Co dominance Lethal genes and multiple Alleles

🧬 1. Co-dominance

✅ Definition

Co-dominance is a genetic condition in which both alleles of a gene express themselves fully and equally in a heterozygous organism.
👉 No allele masks the other (unlike complete dominance).
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🔍 Key Features

Both traits are visible simultaneously

No blending (unlike incomplete dominance)

Seen clearly in heterozygous condition
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🧪 Classic Example: ABO Blood Group System

Controlled by three alleles: , , and

→ produces A antigen

→ produces B antigen
👉 When both are present:

I^A I^B \rightarrow \text{Blood group AB}

✔ Both A and B antigens appear → Co-dominance
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🐄 Another Example: Coat Colour in Cattle

Red (RR) × White (WW)

Offspring (RW) = Roan (red + white hairs together)
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🎯 Important Difference

Co-dominance ≠ Incomplete dominance

Co-dominance → both traits visible

Incomplete dominance → traits blend (pink from red + white)
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☠️ 2. Lethal Genes

✅ Definition

Lethal genes are genes that cause death of an organism when present in a particular genotype.
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🔍 Types of Lethal Genes

1. Recessive Lethal

Causes death only in homozygous condition

Heterozygous individuals survive
2. Dominant Lethal

Causes death even in heterozygous condition

Rare because such genes are usually eliminated quickly
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🧪 Example: Yellow Coat in Mice

Let:

Y = yellow coat (lethal in homozygous form)

y = normal coat
Cross:

Yy \times Yy

Punnett Square Result:

YY → lethal (dies)

Yy → yellow (alive)

yy → normal (alive)
👉 Surviving ratio:

2 yellow : 1 normal (2:1 ratio instead of 3:1)
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⚠️ Key Points

Lethal genes disturb Mendelian ratios

Death may occur:

Before birth (embryonic stage)

After birth
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🔢 3. Multiple Alleles

✅ Definition

When a gene has more than two alternative forms (alleles) in a population, it is called multiple alleles.

👉 Important:

One individual → only 2 alleles

Population → many alleles
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🧪 Best Example: ABO Blood Group System

Alleles:

→ A antigen

→ B antigen

→ no antigen
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🧬 Genotypes and Phenotypes

Genotype Blood Group

, A
, B
AB
O
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🔍 Key Features

More genetic variation in population

Shows both:

Dominance ( , )

Co-dominance ( )
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🔁 Combined Understanding

Co-dominance is often seen in multiple allele systems

Example: ABO blood group shows both concepts together
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📊 Final Comparison Table

Feature Co-dominance Lethal Genes Multiple Alleles

Meaning Both alleles express equally Gene causes death More than 2 alleles exist
Expression In heterozygous condition In specific genotype Population level
Ratio Normal Mendelian Modified (2:1) Variable
Example AB blood group Yellow mice ABO system

Видео Co dominance Lethal genes and multiple Alleles канала Umair Alam