3.14 Understand that a gene is a section of a
molecule of DNA


A gene is a short section of DNA along the
length of the chromosome.
Each gene codes for a specific protein, which
in turn will determine a specific characteristic
or function.
Genes and Alleles


A gene codes for a specific protein (which in
turn determines a characteristic).
Alleles are different forms of the same gene.
Example:
 The gene which codes for eye colour may
have the alleles blue, green, brown, hazel or
black.

For each gene there will be two alleles
present, so for eye colour you may have a
blue allele and a brown allele.
So which colour of eyes do you end up with?



This all depends whether the allele is
dominant or recessive.
Dominant alleles are represented by capital
letters e.g. the allele for Brown eyes (B).
Recessive alleles are represented by lower
case letters e.g. the allele for blue eyes is (b).

If you have a dominant allele – it is ALWAYS
EXPRESSED
Word
Meaning
Dominant
A dominant allele has just as much effect on the
phenotype when there is one if it as when there
are two of it.
Recessive
A recessive allele only affects the phenotype
when there is no dominant allele present.
Homozygous
Possessing two identical alleles.
Heterozygous
Possessing two different alleles.
Phenotype
Characteristics shown by an organism, a result of
interaction between its genotype and its
environment.
Word
Meaning
Genotype
The genes possessed by an organism.
Codominance
The existence of two alleles for a
characteristic where neither is dominant over
the other.
Homozygous dominant
Possessing two dominant alleles
Homozygous recessive
Possessing two recessive alleles
True (pure) breeding
Producing offspring like themselves,
homozygous.


Blue eyes (b) is recessive to brown eyes (B).
What are the chances of parents having blue
eyed children if the father is heterozygous for
brown eyes (Bb) and the mother has blue eyes
(bb)?

Parents Phenotypes Brown
x
Blue

Parents Genotypes
Bb
x
bb

Gametes
Bb
x
bb

Parents Phenotypes

Parents Genotypes

Gametes
Genetic Cross
(Punnett Square)
Brown
Bb
x
x
Bb
Blue
bb
x
bb
b
b
B
Bb
Bb
b
bb
bb

Offsrping Genotype
50% Bb : 50% bb

Offspring Phenotype
50% Brown : 50% Blue