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In fruit flies, red eyes are dominant over white eyes. Show a cross between two white-eye fruit flies.

My question is...

How do I know if the white-eye fruit flies are homozygous or heterozygous?

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This looks like a homework question, you should take a look at the policy on asking homework questions - it is important to demonstrate an attempt at an answer and make sure it is tagged as homework meta.biology.stackexchange.com/questions/266/… –  GriffinEvo Dec 6 '13 at 8:18
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4 Answers 4

In order for any recessive trait to be expressed, it must be homozygous for that recessive trait. If an individual was heterozygous, the recessive allele would be masked by the dominant allele, giving it the dominant phenotype.

Therefore, since white eyes are recessive, fruit flies with white eyes will always be homozygous (aa) for that recessive trait. A fruit fly that is heterozygous (Aa) for this trait would have red eyes.

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In fruit flies, red eyes are dominant over white eyes.

Red is dominant (A) which means we can deduce that white is recessive (a). Therefore AA individuals are red eye, and so to are Aa because the dominant red allele overrides the white recessive allele. Then the only way a fly can be white eyed is with aa.

Show a cross between two white-eye fruit flies.

Both white eyed flies are aa so all four possible outcomes are aa in a single locus punnet square.

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Because red is dominant over white, in order for the flies to express the recessive white eye trait they must be homozygous for the recessive white allele - I.e (no pun intended) rr.

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Just to add an extended perspective to all of the answers submitted so far.

The classic white-eye phenotype in Drosophila is associated with a gene, white (or w) that is carried on the X chromosome (females XX, males XY) i.e. it is a sex-linked phenotype. Male white-eyed flies are therefore technically not homozygous, they are hemizygous since they only have one X chromosome and thus one copy of the gene.

There are other white-eyed phenotypes due to segregation of two genes (brown and scarlet) - this is the standard two-factor cross example in many genetics courses. In this case (no sex linkage is involved) a white-eyed fly is homozygous for the recessive allele at both loci.

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