4.7 Alleles of Sex Linked Genes


In Chapter 4.2 Alleles, we defined alleles as two or more variants (versions) of the same gene that occupy the same position (or locus) on the DNA. We discussed blue and yellow-face blue genes as an example of alleles. Sex-linked genes can also have alleles. For example, in ringnecks ino and pallid are alleles of the same locus and in budgerigars ino and Texas clear-body are also alleles. Let us look at a diagram to understand the relationship between the ino and the Pallid genes. Fig 4.7 shows the X chromosome on which the ino and the pallid genes are located. 


We see that the ino and the pallid gene share the same position on the X chromosome. Therefore one X chromosome can either have the pallid gene OR the ino gene but not both. Keeping this in mind all the possible combinations of the ino and pallid genes are shown in this figure. If a male has two ino genes it is a visual ino, if it has two pallid genes it is a visual pallid, if there is one pallid gene and one ino gene then the bird is called a pallid-ino. Hens can either be ino or pallid but never pallid-ino. Pallid is incomplete dominant over ino, so If a bird has both pallid and ino genes (pallid-ino) it shows a lighter pallid pattern and dark red eyes. Besides there combinations we can have a male split to pallid and a male split to ino. We have already learned that females can never be split to sex-linked mutations.

In budgerigars the relationship between the ino and the Texas Clear-body (TCB) is very similar. You just need to replace the pallid (pd) gene with the TCB gene in the diagram. The key difference is that the Texas clear-body gene is completely dominant over the ino gene and therefore completely hides the effect of the ino. The male bird with both the ino and TCB genes is called a Texas Clear-body split to ino.

Homework
What do you get if you breed a Texas Clear-body split ino cock to a Lutino hen?

Comments

  1. Kashan Ali5 March 2015 at 13:18

    so we will get 25% TCB females 25% Ino females & 25% TCB split to ino males & 25% Ino split to TCB males. m i right sir?

    ReplyDelete
  2. Saud5 March 2015 at 13:48

    You get
    25% TCB hens
    25% ino hens
    25% TCB split ino cocks
    25'% ino cocks

    ReplyDelete
  3. Kashan Ali5 March 2015 at 16:03

    i paired up TCB male with double factor spangle female. could you please tell me the possible outcomes.? i was sure to get some TCB but after reading your useful literature i am doubtful that results will be single factor spangles 1005 what do u say sir.

    ReplyDelete
  4. Saud6 March 2015 at 11:12

    All chicks will be sf spangle
    All females will also be TCB

    ReplyDelete
  5. Unknown4 March 2018 at 01:33

    Hi how are you Mr Saud may I know which pairing is better with TCB budgies. Double fector eno or eno red eyes?

    ReplyDelete

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