Final answer:
If there is no crossing over during meiosis, only two types of gametes are produced by a heterozygous organism carrying two genes: one with dominant alleles (AB) and one with recessive alleles (ab). A Punnett square can be used to visualize the expected genotypes and phenotypic ratios of offspring resulting from gamete fusion, assuming equal probabilities of gamete combinations.
Step-by-step explanation:
If there is no crossing over at the time of gamete formation or meiosis, only two types of gametes are produced, one carrying the dominant alleles (AB) and the other carrying the recessive alleles (ab).
During gamete formation by meiosis, paired alleles always separate and go to different gametes, following Mendel's law of segregation. If no homologous recombination, or crossing over, occurs between linked genes, a heterozygous organism with genotype AaBb would only produce two types of gametes in terms of these genes—those carrying the alleles AB and those carrying the alleles ab—rather than the four types (AB, Ab, aB, ab) that would result from independent assortment of unlinked genes.
The probability of which gametes will join during fertilization is a matter of chance, much like tossing a coin. For example, using a Punnett square, geneticists can predict the different genotypic and phenotypic outcomes of a cross, as well as their respective probabilities. However, the occurrence of crossing over changes these probabilities by creating nonparental or recombinant types of gametes, in addition to the parental types, thus increasing genetic diversity.