La siguiente sesión muestra un/a estudiante que presenta un ejercicio típico sobre cruzamientos genéticos y predicción de genotipos y fenotipos. Aunque dice haber realizado la primera parte del ejercicio, opté por hacer una corrección de esta parte para asegurarme que los conocimientos teóricos y los métodos estuvieran claros. Fue aquí que descubrí un error y recomenzamos el ejercicio para afianzar los conceptos esenciales del tema. 

Student
Help. I don’t know how to do problem 2 & 3! I think I got the first one done already. 1) Cross a wrinkled-pea, short pea plant with a pure round-pea, tall pea plant. Work the cross on a Punnett square to obtain the F1 generation. Give genotypes ad phenotypes as ratios. (Round and tall are dominant traits. 2) Take any two of the offspring of the above cross (F1) and cross them to obtain the F2 generation. Work the cross on a Punnett square. 3) From the F2 Punnett square you did for the previous question, choose two individuals that will permit you to have a pure strain of winkled-pea, tall pea plants. *If you were looking at the phenotypes, would you be able to be sure you had chosen the correct plants to cross? *How would you know when you have obtained a pure strain of wrinkled-pea, tall pea plants?

Tutor
Welcome !
How are you?

Student

I’m fine but sorta struggling

Tutor
Don’t worry! We’ll work on this.
First of all, have you done the 1st part?

Student
I think so

Tutor
Ok, I need some information that is missing there.
There are 2 traits: wrinkle/pure and tall/short
Which is dominant in both?

Student
oh
sorry

Tutor
don’t worry :)

Student
round and tall are dominant

Tutor
I’m asking to check what you’ve done, to see if we can move to the 2nd directly
and how did you call the alleles?
which letter did you use for each one

Student
oh I used wrinkled for w
short for r
W for round
and R for Tall

Tutor
Ok, and how did you write the parental genotypes?
wrinkled and short

Student
wr

Tutor
That means that it has only one allele for each trait?

Student
round and tall is WR

Tutor
Each individual receives one allele from father and one from mother
right?

Student
umm oops
yeah

Tutor:)
how would you write the genotypes then?
The logic is the same, just writing the 2 alleles
wrinkle and short?

Student
wrinkled short-wwrr

Tutor
very good!
pure round and tall?

Student
round and tall: WWRR

Tutor
right, let’s assume that is PURE for both traits
that is, it’s homozygous for both
right?

Student
yeah

Tutor
Ok, can you do the crossing?
in the punnett square, as it asks in the exercise

Student
ok

Tutor
before you start…
oh ok
you did it right
Perfect!

Student
and the rest is the same…

Tutor
very good
how is this phenotype?
remember which is dominant over the other…

Student
it’s the round…

Tutor
good…
height?
tall or short?

Student
short…

Tutor
it’s Rr

Student
i mean tall

Tutor
right :)
So there you have the F1.
The 1st part of the exercise.
Luckily we checked it because if not, there were going to be mistakes
in the next part
We have time to do the 2nd part now

Student
ok

Tutor
Take any two of the offspring of the above cross (F1) and cross them to obtain the F2 generation. Work the cross on a Punnett square.
You got the first, I’m sure you can do this, want to try?

Student
ummm ok?

Tutor
We still have 5 minutes. After that we will have to round up the session, ok?

Student
okType your message here.
i dont get it
sorry

Tutor
DOn’t worry
Let’s see…
Second part
ask you to cross to individuals from the F1
We only have one type right?
So we’ll cross 2 «WwRr»

Student
yeah…

Tutor
which combinations of alleles can this individual give?

Student
ummm
WW, ww, RR, and rr?

Tutor
mmm…it has to be one allele for 1 trait and 1 for the other
It has to mix: one allele for height and one for texture

Student
WW, Ww, WR, and Wr?

Tutor
Why? WW ?
these are 2 alleles for the same trait
do you agree?

Student
oh ok

Tutor
From the genotype:
you take: one allele for the first trait ( W or w) and one for the other (R or r)
The gametes will have half the genotype of the individual

Student
ohhhh

Tutor
and always have alleles for every trait :)

Student
ok

Tutor
so, which are the possible combinations?

Student
Wr, wr, WR,

Tutor
right!

Student
sorry

Tutor
one’s missing

Student
Rw?

Tutor
right
so, both individuals will give these 4 combinations

Student
ok

Tutor
and now…the punnett square!
Ok, fill this.
Oe we finish with this, we’ll round up the session.

Student
ok
wait so do i cross those with those with WwRr?

Tutor
right
but both parents are the same
so the combinations are the same
you have to write the same on both sides of the square

Student
ok…

Tutor
The combinations given by each one are the same :)

Student
am i wrong?

Tutor
not at all
It’s perfect
Now, the typical crossing.
Want to do it now?
Or you think you can finish with this?

Student
ummm… I think i can finish this myself

Tutor
Oh ok:)
Probably you can finish this and the sign back again, to check this and finish the last question, ok?

Student
ok
thank you

Tutor
You’re welcome!
Good job!

Student
ok bye

Colaboradora: Nicole Rosenstock, Tutora Uruguaya de Biología