Question

Monica is heterozygous for brown hair. Her husband, John has blonde hair, which is the recessive trait. Complete a cross and tell the genotype and phenotype.

@zepdrix @agent0smith @mathmate @.Sam.

Answer 1

I used Bb (Brown) as her mom's genes, and dd (Blonde) as her dads
the genotype is: 50% Bd, 50% bd
the phenotype is: 50% Brown, and 50% unknown (they are both recessive, so it is literally a 50-50 situation)

Hope this helped!

Answer 2

Assuming hair colour is simple dominant and recessive situation:
we can make a Punnett square using:
B=dominant allele for brown
b=recessive allele for brown (i.e. blonde)
Father: bb (the recessive trait)
Monika: Bb

So the genotypes are Bb with phenotype brown hair, and
bb and blonde.

Unfortunately, hair colour is not simple like we assumed above. There are multiple genes (and therefore alleles) that affect hair colour, and in general, the influence the intensity of the colour, caused by (two versions of) melanin, a pigment. The net effect is that hair colour is not on/off like a switch.
For an interesting reading and explanation, read:
http://genetics.thetech.org/ask/ask39
Other traits that involve multiple genes include eye colour and blood type.