Biology Tutorials > Genetics and Evolution > Dominance and Crossing Over

Dominance and Crossing Over

Crossing Over

Crossing over between nonsister chromatids of homologous chromosomes

The previous tutorial investigated Gregor Mendel and how he found trends in the phenotypes of offspring produced by true-breeding parents

Dominant and Recessive Alleles

Mendel paved the way to discovering that alleles that code for a particular characteristic, such as the shape of the seeds produced are expressed in dominant and recessive genes.

When dominant genes were present, they would supersede the presence of wrinkled and were deemed the dominant gene. For example;

If the genotype for seeds was Rr (where R is dominant and r is recessive), R would supersede the recessive gene and the plant would express a round seed phenotype.

If the genotype was rr (where both are recessive) there are no dominant genes, therefore, the recessive phenotype for wrinkled seed is expressed

The previous tutorial mentioned that in the first generation all offspring produced were round seeds, and in the second generation for every three that were round seeded there would be one wrinkled seed. This can be expressed in a Punnett square as illustrated below.

All dominant genes are marked in red, and all recessive genes are marked in green. Whenever the dominant gene is present in an organism this will be expressed. We can summarise the above diagram in the following statements

  • Parent 1 possesses 2 dominant genes in its genotype
  • Parent 2 possesses 2 recessive genes in its genotype
  • The gametes produced by parent 1 all contain the dominant gene while parent 2’s gametes all possess the recessive gene.
  • When parent 1 and 2’s gametes are crossed, the genotype is Rr. Since all of them have the dominant gene passed on from parent 1, they all express the dominant phenotype
  • The gametes from the first generation produce 50% R gametes and 50% r gametes
  • The second generation produces 1 RR 2 Rr and 1 rr genotypes in offspring, resulting in 3 round seed phenotypes per 1 wrinkled seed

This is an example of a monohybrid cross, where we are studying one respect of an animal’s genotype. The next tutorial continues to look at dominance and examples of monohybrid and dihybrid crossing plus related info.

Biology Tutorials > Genetics and Evolution > Dominance and Crossing Over

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