Campbell & Reece:
Chapters 14 and 15
1. WHAT IS GENETICS
• Genetics: The study of heredity.
• Heredity is the relations between
• Why do children look a little bit like their
parents but also different?
• What is responsible for these
similarities and differences?
2. MENDEL’S GENETICS
• Gregory Mendel is the father of
• Mendel discovered the basic principles
of heredity by breeding garden peas in
carefully planned experiments.
• Advantages of pea plants for genetic
study: Cross-pollination (fertilization
between different plants) can be achieved by
dusting one plant with pollen from another.
• Cross-pollination (fertilization between
different plants) can be achieved by dusting
one plant with pollen from another.
• He also used varieties that were true-breeding
(organisms with only one variety of a type e.g.
red flowers can only produce red flowers)
• In a typical experiment, Mendel mated two
contrasting, true-breeding varieties, a process
• The true-breeding parents are the P
• The hybrid offspring of the P generation are
called the F1 generation
• When F1 individuals self-pollinate, the F2
generation is produced
• When Mendel crossed contrasting, true-
breeding white and purple flowered pea
plants, all of the F1 hybrids were purple
• When Mendel crossed the F1 hybrids,
many of the F2 plants had purple flowers,
but some had white
• Mendel discovered a ratio of about three
to one, purple to white flowers, in the F2
• Mendel reasoned that only the purple flower
factor was affecting flower color in the F1
• Mendel called the purple flower color a
dominant trait and the white flower color a
• What Mendel called a “heritable factor” is
what we now call a gene
• He did 7 other crosses using different traits
and found the same phenomenon.
• Mendel noted that the gene for flower color for
example exists in two versions, one for purple
flowers and the other for white flowers
• These alternative versions of a gene are now
• Each gene is found at a specific locus (position) on
a specific chromosome.
• The two alleles at a locus on a homologous
chromosome pair may be identical, as in the
true-breeding plants – they are then said to
be homozygous for that trait/gene.
• Alternatively, the two alleles at a locus may
differ – they are said to be heterozygous for
• If the two alleles at a locus differ, then one
(the dominant allele) determines the
organism’s appearance (we refer to it as its
phenotype), and the other (the recessive
allele) has no noticeable effect on
• Mendel then formulated the law of
segregation, states that the two alleles for a
heritable character separate (segregate)
during gamete formation and end up in
• Thus, an egg or a sperm gets only one of the
two alleles that are present in the somatic
cells of an organism.
• An organism traits are indicated via its
genotype and phenotype.
Genotype: The genetic composition of
the gene, indicated by letters e.g. GG,
Gg, gg. (A capital letter represents a
dominant allele, and a lowercase letter
represents a recessive allele)
Phenotype: The external appearance of
the gene e.g. Brown hair, white hair.
3. GENETIC CROSSES
• HOW CAN WE NOW MORE OF LESS
DETERMINE WHAT WILL BE THE
OUTCOME IF 2 ORGANISMS HAVE A
TWO TYPES OF
• MONOHYBRID CROSSES: A cross
between 2 organisms where we only
look an one pair of contrasting traits.
• DIHYBRID CROSS: A cross between 2
organisms where we look at two pairs
of contrasting traits at the same time.
MONOHYBRID CROSS -EXAMPLE
• Determine the outcome/ F1 generation
of a cross between a homozygous tall
plant and a homozygous short plant.
Tall plants are dominant over short
STEPS TO SOLVE A CROSS PROBLEM
1. What trait are we looking at?
2. Choose a letter to represent the trait.
3. See if you can identify which trait is dominant –
allocate the capital letter to that trait.
4. Identify the recessive trait and allocate a lower case
letter to that trait.
5. Determine the genotypes of the parents. –
Homozygous dominant – Two capital letters e.g. GG
Homozygous recessive – Two lower case letter. E.g.
