7.2.2 Genotype and Phenotype

I) Genotype

The genetic makeup of a trait at each locus is called the genotype. A genotype usually consists of a pair of genes or alleles.

Examples:

• Free earlobes: EE or Ee
• Attached earlobes: ee

Homozygous:

- If the genotype has two identical alleles (like EE or ee), it is called homozygous.

Heterozygous:

- If it has two different alleles (like Ee), it is called heterozygous.

True breed:

- An individual with a homozygous genotype is considered a "true breed," as it always produces offspring with the same trait when self-fertilized.

Non-true breed:

- A heterozygous individual is "non-true breed" because it can produce offspring with either form of the trait when self-fertilized.

Examples in pea plants:

• A true breed round seeded pea plant (RR) always produces round seeded offspring upon self-fertilization.
• A non-true breed round seeded plant (Rr) can produce both round seeded and wrinkled seeded plants upon self-fertilization.

Simple explanation: Genotype is the genetic makeup (e.g., EE, Ee, ee). Homozygous means two identical alleles; heterozygous means two different alleles.

II) Phenotype

The physical appearance of a trait is called the phenotype, which is determined by the genotype.

Examples:

• When EE or Ee genotypes are expressed, they produce free ear lobes.
• When ee is expressed, it produces attached earlobe.

Dominant phenotype:

- A phenotype that is visible even in the heterozygous state is called a dominant phenotype.

Recessive phenotype:

- A phenotype that only appears in the homozygous state is called a recessive phenotype.

Representation:

• The gene for a dominant phenotype is usually represented by a capital letter (e.g., E).
• The gene for a recessive phenotype is represented by a lowercase letter (e.g., e).

Simple explanation: Phenotype is the physical appearance (e.g., free earlobes, attached earlobes). Dominant traits appear even with one copy; recessive traits need two copies.

Summary Table

For Your Information

Genome

The complete set of an individual's genes is called the genome. The number of chromosomes that carry the entire genome is referred to as a set of chromosomes, represented by "n" or monoploid. A cell with two sets of chromosomes is called diploid or 2n. In humans, the full genome is spread across 23 chromosomes, which make up one set. The term haploid is used to represent a cell that contains half than the number of chromosomes of somatic cells.

Gene Pool

A gene pool is the complete set of all the genes and their different alleles on specific locus present in all the individuals of a population. For example, total number of earlobe genes or alleles ("E" and "e") in a population of 500 individuals, is the gene pool of earlobe.

Gene frequency, also known as allele frequency, refers to how often a particular allele appears in the gene pool. For example, total number of free earlobe genes (E) in the population is the dominant gene frequency and total number of attached earlobe genes (e) in the population is the recessive gene frequency. These frequencies reflect the genetic diversity of a population and can change over time due to factors like natural selection or mutation.

Intermediate Level

Genotypes and phenotypes are key concepts in genetics:

Key points:

• Genotype determines phenotype.
• Dominant traits mask recessive traits in heterozygotes.
• Recessive traits appear only in homozygotes.
• True breed = homozygous.
• Non-true breed = heterozygous.

Important Points

• - Each characteristic and function of the body is controlled by factor called gene.
• - The parental traits are passed to their offspring in the form of genes.
• - Therefore, a gene is the unit of inheritance.
• - A gene is a specific segment of DNA, comprising a unique sequence of nucleotides that encodes the sequence of amino acid in a particular polypeptide.
• - Mendel was the first person who proposed the idea of gene, he called them "factors" or "elements".
• - Genes are found in chromosomes at specific positions called loci.
• - At each locus on a chromosome, a gene can exist in two or more forms. These different forms or variations of a gene are called alleles.
• - Alleles are versions of the same gene that occupy the same position on a chromosome.
• - The genetic makeup of a trait at each locus is called the genotype.
• - A genotype usually consists of a pair of genes or alleles.
• - If the genotype has two identical alleles (like EE or ee), it is called homozygous.
• - If it has two different alleles (like Ee), it is called heterozygous.
• - An individual with a homozygous genotype is considered a "true breed".
• - A heterozygous individual is "non-true breed".
• - The physical appearance of a trait is called the phenotype, which is determined by the genotype.
• - A phenotype that is visible even in the heterozygous state is called a dominant phenotype.
• - A phenotype that only appears in the homozygous state is called a recessive phenotype.
• - The complete set of an individual's genes is called the genome.
• - A gene pool is the complete set of all the genes and their different alleles on specific locus present in all the individuals of a population.
• - Gene frequency, also known as allele frequency, refers to how often a particular allele appears in the gene pool.

Remember

• - Gene = unit of inheritance
• - Allele = different forms of a gene
• - Locus = position of a gene on a chromosome
• - Genotype = genetic makeup (EE, Ee, ee)
• - Phenotype = physical appearance (free or attached earlobes)
• - Homozygous = two identical alleles (EE or ee)
• - Heterozygous = two different alleles (Ee)
• - Dominant = expressed even with one copy (E)
• - Recessive = expressed only with two copies (e)
• - Genome = complete set of genes
• - Gene pool = all alleles in a population
• - Gene frequency = how often an allele appears

Common Mistakes

• 1. Confusing genotype and phenotype.
• - Genotype = genetic makeup.
• - Phenotype = physical appearance.
• 2. Mixing up homozygous and heterozygous.
• - Homozygous = two identical alleles (EE or ee).
• - Heterozygous = two different alleles (Ee).
• 3. Confusing dominant and recessive.
• - Dominant = expressed even with one copy (E).
• - Recessive = expressed only with two copies (e).
• 4. Thinking alleles are the same as genes.
• - Alleles are different forms of a gene.
• 5. Forgetting the role of the locus.
• - The locus is the position of a gene on a chromosome.
• 6. Believing phenotype is only determined by genotype.
• - Phenotype is determined by genotype and influenced by environment.
• 7. Confusing gene pool and genome.
• - Genome = complete set of an individual's genes.
• - Gene pool = all alleles in a population.

Quick Revision

60-Second Revision

• Gene = unit of inheritance
• Allele = different forms of a gene
• Locus = position of a gene on a chromosome
• Genotype = genetic makeup (EE, Ee, ee)
• Phenotype = physical appearance (free or attached earlobes)
• Homozygous = two identical alleles (EE or ee)
• Heterozygous = two different alleles (Ee)
• Dominant = expressed even with one copy (E)
• Recessive = expressed only with two copies (e)
• Genome = complete set of genes
• Gene pool = all alleles in a population
• Gene frequency = how often an allele appears
• Warning: Do not confuse genotype (genetic makeup) with phenotype (physical appearance). Homozygous means two identical alleles; heterozygous means two different alleles.