Does anyone know how to get an answer in the punnet squares?

Question

Give me examples of how you do the first generation and the second generation. I really need to understand this HELP

Answer 1

Let's take the Blue eyes - brown eyes example. Brown is the dominant trait, so we'll use the capital B letter. Blue is the recessive trait, so we'll use the b letter. A person with blue eyes has the genotype bb (got a b from dad and a b from mom). A person with brown eyes can be homozygous BB (got B from dad and B from mom) or heterozygous Bb (got B from one parent and b from the other parent).
A first generation usually starts with pure breeds, so both homozygous. We'll have BB and bb as the starting parents. Those parents produce gametes in their gonads. Let's say that mom has blue eyes and dad has brown eyes. All her eggs will have the b gene as all dad's spermatozoids will have the B gene. If you want to make the Punnett square, you'll find that all babies will have 100% probability to be Bb
____|__B__|__B__
__b_|_Bb__|_Bb__
__b_|_Bb__|_Bb__
The second generation is made by taking two of those offspring and crossing them. First, you have to find out what alleles will be in the gametes. Half of the eggs will be b and half will be B. Also, half of the spermatozoids will be B and half will be b. In the Punnett square you'll have.
____|__B__|__b__
__B_|_BB_|_Bb_
__b_|_Bb__|_bb__
Here, the babies will have 25% chance to have blue eyes (bb) and 75% chance to have brown eyes (25% homozygotous BB and 50% heterozygotous Bb).

Answer 2

Well I guess I can't draw out the actual squares here, but this is how it works.

On top of the square you write the genotype (for example Bb or bb or BB), and on the sides you write that of the other parent. So now, you have a 2 X 2 square, with the columns labelled with the two alleles of one parent, and the rows labelled with the alleles of the other parent. Now look at the four squares inside the Punnett square. This just works like reading coordinates : a square with B on the top and b on the side is labelled Bb, etc.

At the end, when you've worked out all 4 squares, you get the possible genotypes of the offspring of the two parents you put in. If you have 1 bb, 2 Bb, and 1 BB, for example, you can conclude that 25 % of offspring will be BB, 50% Bb, and 25% bb.

The exact formatting of labelling etc will depend on whoever's marking your exam, so you'll need to check with a teacher or classmate on this.

Take a look at this, might make it clearer :
http://www.athro.com/evo/gen/punexam.html

Answer 3

OK, let's say that the alleles of the dad are Zz and the alleles for the mom are Zz. It doesn't matter which one goes where, but one set of these alleles goes on the top of the square (so the Z goes over the first column and the z goes over the second column) and the other set goes on the left side of the square (Z goes next to top row, z goes over bottom row.) Then, all you need to do is write the letters on top/on the side of each row in their respective rows/columns. If you did this correctly, ZZ should be in the top left corner, Zz should be in the top right and bottom left, and zz should be in the bottom right. This is the result for the first generation. I'm not sure about the second generation, but it's the same concept, just maybe with different starting alleles.

Answer 4

Well I will try, but it is hard to explain without drawing some examples. First, you make a square whose dimensions are parent 1's possible gametes x parent 2's possible gametes. You fill in the squares by combining the two gametes. To find the F2 generation, you take the result two F1 organisms and cross them in the same manner.