The document defines key genetics terminology such as genotype, phenotype, dominant and recessive alleles, locus, homozygous, heterozygous, carrier, and test cross. It explains that some genes have multiple alleles and describes ABO blood groups as an example of codominance and multiple alleles. It also explains sex determination in humans using X and Y chromosomes and defines sex linkage, providing examples of color blindness and hemophilia. The document notes techniques for predicting offspring ratios using monohybrid crosses and deducing genotypes from pedigree charts.

1. 4.3 Theoretical Genetics
Topic 4 Genetics

2. Theoretical Genetics
 4.3.1 Define genotype, phenotype, dominant allele,
recessive allele, codominant alleles, locus, homozygous,
heterozygous, carrier and test cross.
 Genotype: the alleles of an organism.
 Phenotype: the characteristics of an organism.
 Dominant allele: an allele that has the same effect on the
phenotype whether it is present in the homozygous or
heterozygous state.
 Recessive allele: an allele that only has an effect on the
phenotype when present in the homozygous state.
 Codominant alleles: pairs of alleles that both affect the
phenotype when present in a heterozygote.

3. Theoretical Genetics
 Locus: the particular position on homologous
chromosomes of a gene.
 Homozygous: having two identical alleles of a gene.
 Heterozygous: having two different alleles of a gene.
 Carrier: an individual that has one copy of a recessive
allele that causes a genetic disease in individuals that are
homozygous for this allele.
 Test cross: testing a suspected heterozygote by crossing it
with a known homozygous recessive.

4. Theoretical Genetics
 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
humans.
 4.3.6 State that some genes are present on the X
chromosome and absent from the shorter Y chromosome
in humans.

5. Theoretical Genetics
 4.3.7 Define sex linkage.
 4.3.8 Describe the inheritance of colour blindness and
hemophilia as examples of sex linkage.
 Both colour blindness and hemophilia are produced by a
recessive sex-linked allele on the X chromosome. Xb and
Xh is the notation for the alleles concerned. The
corresponding dominant alleles are XB and XH.
 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 recessive alleles.

6. Theoretical Genetics
 4.3.11 Predict the genotypic and phenotypic ratios of
offspring of monohybrid crosses involving any of the
above patterns of inheritance.
 Aim 8: Statisticians are convinced that Mendel’s results
are too close to exact ratios to be genuine. We shall never
know how this came about, but it offers an opportunity to
discuss the need for scientists to be truthful about their
results, whether it is right to discard results that do not fit a
theory as Louis Pasteur is known to have done, and the
danger of publishing results only when they show
statistically significant differences.

7. Theoretical Genetics
 TOK: Reasons for Mendel’s theories not being accepted
by the scientific community for a long time could be
considered. Other cases of paradigm shifts taking a long
time to be accepted could be considered. Ways in which
individual scientists are most likely to be able to convince
the scientific community could be considered, and also the
need always to consider the evidence rather than the views
of individual scientists, however distinguished.

8. Theoretical Genetics
 4.3.12 Deduce the genotypes and phenotypes of
individuals in pedigree charts.
 For dominant and recessive alleles, upper-case and lower-
case letters, respectively, should be used. Letters
representing alleles should be chosen with care to avoid
confusion between upper and lower case. For
codominance, the main letter should relate to the gene and
the suffix to the allele, both upper case. For example, red
and white codominant flower colours should be
represented as CR and Cw, respectively. For sickle-cell
anemia, HbA is normal and Hbs is sickle cell.

9. Theoretical Genetics
 Aim 8: There are many social issues in families in which
there is a genetic disease, including decisions for carriers
about whether to have children, personal feelings for those
who have inherited or passed on alleles for the disease,
and potential problems in finding partners, employment
and health or life insurance. There are ethical questions
about whether personal details about genes should be
disclosed to insurance companies or employers. Decisions
may have to be made about whether or not to have
screening. These are particularly acute in the case of
Huntington disease.

