Critical values cannot be negative. The chi squared formula  will not result in a negative value as the numerator is squared.

If the chi square value is large, this indicates a large difference between the observed and expected values. This will subsequently result in a small p-value when using the critical value table. When the p-value is equal to or smaller than the significance level (alpha), the null hypothesis is rejected.

Epistasis describes the interaction of genes, where the epistatic locus masks the effects of a gene at another locus. In this example, locus E is epistatic. As this is stated to be recessive masking epistasis, when the E locus is homozygous recessive (ee), this locus will mask the effect of the B locus (color). Thus, any combination of B/b with ee will result in yellow fur. When the E locus is heterozygous (Ee) or homozygous dominant (EE), the effect of the B locus will not be masked. Thus, BbEe will result in black fur (as black B is dominant to brown b), and the E locus does not mask the B locus here, as the E locus is heterozygous in this individual (Ee).

Epistasis describes the interaction of genes, where the epistatic gene masks the effects of another gene, called the hypostatic gene.

Epistasis describes the interaction of genes, where the gene at the epistatic locus masks the effects of another gene at the hypostatic locus.

As this is recessive epistasis, only the genotype aa will mask gene B (Aa will not mask B). BB or Bb will give the dominant phenotype as B is dominant to b. Thus, AaBb will yield the dominant phenotype.

Epistasis describes the interaction of genes, where the epistatic locus masks the effects of a gene at another locus. In this example, locus E is epistatic. As this is stated to be dominant epistasis, when the E locus is either Ee or EE this locus will mask the effect of the B locus (color). Thus, any combination of B/b with Ee or EE will result in white squash. When the E locus is homozygous recessive (ee), the effect of the B locus will not be masked. Thus, BbEe will result in yellow squash as ee will not mask the color, and B (yellow) is dominant.

Epistasis describes the interaction of genes, where the epistatic locus masks the effects of a gene at another locus. In this example, locus A is epistatic. As this is stated to be recessive epistasis, when the A locus is aa this locus will mask the effect of the B locus (color). Thus, any combination of B/b with aa will result in white snapdragon. Thus, bbaa will result in white snapdragon, as the epistatic locus will mask the effect of the B locus.

Darwin observed that the finches found on the Galapagos had adapted different sized beaks to eat different diets. By doing this, the birds had evolved to eat diets that would be inedible by other finches. This concept of a "niche" means that different species will exploit and use their environment in different ways.

The inheritance of traits through discrete units called alleles was not a part of Darwin's theory of evolution. A monk named Gregor Mendel developed the theory of inheritance through alleles. Darwin's theory did not propose a method of inheritance, and Darwin was unaware of Mendel’s work. The works of Darwin and Mendel were later combined by scientists to create the modern theory of evolution.