Using Mendel’s theories, we can predict the characteristics of an offspring given the physical characteristics of the parents. A Punnett square uses probability to help us determine an offspring’s genetic makeup and physical appearance. These methods of prediction were found through Gregor Mendel’s discoveries. Codominance and incomplete dominance, however, are considered to be under a non-Mendelian influence.
Gene: Segment of DNA that codes for a single protein or RNA. Controls what characteristics are expressed.
Alleles: Variants of a specific gene.
Dominant Allele: The allele that is expressed as long as a dominant allele is present.
Recessive Allele: The allele that is expressed as long as no dominant alleles are present.
Genotype: The genetic makeup of an organism. An individual’s inherited alleles make up its genotype.
Phenotype: The physical appearance or expression of an inherited genotype.
Probability: The chance or likelihood that an event will occur.
Punnett Square: A chart used to determine the probability of different genotypes in the offspring of two parents.
Homozygote: An organism that inherits two alleles of the same type (BB or bb).
Heterozygote: An organism that inherits two alleles of differing types (Bb).
Monohybrid Cross: A cross dealing with only one characteristic.
Dihybrid Cross: A cross in which two independent traits are being determined by crossing parents that differ in genotype.
Codominance: A phenotype in which both alleles are expressed equally.
Incomplete Dominance: Occurs when the dominant allele is not completely dominant, resulting in an intermediate phenotype.
Polygenic Characteristics: A characteristic (a phenotype or genotype) that is controlled by more than one gene, with the possibility that each gene has two or more alleles. These genes may be located on the same or different chromosome.
From his experiments, Mendel concluded that an offspring inherits two alleles for each gene.
In writing genotypes,
The three possible genotypes are BB, bb, Bb (the order that the alleles are listed does not matter, so bB is the same as Bb)
There are only two phenotypes:
A genotype containing at least one dominant allele will express the dominant allele.
Based off of the parents’ genotypes, we can determine the probability that their offspring has a given genotype and phenotype by using a Punnett square. Use the image here as a visual guide for using Punnett squares in a monohybrid cross.
Both of the parents above were heterozygous. Their offspring, according to the Punnett square, has a 25% chance of having the genotype BB, a 25% chance of having the genotype bb, and a 50% chance of having the genotype Bb.
The dominant allele B dominates over the recessive allele b. In this case, the coloring of the petals is purple whenever B is expressed. The offspring has a 75% chance of receiving the genotype of BB or Bb, or of having the phenotype of purple petals. It has a 25% chance of receiving the genotype of bb, or of having the phenotype of white petals.
The ratio 3:1 is typical for a monohybrid cross where both parents are heterozygous.
Punnett squares do not have to only consider one characteristic at a time. A Punnett square can also be used for a
The ratio 9:3:3:1 is typical for a dihybrid cross between two heterozygous parents.
Inheritance does not always behave like Mendel’s pea plants. Codominance, incomplete dominance, and polygenic characteristics complicate inheritance
Although genes are inherited, their expression can be modified by interaction with the environment. New inheritable characteristics come from new combinations of genes or from gene mutations in reproductive cells, which will change the genetic makeup of the offspring.(See the Mutation biology study guide for more details.)