DETERMINING THE LOCATION OF GENES IN DROSOPHILA MEANOGASTER BY OBSERVING PHENOTYPIC RATIOS OF OFFSPRING July 7, 2014 ABSTRACT The purpose of this study was to find where genes for specific traits are located, either on the autosomes or sex chromosomes. We determined this by examining two different D. melanogaster populations for differences in the phenotypic ratios of offspring. D. melanogaster were chosen for our study because they are easy to care for, and they have a short reproduction cycle. D. melanogaster have only eight chromosomes making them less complex organisms for study. Based on our results, we accepted our autosomal inheritance hypothesis because the ratio observed in the F2 generation was 3.23:1, which shows no statistical difference to our expected ratio of 3:1, indicating that the gene for wing presence is on an autosome. We also accepted the X-linked inheritance hypothesis because the ratio observed in the F2 generation was1.01:1, which showed no statistical difference to our expected ratio of 1:1, indicating that the gene for eye color is on the X chromosome. 1 P:\RMSC\RMS\Group Papers\FruitFly-2014.docx Saved: 10/2/2015 10:14 AM Printed: 10/2/2015 10:14 AM INTRODUCTION Genetics is the branch of biology concerned with heredity and variation (Cumming and Klug, 2000, p.5). Heredity is the passing of traits from a parent to an offspring. For example, handedness, the preference of using one hand over the other, is a trait that can be passed down from parents to their offspring in humans. Variation is the genetic differences among individuals of the same species. For example, offspring could either be left or right handed. These traits are passed through chromosomes. Chromosomes come in homologous pairs called autosomes, and one non-homologous pair called sex chromosomes. The number of chromosome pairs depends on the species. For example, humans have 23 pairs of chromosomes, 22 pairs of autosomes and one pair of sex chromosomes. Fruit flies have four pairs of chromosomes, three pairs of autosomes and one pair of sex chromosomes (Fig. 1). Chromosomes are coiled strands of DNA molecules found in every cell of the body of an organism. DNA molecules are the genetic codes that contain all the information needed to build and maintain a species (Cumming and Klug, 2000, p.6). DNA has segments that represent genes that code for specific traits in individuals (Fig. 2). These genes can either be on autosomes or sex chromosomes. In homologous chromosomes, autosomes, each pair is identical in length and in the location of the centromere, as well as the size and location of genes (Cumming and Klug, 2000, p.6). In non- homologous pairs, sex chromosomes, the chromosomes are not identical because in the pair, one chromosome is larger than the other, and they do not have the same genes. Each type of chromosome contains specific genes that determine the traits of an offspring. 1A 1B Fig. 1. – This figure shows entire sets of chromosomes for humans and fruit flies. Figure 1A shows the 23 pairs of human chromosomes, 22 pairs of autosomes and one pair of sex chromosomes. Figure 1B shows the four pairs of fruit fly chromosomes, three pairs of autosomes and one pair of sex chromosomes. 2 P:\RMSC\RMS\Group Papers\FruitFly-2014.docx Saved: 10/2/2015 10:14 AM Printed: 10/2/2015 10:14 AM Chromosome Gene 1 Gene 2 DNA Fig. 2. – This figure shows a chromosome, the DNA that makes up the chromosome, and the gene segments of the DNA strand that code for specific traits. Genes are segments of DNA molecules that code for specific traits (Cumming and Klug, 2000, p.7). Genes store information about traits and give variation among individuals. For example, humans have a gene location that determines handedness. An allele is an alternative form of a gene. The gene location in a chromosome pair will be the same, but the alleles can be different. The combination of alleles determines the expression of traits. For example, alleles will determine if an offspring is left handed or right handed. In Figure 3, the genes are for the same trait of handedness, but the chromosomes have different alleles for either left handedness or right handedness. The traits that are expressed depend on if the alleles are dominant or recessive. Dominant alleles, in this case right handedness, are those that are always expressed (Fig. 3). Dominant alleles also have the ability to mask other alleles. The alleles that are masked in the presence of a dominant allele are known as recessive. Recessive alleles, in this case left handedness, will only be displayed if there are two recessive alleles present and no dominant alleles present for the trait (Fig. 3). Since the offspring in Figure 3 has one dominant and one recessive allele, they will display the dominant trait of being right handed, with the recessive trait of left handedness being masked. The appearance of an offspring is determined by different combinations of the alleles present. Fig. 3. – This figure shows a homologous pair of chromosomes with the gene location for handedness. One chromosome has the dominant allele for right handedness, and the other chromosome has the recessive allele for left handedness. 