Topic 4.3: Theoretical genetics
Assessment
Statement
• 4.3.1: Define genotype, phenotype, dominant allele, recessive allele,
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codominant alleles, locus, homozygous, heterozygous, carrier and test cross
4.3.2: Determine the genotype and phenotype of the offspring of a
monohybrid cross using a Punnett grid
4.3.3: State that some genes have more than two alleles (multiple alleles)
4.3.4: Describe ABO blood groups as an example of codominance and
multiple alleles
4.3.5: Explain how the sex chromosomes control gender by referring to the
inheritance of X and Y chromosomes in human
4.3.6: State that some genes are present on the X chromosome and absent
from the shorter Y chromosome in humans
4.3.7: Define sex linkage
4.3.8: Describe the inheritance of color blindness and haemophilia as
examples of sex linkages
4.3.9: State that a human female can be homozygous or heterozygous with
respect to sex-linked genes
4.3.10: Explain that female carriers are heterozygous for x-linked reciessive
alleles
4.3.11: Predict the genotypic and phenotypic ratios of offspring of
monohybrid crosses involving any of the above patterns of inheritance
4.3.12: Deduce the genotypic and phenotypic ratios of offspring of
monohybrid crosses involving any of the above patterns of inheritance
4.3.13: Deduce the genotypes and phenotypes of individuals in pedigree
charts
Who is Gregor Mendel
• Published results of his experiments on how
garden pea plants passed on their characteristics
▫ Among the questions Mendel asked:
 How can I be sure that I will get only smooth peas
and no wrinkled ones?
 How can I be sure that the resulting plants will be
short or tall?
 How can I be sure to obtain only flowers of a certain
color?
Who is Gregor Mendel
• Mendel used artificial pollination in a series of
experiments in which he carefully chose the
pollen of various plants to fertilize other plants
Key Terminology
1. Genotype: The symbolic representation of pair
of alleles possessed by an organism, typically
represented by two letters
2. Phenotype: The characteristics or traits of an
organisms
3. Dominant allele: An allele that has the same
effect on the phenotype whether it is paired
with the same allele or different one.
4. Recessive allele: An allele that has an effect on
the phenotype when present in a heterozygote
Key Terminology
1. Codominant alleles: Pairs of alleles that both
affect the phenotype when present in a
heterozygote
2. Locus: The particular position on homologous
chromosomes of a gene. Each gene is found at a
specific place on a specific pair of chromosomes
3. Homozygous: Having two identical alleles of a
gene
4. Heterozygous: Having two different alleles of a
gene
Key Terminology
1. Carrier: An individual who has a recessive
allele of a gene that does not have an effect on
their phenotype
2. Test Cross: Testing a suspected heterozygote
plant or animal by crossing it with a known
homozygous recessive. (aa) Since a recessive
allele can be masked, it is often impossible to
tell if an organism is AA or Aa until they
produce offspring which have the recessive
trait.
Punnett Squares
• Practice!!!!
Multiple alleles
• With two alleles, three different genotypes are
possible which produce two different
phenotypes
▫ Genetics are not always this simple; sometimes
there are three or more alleles for the same gene
 This is the case for the alleles which determine the
ABO blood type in humans
Multiple alleles
• Blood Type
▫ ABO blood type system in humans has four
possible phenotypes
 A, B, AB, and O
 To create these four blood types there are six alleles
of the gene.
 These three alleles can produce six different genotypes
 The gene for the ABO blood type is represented by the
letter I. To represent more than just two alleles (i and
I)
▫ Two capital letters represent codominance
Multiple alleles
• Blood Type
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IAIA or Iai gives phenotype of type A blood
IBIB or Ibi gives phenotype of type B blood
IAIB gives type AB (codominance)
Ii give type O blood
Sex Chromosomes: X and Y
• The 23rd pair of chromosome are called the sex
chromosome because they determine if a person
is a male or a female.
▫ The X chromosome is longer than the Y
chromosome and contains many more genes
 Unlike the other 22 pairs of chromosome, this is the
only pair in which it is possible to find chromosomes
that are very different in size and shape
 XX= female
 XY= male
Sex Chromosomes: X and Y
• Genes carried on the sex chromosome
▫ Because the Y chromosome is significantly smaller
than the X chromosome, it has fewer loci and
therefore fewer genes than the X chromosome
 This means that sometimes alleles present on the X
chromosome have nothing to pair up with.
 For example, a gene whose locus is at an extremity of
the X chromosome would have no counterpart on
the Y chromosome because the Y chromosome does
not extend that far from its centromere
Sex Chromosomes: X and Y
• Sex Linkage
▫ Any genetic trait whose allele has its locus on the
X or the Y chromosome is said to be sex linked
 Often genetic traits which show sex linkage affect
one gender more than the other.
 Two examples of genetic traits which have this
particularity are color blindness and hemophilia
Sex Chromosomes: X and Y
• Alleles and genotypes of sex-linked traits
▫ Since the alleles for both color blindness and
hemophilia are found only on the X chromosome
▫ Xb=recessive allele for color blindness
▫ XB=allele for the ability to distinguish
▫ XBXB=give the phenotype of a non-affected female
▫ XBXb=gives the phenotype of a non-affected female
who is a carrier
▫ XbXb=gives the phenotype of an affected female
▫ XBY=gives the phentype of a non-affected males
▫ XbY=gives the phenotype of an affected male
Sex Chromosomes: X and Y
• Carriers of sex-linked traits
▫ Sex-linked recessive alleles such as Xb are rare in
most populations of humans worldwide
Pedigree Charts
• The term pedigree refers to the record of an
organisms ancestry
• Pedigree charts are diagrams which are
constructed to show biological relationships
▫ In genetics, they are used to show how a trait can
pass from one generation to the next
 Used in this way for humans, a pedigree chart is
similar to a family tree complete with parents,
grandparents, aunts, uncles and cousins
Pedigree Charts
• To build such a chart, symbols are used to
represent people.
▫ Male (affected)
▫ Female (affected)
▫ Male
▫ Female
▫ Vertical line-represents offspring
▫ Horizontal line-represents marriage