Interactions of Genes
BiologyPrinciples of Inheritance and VariationFor NEET aspirants
Interactions of genes explain the results that do not fit Mendel's simple dominance. This page covers incomplete dominance in snapdragon, what dominance really means at the level of enzymes, co-dominance and multiple alleles in the human ABO blood groups, and why one gene can show dominance for one phenotype but not for another. It ends with polygenic inheritance and pleiotropy, as in the NCERT Class 12 chapter Principles of Inheritance and Variation. NEET often asks the 1 : 2 : 1 ratio, blood-group genotypes, and examples of these interactions of genes.
On this page1Beyond dominance2Incomplete dominance3What is dominance?4Co-dominance and ABO5Multiple alleles6Starch grains in pea7Polygenic inheritance8Pleiotropy9Exam essentials10Quick revision11Solved examples12Practice
Key Points at a Glance
- ★ Must learn Incomplete dominance: the F1 is in-between. Snapdragon red RR × white rr gives pink Rr; F2 1 red : 2 pink : 1 white.
- In incomplete dominance the phenotypic and genotypic ratios are the same, 1 : 2 : 1.
- A modified allele may make a normal or less efficient enzyme, a non-functional enzyme, or no enzyme at all.
- ★ Must learn Co-dominance: the F1 resembles both parents. IA IB red blood cells carry both A and B sugars.
- ★ Must learn ABO gene I: three alleles IA, IB, i; 6 genotypes and 4 blood groups (A, B, AB, O).
- IA and IB are completely dominant over i, which makes no sugar.
- Multiple alleles: more than two alleles of one gene; seen only in population studies.
- Pea starch gene B/b: B is dominant for seed shape, but incompletely dominant for starch grain size.
- ★ Must learn Polygenic inheritance: three or more genes with additive effects, plus the environment; human height and skin colour.
- ★ Must learn Pleiotropy: one gene with several phenotypic effects; phenylketonuria in humans.
1. Beyond Complete Dominance
- Mendel's experiments on peas were repeated using other traits in other plants.
- Sometimes the F1 had a phenotype that resembled neither parent and lay in between the two.
- In some other cases the F1 resembled both parents.
| Type | F1 phenotype | Example |
|---|---|---|
| Dominance | Resembles one parent | Tall Tt pea |
| Incomplete dominance | In between the two parents | Pink Rr snapdragon |
| Co-dominance | Resembles both parents | Blood group AB (IA IB) |
Tips and Tricks
Read the F1 to name the interaction. Looks like one parent: dominance. Looks in between: incomplete dominance. Shows both parental traits together: co-dominance.
Key idea
Dominance, incomplete dominance and co-dominance differ in how the heterozygote looks.
2. Incomplete Dominance
★ Very important Incomplete dominance: a condition in which the F1 has a phenotype that does not resemble either parent but is in between the two, because one allele is not completely dominant over the other.
- ★ Exam imp A good example is flower colour in the dog flower (snapdragon, Antirrhinum sp.).
- Cross a true-breeding red-flowered plant (RR) with a true-breeding white-flowered plant (rr).
- The F1 (Rr) is pink.
- Self-pollinate the F1.
- The F2 shows 1 (RR) red : 2 (Rr) pink : 1 (rr) white.
- The genotypic ratios were exactly as in any Mendelian monohybrid cross.
- ★ Exam imp But the phenotypic ratio changed from the 3 : 1 dominant : recessive ratio to 1 : 2 : 1.
- This happened because R was not completely dominant over r.
- So Rr could be told apart as pink from RR (red) and rr (white).
Memory Trick
Incomplete dominance: what you see is what you have. Each genotype has its own colour (red, pink, white), so the phenotypic ratio equals the genotypic ratio, 1 : 2 : 1.
Key idea
In snapdragon the heterozygote is pink, so the F2 phenotypes show the 1 : 2 : 1 genotype ratio directly.
3. What Exactly Is Dominance?
- To see why some alleles are dominant and some recessive, we must understand what a gene does.
- Every gene contains the information to express a particular trait.
- A diploid organism has two copies of each gene, a pair of alleles. The two alleles need not be identical, as in a heterozygote.
- One allele may differ because of changes it has undergone, which modify the information it contains.
