4.3 THEORETICAL GENETICS
ASSESSMENT STATEMENTS
Define genotype, phenotype, dominant allele, recessive allele, codominant allele, locus,
homozygous, heterozygous, carrier, and test cross
Determine the genotypes and phenotypes of the offspring of a monohybrid cross using a
Punnett grid.
State that some genes have more than two alleles (multiple alleles)
Describe ABO blood groups as an example of codominance and multiple alleles
Explain how sex chromosomes control gender by referring to the inheritance of X and Y
chromosomes
State that some genes are present on the X chromosome and absent from the shorter Y
chromosome in humans
Define sex linkage
Describe the inheritance of color blindness and hemophilia as examples of sex linkage
State that a human female can be homozygous or heterozygous with respect to sex
linked genes
Explain that female carrier are heterozygous for X-linked recessive alleles.
Predict the genotypic and phenotypic ratios of offspring of monohybrid crosses involving
any of the above patterns of inheritance.
Deduce the genotypes and phenotypes of individuals in pedigree charts.
Genotype- the alleles possessed by an organism; each allele is
represented by a letter; chromosomes come in pairs and so alleles come
in pairs- a genotype is therefore represented by a pair of letters- such as
TT or Tt.
Phenotype- the characteristics of an organism; a characteristic may be
an external feature, such as hair color or flower color, or internal like
sickle-cell anemia
Dominant allele- an allele that has the same effect on the phenotype
when in either the homozygous or heterozygous state; the dominant
allele is always given a capital letter such as T
Recessive allele- an allele that only has an effect on the phenotype
when in the homozygous state; recessive allele is always given a lower
case of the same letter as the dominant allele such as t
Locus- the specific position of a gene on a homologous chromosome; a
gene locus is fixed for a species- for example, the insulin gene is always
found at the same position on chromosome 11 in humans.
Homozygous- have two identical alleles at a gene locus; the alleles may
both be dominant or both recessive- for example TT or tt.
Heterozygous- having two different alleles at a gene locus- for example
Tt.
Codominant alleles- pairs of alleles that both affect the phenotype when
present in the heterozygous state; these alleles are represented in a
different way in genetic; a capital letter is chosen to represent the gene
and then other (superscript) letters represent the alleles (for example for
human blood types A and B are codominant alleles and are represented as
IA and IB) .
Test Cross- testing a dominant phenotype to determine if it is
heterozygous or homozygous- for example, crossing either TT or Tt with
tt; if there are any offspring with recessive phenotypes, then the parent
with the dominant phenotype must be heterozygous (Tt).
Carrier- an individual with one copy of a recessive allele which causes a
genetic disease in individuals that are homozygous for this allele.
You must understand and remember these terms!!
PUNNETT SQUARES
Punnett Squares can be used to work out all the possible
combinations of alleles that van be presented in the
offspring of two parents with known genotypes.
Punnett squares show combinations and also help to deduce
the probabilities of each one occurring.
STEPS FOR PUNNETT SQUARES
1. Choose a letter to represent the gene. Choose one that has a
distinctly different upper and lower cases. The letter should be
based on the dominant allele.
2. Represent the genotype of each parent with a pair of letters.
Use a single letter surrounded by a circle to represent the
genotype of each gamete.
3. Combine pairs of the letter representing the gametes to give
all the possible genotypes of the offspring.
4. From the possible genotypes, workout the possible phenotypes
of the offspring.
CODOMINANCE AND MULTIPLE ALLELES
Before when looking at inheritance one of the alleles completely
dominates the other, so in a heterozygous genotype the phenotype
is determined by the dominant allele
In codominance, both alleles have an affect on the phenotype.
This Punnett Square represented smooth or wrinkled peas is
known as a monohybrid cross because it involves just one gene
with two alleles: A for round and a for wrinkled.
There are many cases where genes have more than two
alleles.
One example is human blood groups.
The ABO human blood grouping is an example of both
codominance and multiple alleles.
There are three alleles IA ,IB , and i
IA and IB are codominant and both are dominant to i.
This results in four different phenotypes or blood groups
Genotype
Phenotype or Blood Group
IA IA
A
IA i
A
IB IB
B
IB i
B
IA IB
AB
ii
O
A person’s blood group depends on which combination of
alleles they receives. Each person has only two of the three
alleles and they are inherited just as though they are
alternative alleles of a pair.
Example
Hair shape in humans is a codominance characteristic. Straight
hair and curly hair are codominant alleles and the heterozygous
has wavy hair. Popeye and Olive Oil both have wavy hair. Deduce
the probabilities that their children will have straight hair, curly
hair or wavy hair.
HS - straight hair
HC – curly hair
Since both Popeye and Olive Oil have wavy hair their
genotypes must be HSHC
SEX CHROMOSOMES AND THE CONTROL OF GENDER