Mendelian Genetics

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Homozygous

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means that there are two identical alleles for a particular gene (true breeding)

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What are the two components of homozygous?

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Homozygous dominant and Homozygous recessive

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Heterozygous

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Heterozygous means that there are two different alleles for a particular gene

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Punnett square

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Diagrammatic way to predict allele composition of offspring from a cross of known genetic origin.
can be used to assess probability in genetics

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Monohybrid Cross

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Parents are monohybrids i.e. heterozygous for one character
Resulting Pheotypic Ratio is 3:1
Resulting Genotypic Ratio is 1:2:1
Genotypic ratios can be different from phenotypic ratios

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Monohybrid Cross: Resulting phenotypic ratio

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3:1

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Monohybrid Cross: genotypic ratio is

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1:2:1

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Testcross

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is used to determine if a group exhibiting a dominant trait is homozygous or heterozygous for that trait. More simply put, test crosses determine the genotype of an individual with a dominant phenotype.
Test crosses involve breeding the individual in question with another individual that expresses a recessive version of the same trait. If all offspring display the dominant phenotype, the individual in question is homozygous dominant; if the offspring display both dominant and recessive phenotypes, then the individual is heterozygous.
If the individual being tested produces any recessive offspring (except in cases of incomplete penetrance) its genotype is heterzygous. If all the offspring are phenotypically dominant, its genotype is homozygous.

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Dihybrid Cross Demonstrates Independent Assortment

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allele pairs separate independently during the formation of gamates. This means that traits are transmitted to offspring independently of one another. Ratio of 9:3:3:1

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Resulting phenotypic Ratio: Dihybrid Cross Demonstrates Independent Assortment

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Ratio of 9:3:3:1

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Degrees of Dominance:

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Alleles can show different degrees of dominance and recessiveness with relation to each other

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Incomplete Dominance

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The phenotype of F1 hybrids is somewhere between the phenotypes of the two parental varieties
For example, neither red or white allele is a completely dominant. The original traits show up again when you cross the F1s.
A type of inheritance in which one allele for a specific trait is not completely dominant over the other allele. This results in a combined phenotype. For example, if you cross pollinate red and white snapdragon plants, the dominant allele that produces the red color is not completely dominant over the recessive allele that produces the white color. The resulting offspring are pink.

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Epistasis

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A phenomenon in which the expression of one gene depends on the presence of one or more modifier genes. A gene whose phenotype is expressed is called epistatic, while one whose phenotype is altered or suppressed is called hypostatic.
An interaction between nonallelic genes, especially an interaction in which one gene suppresses the expression of another SEE CHICKEN EXAMPLE