Dihybrid Cross Involving Sex-Linked Traits Consider a dihybrid cross between two fruit flies. One parent is heterozygous for body color (Bb, where B is gray and b is black) and is also a carrier for a sex-linked trait of Fragile X Syndrome (XAXA). The other parent is black (bb) and not a carrier for the sex-linked trait (XY). Questions: Construct a Punnett square to show the possible genotypes of the offspring. What are the possible genotypes and phenotypes of the offspring produced by each parent? What is the expected genotypic and phenotypic ratio of the offspring?
Answer (1)
Answer:Here's a breakdown of the dihybrid cross involving sex-linked traits, addressing each question: 1. Punnett Square: Because this involves a sex-linked trait, we need to consider the sex chromosomes (X and Y) separately from the autosomal body color genes (B and b). The Punnett square will be larger than a typical dihybrid cross. Parent 1 (heterozygous gray body, carrier for Fragile X): Bb XAXaParent 2 (black body, not a carrier): bb XY Table
B XA B Xa b XA b Xa bX Bb XAX Bb XaX bb XAX bb XaX bX bY Bb XAY Bb XaY bb XAY bb XaY bY 2. Possible Genotypes and Phenotypes of Offspring from Each Parent: It's important to remember that the sex-linked trait (Fragile X) is only expressed differently in males and females. Females can be carriers (XAXa) without showing symptoms, while males (XAY or XaY) will express the trait if they inherit the affected allele (Xa). - Parent 1 (Bb XAXa): Contributes either B or b for body color and either XA or Xa for Fragile X.- Parent 2 (bb XY): Contributes b for body color and either X or Y for sex. 3. Expected Genotypic and Phenotypic Ratio of Offspring: Let's analyze the Punnett square results to determine the ratios: - Body Color:- Gray (B_): Bb XAX, Bb XaX, Bb XAY, Bb XaY = 4/8 = 1/2- Black (bb): bb XAX, bb XaX, bb XAY, bb XaY = 4/8 = 1/2- Fragile X Syndrome:- Female Carriers (XAXa): 2/8 = 1/4- Females without Fragile X (XAXA): 1/8- Males with Fragile X (XaY): 2/8 = 1/4- Males without Fragile X (XAY): 2/8 = 1/4 Therefore: - Genotypic Ratio: This is complex due to the combination of autosomal and sex-linked traits and cannot be expressed with a simple ratio. You would need to list each unique genotype and its frequency (e.g., Bb XAX: 1/8, etc.).- Phenotypic Ratio (simplified):- Gray body, female carrier for Fragile X: 1/4- Gray body, female without Fragile X: 1/8- Gray body, male without Fragile X: 1/4- Black body, female carrier for Fragile X: 1/4- Black body, female without Fragile X: 1/8- Black body, male with Fragile X: 1/4- Black body, male without Fragile X: 1/4 Important Note: These ratios are theoretical probabilities. Actual offspring ratios may vary due to random chance.
