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Dominant Allele
1
Heterozygous Red
Homozygous Red
Homozygous White
Monohybrid cross
Recessive Allele
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drag and drop answer hete
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RR
Rr
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Rr

Heterozygous Red- Rr

Homozygous Red-- RR

Homozygous white-- rr

Monohybrid cross- Rr

Recessive allele- homozygous white i.e rr

Explanation:

Heterozygous have 2 different alleles of a gene as both Red and white alleles are present in the example Rr.

Homozygous: If the 2 alleles at a locus is same then they are homozygous, it can be dominant as RR or recessive as rr.

Recessive gene are the ones that can be masked by the dominant genes as red is dominant on white.

Monohybrid cross is the result of cross between the individual having either homologous dominant or recessive genes or genotype. The result is generally masking the recessive allele resulting in dominant phenotypic trait but the genotype changes.

From the example given in question:

A cross between dominant red as RR

and a cross between recessive white as rr

The punnet square shows:

  r     r

R Rr  Rr

R Rr   Rr

it can be seen that all progeny are red but genotype changed to heterozygous red as Rr

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.