Mendelian Genetics
Gregor Mendel (1822-84)
• Known as the ‘father of genetics’
• Austrian monk who carried out work
using pea plants to study the inheritance
patterns of a number of traits
• Formulated a number of ideas about the
inheritance of characteristics – these are
known as Mendel’s Laws of Inheritance
Mendel’s Laws of Inheritance
• Theory of Particulate Inheritance
– Mendel recognised that traits are determined by discrete units
that are inherited intact down through generations – we now
know these units are genes.
– This model explained many observations that could not be
explained by the idea of blended inheritance that was accepted
at the time.
• Law of Segregation
– Two alleles separate unchanged during gamete formation i.e. (a
Tt parent can produce both T sperm, and t sperm)
• Law of Independent Assortment
– Pairs of alleles on different chromosomes will segregate
independently
Genotype and Phenotype
• Genetic inheritance involves the genotype – the
particular set of alleles possessed by an organism
• The observable inherited characteristics of an
organism are referred to as it’s phenotype
• It is important to realize that the phenotype has
both inherited and environmental components
GENOTYPE + ENVIRONMENT = PHENOTYPE
The 3Hs
• An organism that has two copies of the same allele is
said to be homozygous for that allele and the organism
is referred to as a homozygote. These organisms are
also referred to as pure-breeding strains for the locus
in question.
• An organism that has two different alleles of a particular
gene is said to be heterozygous for that allele and the
organism is referred to as a heterozygote.
• The above statements are true for autosomes, as
individuals will carry two copies of the genes. The X
chromosome is a different story.
• Human males only carry one X chromosome so they
can only have one copy of any allele found on the X
chromosome. They are referred to as being
hemizygous.
Dominant and Recessive Phenotypes
• An important property of phenotypes is dominance - genes are not
recessive or dominant.
• Phenotypes may be completely dominant, incompletely dominant,
codominant or recessive depending on their appearance in the
heterozygote.
• A dominant phenotype is one that is visible in the heterozygote
and homozygote.
• A recessive phenotype is one that is visible only in the
homozygote.
• Scientific convention dictates that the allele associated with a
dominant phenotype is assigned an uppercase symbol (e.g. B) and
the allele associated with the recessive phenotype is assigned the
lowercase symbol (e.g. b)
• Dominant does not mean the most common, and a dominant trait
is not necessarily most frequently represented in pedigree.
Complete Dominance
• In complete dominance homozygous and heterozygous
individuals are generally indistinguishable.
• The alternative phenotype, expressed only in the
homozygote is said to be recessive.
• Example: Eye colour
– Brown eye phenotype is dominant over blue eyes
– Assign B to allele associated with brown and brown and b to
allele associated with blue
– Homozygous individual BB has two copies of allele for brown
eyed phenotype and will have brown eyes
– Homozygous individual bb has two copies of allele for blue
eyed phenotype and will have blue eyes
– Heterozygous individual Bb has one copy of each allele and will
have brown eyes
Incomplete dominance
• With incomplete dominance, the phenotype is only
partially expressed in the heterozygote, so the
heterozygote has a different phenotype from either
homozygote.
• Example: Colour of snap dragon flowers
– Snap dragons have a dark red phenotype or a white phenotype
– Assigning R1R1 as genotype for red phenotype and R2R2 as
genotype for white phenotype
– Heterozygote has genotype of R1R2 and has pink phenotype.
– In this case R1 codes for enzyme that promotes reaction that
produces red pigment while R2 codes for defective enzyme.
Flowers are white due to complete absence of enzyme in R2R2
homozygote, and pink in R1R2 heterozygote due to half the
amount of enzyme being produced.
Codominance
• With co-dominant phenotypes, the phenotypes of both
homozygotes are expressed equally in the heterozygote.
• Example: ABO blood system
– There are three different alleles found at the locus for the ABO
blood group, and individuals can have A, B, AB or O
phenotypes.
– People with AB blood groups are heterozygous. They have one
allele that produces A antigen and one that produces B antigen.
– A and B phenotypes are co-dominant.
