Genes, Genotype, and Phenotype – MCAT Biology | MedSchoolCoach

Genes, Genotype, and Phenotype

MCAT Biology

Sample MCAT Question: Genes, Genotype and Phenotype

A set of alleles in an organism is called:

a) Phenotype

b) Locus

c) Heterozygote

d) Genotype

D is correct. Genotype. In Mendelian genetics, an allele is a variant form of a gene. This can be a dominant allele, designated by “R”, or a recessive allele, designated by “r.” The genotype refers to a set of alleles for a particular gene in an organism. Genotypes can be designated as, “RR”, “Rr”, or “rr”. Choice A is incorrect; the phenotype is the set of observed characteristics in an organism. Choice B is incorrect; a locus is the position of a gene on a chromosome, at which multiple alleles for that gene might be observed, e.g. either “R” or “r”. Choice C is incorrect; a heterozygote is an organism that possess two different alleles of a gene at a given locus, e.g. an organism with the genotype “Rr”.


Genes are the basic units of heredity. For an organism to pass on its traits to its offspring, it must pass on its genes. DNA is the genetic material in cells, which means that genes are DNA sequences. Specifically, genes are DNA sequences that code for some sort functional molecule. These molecules can include mRNA molecules that code for proteins or non-coding RNAs that do not code for proteins but have vital functions (such as tRNA).


Genes represent only around 1% of the total DNA found in human cells. This means that 99% of the genome consists of non-coding DNA. In the past, it was thought that this non-coding DNA had no purpose or function. Recently, however, it has been discovered that non-coding DNA has a role in regulating gene expression. Studies on the role of non-coding DNA are currently ongoing.

Genotype vs Phenotype

Before we can understand the concepts of phenotype and genotype, we must first clarify some terms. The first term we must comprehend is “”locus””. A gene’s locus is its position or location on a chromosome. For example, the locus of the p53 gene is in region 13 of the long arm of chromosome 17. You can think of a gene’s locus as its “”address”” on the chromosome. Another term to know is “”allele””. An allele is a variant form of a gene. For example, in Mendel’s experiment with pea plants, some peas were round and some were wrinkled. The reason for this difference is that there are two alleles for shape-determing gene: the R allele, which codes for the round pea shape, and the r allele, which codes for the wrinkled pea shape.


With this in mind, we can define genotype as the set of alleles possesed by an organism. Organisms that reproduce sexually have two copies (alleles) for each gene. One allele is inherited from the mother, the other from the father. At the locus for the shape-coding gene in peas, possible genotypes are RR, Rr, and rr.


Phenotype, on the other hand, is the set of observed characteristics of an organism. In terms of shape, peas can have either a round or wrinkled phenotype. Pea plants with an RR or Rr genotype have a round phenotype, whereas peas with an rr genotype have a wrinkled phenotype.

Homozygotes vs Heterozygotes

Homozygotes are organisms with two identical alleles at a given loci. Pea plants with an RR or rr genotype are homozygous for the shape-coding gene. Heterozygotes, on the other hand, are organisms with two different alleles at a given loci. Pea plants with an Rr genotype are heterozygous for the shape-coding gene.

Dominant Alles vs Recessive Alleles

An allele is said to be dominant if it determines the phenotype in heterozygotes. For the shape-determining gene in pea plants, the dominant allele is R, since a heterozygous Rr individual will have the same round phenotype as a homozygous RR individual. The r allele, meanwhile, is recessive, since the heterozygote has a round phenotype while a homozygous rr individual will have a wrinkled phenotype. Thus, a recessive allele only determines the phenotype of an organism if it is present in two copies. Keep in mind that Mendelian genetics focuses on this simple distinction between dominant and recessive alleles. In reality, dominance is more complex and there are three different types of dominance called complete dominance, incomplete dominance, and codominance.

Wild-type vs Mutant

The term “wild type” is used to refer to the normal phenotype found in nature. Thus, wild-type organisms have the standard phenotype found in nature. This is in contrast to mutants, which have an abnormal phenotype. As an example, Arg+ bacteria, which can synthesize their own arginine amino acids, are wild-type organisms. (A ‘+’ next to the symbol for an allele denotes that that allele is wild type.) This means that Arg bacteria, which cannot synthesize their own arginine, are mutants.

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