1/16
 6/16 or 3/8
Summary of Basic Mendelian Genetics
• We cannot predict with certainty the genotype or phenotype of
any particular seed from the F2 generation of a dihybrid cross,
but we can predict the probabilities that it will fit a specific
genotype of phenotype.
• Mendel’s experiments succeeded because he counted so many
offspring and was able to discern this statistical feature of
inheritance and had a keen sense of the rules of chance.
• Mendel’s laws of independent assortment and segregation
explain heritable variation in terms of alternative forms of
genes that are passed along according to simple rules of
probability.
Extending Mendelian Genetics
• The inheritance of characters by a single gene
may deviate from simple Mendelian patterns
• Inheritance patterns are often more complex
than predicted by simple Mendelian genetics
• The relationship between genotype and
phenotype is rarely simple
• But we can extend Mendelian principles to
patterns of inheritance more complex than
Mendel described
The Spectrum of Dominance
• Complete dominance occurs when the
phenotypes of the heterozygote and dominant
homozygote are identical
• In incomplete dominance the phenotype of F1
hybrids is somewhere between the phenotypes
of the two parental varieties
Red and White Snapdragons
Incomplete Dominance
P Generation
White
CWCW
Red
CRCR
Gametes
CR
CW
Incomplete Dominance
P Generation
White
CWCW
Red
CRCR
Gametes
CR
CW
F1 Generation
Gametes 1/2 CR
Pink
CRCW
1/
2
CW
Incomplete Dominance
P Generation
White
CWCW
Red
CRCR
Gametes
CR
CW
F1 Generation
Pink
CRCW
1/
Gametes 1/2 CR
2
CW
Sperm
F2 Generation
1/
2
CR
2
CW
Eggs
1/
1/
2
CR
1/
2
CW
CRCR CRCW
CRCW CWCW
The Spectrum of Dominance
• In codominance two dominant alleles affect
the phenotype in separate, distinguishable
ways
• The human blood group MN is an example of
codominance
MN Blood Groups
The Relation Between Dominance
and Phenotype
• Dominant and recessive alleles
– Do not really “interact”
– Lead to synthesis of different proteins that
produce a phenotype
Tay-Sachs Disease
• Humans with Tay-Sachs disease produce a non-functioning
enzyme to metabolize gangliosides (a lipid) which then
accumulate in the brain, harming brain cells, and ultimately
leading to death. Tay-Sachs most common in Ashkenazic
Jews (from Central Europe)
• Children with two Tay-Sachs alleles have the disease.
• Heterozygotes with one working allele and homozygotes
with two working alleles are “normal” at the organismal
level, but heterozygotes produce less functional enzymes.
• However, both the Tay-Sachs and functional alleles
produce equal numbers of enzyme molecules, codominant
at the molecular level.
Tay-Sachs Disease
Frequency of Dominant Alleles
• Dominant alleles are
not necessarily more
common in
populations than
recessive alleles
• Polydactyly is a
dominant trait –
Antonio Alfonseca
• 399 out of 400
people have 5 digits
Dominance/recessiveness
relationships
• Range from complete dominance through
various degrees of incomplete dominance to
codominance
• Reflect the mechanisms by which specific
alleles are expressed in the phenotype and do
not involve the ability of one allele to subdue
another at the level of DNA
Multiple Alleles
(a) The three alleles for the ABO blood groups and their
carbohydrates
Allele
Carbohydrate
IA
IB
i
none
B
A
(b) Blood group genotypes and phenotypes
Genotype
IAIA or IAi
IBIB or IBi
IAIB
ii
A
B
AB
O
Red blood cell
appearance
Phenotype
(blood group)
Pleiotropy – gene affects more than one
phenotypic trait
Sickle-cell Anemia
Pleiotropy - Phenotypic traits
affected by sickle-cell anemia
•
•
•
•
•
•
•
Sickled red-blood cells
Anemia
Heart failure
Brain damage
Spleen damage
Rheumatism
Kidney failure
Coat color in Labrador Retrievers
BbEe
Epistasis –
a gene at
one locus
alters the
phenotypic
expression
of a gene at
another
locus
Eggs
1/
1/
1/
1/
4 BE
4 bE
4 Be
4
Sperm
1/ BE
4
1/
BbEe
4 bE
1/
4 Be
1/
4 be
BBEE
BbEE
BBEe
BbEe
BbEE
bbEE
BbEe
bbEe
BBEe
BbEe
BBee
Bbee
BbEe
bbEe
Bbee
bbee
be
9
: 3
: 4
PolygenicTrait,
Quantative
Characters –
Human height in 175
students at Connecticut
Agricultural College
PolygenicTrait,
Quantative Characters –
How human skin color
might work
Sperm
1/
1/
8
8
1/
1/
Eggs
AaBbCc
AaBbCc
8
1/
1/
8
8
1/
8
1/
1/
8
8
8
8
1/
8
1/
8
1/
1/
8
1/
8
1/
8
1/
8
Phenotypes:
Number of
dark-skin alleles:
1/
64
0
6/
64
1
15/
64
2
20/
64
3
15/
64
4
6/
64
5
1/
64
6
Figure 14.UN03
Relationship among
alleles of a single gene
Complete dominance
of one allele
Description
Heterozygous phenotype
same as that of homozygous dominant
Incomplete dominance Heterozygous phenotype
intermediate between
of either allele
the two homozygous
phenotypes
Codominance
Both phenotypes
expressed in
heterozygotes
Example
PP
Pp
CRCR CRCW CWCW
IAIB
Multiple alleles
In the whole population, ABO blood group alleles
some genes have more
IA, IB, i
than two alleles
Pleiotropy
One gene is able to affect Sickle-cell disease
multiple phenotypic
characters
Figure 14.UN04
Relationship among
two or more genes
Epistasis
Description
The phenotypic
expression of one
gene affects that
of another
Example
BbEe
BE
BbEe
bE
Be
be
BE
bE
Be
be
9
Polygenic inheritance
A single phenotypic
character is affected
by two or more genes
AaBbCc
:3
:4
AaBbCc