Final answer:
The statement is true; both parents contribute an allele of the same gene to their progeny, resulting in either homozygous (identical alleles) or heterozygous (different alleles) genotypes, as demonstrated by Mendelian genetics and Punnett squares.
Step-by-step explanation:
The statement that if both parents contribute to a trait in the progeny, they must be contributing a copy of the same gene, is true. In genetics, for each characteristic, an organism inherits two alleles, one from each parent. These alleles can be the same, resulting in homozygous organisms, or different, leading to heterozygous organisms.
The variation within species is due to the specific combination of genes inherited from both parents. For instance, blood type in humans is determined by the two alleles of the marker gene that are inherited. Individuals can have two identical alleles (e.g., AA, BB, or OO) or two different alleles (e.g., AB, AO, or BO), which results in the traits of the progeny.
Using a Punnett square to predict the outcomes of a genetic cross, it can be seen that each parent contributes one allele to the offspring.
The resulting genotype can be homozygous or heterozygous, depending on whether the inherited alleles are the same or different. This model is based on Mendelian genetics, where traits are passed from parents to offspring through alleles.