Part of Extensions of Mendelian Genetics
Mendel imagined each gene comes in just two flavours (e.g. tall vs short). But a gene is a stretch of DNA — and DNA can mutate at many positions. So in a whole population , the same gene can exist in more than two alternative forms. These are multiple alleles . The catch that students always forget: a single diploid individual still carries only two of them (one per homologous chromosome). The "many" lives in the population, not in one organism.
Definition Multiple allelesA gene is said to have multiple alleles when three or more alternative forms (alleles) of that gene exist within a population at the same locus.
Population level: 3, 4, ... many alleles possible.Individual (diploid) level: still only 2 alleles max — one on each homologous chromosome.Gamete (haploid) level: exactly 1 allele.
Intuition WHY only two per person?A diploid organism has two copies of each chromosome → two copies of each locus → room for only two alleles. The pool of possibilities is huge, but the seats on your chromosomes are limited to two.
An "allele" is just one specific DNA sequence at a locus. Ask: how many different sequences are possible?
A locus of length n n n A , T , G , C A,T,G,C A , T , G , C N possible = 4 n N_{\text{possible}} = 4^{\,n} N possible = 4 n Why this step? Each of the n n n
Even for a tiny n = 10 n=10 n = 10 4 10 ≈ 10 6 4^{10} \approx 10^6 4 10 ≈ 1 0 6 biological surprise is not that multiple alleles exist — it's that Mendel's two-allele model worked at all. Multiple alleles are the rule ; two alleles are a special case.
Worked example Check with the ABO blood group (k = 3 k=3 k = 3 I A , I B , i I^A, I^B, i I A , I B , i G ( 3 ) = 3 ⋅ 4 2 = 6 genotypes G(3)=\frac{3\cdot 4}{2}=6 \text{ genotypes} G ( 3 ) = 2 3 ⋅ 4 = 6 genotypes Why? to see the formula is real:
I A I A , I A I B , I A i , I B I B , I B i , i i I^AI^A,\; I^AI^B,\; I^Ai,\; I^BI^B,\; I^Bi,\; ii I A I A , I A I B , I A i , I B I B , I B i , ii 6 . ✔
Worked example ABO system — multiple alleles+ codominance + dominance, all at once
Gene I has three alleles in the human population:
I A I^A I A A on red cells.I B I^B I B B .i i i no antigen (recessive to both).
Dominance relations: I A = I B > i I^A = I^B > i I A = I B > i I A , I B I^A, I^B I A , I B codominant with each other.
Genotype
Phenotype (blood group)
Why?
I A I A , I A i I^AI^A,\ I^Ai I A I A , I A i A I A I^A I A i i i
I B I B , I B i I^BI^B,\ I^Bi I B I B , I B i B I B I^B I B i i i
I A I B I^AI^B I A I B AB I A I^A I A I B I^B I B codominant → both antigens
i i ii ii O no antigen
Result: 6 genotypes → only 4 phenotypes (A, B, AB, O).
Worked example Forecast-then-Verify: can two type-O parents have a type-A child?Forecast: No.
Verify: Type O = i i ii ii i i i i i ii ii O . ✔ Confirmed impossible — a real fact used in (historical) paternity disputes.
Common mistake "An individual can have many alleles of one gene."Why it feels right: We just said the gene has many alleles, so surely one person carries them all.
The fix: Many alleles exist in the population . A diploid individual has only 2 chromosome copies → at most 2 alleles. "Multiple" is a gene/population property, not an individual one.
Common mistake "Multiple alleles means polygenic inheritance."Why it feels right: Both have the word "multiple" and both complicate Mendel.
The fix: Multiple alleles = one gene, many versions (one locus). Polygenic = many genes each affecting one trait. Opposite directions! ABO = one gene, three alleles. Skin colour = many genes.
I A I B I^AI^B I A I B Why it feels right: Two different alleles, neither "wins" → sounds like blending.
The fix: It's codominance , not incomplete dominance. Both antigens A and B appear fully and separately (group AB), not a mixed/intermediate product. No blending.
2 k 2^k 2 k k 2 k^2 k 2 Why it feels right: Powers look like the natural "combinations" answer.
The fix: Order of the two alleles doesn't matter, so it's unordered pairs with repetition : k ( k + 1 ) 2 \dfrac{k(k+1)}{2} 2 k ( k + 1 ) not k 2 k^2 k 2 I A I B I^AI^B I A I B I B I A I^BI^A I B I A
Worked example More multiple-allele systems
Coat colour in rabbits: C > c c h > c h > c C > c^{ch} > c^h > c C > c c h > c h > c 4 alleles , a dominance series .Self-incompatibility in plants: S 1 , S 2 , S 3 , … S_1, S_2, S_3, \ldots S 1 , S 2 , S 3 , … S S S Human HLA genes: hundreds of alleles → why organ-transplant matching is hard.
Multiple alleles = 3+ forms of one gene in a population ; 2 max per individual .
Origin = mutation of the same locus.
