These male offspring can not be carriers. These carrier females have a 50% chance of passing the recessive alleles to their male offspring. Females can be heterozygous for a trait and therefore carry the recessive allele without expressing it. X-linked recessive phenotypes are more commonly observed in males because males are hemizygous for sex-linked traits. In both reciprocal crosses, patterns of inheritance beyond the F2 generation vary depending on which F2 individuals are chosen for the cross. The male offspring all received their single X-chromosome from the heterozygous female parent, so half received a red allele, and half received a white allele.įirst three generations of the second reciprocal cross.Ĭrossing these F1’s again results in a 1:1 ratio of red and white-eyed individuals, but in the F2, half the female offspring and half the male offspring have red eyes. No females have white eyes because they received one of their X-chromosomes from their hemizygous dominant, red-eyed father. This cross produces a 3:1 ratio of red-eyed to white-eyed individuals, but all white-eyed individuals are male. The differences between the sexes become more apparent in a cross using the red-eyed F1 male and red-eyed F1 females. The females’ phenotype and genotype are consistent with the patterns discovered by Mendel, but the males, as hemizygotes, are not. Males are red-eyed because they only have one copy of the gene, and that copy is for the red allele. Since the female parent is homozygous, whichever allele the males get, they will receive a red-eye allele.įemales are red-eyed because the presence of the recessive copy is masked. Male offspring only have one X-chromosome, which they received from their female parent. The presence of the red allele from the mother masks the white allele. However, with an X-linked gene, the reason for red eyes differs between males and females.Īll the female offspring are heterozygous, receiving an X-chromosome with a red allele from their mother and an X-chromosome with the white allele from their father. 100% of the F1 generation having red-eyes is consistent with what would be predicted based on Mendelian inheritance of a recessive allele. Performing the first reciprocal cross: a true-breeding red-eyed female (homozygous dominant) with a true-breeding white-eyed male (hemizygous recessive) results in an F1 generation comprised entirely of red-eyed individuals. Two reciprocal crosses are possible A) a true-breeding red-eyed female with a white-eyed male and B) a true-breeding white-eyed female with a red-eyed male. Homozygous recessive with two white encoding alleles - genotype X wX w phenotype white eyes.Heterozygous - genotype X WX w phenotype: red eyes.Homozygous dominant for the red encoding allele - genotype: X WX W phenotype: red eyes.In females, the presence of one dominant red encoding allele (X W) will produce red eyes even if the individual is heterozygous for the white allele. Since this particular gene that controls eye color is on the X-chromosome, females (XX) carry two copies, and males (XY) only carry one. Select one male and one female individual for the P 1 generation and click 'begin' to explore eye color inheritance patterns in fruit flies: However, the gene is on the X-chromosome, making it an excellent illustration of sex-linked inheritance patterns. If the white gene were on an autosome, it would exhibit classical Mendelian inheritance patterns. As a recessive trait, the white eye phenotype is masked by the presence of a wild-type (red encoding) allele. Wild-type fruit flies have dark red eyes, but there are recessive alleles of this eye color gene (called the white gene) that cause individuals to have white eyes. The X-chromosome is larger and contains more genes than the Y-chromosome, so most sex-linked traits are X-linked traits. These patterns cause expression patterns of sex-linked traits to differ between male and female offspring. In other words, their phenotypes always match their genotype *s.įemales get two copies of X-linked genes, demonstrating the more typical dominant-recessive expression patterns of non-sex linked traits. Since males have only one copy of each sex chromosome, they are hemizygous for all sex-linked genes, and they always express the phenotype * of the allele * they get. With both an X and a Y-chromosome, males inherit both X and Y-linked traits, while females only inherit X-linked traits.
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