SEX Punnet Squares

Test Preparation
1

<p>I have a question about determining the probability of a specific sex-linked trait.</p>

<p>Since the results of the punnet square yields half boy and half girl phenotypes in addition to the X linked phenotype under analysis, how do you calculate the probability a girl/boy will have the phenotype? If the punnett is:</p>

<p><em>X^I</em> X
_<em>X X^IX</em>XX
_<em>Y X^IY</em>XY</p>

<p>Is the probability for a boy having the dominant X linked phenotype 50% or 25%?</p>

<p>(I was thinking 25% because you have 50% for a boy and 50% of a boy getting the allele, so 50% x 50% is 25%)</p>

<p>So to sum it up, the question is: Is the probability of a sex linked phenotype appearing in a particular individual based on their sex and % chance of getting the trait, or is it just their % chance of getting the trait?</p>

2

<p>Well, personally, I do not understand your question.</p>

3

<p>I mean, in a cross between XY and X^i X, is the chance of a having a girl that is a carrier 50% or 25%?</p>

<p>Also is the ratio for the phenotype 1:1:1:1 or 1:1?</p>

4

<p>The chance for a specific girl being a carrier is 50%.</p>

<p>The chance of having a girl who is a carrier is, as you say, 25%.</p>

<p>Generally, though, (afaik) you assume the sex of the offspring to be known, ie, you know its a girl.</p>

5

<p>Thanks shash rao! So would the phenotypic ratio be 1:1:1:1?</p>

6

<p>bump......</p>

7

<p>What shash_rao said is right.
To add, the probability of a boy having the dominant sex linked phenotype is 50%.
The probability of having a boy with the dominant sex linked phenotype is 25%. (50% of having a boy X 50% of the boy having the phenotype)</p>

<p>Assuming the X^I is a recessive X-linked allele, the phenotypic ratio is 1:1:1:1 for normal male: mutated male: normal female: carrier female</p>

<p>If X^I is a dominant X-linked allele, then the ratio is 1:1:1:1 for normal male: mutated male: normal female:mutated female (the difference being there is no carrier if the mutation is dominant)</p>

8

<p>yeah....................what he said. lol</p>

9

<p>This a case where you have to read the question carefully. If it is properly written, you can figure out which is the correct answer.
1. What is the probability of having a child who is a boy and displays the trait? (25%)</p>

<ol>
<li> What is the probability of a son displaying the trait? (50%)</li>
</ol>

<p>Most of the other ways of writing the question are ambiguous and can lead to problems.</p>

10

<p>Thanks guys.</p>