Concepts in Inheritance - Grade 12

 

1. Chromatin

What it is:

• Chromatin is like a spaghetti of very thin thread-like structures inside the nucleus of a cell.
• It's made of DNA and proteins.
• It’s not tightly packed — it looks messy when you see it under a microscope.

Example:
Before a cell divides, chromatin coils up tightly to form chromosomes (those X-shaped structures you often see in diagrams).

Imagine a bowl of spaghetti → that's chromatin.
Twist and pack the spaghetti tightly → that forms chromosomes.

2. Chromatid

What it is:

• When a chromosome copies itself (for example before cell division), each half is called a chromatid.
• So, one chromosome has two chromatids joined at a point called the centromere.

Example:
Think of a pair of shoes:

• Left shoe = 1 chromatid
• Right shoe = 1 chromatid
• Together, they form a full pair (replicated chromosome).

3. Gene

What it is:

• A gene is a small piece of DNA that gives instructions for a specific trait (like eye color, hair type, or blood type).

Example:

• There’s a gene that decides whether you have brown or blue eyes.
• The gene is part of the DNA strand inside your chromosomes.

4. Alleles

What it is:

• Alleles are different versions of the same gene.
• They are found at the same position (locus) on a pair of chromosomes.

Example:
For eye color:

• One allele might code for brown eyes.
• Another allele might code for blue eyes.

Now, within alleles, we get:

5. Dominant Allele

What it is:

• A dominant allele shows up in the organism's appearance (phenotype) even if there is only one copy.

Example:

• If "T" = Tall and "t" = Short in plants:
• "Tt" (one Tall allele, one Short allele) → the plant will be Tall because "T" is dominant.

6. Recessive Allele

What it is:

• A recessive allele is hidden when there’s a dominant one present.
• It only shows when there are two copies of it.

Example:

• "tt" (two short alleles) → the plant will be Short.
• "Tt" → the plant will still be Tall because the dominant "T" hides the recessive "t".

7. Phenotype

What it is:

• Phenotype = What you see — the physical traits.

Example:

• Tall plant
• Blue eyes
• Curly hair

Note: The phenotype is determined by the genes (genotype) but also can be influenced by the environment.

8. Genotype

What it is:

• Genotype = What the genes say — the genetic code inside the cells.

Example:

• TT = two dominant genes for Tall
• Tt = one dominant, one recessive
• tt = two recessive genes for Short

So, Genotype decides Phenotype!

9. Homozygous

What it is:

• Homo = Same
• Two identical alleles for a trait.

Example:

• TT (homozygous dominant — tall)
• tt (homozygous recessive — short)

10. Heterozygous

What it is:

• Hetero = Different
• Two different alleles for a trait.

Example:

• Tt → One Tall (T), one Short (t).

Even though it has a 't', the plant will still be Tall because "T" is dominant.

11. Monohybrid Cross

What it is:

• A genetic cross that looks at only one characteristic.

Example:

• Cross between two plants for height only (Tall x Short).

12. Dihybrid Cross

What it is:

• A genetic cross that looks at two characteristics at the same time.

Example:

• Crossing two plants looking at height (Tall or Short) and color (Green or Yellow seeds) together.

13. Karyotype

What it is:

• A picture or map showing all the chromosomes of a cell arranged by size and shape.
• It shows the total number, shape, and arrangement of chromosomes.

Example:

• Humans have 46 chromosomes (23 pairs).
• A karyotype can show if someone has an extra chromosome, like in Down syndrome (an extra chromosome 21).