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Supplementary Material
Lecture 2: the physical result of
Inheritance
Inheritance
• If a gene has a two or more variants, then
these are called alleles; alleles are the result of
mutations of one of the genes.
• The presence of such alleles is the basis of
differences within members of a species; e.g.
Tall/dwarf [in certain plants] .
• Therefore each physical trait (phenotype) has
two alleles associated with it. One on the
chromosome from the male and one from the
female; or one on each chromosome [in the
chromosome pair]
Types of alleles-> Phenotype
• Dominant/recessive system
– the dominant allele is capitalised/ recessive is lower case
– In heterozygous only the dominant trait is seen.
– In the homozygous it depends it can be either.
• Homozygous dominant: DD (Tall)
• Homozygous recessive: dd (dwarf )
• Heterozygous: Dd (Tall)
• Incomplete / semi-dominance (snap dragon)
– No allele dominant and mixed phenotype (red and white
giving pink)
• Co-dominant (e.g. blood groups)
– The phenotype of both alleles are equally expressed; AB,
AA, BB, OO
Classical (autosomal) Mendelian
Inheritance
Somatic Monohybrid cross Adapted from ref [1] p42
Inheritance: Questions
• This is a
dominant/recessive
inheritance system.
• F1: stands for crosspollination.
• What conclusion can
you draw from F1
results?
• F2 is self pollination:
• How the ratios are
obtained.
• For each example
determine:
– Which is the
dominant/recessive
trait.
Adapted from ref [1] p. 39
X-linked inheritance
• Haemophilia: (a classical case is son Alexei of last
tzar of Russia who was related to queen Victoria)
–
–
–
–
–
X chromosome has the normal/defective gene (H/h)
Y chromosome has no gene (smaller in size)
Defective allele is recessive
Male is XY and Female is XX
Homozygous defective results in the disease
• This includes a defective allele in males
– Homozygous/heterozygous normal results in no
physical effects.
Illustration of royal disease