10. Terminology
 There are some terms you need to be able to define:
 Genotype
 Phenotype
 Dominant allele
 Recessive allele
 Codominant allele
 Locus
 Homozygous
 Heterozygous
 Carrier
 Test cross

11. Punnett Grids
 A punnett grid is a way of finding the
expected ratio of the offspring, given
certain parental phenotypes.
 Punnett grid can be constructed looking
at one or two characteristics:
 One chacteristic – a monohybrid
cross
 Two characteristics – a dihybrid cross
 Standard level only need to do
Monohybrid crosses.
 To the right is a simple punnett grid for
sex determination in humans.
Ref: Biology, Weem

12. Pedigrees
 A pedigree chart shows the members of a family and how they are
related to each other.
 It can also show ancestral history of a group of related individuals.
 Pedigree charts can also be used to study the inheritance of a
characteristic.
 By convention:
 Circles represent females and squares represent males
 Shaded circles or shaded squares represent affected individuals;
unshaded circles or squares represent unaffected individuals.
 Two parents are linked by a horizontal line joining a circle and a
square.
 Vertical lines run down from parents to children.
 The children in one family are linked by a horizontal line above them.

13. Pedigrees
A pedigree showing the inheritance of
haemophillia in Queen Victoria’s family
Ref: Advance Biology, Kent

14. Multiple Alleles
 Alleles always occur in pairs because chromosomes are in
pairs and so the alleles occupy the pair of gene loci on
homologous chromosomes.
 Usually within a population there are many different
alleles of a gene and this is called Multiple alleles.
 But any one individual can only have two of these alleles
because it only has one pair of loci for any given gene.

15. Codominance
 Codominance means that both alleles have an effect on
the phenotype.
 ie: the phenotype is some form of mixture of the two
characteristics.
 Neither gene is dominant or recessive.

16. The ABO Blood Groups
 The ABO blood grouping is an example of multiple
alleles and codominance.
 To determine you blood type, there are three alleles:
 IA
, IB
and i.
 This is an unusual case because they also show
codominance:
 Alleles IA
and IB
are codominant.
 This results in four different phenotypes.

17. The ABO Blood Groups
Phenotype or blood
group
Genotypes
A IA
IA
or IA
i
B IB
IB
orIB
i
AB IA
IB
O ii

18. Sex Determination
 Two chromosomes determine the gender of a child (whether it is
male or female). These are called the sex chromosomes.
 The X chromosome is relatively large and carries many genes.
 The Y chromosome is much smaller and carries only a few genes.
 If two X chromosomes are present in a human embryo, it develops
into a girl.
 If one X and one Y chromosome are present in a human embryo, it
develops into a boy.
 When women produce gametes, they pass on one X chromosome in
each egg.
 When men produce gametes, they pass on either one X or one Y
chromosome in the sperm so the gender of the child depends on
whether the sperm that fertilises the egg is carrying an X or a Y
chromosome.

19. Sex Determination
This also
shows that
there is a
50:50 chance
of a child
being a boy
or a girl.
Ref: Biology for the IB Diploma, Allott

20. Sex Linkage
 Genes that are located on one of the sex chromosomes
are said to be sex-linked.
 Because the X chromosome is much larger than the Y
chromosome, most sex-linked genes are located on the
X chromosome.
 As well as carrying genes for sex characteristics, the X
chromosome also carries genes for non-sexual
characteristics.
 Two well studied sex-linked examples are:
 Haemophilia
 Colour blindness

21. Sex-linkage in Males & Females
 In respect to sex-linked genes human females can be:
 Homozygous XN
XN
or Xn
Xn
 Heterozygous XN
Xn
 Females who are heterozygous for X-linked recessive
alleles are said called Carriers.
 Males only have one X chromosome so their inheritance
is a little different. They can be:
 Normal XN
Y
 Affected Xn
Y

22. Haemophilia
 Haemophilia is a condition in which the blood does not
clot normally.
 It normally results in excessive bleeding both internally
and externally.
 The condition is due to the lack of one or more clotting
factors, the most common is a lack of Factor VIII.
 Individuals can lead a normal life with regular injections
of factor VIII.
 Haemophilia is a sex-linked characteristic caused by a
recessive allele carried on the X chromosome.

