In sheep, coat color is influenced by two genes. gene a influences pigment production, while gene b produces black or brown pigment. if two heterozygous white sheep resulted in 12 white sheep, 3 black sheep, and 1 brown sheep, which genotype(s) of the white sheep explain this data? a) the white sheep could be a_b_ or aab_. b) the white sheep could be a_b_ or aabb. c) the white sheep could be a_b_ or a_bb

c) The white sheep could be A_B_ or A_bb.

Explanation:

This is a case of dominant epistasis which falls under the category of intergenic interaction (gene interaction) and produces phenotypes in 12:3:1 ratio rather than producing ideal 9:3:3:1 ratio of Mendelian inheritance for dihybrid crosses.

The progeny produced after such dihybrid cross and their phenotypes will be as under:

AABB - 1 white

AABb - 2 white

AAbb - 1 white

AaBB - 2 white

AaBb - 4 white

Aabb - 2 white

aaBB - 1 black

aaBb - 2 black

aabb - 1 brown

In the example as shown above, gene A is masking the expression of gene B and influencing the coat color.

Whenever gene A is present in dominant and heterozygous form, the coat color is white but in the progeny where gene A is homozygous recessive the coat color is either black or brown depending upon whether gene B is present or not. When gene A is absent and gene B is present in dominant and heterozygous form, the coat color is black but when gene B is also absent then the coat color is brown.  

So it is clear that the product of gene A will cause the product of gene B to be white always as a result of which the product of gene B will express only in the absence of gene A.

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explanation:

plate tectonics and volcanic euptions

over time, plate tectonic processes cause continents to move to different positions on the globe.

for example, britain was near to the equator 300 million years ago, and therefore was hotter than it is today. for further details look at our climate through time poster or online map.

the movement of the plates also causes volcanoes and mountains to form and these too can contribute to a change in climate.

mountain ranges formed by plate tectonics. large mountain chains can influence the circulation of air around the globe, and consequently influence the climate.   warm air might be deflected somewhere cooler by the mountains.

well, i believe that the answer is 2. : 3