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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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.
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.
In fruit flies, red eyes are dominant over white eyes.
Red is dominant (
A) which means we can deduce that white is recessive (
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
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.
If the question allows you to assume that eye-colour is a typical gene with two alleles, then we can figure out the answer. If flies' eye-colour has two alleles - dominant and recessive - then the phenotype will be dominant (red - A) or recessive (white - a). Dominant traits need only one dominant allele, so the genotype can be Aa or AA. The only other combination is aa, and that codes for the recessive phenotype of white eyes. There can be no heterozygous white eyed fly.