– Those with phenotype A or B may be homozygous or
heterozygous, as the allele for the O phenotype does not code
for expression of an antigen. This means that you can carry an
allele corresponding to A phenotype and an allele
corresponding to O phenotype but your phenotype will be A.
– This means that O phenotype is recessive.
ABO Blood System
Other factors to consider
• Multiple alleles at a single locus
– Many genes have more than two alleles e.g. ABO blood systems and
HLA antigens (markers of self)
– Dominance relationships can only be established for a pair of alleles
and these relationships can sometimes change due to environmental
factors
• Single genes with multiple effects
– Some genes affect more than one phenotype.
– These genes tend to be regulatory genes that control other genes that
dictate development of a particular trait.
– The lozenge gene in Drosophila controls differentiation of a number of
tissues, and mutation can lead to a number of different phenotypic
effects in the recessive homozygote in addition to the eye defect for
which it was originally named.
– The Tail-less mutation in mice is dominant, leading to mice with short
or no tails. This mutation is also a recessive lethal, meaning that the
homozygote for this mutation does not survive. This shows that the
gene has at least one other function and that this function is essential
for normal development of the organism.
Relationship between environment
and phenotype
• Identical twins are genetically identical as they arise from the
separation of the daughter cells of a single fertilised egg.
• Studies have shown that while their genotypes may be identical,
their phenotypes can show differences. These differences must be
attributable to different environments.
• Better defined examples of the potential influence of environment
of phenotype are provided by phenylketonuria (PKU), fur colour in
Himalayan rabbits, and sickle cell anaemia.
– Individuals with the disorder PKU are prone to mental retardation
which can be prevented if diagnosed at birth and provided with a
modified diet (environmental factor)
– When a small region of white fur is shaved from the back of a
Himalayan rabbit, the fur will grow back black if the animal is kept at
low temperatures, but white if the animal is kept at high temperatures.
Effect of environment on phenotype of individuals carrying
alleles for faulty b-haemoglobin productions
Monohybrid Cross
• A monohybrid cross is a cross between two individuals
involving a single genetic locus.
• Procedure:
– Select two strains that are pure-breeding for differing
phenotypes (ie two homozygous organisms with different
phenotypes). This is the parental generation.
– Breed these to produce offspring. This is the first filial
generation (F1). These individuals will all be heterozygous.
– Breed F1 offspring to each other to produce second filial
generation (F2). Examining the appearance offspring resulting
from these crosses give us information about the about the
pattern of inheritance of the trait being studied.
– Punnet squares are used to illustrate the random combination
of gametes and the resulting offspring in each generation
Examples of monohybrid crosses
Complete dominance
•Parental Phenotypes:
red eyes or white eyes
•F1 Phenotypes:
all red eyes
•F2 Phenotypes:
3 red eyes, 1 white eyes
Incomplete dominance
•Parental Phenotypes:
red or white flowers
•F1 Phenotypes:
all pink flowers
•F2 Phenotypes:
1 red, 2 pink, 1 white
Rules for Monohybrid Crosses
• A 3:1 phenotypic ratio will be observed in F2 generation
if the following conditions apply:
–
–
–
–
The variation in the trait is controlled by a single gene.
The gene is on an autosome.
There are two alleles of the gene.
One phenotype is dominant.
• A 1:2:1 phenotypic ratio will be observed in F2
generation if the following conditions apply:
–
–
–
–
The variation in the trait is controlled by a single gene.
The gene is on an autosome.
There are two alleles of the gene.
One phenotype is incompletely dominant or the phenotypes are
co-dominant
Dihybrid Cross
• A dihybrid cross is a cross between two individuals
involving a two genetic loci.
• Procedure:
– Select two strains that are pure-breeding for both genetic loci.
This is the parental generation.
– Breed these to produce offspring. This is the first filial
generation (F1). These individuals will all be heterozygous and
have the same genotype.
– Breed F1 offspring to each other to produce second filial
generation (F2). Examining the appearance offspring resulting
from these crosses give us information about the about the
pattern of inheritance of the trait being studied.
