WHAT IS THE EFFECT OF THE PRESENCE OF A RECESSIVE ALLELE IN AN INDIVIDUAL WHOE OTHER ALLELE IS A DOMINANT ALLELE

This makes it a heterozygous allele and the dominant allele with be expressed.

For example:

T is for tall and t is for short

The heterozygous allele is Tt, which makes the allele "heterozygous tall"

a: the sheep and waste material both release carbon into the atmosphere.

i think a) betsy can donate only blood to mark is correct because, mark and betsy have the same blood type. whereas nobody else has b+ blood so betsy can't donate to them.