Question

Jenny has red hair and freckles. We know that non-red hair (R) is dominant to red hair (r), and that freckles (F) are dominant to plain skin (f). She tells you that her mom has brown hair and freckles, and that her dad has red hair and plain skin. Furthermore, she says that her maternal grandmother (her mom’s mom) had red hair and plain skin. What is Jenny’s genotype?

Answer 1

Hello Milan!

Answer:

rrFf

Explanation:

In this question, we’re talking about two traits, which means that we will represent their genotypes with four letters ( two letters for each trait, since one trait is present in two chromosomes -pair-, one from the mother, and one from the father, revise if you don't get it, or just ask me).

First, let’s make everything clear:

Capital letters are for dominant traits (if one dominant trait is present in the genotype, it will be shown as the phenotype of a person)

Small letters are for recessive traits (if both chromosomes have the recessive trait, it will be shown as the phenotype)

( one letter = one allele )

R : not-red hair         r : red hair

F : freckles                f : plain skin

Second, let's write down every possible genotype for each person:

Let's start with Jenny, she has a recessive trait (red hair) and a dominant trait (freckles). As I mentioned before, we will represent recessive traits in two small letters ( rr for Jenny's red hair ). And in the case of our dominant trait ( freckles), we MUST have at least one capital letter (for this trait: F ). That means we have two possible genotypes for freckles: either Ff or FF.

Now as easy as it sounds, we should write Jenny's possible genotypes like this:

                                    Either rrFf or rrFF

Jenny's mother has brown hair and freckles (two dominant traits), again, as I mentioned before, dominant traits MUST be represented by at least one capital letter.

               Possible genotypes for brown hair:

             either RR or Rr

              Possible genotypes for freckles:

            either FF or Ff

Here, we have the phenotype of one parent of Jenny's mother's parents. Lucky for us, Jenny's grandmother has two recessive traits that can only be presented as rrff.

Why are we lucky? Because Jenny's mother must have inherited one allele of her grandmother, since both of Jenny's grandmother's alleles are the same, we know that Jenny's mother has one r allele and one f allele. We can write down Jenny's mother's genotype like this: RrFf.

Now I ask you to stop reading and try to figure out Jenny's father's genotype.

It is rrff, just like her grandmother.

Finally, let's make a Punnett square for Jenny's genotype:

 

                  mother:     father:

                    RrFf     x    rrff

( I'm not gonna show you how to make a Punnett square because that's gonna be a tooooooo long explanation, watch this video: https://youtu.be/qIGXTJLrLf8 )

         possible genotypes for Jenny and her siblings:

  RrFf  (phenotype: non-red hair & freckles)

or  Rrff  (phenotype: non-red h & plain skin)

or  rrFf  (phenotype: red h & freckles)

or  rrff  (phenotype: red h & plain face)

If Jenny has red hair and freckles, she MUST have a genotype of rrFf. If not, her mother is cheating XD (or a genetic mutation had happened).

I know I took too long explaining and answering but I hope I helped you,

keep it up!!