- Take a gene that carries the information for an enzyme. The normal allele produces the normal enzyme needed to transform a substrate S.
- Theoretically, the modified allele could produce one of three things:
| The modified allele produces | Effect on the phenotype |
|---|---|
| (i) The normal or a less efficient enzyme | Equivalent to the unmodified allele; same phenotype (S is still transformed). Such equivalent allele pairs are very common. |
| (ii) A non-functional enzyme | Phenotype depends only on the unmodified allele; the modified allele is recessive |
| (iii) No enzyme at all | Phenotype depends only on the unmodified allele; the modified allele is recessive |
★ Very important The unmodified (functioning) allele, which represents the original phenotype, is the dominant allele. The modified allele is generally the recessive allele. A recessive trait is seen because of a non-functional enzyme, or because no enzyme is produced.
Key idea
An allele behaves as recessive when its product does not work; the working allele is dominant.
4. Co-dominance and the ABO Blood Groups
★ Very important Co-dominance: a condition in which the F1 generation resembles both parents, because both alleles express themselves fully in the heterozygote.
- ★ Exam imp A good example is the different types of red blood cells that determine ABO blood grouping in human beings.
- ABO blood groups are controlled by the gene I.
- The plasma membrane of red blood cells has sugar polymers that protrude from its surface; the kind of sugar is controlled by the gene.
- The gene I has three alleles: IA, IB and i.
- IA and IB produce slightly different forms of the sugar; i does not produce any sugar.
- Humans are diploid, so each person has any two of the three alleles.
- IA and IB are completely dominant over i: in IA i only IA expresses (as i makes no sugar), and in IB i only IB expresses.
- ★ Exam imp When IA and IB are present together, both express their own sugars: this is co-dominance. The red blood cells then carry both A and B sugars.
- Three alleles allow six different combinations, so there are six genotypes of the human ABO blood types.
Genetic basis of blood groups in the human population
| Allele from parent 1 | Allele from parent 2 | Genotype of offspring | Blood type of offspring |
|---|---|---|---|
| IA | IA | IA IA | A |
| IA | IB | IA IB | AB |
| IA | i | IA i | A |
| IB | IA | IA IB | AB |
| IB | IB | IB IB | B |
| IB | i | IB i | B |
| i | i | ii | O |
- The six genotypes give only four phenotypes: blood groups A, B, AB and O.
| Blood group | Genotypes | Sugar on red blood cells |
|---|---|---|
| A | IA IA, IA i | A sugar |
| B | IB IB, IB i | B sugar |
| AB | IA IB | Both A and B sugars |
| O | ii | No sugar |
Memory Trick
3, 6, 4: the ABO gene has 3 alleles, 6 genotypes and 4 blood groups. Only AB and O have a single genotype each; A and B have two each (homozygous and with i).
NEET Focus
One gene shows two kinds of interaction. IA and IB are co-dominant with each other, but each is completely dominant over i. Statements such as “IA is dominant over IB” or “i makes the O sugar” are false: i makes no sugar.
Key idea
In IA IB both alleles show up, so blood group AB is the classic case of co-dominance.
5. Multiple Alleles
- ABO blood grouping is also a good example of multiple alleles.
- ★ Exam imp Multiple alleles: more than two alleles (here three) governing the same character.
- An individual can carry only two alleles of a gene. So multiple alleles can be found only when population studies are made.
Tips and Tricks
Do not confuse multiple alleles (many forms of one gene, as IA, IB, i) with polygenic traits (several genes for one trait, as skin colour). One person shows at most two alleles of a gene, but a polygenic trait involves several genes in every person.
Quick Recall: tap to check
Which ABO allele produces no sugar?
The allele i.
How many genotypes and phenotypes does the ABO system have?
Six genotypes and four phenotypes (A, B, AB, O).
Why can multiple alleles be found only in population studies?
Because a diploid individual carries only two alleles of a gene.
Key idea
Three ABO alleles exist in the population, but each person carries only two of them.
6. One Gene, More Than One Effect: Starch Grains in Pea
- Occasionally, a single gene product produces more than one effect.
- Example: starch synthesis in pea seeds is controlled by one gene with two alleles, B and b.
