Monohybrid Cross
Higher Human Biology
Lesson Aims
• To revise and consolidate understanding
of monohybrid crosses
• To examine Rhesus and Rhesus- blood
groups
• To learn about different conditions caused
by genetic mutations
• To find out the difference between
incomplete dominance and co-dominance
History
• Gregor Mendel - The Father of Genetics
• 1. Monk who used science and maths to establish
patterns in how traits were inherited
2. Year 1857
3. He used the garden pea as his test subjects
• Some Vocabulary
• Character - a heritable feature (eg flower colour)
Trait - a variant of each character (eg purple or white)
Cross Pollination - one plant fertilizes a different plant
Self Pollination - a plant fertilizes itself
True-Breeding - plants that over several generations only
produce plants like themselves
• Mendel’s Experiments - a Monohybrid Cross
• cross involved plants that differed for a single
character: tall x short, purple flower x white
flower, round seed x wrinkled seed
• P (Parental Generation) True breeding plants
F1 (First Filial) The offspring of the P generation
--> they always displayed a single trait, the
dominant one
F2 (Second Filial) The offspring of the F1
generation, self fertilized --> always had a 3:1
ratio
Mendel’s Law of Segregation
• The Law of Segregation: two alleles
separate during gamete formation (a Tt
parent can produce both T sperm, and t
sperm)
• A punnet square is a representation of
this law, showing how gametes separate
and then come together during fertilization
• Locus - spot on the chromosome where
an allele (gene) is located.
•
Monohybrid Inheritance in Humans
• Tongue rolling is inherited as a simple
Mendelian trait
• R is the allele for roller
• r is the allele for non-roller
Genetics of Tongue Rolling
Rhesus D Antigen
• Antigen D is found on
the surface of some
people’s red blood
cells
• Rh+ (rhesus positive)
posses this antigen
• Rh- (rhesus negative)
do not possess this
antigen
• If a Rh- person is given Rh+ red blood
cells during a transfusion the persons
immune system responds by producing
anti-D antibodies. This leaves the person
sensitised.
• If this person receives more Rh+ red blood
cells they suffer from severe or fatal
agglutination.
Agglutination of Red Blood Cells
• Presence of Antigen D is genetically
dominant (D)
• Lack of antigen D is due to a recessive
allele (d)
P
DD x dd
(Rh-) (Rh+)
F1 all Dd
(Rh+)
or
P
dd x Dd
(Rh-) (Rh+)
F1 Dd and dd
(Rh+) (Rh-)
• Albinism - inability of the body to make
melanin - inherited as simple Mendelian
recessive trait
• Cystic Fibrosis - disorder of the mucus
secreting glands - simple Mendelian
recessive trait
• PKU – inborn error of metabolism – simple
Mendelain recessive trait
Huntingdon’s Chorea
• Degeneration of the nervous system which
leads to premature death
• Determined by dominant allele
• Allele not expressed in phenotype until
about 38 years of age when sufferer will
probably have had a family and passed on
the allele
Incomplete Dominance
• Occurs when the recessive allele has
some effect on the heterozygote.
• An example of incomplete dominance is
illustrated in the condition known as sickle
cell anaemia.
• The haemoglobin produced is an unusual
type called haemoglobin S which is an
inefficient carrier of oxygen
• HH x HH all offspring HH - survive
• HH x HS offspring HH, HH, HH, HS survive
• HS x HS offspring HH, HS, HS, SS – 75%
survive (SS dies)
Co-dominance
• Describes the situation where two alleles
can be expressed in the heterozygote,
neither suppressing the other.
• This is seen in the MN blood grouping.
MN Blood Group
• M and N alleles are co-dominant (both
alleles expressed in the phenotype of the
heterozygote
• MM blood group have M antigens on rbc
• NN blood group have N antigens on rbc
• MN blood group have both M and N
antigens on rbc
ABO Blood Group
• Antigens coded by a gene that has three
alleles A, B and O.
• Allele A produces antigen A
Allele B produces antigen B
Allele O produces no antigens
• Alleles A and B are co-dominant to one
another and completely dominant over
allele O
The Facts You Need To Know
• p6
• from “monohybrid inheritance depends
on…”
• to “if an individual has the genotype AO…”