Males have one X and one Y-chromosome while females have two X-chromosomes. Due to the differences between the X and Y-chromosomes, the number and type of genes inherited by an individual depends on its sex.
To test prediction 2, we must perform the following cross: If we draw this out in a Punnett Square, we obtain this: The flies represented in the bottom row will have brown eyes. Two squares in the top row, two in the bottom row, and we get What do we find when we do the cross?
This result is consistent with our expectation. Back to top The Cross Among Siblings inter se cross: We expect 1 out of 4 offspring to be homozygous for brown1 lower right corner of the table.
We obtain the results we predict, within statistical variation: The data we obtain from these crosses appear to support our hypothesis--our understanding of how the brown gene is inherited. This allele appears to be inherited as a simple recessive allele.
There are several models that we can imagine--from brown1flies failing to produce a red pigment, to brown1 flies over-producing a brown pigment due to some kind of biochemical problem. We need more information before we can understand how the brown1 mutation causes fly eyes to be brown.
TH Morgan named the gene white. Again, we have no understanding of how this mutation might be inherited, so there is little point in guessing. Again, we will ask this question experimentally, by crossing wild type, red-eyed flies with mutant, white-eyed flies.
The results are as follows: A The cross using a white-eyed male B The cross using a white-eyed female The results of cross A are what we would expect if the white1 allele is inherited as a simple recessive. But the "reverse" cross, Bis very different. Somehow, the sex of the fly matters.
To figure this out, it might be necessary to look into the mechanisms that determine sex. Back to top Sex Determination In crocodiles and some species of turtles, sex is determined by the temperature at which the eggs develop.
In some species, a warm nest produces all females, and a cold nest produces all males. But in fruit flies and mammals, the temperature has no effect.
Sex determination occurs genetically, in response to certain genes that control embryological development. In species with genetic sex determination, there is often a visible difference in the chromosomes of males and females.
This is true in humans and in Drosophila. For both of us, most of the chromosomes are identical in both males and females though they may carry slightly different alleles.
This is easiest to see in Drosophila, which have only 4 chromosome pairs compared to humans, with 23 pairs. The image shown here redrawn from J. In Drosophila, mitotic chromosomes often associate in pairs, with the tiny chromosome 4 in the center of the cluster. It is evident from looking at the chromosomes that females have two X chromosomes, while males have only one.
The situation is similar in humans: XX individuals develop as females, and XY individuals develop as males. What happens with genes that are located on the X chromosome or on the Y chromosome?
But what if genes are on the sex chromosomes? In humans, there is a gene on the Y chromosome that regulates the "hair-growth program" specifically in ears. Most alleles of this gene result in little or no ear hair. Some alleles, however, activate the hair growth program, resulting in hairy ears.
The image on the right is hosted at http: Because the Y chromosome also carries the gene that determines maleness, the hairy-ear phenotype rarely, if ever, occurs in women.
The white-eye phenotype in Drosophila does occur in female flies.We now call this "sex linked" or "X-linked" inheritance, although Morgan called it sex limited.
A Punnett square can be seen in Figure 1. Figure 1: This Punnett square shows a cross between Morgan's original, mutant white-eyed male fly with a red-eyed heterozygous female fly from the F1 progeny.
The wild-type eye color of Drosophila is dull red, but all the white-eyed flies are males. This inheritance pattern is explained by the alleles being located on the differential region of the X chromosome; in other words, Sex chromosomes and sex-linked inheritance - An Introduction to Genetic Analysis.
In Mendelian pattern of inheritance, the genes for contrasting characters were located on autosomes but not on the sex chromosomes.
Secondly, the result of reciprocal cross is same as normal cross which is not the case with sex linked inheritance. Sex linked Traits In sex-linked inheritance, alleles on sex chromosomes are inherited in predictable patterns. In Drosophila the locus for eye color is located on the X chromosome.
The allele for red eye color, which is normal in wild flies, is dominant to the mutant allele for white eyes. In sex-linked inheritance, alleles on sex chromosomes are inherited in predictable patterns. For example, in Drosophila the locus for eye color is located on the X chromosome.
The allele for red eye color, which is normal in wild flies, is dominant to the mutant allele for white eyes. Abstract. This was a study on determining the inheritance mechanisms for different physical traits found in Drosophila melanogaster.
In order to accomplish this, a reciprocal cross was performed by mating wild type male flies with mutant female flies and then wild type female flies with mutant male flies.