Theory of Blending inheritance
Offspring have traits that are intermediate to their parents. Problematic because variation would be reduced over time and does not reflect constant presence of variation in populations
Lamarck’s theory of inheritance of acquired traits
Favourable traits acquired by parents are passed to offspring (problematic theory - directed evolution)
Gregor Mendel
Tested blending vs particulate inheritance using true breeding varieties of peas. Focused on one trait at a time and counted each individual
True breeding
Always give the same phenotype when self crossed
Po
Parental generation
F1
First filial generation
Dominant vs recessive
Often the dominant allele codes for a functional protein whereas the recessive allele does not. Ex. dominant pea colour breaks down chlorophyll
Monohybrid cross
Self fertilize F1 generation. Probability of a gamete inheriting one of the two alleles is random (random alignment of chromosomes at the metaphase plate). Probability of a genotype determined by the product of the probabilities or acquiring each gamete.
Mendel’s principle of segregation
Each organism is diploid (has 2 alleles)
Homozygous
2 alleles are the same, produce only one type of gamete
Heterozygous
2 alleles are different, produces 2 types of gametes
Expected ratio of dominant to recessive phenotypes
3:1
Expected ratio of A/A : A/a : a/a genotypes
1:2:1
F2 generation with recessive phenotype…
Must be homozygous recessive
Test cross
Mating an individual with the homozygous recessive genotype. If heterozygous the phenotype ratio will be 1:1. If homozygous dominant then all offspring will have the dominant phenotype
Mendel’s hypothesis
-adults carry 2 copies of factors that govern inheritance of a character
-dominant masks recessive with different alleles
-the pair of alleles that control a character separate as gametes (meiosis), half carry one allele and the other half carry the other (haploid). Diploid organisms get one allele from each parent
Product rule
Probability of 2 independent events occurring in succession. Individual probabilities are multiplied
Sum rule
Probability of an outcome that can be achieved by two or more mutually exclusive events. Individual probabilities are added
Chromosomal theory of inheritance (Walter Sutton)
Chromosomes occur in pairs, separated in gametes, separation of each pair is independent of other pairs
When do chromosomes separate?
Anaphase I
Incomplete dominance
One allele is not completely dominant. Heterozygote exhibits intermediate phenotype
Genotype ratio of incomplete dominance
1:2:1
Phenotype ratio of incomplete dominance
1:2:1
Co dominance
Co dominant alleles have equal effects. Heterozygotes express both homozygous phenotypes
