Blending Theory of Inheritance
Hereditary traits blend evenly in offspring through the mixing of the parents’ blood like coffee and cream
What is a Character?
A heritable characteristic (i.e fur colour)
What is a Trait?
A particular expression of a character in an individual i.e brown fur
What is the principle of segregation?
Mendel’s hypothesis that the pairs of alleles that control a character separation as gametes formed; half the gametes carry one allele and the other half carry the other allele
What is a genotype?
A particular combination of alleles present in a given organism
What is a phenotype?
The expression of a trait (or traits) in an individual
How is variation in phenotypes generated?
By simple to complex variation in an organism’s genotype
What is transmission genetics?
Deals with the way genetic differences among individuals are passed from generation to generation
Why is the blending theory of inheritance problematic?
The variation would be lost over time
Mendel’s Work
- tested hypothesis of blending vs. particulate inheritance
- used true-breeding varieties of peas
- many varieties with different characters
Three Important Distinctions of Mendel’s Work
- True breeding strains (“good” phenotypes)
- Focused on a single trait at a time
- Quantitative: counted the progeny
Mendel’s Hypotheses
- Adult plants carry two copies of factors (genes) that govern the inheritance of a character
- If an individual’s pair of genes consists of different alleles, one allele is dominant over the other
- Diploid organisms get one allele from each parent
What is a monohybrid cross?
A cross between two heterozygotes (for a single gene)
What does homozygous mean?
The two alleles are the same, therefore produce only one type of gamete
What does heterozygous mean?
The two alleles are different, therefore produces two types of gametes
What is the gene responsible for Mendel’s white and purple pea flowers?
- Gene encodes a transcription factor required to activate genes that synthesize pigments to make purple pea flowers
- White flowers contain a nonsense mutation in this gene
How do you determine if you have a heterozygote?
- A test cross is mating to an individual with the homozygous recessive genotype
- Highly informative: homozygous individuals carry only the recessive allele
Test Cross Steps
- Procedure: cross individual with dominant phenotype to a homozygous recessive
- If heterozygous (Pp), then see 1:1 of dominant and recessive phenotypes
- If homozygous (PP), then see all dominant phenotype
What is Incomplete Dominance?
- One allele is not completely dominant to the other allele
- Heterozygote exhibits an intermediate phenotype compared to the homozygous phenotypes
What is Co-dominance?
- Codominant alleles have equal effects (dominant alleles fight for expression) - heterozygote exhibits both homozygote phenotypes i.e white + black feathers instead of solid white or black
Discrete Trait Variation
Traits that can be sorted into bins/categories
Continuous Trait Variation
Traits of individuals can be anywhere along a continuum i.e height
