Types of inheritance

Two types of different genetic inheritance are distinguished, depending on how the alleles work:

Dominant inheritance and recessive inheritance

If one of the two alleles of the chromosomes pair has more power to express itself than the other, it is called the dominant allele. The recessive allele is the one with the least strength, and it only manifests itself if there is no other dominant allele to hide it. Thus, if there is a dominant and a recessive allele, the character determined by the dominant is expressed. Only if both alleles are recessive will that trait be expressed.

For example, the gene that determines eye color has an allele for dark eyes that dominates over the allele that determines light eyes. The individual will manifest the dark-eyed phenotype if at least one of the two alleles is dominant. For the phenotype to be light-eyed, the two alleles must be recessive, so the individual must be homozygous recessive.

We will represent the dominant alleles with a capital letter, and the recessive ones with the same letter, but lower case.

For example:

• Character: eye color.
• Phenotypes: dark eye color (dominant) and light eye color (light).
• Alleles: A (dark eye color) and a (light eye color).
• Genotypes:
  • AA (homozygous dominant): dark eye color.
  • Aa (heterozygous): dark eye color.
  • aa (homozygous recessive): light eye color.

Intermediate inheritance and codominance

It can also happen that two alleles have the same strength and that neither dominates the other, being codominant alleles. In this case, a new intermediate phenotype will appear. There can be two cases:

Intermediate inheritance

For example, if when we cross a plant with white flowers with another with red flowers, we obtain an intermediate phenotype between the two, pink flowers, since neither allele completely cancels the other.

Since neither allele dominates the other, they are represented by different capital letters.

For example:

• Character: Flower color.
• Phenotypes: White, red and pink color.
• Alleles:
  • B = white color.
  • R = red color.
  • BR = pink color.
• Genotypes:
  • BB = white color (homozygous).
  • RR = red color (homozygous).
  • BR = pink color (heterozygous).

Modificado a partir de Spencerbaron (Own work) [CC BY-SA 3.0], via Wikimedia Commons

Codominance

It occurs when both characters are manifested at the same time. For example, crossing a brown cow with a white bull, and offspring with brown and white spots are obtained.

 By Hhughes15 (Own work) [CC BY-SA 4.0], via Wikimedia Commons

Genetic problems: Codominance and intermediate inheritance. Solved exercises.