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UNIT 4:
GENETICS AND
INHERITANCE
Campbell & Reece:
Chapters 14 and 15

1. WHAT IS GENETICS
• Genetics: The study of heredity.
• Heredity is the relations between
successive generations.
• 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
Genetics.
• 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
called hybridization
• The true-breeding parents are the P
generation.
• 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
generation.

Unit 4 genetics and inheritance

• Mendel reasoned that only the purple flower
factor was affecting flower color in the F1
hybrids.
• Mendel called the purple flower color a
dominant trait and the white flower color a
recessive trait
• 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
called alleles
• 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
that gene/trait.
• 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
different gametes
• 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
BABY?

TWO TYPES OF
GENETIC CROSSES
• 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
plants.

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.
gg
Heterozygous – One capital letter and one lower
case letter e.g. Gg
1. Start with cross

SOLUTION
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
Why?
Tall is dominant over short plants – Babies have both
alleles: tall and short

Cross shown as a punnet square

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
pink eyes?

SOLUTION
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
eyes.

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 of each other pair of alleles
during gamete formation.
• Strictly speaking, this law applies only to
genes on different, nonhomologous
chromosomes
• Genes located near each other on the
same chromosome tend to be inherited
together.

4. DEGREES OF DOMINANCE
•Complete dominance One allele
suppresses the expression of the other
allele.
• 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
allelic forms.
 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.

6. PLEIOTROPY
 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
disease

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
inheritance.

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
chromosome
 Only the ends of the Y chromosome have
regions that are homologous with the X
chromosome
 The SRY gene on the Y chromosome codes
for the development of testes.

 Females are XX, and males are XY
 Each ovum contains an X
chromosome, while a sperm may
contain either an X or a Y
chromosome.

DIAGRAM TO DETERMINE THE SEX OF
A BABY

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
• Hemophilia

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Unit 4 genetics and inheritance

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