What is Homozygous?

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Grade Level:

Class 12

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Definition

What is it?

Homozygous describes an organism that has two identical copies of a gene for a particular trait. This means both alleles (versions of a gene) at a specific location on homologous chromosomes are exactly the same.

Simple Example

Quick Example

Imagine your favourite chai shop has two types of sugar: 'extra sweet' (S) and 'less sweet' (s). If a chai recipe is homozygous for 'extra sweet', it means it always uses two 'extra sweet' instructions (SS). It will consistently make extra sweet chai, never less sweet.

Worked Example

Step-by-Step

Let's say the gene for a plant's flower colour has two alleles: 'Red' (R) and 'white' (r). --- STEP 1: A plant inherits one allele for flower colour from each parent. --- STEP 2: If a plant inherits 'R' from one parent and 'R' from the other parent, its genetic makeup for flower colour is RR. --- STEP 3: Since both alleles (R and R) are identical, this plant is homozygous for the red flower colour. --- STEP 4: Similarly, if a plant inherits 'r' from one parent and 'r' from the other, its genetic makeup is rr. --- STEP 5: Since both alleles (r and r) are identical, this plant is homozygous for the white flower colour. --- ANSWER: A plant with genotype RR or rr is homozygous for flower colour.

Why It Matters

Understanding homozygosity is crucial in Medicine for predicting genetic diseases and in Biotechnology for breeding plants or animals with desired traits. It helps genetic counsellors advise families and scientists develop new medicines or improve crop yields.

Common Mistakes

MISTAKE: Thinking homozygous means having two different alleles. | CORRECTION: Homozygous specifically means having two IDENTICAL alleles for a gene.

MISTAKE: Confusing homozygous with dominant or recessive. | CORRECTION: Homozygous refers to the sameness of alleles (e.g., AA or aa), while dominant/recessive describes how those alleles express a trait (e.g., A is dominant over a).

MISTAKE: Believing homozygous only applies to visible traits. | CORRECTION: Homozygosity applies to any gene, whether its trait is visible (like flower colour) or not (like a gene for an enzyme in our body).

Practice Questions

Try It Yourself

QUESTION: If a person has the genotype 'BB' for a specific gene, are they homozygous or heterozygous? | ANSWER: Homozygous

QUESTION: A farmer wants to breed cows that consistently produce high-quality milk. If the gene for high milk quality (M) is dominant over low milk quality (m), what homozygous genotype should the farmer aim for in the cows? | ANSWER: MM

QUESTION: A geneticist is studying a plant where the allele for tallness (T) is dominant and shortness (t) is recessive. If a plant is homozygous for tallness, what is its genotype? If another plant is homozygous for shortness, what is its genotype? | ANSWER: Homozygous tallness: TT. Homozygous shortness: tt.

MCQ

Quick Quiz

Which of the following genotypes represents a homozygous individual?

The Correct Answer Is:

Homozygous means having two identical alleles. In option B, 'BB' shows two identical 'B' alleles. Options A, C, and D show two different alleles (e.g., A and a).

Real World Connection

In the Real World

In India, understanding homozygous traits is vital in agriculture. For example, plant breeders at institutes like ICAR (Indian Council of Agricultural Research) select homozygous seeds for specific traits like disease resistance or high yield in crops like rice or wheat. This ensures that the next generation of plants consistently carries these desired characteristics, helping farmers produce better food.

Key Vocabulary

Key Terms

ALLELE: A different version of a gene | GENOTYPE: The genetic makeup of an organism | HETEROZYGOUS: Having two different alleles for a gene | TRAIT: A specific characteristic of an organism

What's Next

What to Learn Next

Now that you understand homozygous, you should learn about 'Heterozygous'. It's the opposite concept and understanding both will give you a complete picture of how genes are passed on and expressed.