In a breed of domestic cattle, horns can appear on males and on females. Males and females can also be hornless. The following crosses are performed with parents from pure-breeding lines.

Explain the inheritance of this phenotype in cattle, and assign genotypes to all cattle in each cross.

Verified step by step guidance

1. Step 1: Understand the phenotype and the inheritance pattern. Horn presence in cattle can be influenced by sex and genotype, often involving sex-influenced or sex-limited traits. Determine if the trait is autosomal or sex-linked, and whether dominance differs between males and females.
2. Step 2: Define the possible genotypes. For example, let 'H' represent the allele for horns and 'h' for hornlessness. Since the trait appears in both sexes but with different expressions, consider that 'H' might be dominant in males but recessive in females, or vice versa.
3. Step 3: Analyze the pure-breeding parents. Pure-breeding horned males and females would be homozygous for the horn allele (e.g., HH), and pure-breeding hornless individuals would be homozygous recessive (hh). Assign genotypes accordingly to each parent in the crosses.
4. Step 4: Predict the genotypes and phenotypes of the offspring from each cross using Punnett squares. Consider sex-specific expression by applying the dominance pattern differently in males and females to determine which offspring will have horns or be hornless.
5. Step 5: Summarize the inheritance pattern based on the crosses. Explain how the genotype relates to the phenotype in each sex, confirming whether the trait is sex-influenced (same gene, different dominance in sexes) or sex-limited (expressed only in one sex).

Key Concepts

Sex-Influenced Inheritance

Sex-influenced traits are those where the phenotype expression differs between males and females despite the same genotype. In cattle horn inheritance, the presence or absence of horns can be dominant in one sex but recessive in the other, causing males and females to show different patterns of horn development.

Pure-Breeding Lines and Genotype Assignment

Pure-breeding lines are populations that consistently produce offspring with the same phenotype, indicating homozygosity for the trait. Understanding these lines helps assign genotypes to parents and predict offspring genotypes by applying Mendelian inheritance principles.

Dominance Relationships in Horn Phenotype

The inheritance of horns involves dominance where horned or hornless alleles may show different dominance depending on sex. Recognizing which allele is dominant in males versus females is essential to explain the observed phenotypes and correctly assign genotypes in crosses.