- BB homozygotes synthesise starch effectively and so produce large starch grains.
- bb homozygotes are less efficient at starch synthesis and produce smaller starch grains.
- After the seeds mature, BB seeds are round and bb seeds are wrinkled.
- Heterozygotes (Bb) produce round seeds, so B seems to be the dominant allele.
- ★ Exam imp But the starch grains in Bb seeds are of intermediate size. If starch grain size is taken as the phenotype, the alleles show incomplete dominance.
| Genotype | Starch synthesis | Starch grain size | Seed shape |
|---|---|---|---|
| BB | Effective | Large | Round |
| Bb | - | Intermediate | Round |
| bb | Less efficient | Small | Wrinkled |
★ Very important Dominance is not an autonomous feature of a gene or of the product it codes for. It depends as much on the gene product and how it produces a phenotype as on the particular phenotype we choose to examine, when the same gene influences more than one phenotype.
Key idea
The same B/b pair is dominant for seed shape but incompletely dominant for starch grain size.
7. Polygenic Inheritance
- Mendel's studies mainly described traits with distinct alternate forms, such as flower colour, which is either violet or white.
- Many traits are not so distinct; they are spread across a gradient.
- Example: humans are not just tall or short; there is a whole range of possible heights.
- ★ Exam imp Such traits are generally controlled by three or more genes, and are called polygenic traits.
- Besides several genes, polygenic inheritance also takes into account the influence of the environment.
- Human skin colour is another classic example.
- ★ Exam imp In a polygenic trait, the phenotype reflects the contribution of each allele: the effect of each allele is additive.
7.1 A model of skin colour
- Assume that three genes, A, B and C, control skin colour in humans.
- The dominant forms A, B and C are responsible for dark skin colour, and the recessive forms a, b and c for light skin colour.
- The genotype with all dominant alleles (AABBCC) has the darkest skin colour.
- The genotype with all recessive alleles (aabbcc) has the lightest skin colour.
- A genotype with three dominant and three recessive alleles has an intermediate skin colour.
- So the number of each type of allele in the genotype decides how dark or light the skin of an individual is.
Extra Depth: In this three-gene model, a cross AaBbCc × AaBbCc gives seven classes of offspring with 0 to 6 dominant alleles, in the ratio 1 : 6 : 15 : 20 : 15 : 6 : 1 (out of 64). The middle class, with three dominant alleles, is the most common, which is why most people fall near the average.
Memory Trick
Count the capitals. In the skin-colour model, only the number of capital letters matters: AABbcc, AaBbCc and aaBBCc each have three, so all have the same intermediate colour.
Key idea
Polygenic traits vary in small steps because several genes add their effects, and the environment adds more variation.
8. Pleiotropy
- So far we have seen the effect of a gene on a single phenotype or trait.
- ★ Exam imp Some genes show multiple phenotypic expression. Such a gene is called a pleiotropic gene.
- In most cases, the underlying mechanism is the effect of a gene on metabolic pathways that contribute to different phenotypes.
- Example: the disease phenylketonuria in humans.
- It is caused by a mutation in the gene that codes for the enzyme phenylalanine hydroxylase (a single gene mutation).
- It shows up as several phenotypes: mental retardation and a reduction in hair and skin pigmentation.
Polygenic inheritance
- Several genes, one trait
- Effects of alleles are additive
- Environment also acts
- Example: human height, skin colour
Pleiotropy
- One gene, several traits
- Gene acts on metabolic pathways
- A single gene mutation
- Example: phenylketonuria
Memory Trick
Poly-GENIC = many GENES; pleio-TROPY = many effects. Many genes make one trait (skin colour); one gene makes many effects (phenylketonuria).
Quick Recall: tap to check
What is the phenotype of Rr in snapdragon?
Pink, in between red (RR) and white (rr).
Name the three possible products of a modified allele.
A normal or less efficient enzyme, a non-functional enzyme, or no enzyme at all.
Which enzyme is coded by the gene mutated in phenylketonuria?
Phenylalanine hydroxylase.
Key idea
Polygenic inheritance and pleiotropy are opposites: many genes for one trait versus one gene for many traits.
