Biology · Principles of Inheritance and Variation · NEET
A parental type offspring carries the same combination of alleles that was present in one of the parents. If a parent gives gametes with genes AB and ab, then offspring showing AB together or ab together are parental types. Nothing was mixed up. Their gene arrangement is just a repeat of what the parent had.
A recombinant type offspring carries a NEW combination of alleles that neither parent showed as a set. From a parent with AB and ab, the new mixes Ab and aB are recombinant types. These new sets are formed when crossing over swaps segments between homologous chromosomes during meiosis (pachytene of prophase I).
When two genes lie close together on the SAME chromosome, they are 'linked' and usually travel together into the gamete. Crossing over between them happens only sometimes, so most offspring keep the old (parental) combinations and only a few become recombinant. So a HIGH parental proportion is the signal of linkage. This is exactly what NEET 2016 tested.
If the two genes are on DIFFERENT chromosomes, they assort independently (Mendel's law of independent assortment). Then parental and recombinant types come out in roughly equal numbers (about 50:50 in a testcross). Unequal numbers, with parental higher, means the genes are linked on the same chromosome.
In a dihybrid testcross, the F1 (AaBb) is crossed with a fully recessive parent (aabb). The recessive parent only gives ab gametes, so the offspring phenotype directly shows the F1's gamete. Counting offspring types tells you how many are parental and how many are recombinant, and the ratio reveals linkage.
Not exactly. Crossing over is the PROCESS (exchange of segments between non-sister chromatids of homologous chromosomes). A recombinant offspring is the RESULT of that process being passed on. Recombination frequency (percent of recombinant offspring) is also used by Sturtevant to measure distance between genes.
In a testcross involving F1 dihybrid flies, more parental-type offspring were produced than the recombinant-type offspring. This indicates:
Try the real previous-year questions from this chapter — each with the answer and a full solution.
Parental type is more. Linked genes stay together most of the time, so recombinant types stay in the minority.
Crossing over during meiosis. It exchanges chromosome segments between non-sister chromatids of homologous chromosomes, making new allele combinations.
Only when the genes are on different chromosomes (independent assortment). For linked genes on the same chromosome, parental types are far more common.
NEET repeatedly links this to LINKAGE: a parental majority proves genes are on the same chromosome. It also connects to recombination frequency and gene mapping, which are common NEET questions.