– Punnet squares are used to illustrate the random combination
of gametes and the resulting offspring in each generation
Example of a dihybrid cross
•
•
•
•
•
Coat color is indicated by B
(brown, dominant) or b (white).
Tail length is indicated by S
(short, dominant) or s (long).
Parents are homozygous for each
trait (SSbb and ssBB).
F1 generation are heterzygous at
both loci and only show the
dominant phenotypes.
F2 generation all combinations of
coat color and tail length occur:
– 9 are brown/short (purple
boxes),
– 3 are white/short (pink boxes)
– 3 are brown/long (blue boxes)
– 1 is white/long (green box).
Rules for Dihybrid Crosses
• A 9:3:3:1 phenotypic ratio will be
observed in the F2 generation if the
following conditions apply:
–
–
–
–
–
The two genes control two distinct traits
There are two alleles for each of the genes
For each trait one phenotype is dominant
Both genes are on autosomes
The two genes assort independently
Back crosses and test crosses
• A back cross is a cross between the F1 heterozygote and
either one of the pure-breeding (homozygous) parental
strains.
• A test cross is a cross between F1 heterozygote and the
parental strain showing the recessive phenotype.
• Test crosses are carried out to determine whether an
individual of dominant phenotype is homozygous or
heterozygous.
• Test crosses also provide an answer as to whether the
phenotypic variation we observe is due to the
segregation of alleles (meaning phenotype is controlled
by one gene) or independent assortment (indicating
phenotype is controlled by two genes).
Examples of test crosses
Test cross involving single gene
Offspring phenotypic ratio 1:1
Test cross involving two gene
Offspring phenotypic ratio 1:1:1:1
Interpreting phenotypic ratio results
in F2 and testcross offspring
• F2 ratio 3:1
Testcross ratio 1:1
– One phenotype is dominant, the other recessive.
– Trait is inherited by segregation of alleles of single autosomal
gene
• F2 ratio 1:2:1
Testcross ratio 1:1
– The heterozygote is of distinct phenotype because of
incomplete dominance or codominance.
– Trait is inherited by segregation of alleles of single autosomal
gene
• F2 ratio 9:3:3:1
Testcross ratio 1:1:1:1
– For each of the two traits there is a dominant and a recessive
phenotype.
– Each trait is determined by a single autosomal gene.
– The alleles of each gene assort independently.
Gene interactions
• Aspects of phenotype are often influenced by more than
one gene.
• Two or more genes may interact to produce a particular
phenotype.
• One such example is the inheritance of a type of
deafness in humans.
– This hereditary deafness is due to the action of two unlinked
genes, each with two alleles.
– The allele pairs can be represented as D and d, and E and e.
– At least one D and one E is required for normal hearing.
– If either or both loci are homozygous, dd or ee, deafness
results.
– The following genotypes will produce deaf individuals: DD ee,
Dd ee, dd EE, dd Ee, dd ee
Gene interactions in human deafness
• Consider a family in
which both parents are
heterozygous for these
genes and have normal
hearing:
– DdEe x DdEe
• The genotypes look the
same as in a standard
dihybrid cross, but the
resulting phenotypes do
not give the typical 9:3:3:1
ratio
• There is a probability of
9/16 (unshaded) normal
hearing and 7/16
(shaded) deafness.
gamete
s
¼
DE
¼
De
¼
dE
¼
de
¼
DE
¼
De
¼
dE
¼
de
1/16
DDEE
1/16
DDEe
1/16
DDdE
1/16
DdEe
1/16
DDEe
1/16
DDee
1/16
DdEe
1/16
Ddee
1/16
DdEE
1/16
DdEe
1/16
ddEE
1/16
ddEe
1/16
DdEe
1/16
Ddee
1/16
ddEe
1/16
ddee
Handy Hint
• If you are working out probabilities when
the phenotype results form the interaction
of two or more genes, you cannot rely on
typical ratios you might expect in the
cross.
• You need to work the example out in a
punnet square and identify the phenotypes
from the resulting genotypes.