Q.Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in F1 generation for a dihybrid cross?

When two heterozygous parents are crossed, and the two loci controlling the traits are linked, the genes are located on the same chromosome and tend to be inherited together. This phenomenon, known as linkage, significantly alters the expected phenotypic distribution in the offspring.

Instead of independent assortment, the linked genes cause the heterozygous parents to produce a higher proportion of gametes carrying the original parental combinations of alleles. Recombinant gametes, formed due to crossing over between the linked genes, will be produced in much lower proportions. …

A dihybrid cross involves studying the inheritance of two different traits simultaneously. When we speak of "two heterozygous parents," it means each parent carries two different alleles for each of the two genes being studied. For example, if we consider genes for seed shape (R/r) and seed colour (Y/y), a heterozygous parent would have the genotype RrYy. When two such parents (RrYy x RrYy) are crossed, we are typically looking at the F2 generation in classical Mendelian genetics, but the question refers to the offspring of this specific cross as the "F1 generation."

The crucial aspect here is that the two loci (genes) are linked. Linkage refers to the phenomenon where two or more genes are located on the same chromosome and tend to be inherited together. This physical association on a chromosome prevents the genes from assorting independently during meiosis, which is a fundamental principle observed by Mendel for unlinked genes.

When genes are linked, their inheritance pattern changes dramatically because they do not assort independently. Instead, they tend to be passed on together as a unit. This has a direct impact on the types and proportions of gametes produced by the heterozygous parents.

Consider a heterozygous parent with genotype AaBb, where genes A and B are linked.

• If the alleles A and B are on one chromosome, and a and b are on the homologous chromosome (this is called the coupling or cis phase), then the parent primarily produces gametes carrying AB and ab.
• If the alleles A and b are on one chromosome, and a and B are on the homologous chromosome (this is called the repulsion or trans phase), then the parent primarily produces gametes carrying Ab and aB.

The distribution of phenotypic features in the F1 generation (offspring of AaBb x AaBb) will depend on the strength of linkage:

• Complete Linkage (No Crossing Over): If the genes are completely linked, meaning they are very close together on the chromosome and no crossing over occurs between them, then a heterozygous parent (e.g., AaBb in coupling phase) will produce only two types of gametes: AB and ab. No recombinant gametes (Ab or aB) will be formed. In a cross between two such parents (AaBb x AaBb), the offspring would effectively behave like a monohybrid cross for a single "linked unit." The phenotypic distribution would be:
  • 3 individuals showing both dominant traits (A_B_)
  • 1 individual showing both recessive traits (aabb) This results in a 3:1 phenotypic ratio, which is a stark deviation from the 9:3:3:1 ratio expected for unlinked genes. The intermediate phenotypic combinations (A_bb and aaB_) would be absent. …

A clean way to see the effect of linkage is to tabulate the four possible gamete types (AB, Ab, aB, ab) with a 'parental' or 'recombinant' label next to each, then note that when genes are linked, the parental-type gametes vastly outnumber the recombinant ones (and, if linkage is complete, the recombinant gametes drop to zero). Once you see the gamete table skewed …