1. BIOLOGY FORM 5: CHAPTER 5 –
INHERITANCE
The hereditary factor that determines a
certain characteristic is called gene.
Each charateristic is controlled by a pair of
alleles.
An allele is one of a pair of ganes that is
located at the same locus in apair of
homologous chromosomes.
If an organism has two similar alleles for a
particular characteristic the organism is
homozygote.
Genotype is the combination of alleles in
an organism.
The phenotype of an organism is its
observable characteristic.The phenotype
of an organism is determined by its
physical appearance,which is an
observable characteristic.

2. The phenotype of an organism is
determined by its genes and is influenced
by the environment.
Examples of phenotype observed in the
garden pea of Mendel are the seed colour
(either yellow or green) and the flower
colour ( either purple or white). In humans,
the observable characteristics (phenotype)
are height (either tall or short), skin colour
(either fair or dark), and hair type (either
curly or straight)
Table show the characteristics and traits of
pea plants (Pisum sativum) studied by
Gregor Mendel in this experiments on
inheritance.

3. Dominant allele will express its trait when
the genotype is heterozygous recessive,
with the absence of a dominant allele.
Pure line (pure-breed) refers to the
population of organism where the
genotype remained unchanged for many
generations and is homozygous for both
dominant and recessive traits.

4. Mendel’s First Law of Law of Segregation
states that a characterisitc of an organism
is determined by a pair of alleles. In the
formation of gamete, the pair of alleles
segregate during meiosis I and only one of
each pair of alleles can be present in a
single gamete.

5. Mendel’s First Law showing alleles
segregation
Mendel’s Second Law or Law of
Independent Assortment states that during
the formation og gametes,two or more
pairs od alleles segregate independently of
each other.
of time.
The inheritance of ABO blood group is
controlled by there alleles;namely allele A,
allele B and allele O.

6. Rhesus positive (Rh+) is a dominant trait
while Rhesus negative (Rh -) is a
recessive tarit.
Problems will arise during pregnancy when
the mother is Rhesus negative and the
foetus is Rhesus positive.
In humans, there are two types of
chromosomes ; namely autosomes and
sex chromosomes.Chromosomes are
paired up based on the location of
centromere and size of chromosome to
form pairs of homologous chromosomes.
Humans have 46 chromosomes : 22 pairs
of autosomes and a pair of sex
chromosome (XX or XY)
The genotype of a male is 44 + XY while
the genotype of a femele is 44 + XX.
There are two types of sperms; sperms
with X or Y chromosome, but there is only
one type of ovum.All ovum has X
chromosome.
Sex of an offspring is determined by the
type of sperm that will fertilise an ovum.An

7. induvidual with Down’s syndrome has 47
choromosomes.This induvidual has three
21 chromosomes instead of two.
Haemophilia and colour- blindness are
examples of sex-linked diseases.The allele
for these two diseases is located on the X
choromosome.
Chromosome contain genes that carry
genetic information which controls the
synthesis of enzymes and structural
protein or polypeptide chain.Each species
has its own specific number of
chromosomes.Each choromosomes is
made up of a DNA molecule coiled around
protein molecules.
Gene is the sequence of nucleotides of a
DNA.Gene is the basic unit of inheritance
and occupies a specific locus in a
chromosome.
DNA consists of basic inits called
nucleotides. Each nucleotide is made up of
deoxyribose sugar, a nitrogenous base
and a phosphate group.

8. The deoxyribose sugar of one nucleotide is
linked to a phosphate group of another
nucleotide to form a long polynucleotide
strand.The nitrogenous bases of each
polynucleotide strand are binded together
by hydrogen bond.
In a nucleotide, the deoxyribose sugar is
linked to the phosphate and the
nitrogenous base.
Determination of a trait an organism from
the gene:

9. The sequence of nitrogenous base in the
molecule of a DNA is called gene. A DNA
molecule consists of two polynucleotide
strands that coil together to form a double
helix. This strusture enable the DNA to
replicate during cell division.
Knowlwdge in genetics is widely used in the following fields :
a) seletive breeding b) genetic engineering i) gene therapy ii)
genetically modified organisms ( GMO ) iii) medicinal
purposes iv) genetically modified food (GMF) c) DNA
fingerprinting d) Human Genome Project e) stem cell research