Biology
The prefix homo means same and the prefix hetero means different.
Dominant refers to the allele that is expressed (in charge) in the presence of a recessive allele.
Recessive refers to the allele that is masked or hidden in the presence of a dominant allele but
shows itself in the presence of another recessive one.
Genotype refers to the genes of an organism; which we use alleles (letters) to represent, while
Phenotype refers to physical characteristics of an organism; which we use words to represent.
A species which is heterozygous is represented by one large and one small allele (two
different alleles) eg. Kk while homozygous ones can be of two types: homozygous dominant
eg. HH and homozygous recessive bb.
Using the allele L
Homozygous dominant - LL
Homozygous recessive - ll
Heterozygous - Ll
If black hair is represented by the allele Q and black hair is more dominant than brown hair,
then brown hair will be represented by the allele q.
The phenotype of QQ = black hair
The phenotype of qq = brown hair
The phenotype Qq = black hair
Practice
Determine the phenotypes of the following:
1. Red hair is dominant to black hair.
Rr -
RR -
rr -
2. Curly hair is dominant to straight hair
hh -
Hh -
HH - curly hair
Determine the genotypes (genes aka letters) and phenotypes (characteristics aka words) of the
following:
1. Straight nose is recessive to broad nose
Broad nose -
Straight nose -
Broad nose -
2. Tallness is dominant to shortness (L)
Short person -
Tall person -
Tall person -
3. Pink flowers are dominant to white flowers
a) WW –
b) Ww -
c) ww –
4. Brown eyes are dominant to grey eyes
a) Bb –
b) bb –
c) BB –
5. In rats, white fur is dominant to grey fur
a) Grey fur –
b) White fur –
c) White fur –
6. Round seeds are dominant to wrinkled seeds
a) Round seeds -
b) Wrinkled seeds –
c) Round seeds –
Genetic crossing
Let B represent the dominant allele for brown eyes
Let b represent the recessive allele for grey eyes
Mother’s phenotype: Brown eyes
Father’s phenotype: Grey eyes
Mother’s genotype: BB
Father’s genotype: bb
Gametes: B B x b b
Bb Bb Bb Bb
Offspring’s genotypes: 100% or 4/4 Bb
Offspring’s phenotypes: 100% brown eyes
Punnett squares (another method of carrying out genetic crosses)
Let B represent the dominant allele for brown eyes
Let b represent the recessive allele for grey eyes
Mother’s phenotype: Brown eyes
Father’s phenotype: Grey eyes
Mother’s genotype: BB
Father’s genotype: bb
Gametes: Mother’s genotype
B B
b Bb Bb
b Bb Bb
Offspring’s genotypes: 100% Bb
Offspring’s phenotypes: 100% brown eyes
Curly hair is dominant to straight hair. Using the alleles Ff carry out a cross between a
heterozygous mother and a homozygous recessive father.
Let F represent the dominant allele for curly hair
Let f represent the recessive allele for straight hair
Mother’s phenotype: curly hair
Father’s phenotype: straight hair
Mother’s genotype: Ff
Father’s genotype: ff
Gametes: F f x f f
Ff ff ff Ff
Offspring’s phenotype: 50% straight hair & 50% curly hair
Offspring’s genotype: 50% ff & 50% Ff
Using a Punnett square, carry out a cross between a mother who is heterozygous and a
father who is homozygous dominant for blue eyes. Blue eyes is dominant to brown eyes.
Let B represent the dominant allele for blue eyes
Let b represent the recessive allele for brown eyes
Mother’s phenotype: blue eyes
Father’s phenotype: blue eyes
Mother’s genotype: Bb
Father’s genotype: BB
B b
B BB Bb
B BB Bb
Offspring’s genotype: 50% BB & 50% Bb
Offspring’s phenotype: 100% blue eyes
Monohybrid inheritance
Monohybrid inheritance - is the inheritance of characteristics controlled by a single gene
(mono = one)
Monohybrid autosomal recessive inheritance – albinism (aa), sickle cell anaemia (ss), cystic
fibrosis (cc)
Albinism – an autosomal recessive condition in which the skin, hair and eye pigment
completely or partially lack melanin. An offspring must inherit two recessive alleles e.g. (aa)
to be termed an albino. This means that a dominant allele controls the normal skin pigment.
https://www.youtube.com/watch?v=cHRM2S_fBOk
Cystic fibrosis and sickle cell anaemia are two other conditions inherited in an autosomal
recessive manner like albinism.
Cystic fibrosis – a disease in which the lungs and pancreas are affected by a blockage of
thick, sticky mucus.
A carrier is an organism that has inherited one recessive allele for a trait but does not show
symptoms of the trait/disease. A carrier is a heterozygote e.g. (Aa)
H/W
FIVE (5) signs/symptoms of: (a) albinism (b) cystic fibrosis (c) sickle cell anaemia
Now complete the following:
1. A sickle cell mother mates with a heterozygous father. Determine the phenotypes of
their offspring.
Let S represent the dominant allele for normal blood
Let s represent the recessive allele for sickle cell
Mother’s phenotype: sickle cell
Father’s phenotype: normal
Mother’s genotype:
Father’s genotype:
Gametes:
Offspring’s genotypes:
Offspring’s phenotypes:
2. If two carriers (heterozygous persons) of cystic fibrosis mate, what is the
probability that their offspring will have the condition? (Your answer should be
given as a fraction)
Let Q represent the dominant allele for normal lungs
Let q …
Parents’ phenotypes:
Parents’ genotypes:
Gametes:
Offspring’s genotype:
Offspring’s phenotype:
3. What are the possible genotypes of the children of two parents who are carriers
(heterozygous) but not albinos?
