Heterozygous Traits

Heterozygous Traits
Flies with the heterozygous genotype (Ww) exhibit normal wings. National Institutes of Health, National Human Genome Research Institute

An organism that is heterozygous for a trait has two different alleles for that trait. An allele is an alternative form of a gene (one member of a pair) that is located at a specific position on a specific chromosome. These DNA codings determine distinct traits that can be passed on from parents to offspring through sexual reproduction. Having different versions of alleles, or different genotypes allows for variations in exhibited traits.

An example of this can be seen in the inheritance of wing types in flies. Flies that inherit the allele for the dominant normal wing trait have normal wings. Flies that do not inherit the dominant allele have wrinkled wings. Flies that are heterozygous for the trait, having one dominant and one recessive allele, exhibit normal wings.

Mendel's Law of Segregation

The process by which alleles are transmitted was discovered by Gregor Mendel and formulated in what is known as Mendel's law of segregation. The four main concepts of gene segregation include: (1) genes exist in various forms (alleles), (2) paired alleles are inherited, (3) alleles are separated during meiosis and united at fertilization, and (4) when alleles are heterozygous, one allele is dominant. Mendel made this discovery through studying various characteristics of pea plants, one of which was seed color. The gene for seed color in pea plants exists in two forms.

There is one form, or allele for yellow seed color (Y) and another for green seed color (y). One allele is dominant and the other is recessive. In this example, the allele for yellow seed color is dominant and the allele for green seed color is recessive. Since organisms have two alleles for each trait, when the alleles of a pair are heterozygous (Yy), the dominant allele trait is expressed and the recessive allele trait is masked.

Seeds with the genetic makeup of (YY) or (Yy) are yellow, while seeds that are (yy) are green.

Heterozygous Genotypic Ratios

When organisms that are heterozygous for certain traits reproduce, expected ratios of these traits can be predicted in the resulting offspring. The expected genotypic (based on genetic makeup) and phenotypic (based on observable characteristics) ratios vary depending on parental genes. Using flower color as the example trait, the allele for purple petal color (P) is dominant to the white petal (p) trait. In a monohybrid cross between heterozygous plants for purple flower color (Pp), the expected genotypes are (PP), (Pp), and (Pp).

Heterozygous Cross
 Pp   
PPPPp
pPppp

The expected genotypic ratio is 1:2:1. Half of the offspring will be heterozygous (Pp), one-fourth will be homozygous dominant (PP), and one-fourth will be homozygous recessive. The phenotypic ratio is 3:1. Three-fourths of the offspring will have purple flowers (PP, Pp) and one-fourth will have white flowers (pp).

In a cross between a heterozygous parental plant and a recessive plant, the expected genotypes observed in the offspring will be (Pp) and (pp). The expected genotypic ratio is 1:1.

Heterozygous Cross
 Pp   
pPppp
pPppp

Half of the offspring will be heterozygous (Pp) and half will be homozygous recessive (pp). The phenotypic ratio will also be 1:1. Half will exhibit the purple flower (Pp) trait and half will have white flowers (pp).

When the genotype is unknown, this type of cross is performed as a test cross. Since both heterozygous organisms (Pp) and homozygous dominant organisms (PP) exhibit the same phenotype (purple petals), performing a cross with a plant that is recessive (pp) for the observable trait (white) can be used to determine the phenotype of the unknown plant. If the genotype of the unknown plant is heterozygous, half of the offspring will have the dominant trait (purple), and the other half will exhibit the recessive trait (white). If the genotype of the unknown plant is homozygous dominant (PP), all of the offspring will be heterozygous (Pp) and have purple petals.

Key Takeaways

  • Heterozygous refers to having different alleles for a particular trait.
  • When alleles are heterozygous in complete dominance inheritance, one allele is dominant and the other is recessive.
  • The genotypic ratio in a heterozygous cross where both parents are heterozygous for a trait is 1:2:1.
  • The genotypic ratio in a heterozygous cross where one parent is heterozygous and the other is homozygous for a trait is 1:1.

Sources

  • Reece, Jane B., and Neil A. Campbell. Campbell Biology. Benjamin Cummings, 2011.