3 P:\RMSC\RMS\Group Papers\FruitFly-2014.docx Saved: 10/2/2015 10:14 AM Printed: 10/2/2015 10:14 AM The phenotype is the physical appearance of a trait that is determined by the alleles present in an offspring. For example, the phenotype would be if an offspring writes with his or her left hand or right hand. Hand preference is the expression of the alleles in the individual. During reproduction, each parent gives the offspring half of a set of chromosomes. When chromosomes are passed, the genes from each parent are passed as well, contributing a different set of alleles that determine the traits expressed in the offspring. Offspring can either be homozygous or heterozygous for traits. A homozygous dominant offspring (RR) contains two dominate alleles and will express the dominant trait of right handedness (Fig. 4A). A homozygous recessive offspring (rr) contains two recessive alleles. Since there are no dominant alleles to mask the recessive alleles, the individual will express the recessive trait of left handedness (Fig. 4B). A heterozygous offspring (Rr) contains both a dominate and recessive allele. Since a dominant allele is present, the recessive allele will be masked, making the individual right handed (Fig. 4C) . 4A 4B R r Homozygous dominant Homozygous recessive (RR) (rr) R r 4C R Heterozygous (Rr) r Fig. 4. – This figure shows the different allele combinations for handedness on pairs of homologous chromosomes. Figure 4A shows a homozygous dominant (RR) pair. Figure 4B shows a homozygous recessive (rr) pair. Figure 4C shows a heterozygous (Rr) pair. 4 P:\RMSC\RMS\Group Papers\FruitFly-2014.docx Saved: 10/2/2015 10:14 AM Printed: 10/2/2015 10:14 AM Most genes are passed on the autosomes of an organism. Autosomes come in homologous pairs with the same size and location of genes. Because of this, offspring receive two alleles for each trait, one from each parent. Alleles are also passed on the sex chromosomes of an organism. There is only one pair of sex chromosomes in each set. Sex chromosomes determine the gender of the offspring. Theses chromosomes will be either be X or Y. Females will receive an X chromosome from both parents, while males will receive an X from the mother and Y from the father as shown in Figure 5. The X chromosome is much larger and has more genes than the Y chromosome. There is only a small homologous pairing region where the genes are the same on both chromosomes. For some traits, the X chromosome carries an allele that the Y chromosome does not (Fig.6). Females will receive two total alleles, one on each X chromosome. However, males will only receive one allele, which is on the X chromosome, because the Y chromosome is not large enough to hold a matching allele (Cain at el, 2008, Pg.289) (Fig. 6). The allele that is present on the male X chromosome must always be expressed since there is no matching allele on the Y chromosome. Female sex chromosomes Male sex chromosomes Fig. 5. – This figure shows the sex chromosomes for male and female offspring. Fig. 6. – This figure shows the differences in the X and Y chromosome including size and number and location of genes present on each. The green segment on the X chromosome represents a gene that is present on the X chromosome, but not on the Y chromosome. The purpose of this study is to find where genes for specific traits are located, either on the autosomes or sex chromosomes, by examining the phenotypic ratios of the offspring. We will accomplish this by comparing our observed ratios to the ratios determined by previous studies on autosomal and sex-linked inheritance. In previous studies it has been shown that if the gene for a trait is located on an autosome, the phenotypic outcome in the F2 generation will show a three dominant: one recessive ratio when the P1 (parental) generation has one homozygous 5 P:\RMSC\RMS\Group Papers\FruitFly-2014.docx Saved: 10/2/2015 10:14 AM Printed: 10/2/2015 10:14 AM dominant (RR) parent and one homozygous recessive (rr) parent (Fig. 7). In studies on X-linked inheritance, it has been shown that if the gene is located on the X chromosome, the phenotypic ratio in the F2 generation will show r r one dominant: one recessive ratio when the female in the P1 generation is homozygous recessive (X X ) and the male is dominant (XRY) (Cumming and Klug, 2000, p.93-94) (Fig.