Crossing-over events occur during the prophase 1. After anaphase 2, telophase 2, and citokynesis, 4 gametes are formed. Each gamete has different genetic information. Alleles are labeled in the attached file.
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Since I do not have the parentals genotype, I will consider two traits, each coded by a single diallelic gene.
Gene 1:
- a is the recessive allele
- Located in the superior arm of the chromosome
Gene 2:
- b is the recessive allele
- Located in the inferior arm of the chromosome
I will assume that both parents are heter0zyg0us for both traits, so their genotypes are AaBb.
During gamete formation, the pairs of h0m0logous chromosomes separate after crossing over. Then, sister chromatids also separate. The result is the formation of four gametes.
Each gamete has different information from the combination of h0m0logous chromosomes belonging to the father and to the mother.
Mother
During the prophase of meiosis 1, the mother's homologous chromosomes express two crossing-over events. One of them in the superior arm and the other one in the inferior arm.
The segments interchange between chromosomes involves Gene 1 and Gene 2.
During anaphase 2, sister chromatids separate, and after telophase and cytokinesis, four haploid gametes are formed carrying the following chromosomes,
- AB → Parental
- ab → Recombinant
- AB → Recombinant
- ab → Parental
Father
During the prophase of meiosis 1, the father's homologous chromosomes express one crossing-over event in the inferior arm.
The segments interchange between chromosomes involves Gene 2.
During anaphase 2, sister chromatids separate, and after telophase and cytokinesis, four haploid gametes are formed carrying the following chromosomes,
- AB → Parental
- Ab → Recombinant
- aB → Recombinant
- ab → Parental
Gametes' destiny is to randomly merge in the process of fecundation, during which a new diploid cell called zygote emerges through fertilization.
The zygote is a complete cell that suffers successive mitosis to form the new organism.
In the exposed example,
Offspring 1 → Parental gamete ab from the mother merged recombinant
gamete aB from the father → genotype aaBb
Offspring 2 → Parental gamete AB from the mother merged recombinant
gamete aB from the father → genotype AaBB
Offspring 3 → Recombinant gamete ab from the mother merged
recombinant gamete Ab from the father → genotype Aabb
Offspring 4 → Parental gamete ab from the mother merged recombinant
gamete ab from the father → genotype aabb
For a better understanding, you can see the image in the attached files in which alleles are labeled.
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You can learn more about crossing-over at
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