Mendel used the scientific approach to identify two laws of inheritance
Two laws of inheritance:
Mendel’s Experimental, Quantitative Approach
Character: a heritable feature that varies among individuals (i.e. hair color)
Trait: variants of character (i.e. red, black, or blonde)
Peas were good because they were simple, had clear variation, a short generation time, and produced many offspring
Mendel used true-breeding parents (they produced the same trait over many generations) and controlled flower fertilization
Mendel crossed for many generations and used large sample sizes
The Law of Segregation
Disproved the blending theory as traits reappeared after not being expressed in one generation
Alternative versions of genes account for variations in inherited characters. Alleles arise from slight differences in the nucleotide sequences at a locus.
For each character, an organism inherits two copies of a gene, one from each parent.
If two alleles at a locus differ, then one, the dominant allele, determines the appearance; the other, the recessive allele, has no noticeable effect on the organism’s appearance.
Law of Segregation: Two alleles from gametogenesis separate into different gametes.
The test-cross is used to determine if an organism is heterozygous or homozygous dominant for a particular trait, cross the organism with another organism that is homozygous recessive.
If there is a 1:1 ratio of dominant to recessive phenotypes, then the unknown organism is heterozygous
If there is a 1:0 ratio of dominant to recessive phenotypes, then the unknown organism is homozygous dominant
The Law of Independent Assortment
Monohybrid: organisms that are heterozygous for ONE trait
Dihybrid: organisms that are heterozygous for TWO traits
Alleles for one gene assort independently. Only applies to genes that are on different chromosomes or are far apart on a single chromosome.
Probability laws govern Mendelian inheritance
Multiplication and Addition Rules
Multiply probabilities for one or more independent affairs
Add probabilities for two or more mutually exclusive wheels
Inheritance patterns are often more complex than predicted by simple Mendelian genetics
Extending Mendelian Genetics for a Single Gene: Simple Mendelian genetics do not apply when there is incomplete dominance/recession, polygenic traits or when a single gene controls multiple phenotypes
Degrees of Dominance
Incomplete Dominance: Neither allele is completely dominant nor recessive
Codominance: This is where both alleles are EQUALLY EXPRESSED
The Relationship Between Dominance and Phenotype
Dominant and recessive allele plays into effect solely from genotype to phenotype
Frequency of Dominant Alleles
Dominant alleles are not inherently more common than recessive alleles or vice versa
Most genes exist in more than two allelic forms
For example, blood type (IA, IB, and i)
Most genes have multiple phenotypic effects
For example, in peas, the gene for flower color also controls the color of the seed coating
Extending Mendelian Genetics for Two or More Genes
Epistasis: one gene affects another because the gene products are related
Polygenic Inheritance: multiple genes independently affect a single trait
Quantitative Characters: traits that are not A or B, rather they exist as a gradient
For example, skin color and hair color.
Genotypes are most commonly associated with a range of phenotypes, not typically a single phenotype