Heterozygous – One capital letter and one lower
case letter e.g. Gg
1. Start with cross
1. Trait – Size of plant.
2. Letter chosen to represent size of plant = T/t
3. Tall plants are dominant. (Given in problem) –
Given the – “T” (capital T)
4. Short plants are recessive – given the “t”
(lower case t)
5. One parent is homozygous tall – TT
other parent is homozygous short - tt
CROSS SHOWN AS A GENETIC DIAGRAM
Tall is dominant over short plants – Babies have both
alleles: tall and short
CROSS BETWEEN F1 GENERATION
INDIVIDUALS (INTERBREED F1 GENERATION)
MONOHYBRID CROSS –EXAMPLE 2
A heterozygous blue eyed rabbit is crossed
with a rabbit with pink eyes. What is the
possibility of the babies being born with
1. Trait: eye colour of rabbit.
2. Letter used: E/e
3. Dominant trait: Blue eyes (Why? The first rabbit is
heterozygous – both alleles – but blue is being
expressed in rabbit eyes.) = E
4. Recessive trait: pink eyes = e
5. Rabbit one – heterozygous: Ee
Rabbit two – homozygous: ee (why?)
The only way that a rabbit can have pink eyes
expressed externally is if both alleles code for pink
EXAMPLE OF A DIHYBRID CROSS
Determine the F2 generation of a cross
between yellow round seeded peas and
wrinkled green seeded peas. Yellow and
round seeds are dominant.
• Using a dihybrid cross, Mendel developed
the law of independent assortment
• The law of independent assortment states
that each pair of alleles segregates
independently from another pair of alleles
during gamete formation.
• Strictly speaking, this law applies only to
genes on different, nonhomologous
• Genes located near each other on the
same chromosome tend to be inherited
• In humans there is a disease called Phenylketonuria
(PKU) which is caused by a recessive allele. People with
this allele have a defective enzyme and cannot break
down the amino acid phenylalanine. This disease can
result in mental retardation or death. Let “E” represent
the normal enzyme. Also in humans in a condition
called galactose intolerance or galactosemia, which is
also caused by a recessive allele. Let “G” represent the
normal allele for galactose digestion. In both diseases,
normal dominates over recessive.
• If two adults were heterozygous for both traits, what
are the chances of having a child that is completely
• Has just PKU?
• Has just galactosemia?
• Has both diseases?
4. DEGREES OF DOMINANCE
•Complete dominance One allele
suppresses the expression of the other
• Incomplete dominance: phenotype of F1
hybrids is somewhere between the
phenotypes of the 2 parental varieties –
neither allele completely dominant
(White x Red = Pink)
• Codominance, 2 dominant alleles affect
the phenotype in separate,
distinguishable ways. (Red and white
flowers = White and red visible.)
5. MULTIPLE ALLELES
Most genes exist in populations in more than two
For example, the four phenotypes of the ABO
blood group in humans are determined by three
alleles for the enzyme (I) that attaches A or B
carbohydrates to red blood cells: IA, IB, and i.
The enzyme encoded by the IA allele adds the A
carbohydrate, whereas the enzyme encoded by
the IB allele adds the B carbohydrate; the enzyme
encoded by the i allele adds neither.
Most genes have multiple phenotypic effects, a
property called pleiotropy
For example, pleiotropic alleles are responsible
for the multiple symptoms of certain hereditary
diseases, such as cystic fibrosis and sickle-cell
7. Polygenic Inheritance
Polygenic inheritance is an additive effect of two
or more genes on a single phenotype
Skin color in humans is an example of polygenic
8. DETERMINING THE SEX OF A BABY
In humans and other mammals, there are
two varieties of sex chromosomes: a larger
X chromosome and a smaller Y
Only the ends of the Y chromosome have
regions that are homologous with the X
The SRY gene on the Y chromosome codes
for the development of testes.
9. Inheritance of Sex-Linked Genes
The sex chromosomes have genes for many
characters unrelated to sex
A gene located on either sex chromosome is
called a sex-linked gene
In humans, sex-linked refers to a gene on the
larger X chromosome.
For a recessive sex-linked trait to be expressed
A female needs two copies of the allele
A male needs only one copy of the allele.
Sex-linked recessive disorders are much more
common in males than in females.
Some disorders caused by recessive alleles on
the X chromosome in humans:
• Color blindness
• Duchenne muscular dystrophy