3.3 Sex Linked Inheritence


We have learned that a male bird has two copies of the X chromosome. A female bird has only one copy of the X chromosome and a shorter Y chromosome. A single copy of a sex linked gene is sufficient to express the mutation in the female where as two copies of the sex-linked gene are required for the male to show the same mutation. We will now learn how we can use this knowledge to predict outcomes of birds carrying sex-linked genes. We will denote a normal X chromosome with the letter X. An X chromosome with a ino gene will be denoted by XL as shown in the figure below.

 
Let us consider the pairing of a Lutino male to a Normal female
The Lutino male is represented by XL XL as we know that for a male to appear Lutino it should have two X chromosomes with the ino gene. The normal female is represented by X Y.
So the pairing becomes
XL XL with X Y
Putting these combinations in a Punnett square we get

 
To interpret the results above we must remember that the birds with two X chromosomes are males and birds with X and Y chromosomes are females. The first column shows the males and the second column shows the females. We see that we get 50% XL X. These are males with one chromosome containing the ino gene. We know that with only one ino gene the males will not show the effect of the ino gene and would appear green. Therefore we say that 50% of the chicks are green males split to ino. From the table we see that the other 50% chicks have XL Y. These are all females with an X chromosome containing the ino gene. A single ino gene is sufficient in female birds to change the appearance of the bird. These chicks are therefore Lutino. So our overall result becomes
Males: 100% Green split ino
Females: 100% Lutino

Let us try another one
Male: Green Split ino paired with Female: Lutino
 

Homework
Interpret the results of the above Punnett squares.

Comments

  1. Kashan Ali5 March 2015 at 15:09

    25% ino males
    25% ino females
    25% green split to ino males
    25% normal females

    ReplyDelete

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