Xy+w+m+ / Xywm
Sepia: se+ / se
Heterozygous at all four loci.
Interactive tutorial · independent assortment
One gene.
One branch.
One probability.
Complete the Punnett squares, enter phenotype proportions, and build a fork line. Every answer refers to all offspring.
yellow · X-linkedwhite · X-linkedminiature · X-linkedsepia · autosomal
Use proportions, not ratios.
Enter answers such as 1/2, 3/4 or 0.25. All mutant alleles are recessive. White is epistatic to sepia: white eyes conceal the sepia phenotype.
01 · The cross
Start with the alleles.
Wild-type alleles have a + and appear before mutant alleles in heterozygous genotypes. X-chromosome labels use complete gene symbols in superscript.
Xywm / Y
Sepia: se+ / se
Carries the recessive yellow, white and miniature alleles; heterozygous at sepia.
Independent-assortment teaching model
Recombination is allowed. For this exercise, maternal y, w and m alleles are sampled independently, giving eight equally likely X combinations. Sepia also segregates independently. This deliberately models no detectable genetic linkage; it does not represent the real linkage distances of these named Drosophila genes. Allowing recombination alone does not guarantee independent assortment.
Explore the eight maternal X combinations
Each combination has probability 1/8 in this model. The six mixed combinations are recombinant relative to the starting chromosomes.
Xy+w+m+ · 1/8Xy+w+m · 1/8Xy+wm+ · 1/8Xy+wm · 1/8Xyw+m+ · 1/8Xyw+m · 1/8Xywm+ · 1/8Xywm · 1/8
Within each chromosome the gene order stays y–w–m, so the loci remain aligned.
02 · Punnett squares
Combine one allele from each parent.
Click an empty cell and choose its genotype. Rows show eggs; columns show sperm. Each box represents 1/4 of all offspring. Each single-gene square shows only the locus being studied.
yellow X-linked
Egg ↓
Sperm →XyYXy+Xy
Sperm →XyYXy+Xy
Each cell = 1/4. Click a cell to set or change its genotype.
white X-linked
Egg ↓
Sperm →XwYXw+Xw
Sperm →XwYXw+Xw
Each cell = 1/4. Click a cell to set or change its genotype.
miniature X-linked
Egg ↓
Sperm →XmYXm+Xm
Sperm →XmYXm+Xm
Each cell = 1/4. Click a cell to set or change its genotype.
sepia Autosomal
Egg ↓
Sperm →se+sese+se
Sperm →se+sese+se
Each cell = 1/4. Click a cell to set or change its genotype.
03 · Phenotype proportions
Count the boxes. Divide by four.
Enter the wild-type and mutant proportions for each locus across all offspring. For the two eye-colour loci, first consider each square on its own; we combine their effects in the next step.
yellow
white
miniature
sepia
04 · Epistasis
White masks sepia.
Sepia eyes require both a functioning white gene and se/se. With the recessive white phenotype, eye colour is white regardless of the sepia genotype.
Functional white + se/seSepia eyes are visible.
Recessive white phenotype + any sepia genotypeEyes are white. Sepia is masked.
Check the actual eye-colour proportions
The se/se proportion from its square is 1/4. The proportion with visible sepia eyes must also include functional white.
Need a hint?
White: use the recessive white proportion. Sepia: P(functional white) × P(se/se). Red: P(functional white) × P(se+/–).
05 · The fork-line method
Choose a phenotype. Follow its branches.
Multiply the independent per-gene probabilities for the desired outcome. Eye-colour branches must account for epistasis.
For visible sepia eyes, choose functional white AND se/se.
1. yellow
2. white
3. miniature
4. sepia
How to draw the full fork line on paper — view the diagram
Start with yellow: wild-type body (1/2) or yellow body (1/2). From each outcome, draw white branches: functional white (1/2) or white eyes (1/2). Repeat for miniature: normal wings (1/2) or miniature wings (1/2), then sepia: se+/– (3/4) or se/se (1/4).
Multiply along a path. Add paths that lead to the same visible phenotype. For white eyes, the sepia branches merge because both lead to white: 3/4 + 1/4 = 1.