Mendelian Inheritance
Conclusions:
Purple colour is dominant
White colour is recessive
The cause of the white colour
is not diluted, but co-exists
with the purple colour
Mendel’s Model
• Alternative versions of genes account for
variations in inherited characteristics
– Purple or white flowers
• For each characteristic, an organism
inherits two alleles – one from each parent
• If two alleles at a locus differ, one is
dominant and the other is recessive
• The two alleles separate during gamete
formation
Helpful definitions
• Gene
– Segments of DNA on a chromosome in the genome that
specify the sequence of a protein
• Locus
– The specific location of a gene on a chromosome in the
genome
• Allele
– any one of a number of versions of a gene occupying a
given locus
– Each cell has 2 copies of a chromosome
» Therefore there are two possible alleles for every
gene
More definitions
• Homozygous
– Organism with a pair of identical alleles
• Heterozygous
– Organism with two different alleles for a particular gene
• Genotype
– The genetic make-up of an individual
• Phenotype
– The displayed traits of an individual
• Pleiotropy
– occurs when a single gene influences multiple phenotypic traits
Genotype vs. Phenotype
Performing a Testcross
Independent Assortment
But….then there is incomplete dominance
Or….co-dominance…or even multiple alleles
Epistasis - the gene at one locus alters the phenotype resulting from another
gene at a separate locus
B – black
b – brown
C – colour
c – no colour
Black (B) is dominant
Colour (C) is dominant
cc will be albino (no colour)
• Quantitative trait locus
– a region of DNA that is associated with a particular
phenotypic trait
– underlie continuous traits - the trait could have any value
within a range (e.g. height)
– a single phenotypic trait is usually determined by many
genes
– QTLs are often found on different chromosomes
• Nature vs. Nurture
Pedigree Charts