By: M (athilde) dawg, L (iam) dawg, and G (eorge) dawg
• 4.3.3 – Multiple alleles
• 4.3.4 – Describe ABO blood groups as an example of
co-dominance and multiple alleles.
• 4.3.5 – Sex Determination
• 4.3.6 – State that some genes are present on the Xchromosome and absent from the shorter Y
chromosome in humans.
• Some genes have more than two alleles.
• An individual can only possess two alleles.
• The population may contain many alleles for a given
• Multiple alleles increases the number of different
• Multiple alleles can be dominant, recessive or codominant to each other.
-The ABO blood type system in humans has 4 possible phenotypes: A, B, AB,
and O.
-The red blood cell antigen is coded for by the gene I
-I has three alleles three alleles IA, IB and IO
-IA and IB are codominant, while IO is recessive.
The Possible Genotypes
and Phenotypes
The cross below shows how all four blood groups can
arise from a cross between a group A and a group B
• Determined by sex chromosomes
• X and Y
• In humans:
- Homologous (XX) are females
- Non- Homologous (XY) are males
Monohybrid cross
• Demonstrates inheritance of X and Y chromosomes
• There will be 1:1 ratio of male to females
X and y
• Sex is determined solely by the sperm
• Technique for separating X and Y sperm is using IVF
• These are called sex or linked X-linked genes
• Female sex chromosomes are XX
• Male sex chromosomes are XY
• The Y chromosomes does not carry an allele