9. Exam Essentials
Pairs to Match
| Example | Matches with |
|---|---|
| Snapdragon (dog flower) flower colour | Incomplete dominance |
| Antirrhinum sp. | Snapdragon, the dog flower |
| Pink Rr snapdragon | Heterozygote of incomplete dominance |
| Blood group AB (IA IB) | Co-dominance |
| IA i gives blood group A | Complete dominance of IA over i |
| Allele i | Produces no sugar |
| ABO blood group gene I | Multiple alleles |
| Pea seed shape (B/b) | Dominance (round over wrinkled) |
| Pea starch grain size (Bb) | Incomplete dominance (intermediate grains) |
| Human skin colour | Polygenic inheritance |
| Human height | Polygenic inheritance |
| AABBCC | Darkest skin colour |
| Phenylketonuria | Pleiotropy |
| Phenylalanine hydroxylase | Enzyme coded by the gene mutated in phenylketonuria |
Exceptions
- In incomplete dominance the F2 phenotypic ratio is not 3 : 1; it is 1 : 2 : 1.
- IA and IB are dominant over i, but not over each other.
- The allele i produces no sugar.
- Multiple alleles cannot be studied in one individual, which has only two alleles; only populations show them.
- Polygenic traits depend on the environment as well, unlike Mendel's distinct pea traits.
- The pea starch gene is dominant for one phenotype but not for another: dominance is not autonomous.
Numbers to Remember
- Snapdragon F2: 1 : 2 : 1 for both phenotype and genotype.
- ABO: 3 alleles, 6 genotypes, 4 phenotypes; each person carries 2 alleles.
- A modified allele can have 3 outcomes: normal or less efficient enzyme, non-functional enzyme, no enzyme.
- Polygenic traits: generally three or more genes; skin-colour model uses 3 genes (6 alleles).
NEET Focus
For a blood-group cross, write the parents' genotypes first. Parents of groups A and B can have children of all four groups, but only if both are heterozygous (IA i × IB i). A child of group O (ii) proves that each parent carries i.
10. Quick Revision
- Incomplete dominance: F1 in between; snapdragon red × white gives pink.
- Snapdragon F2: 1 red (RR) : 2 pink (Rr) : 1 white (rr); genotypes as in Mendel, phenotypes changed.
- Each gene carries information for a trait; a diploid has two alleles.
- Modified allele: normal or less efficient enzyme (equivalent), non-functional enzyme or no enzyme (recessive).
- The functioning allele is dominant; the recessive trait shows because of a non-functional enzyme or no enzyme.
- Co-dominance: F1 resembles both parents; ABO blood groups.
- Gene I: IA, IB make slightly different sugars; i makes none.
- IA, IB completely dominant over i; IA IB has both sugars.
- Six genotypes, four blood groups: A, B, AB, O.
- Multiple alleles: more than two alleles of a gene; seen only in populations.
- Pea starch gene: BB large grains, round; bb small grains, wrinkled; Bb round with intermediate grains.
- Dominance depends on the gene product and the phenotype examined.
- Polygenic: three or more genes, additive effects, environment; height and skin colour.
- Skin model: AABBCC darkest, aabbcc lightest, three dominant alleles intermediate.
- Pleiotropy: one gene, many effects; phenylketonuria (phenylalanine hydroxylase).
11. Solved Examples
Solved Example 1
Match List I with List II.
List I: A. Snapdragon flower colour, B. ABO blood group AB, C. Human skin colour, D. Phenylketonuria
List II: I. Pleiotropy, II. Polygenic inheritance, III. Incomplete dominance, IV. Co-dominance
Choose the correct answer:
(A) A-III, B-IV, C-II, D-I
(B) A-IV, B-III, C-II, D-I
(C) A-III, B-IV, C-I, D-II
(D) A-II, B-IV, C-III, D-I
List I: A. Snapdragon flower colour, B. ABO blood group AB, C. Human skin colour, D. Phenylketonuria
List II: I. Pleiotropy, II. Polygenic inheritance, III. Incomplete dominance, IV. Co-dominance
Choose the correct answer:
(A) A-III, B-IV, C-II, D-I
(B) A-IV, B-III, C-II, D-I
(C) A-III, B-IV, C-I, D-II
(D) A-II, B-IV, C-III, D-I
Solution:
Answer: (A). Snapdragon shows incomplete dominance (III), AB shows co-dominance (IV), skin colour is polygenic (II), and phenylketonuria shows pleiotropy (I).