23. Haemophilia
 Females can be either homozygous dominant or
heterozygous for haemophilia.
 XH
XH
normal
 XH
Xh
carrier
 They cannont be homozygous recessive because the
haemophilia allele is homozygous lethal.
 Ie: Xh
Xh
does not happen because they die.
 Males can be either normal or affected:
 XH
Y normal
 Xh
Y affected - haemophiliacs

24. Ref: Biology for the IB Diploma, Allott

25. Colour Blindness
 Another X-linked condition is colour blindness.
 Females:
 XB
XB
Normal vision (homozygous dominant)
 XB
Xb
Normal vision – carrier (heterozygous)
 Xb
Xb
Colour Blind (homozygous recessive)
 Males
 XB
Y Normal vision
 Xb
Y Colour Blind
 The most common form of colour blindness is red-green colour
blindness.
 6-8% of Caucasian males are red-green colour blind while only 0.5%
of women have this condition

26. Using Punnett Grids
 In choosing symbols for alleles in Punnett grids, these
rules are usually followed:
 Dominant and recessive alleles or genes.
 One letter of the alphabet is chosen. The dominant allele is
represented by the upper-case letter and the recessive allele by the
lower-case letter.
 e.g. A and a
 Codominant alleles.
 One letter of the alphabet is chosen. This letter and a superscript
letter represent each allele
 e.g. Cw
and Cr
 Sex-linked dominant and recessive alleles
 The letter X is used to symbolise the X chromosome. Each allele is
shown superscripted.
 e.g. XH
and Xh

27. Using Pedigree Charts
 You can use pedigree charts to study the inheritance of a
characteristic.
 Pedigree charts can also be used to determine if:
 The characteristic is dominant or recessive.
 The characteristic is sex-linked or not.
Look at the examples on the handout and try the questions.

28. IBO Guide:
 4.3.1 Define genotype, phenotype, dominant allele,
recessive allele, codominant alleles, locus, homozygous,
heterozygous, carrier and test cross.
 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
humans.

29. IBO Guide:
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 colour blindness and
hemophilia as examples of sex linkage.
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 recessive alleles.

30. IBO Guide:
 4.3.11 Predict the genotypic and phenotypic ratios of
offspring of monohybrid crosses involving any of the
above patterns of inheritance.
 Aim 8: Statisticians are convinced that Mendel’s results
are too close to exact ratios to be genuine. We shall never
know how this came about, but it offers an opportunity to
discuss the need for scientists to be truthful about their
results, whether it is right to discard results that do not fit a
theory as Louis Pasteur is known to have done, and the
danger of publishing results only when they show
statistically significant differences.

31. IBO Guide:
 4.3.12 Deduce the genotypes and phenotypes of
individuals in pedigree charts.
 For dominant and recessive alleles, upper-case and lower-
case letters, respectively, should be used. Letters
representing alleles should be chosen with care to avoid
confusion between upper and lower case. For
codominance, the main letter should relate to the gene and
the suffix to the allele, both upper case. For example, red
and white codominant flower colours should be
represented as CR and Cw, respectively. For sickle-cell
anemia, HbA is normal and Hbs is sickle cell.

32. IBO Guide:
 Aim 8: There are many social issues in families in which
there is a genetic disease, including decisions for carriers
about whether to have children, personal feelings for those
who have inherited or passed on alleles for the disease,
and potential problems in finding partners, employment
and health or life insurance. There are ethical questions
about whether personal details about genes should be
disclosed to insurance companies or employers. Decisions
may have to be made about whether or not to have
screening. These are particularly acute in the case of
Huntington disease.