ABO is the showcase: I A , I B , i I^A, I^B, i I A , I B , i I A = I B > i I^A=I^B>i I A = I B > i I A / I B I^A/I^B I A / I B 6 genotypes → 4 phenotypes for ABO.Genotype count: k ( k + 1 ) 2 \dfrac{k(k+1)}{2} 2 k ( k + 1 )
Recall Test yourself (cover the answers)
How many alleles can one diploid individual carry at a locus? → 2.
Where do "multiple" alleles actually reside? → the population.
Phenotype of I A i I^Ai I A i Group A.
Dominance relationship of I A I^A I A I B I^B I B codominant.
Genotypes for k = 4 k=4 k = 4 4 ⋅ 5 2 = \frac{4\cdot5}{2}= 2 4 ⋅ 5 = 10.
Recall Feynman: explain to a 12-year-oldImagine a gene is a slot for a trading card . There might be lots of different cards in the world (red, blue, gold...). But you only have two card slots — one from mum, one from dad. So even though the world has many cards, you can only hold two at a time . Blood type is like this: the world has three "blood cards" (I A I^A I A I B I^B I B i i i both — that's blood group AB.
"Many in the pool, two in the cell, one in the gamete."
For ABO: "A and B are codominant Bosses; little i is the Invisible servant."
Codominance — explains the I A I B I^AI^B I A I B Incomplete Dominance — contrast: blending vs separate expression.Polygenic Inheritance — opposite concept (many genes, one trait).Gene Mutation — the source of new alleles.ABO Blood Group System — the canonical worked example.Punnett Square — tool used for the crosses above.Law of Segregation — why a gamete carries only one allele.
What defines "multiple alleles"? Existence of three or more alleles of one gene at the same locus within a population.
How many alleles of a gene can a single diploid individual carry? At most two — one on each homologous chromosome.
How many alleles does a gamete carry at a locus? Exactly one (gametes are haploid).
What are the three alleles of the human ABO gene? I A I^A I A ,
I B I^B I B , and
i i i .
State the dominance relationship in ABO. I A = I B > i I^A = I^B > i I A = I B > i ;
I A I^A I A and
I B I^B I B are codominant to each other, both dominant over
i i i .
Genotype(s) giving blood group A? I A I A I^AI^A I A I A and
I A i I^Ai I A i .
Genotype giving blood group AB and why? I A I B I^AI^B I A I B ; because
I A I^A I A and
I B I^B I B are codominant, both antigens are expressed.
Genotype giving blood group O? i i ii ii (homozygous recessive, no antigen).
Formula for number of genotypes with k alleles? G ( k ) = k ( k + 1 ) 2 G(k)=\dfrac{k(k+1)}{2} G ( k ) = 2 k ( k + 1 ) .
How many genotypes and phenotypes does ABO have? 6 genotypes, 4 phenotypes.
Can two type-O parents have a non-O child? No;
i i × i i ii \times ii ii × ii gives only
i i ii ii (O).
Difference between multiple alleles and polygenic inheritance? Multiple alleles = one gene, many versions; polygenic = many genes affecting one trait.
What is the origin of multiple alleles? Mutations at the same gene locus over time in a population.
Name a 4-allele multiple-allele system. Rabbit coat colour:
C > c c h > c h > c C > c^{ch} > c^h > c C > c c h > c h > c .
DNA can mutate at many positions
3+ allele forms at one locus
Diploid individual = 2 alleles max
Two homologous chromosomes
N = 4^n possible sequences
Heterozygotes = k choose 2
ABO blood group = 6 genotypes
Codominance and dominance
Intuition Hinglish mein samjhoDekho, Mendel ne socha tha ki har gene ke sirf do roop (alleles) hote hain — jaise tall ya short. Lekin asliyat mein gene ek DNA ka tukda hai, aur DNA kahin bhi mutate ho sakta hai. Isliye ek poori population mein ek hi gene ke teen ya zyada roop ho sakte hain — yahi hai multiple alleles . Bada confusion point: ek aadmi (diploid) ke paas chahe kuch bhi ho, sirf do alleles hi hote hain — ek maa se, ek papa se. "Multiple" ka matlab population mein multiple, har banda nahi.
Best example hai ABO blood group . Yahan gene I ke teen alleles hain: I A I^A I A I B I^B I B i i i I A I^A I A I B I^B I B i i i codominant hain — yani agar dono saath ho (I A I B I^AI^B I A I B AB banta hai. i i ii ii O . Isiliye 6 genotypes lekin sirf 4 phenotypes (A, B, AB, O) milte hain.
Counting ka shortcut: agar k k k k ( k + 1 ) 2 \frac{k(k+1)}{2} 2 k ( k + 1 ) k = 3 k=3 k = 3 3 × 4 2 = 6 \frac{3\times4}{2}=6 2 3 × 4 = 6 O group parents ka bachcha kabhi A/B/AB nahi ho sakta, kyunki i i × i i ii \times ii ii × ii i i ii ii
Yaad rakhne ka tarika: "Many in the pool, two in the cell, one in the gamete." Yeh ek line poora concept pakad leti hai. Aur multiple alleles ko polygenic se mat mila dena — multiple alleles matlab ek gene ke kai roop , polygenic matlab kai gene ek trait par . Bilkul ulta cheez hai.
