Inheritance
Genotypes & phenotypes.
Inheritance
Inheritance is the passing of traits from one generation to the next.
Traits that are expressed in an individual are referred to as the phenotype.
The phenotype is determined by the:
• genes present.
• environment.
Variation
Variations between and within new
generations are due to:
• Random fertilisation (which sperm
fuses with which egg is by chance).
• Genes being present in more than one
form – alleles.
Genotype
An individual’s genotype identifies the alleles present for a particular gene.
Example: Gene B has the alleles B & b.
Individuals are classified as either:
• Homozygous (pure)
the pair of alleles are the same.
• Heterozygous (hybrid)
the pair of alleles are different.
Complete dominance
In a heterozygote (hybrid) some traits are
hidden in the presence of others.
The trait that is hidden is recessive.
The trait that is observed is dominant.
Example:
Gene: flower colour
Alleles: R & r
RR = red
rr = white
Rr = red
Red is dominant; white is recessive.
Sometimes the hybrid does not look like either of the pure parents.
Incomplete dominance
Codominance
• Incomplete dominance: the hybrid’s
phenotype is intermediate between
that of the pure parents.
• Codominance: the hybrid’s phenotype
shows trait from both pure parents.
Example: Pure red x Pure white  Pink offspring
Example: Pure blood group A x Pure blood group
B  AB offspring
RR x WW  RW
AA x BB  AB