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

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.

Since the short trait is recessive, we know that the short plant must have two short alleles from its "parents". The tall plant could have two tall, or one tall and one short allele, since the tall trait is dominant.

I'll show both examples. First, let's look at a short plant, which would have two short alleles, this is also called homozygous recessive. We'll use lower case t to mean the recessive trait.

And we'll cross it with a tall plant that is homozygous dominant, or it has two tall alleles. We'll use upper case T to mean the dominant trait. So we make a table separating the two alleles from both plants, one across the top and one down the left side.

We fill in the table by using the allele at the top of each column in each spot below it and using the allele at the left of the row in each spot to the right. Sorry, you'll have to imagine lines in between the columns and rows.

What you end up with is what would be the genotypes the two alleles of the progeny the children. When you cross this example, each progeny ends up with Tt, which means it has one tall and one short allele this is called heterozygous. Again, since tall is dominant, the plants will look tall their phenotypes. This time, half of the progeny children will have the Tt genotype heterozygous , while the other half have the tt genotype homozygous recessive. The Tt half will look tall since tall is dominant, while the tt half will look short.

Just like all probability, these numbers aren't exact in real life, but the more you experiment with, the closer it gets to those numbers. It can also be used to predict the most likely phenotype of how the trait will be expressed. A Punnett square is made by drawing a square divided into four smaller squares 2x2. The male's allele is written across the top. The female's allele is written down the side.

In each of the four smaller squares is written the combination of one male allele and one female allele. A Punnett square is used to discover what the organism will turn out to be when two organisms are "crossed".


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Punnett squares are regularly used by geneticists to predict outcomes of crossings between individuals. A geneticist can provide probabilities for certain genotypes and phenotypes before the breeding takes place. Homework Help; Specialized Programs. ADD/ADHD Tutoring Programs; Determining Genotypes and Phenotypes using Punnett Squares.

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We explain Punnett Square with video tutorials and quizzes, using our Many Ways(TM) approach from multiple teachers. This lesson will introduce how to use a punnett square to determine genetic traits.

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