Solved Example 2
Read the statements about the ABO blood groups.
A. The gene I has three alleles.
B. The allele i produces a third type of sugar.
C. IA and IB are completely dominant over i.
D. There are six genotypes and four phenotypes.
E. Every person carries all three alleles.
Choose the correct answer:
(A) A, C and D only
(B) A, B and D only
(C) B, C and E only
(D) A, C, D and E only
A. The gene I has three alleles.
B. The allele i produces a third type of sugar.
C. IA and IB are completely dominant over i.
D. There are six genotypes and four phenotypes.
E. Every person carries all three alleles.
Choose the correct answer:
(A) A, C and D only
(B) A, B and D only
(C) B, C and E only
(D) A, C, D and E only
Solution:
Answer: (A). B is wrong: i produces no sugar. E is wrong: a diploid person carries only two of the three alleles. A, C and D are correct.
Solved Example 3
Arrange the steps of Mendel-style analysis of snapdragon flower colour in order.
A. Self-pollinate the pink plants
B. Cross true-breeding red and white plants
C. Obtain red, pink and white plants in a 1 : 2 : 1 ratio
D. Obtain all-pink F1 plants
Choose the correct answer:
(A) B, D, A, C
(B) B, A, D, C
(C) D, B, A, C
(D) B, D, C, A
A. Self-pollinate the pink plants
B. Cross true-breeding red and white plants
C. Obtain red, pink and white plants in a 1 : 2 : 1 ratio
D. Obtain all-pink F1 plants
Choose the correct answer:
(A) B, D, A, C
(B) B, A, D, C
(C) D, B, A, C
(D) B, D, C, A
Solution:
Answer: (A). Red × white (B) gives pink F1 (D); selfing them (A) gives the 1 : 2 : 1 F2 (C).
Solved Example 4
A man of blood group A and a woman of blood group B have a child of blood group O. What is the probability that their next child has blood group AB?
(A) 0
(B) 1/4
(C) 1/2
(D) 3/4
(A) 0
(B) 1/4
(C) 1/2
(D) 3/4
Solution:
Answer: (B). The O child is ii, so the parents are IA i and IB i. Their children are IA IB, IA i, IB i and ii in equal proportion, so the chance of AB is 1/4.
Solved Example 5
Which of the following is NOT correct about the starch-synthesis gene in pea?
(A) BB seeds have large starch grains
(B) bb seeds are wrinkled
(C) Bb seeds are wrinkled because their grains are intermediate
(D) For starch grain size, B and b show incomplete dominance
(A) BB seeds have large starch grains
(B) bb seeds are wrinkled
(C) Bb seeds are wrinkled because their grains are intermediate
(D) For starch grain size, B and b show incomplete dominance
Solution:
Answer: (C). Bb seeds are round, so B appears dominant for seed shape; only the grain size is intermediate.
Solved Example 6
Statement I: Polygenic traits are generally controlled by three or more genes.
Statement II: In a polygenic trait, the effect of each allele is additive.
(A) Both Statement I and Statement II are correct
(B) Statement I is correct but Statement II is incorrect
(C) Statement I is incorrect but Statement II is correct
(D) Both Statement I and Statement II are incorrect
Statement II: In a polygenic trait, the effect of each allele is additive.
(A) Both Statement I and Statement II are correct
(B) Statement I is correct but Statement II is incorrect
(C) Statement I is incorrect but Statement II is correct
(D) Both Statement I and Statement II are incorrect
Solution:
Answer: (A). Both are correct; the environment also influences polygenic traits.
12. Practice Questions
Practice Questions
- Match List I with List II.
List I: A. IA IA, B. IA IB, C. IB i, D. ii
List II: I. Blood group O, II. Blood group B, III. Blood group AB, IV. Blood group A
(A) A-IV, B-III, C-II, D-I (B) A-III, B-IV, C-II, D-I (C) A-IV, B-II, C-III, D-I (D) A-IV, B-III, C-I, D-IIAnswer: (A). IA IA is A, IA IB is AB, IB i is B and ii is O. - Read the statements.
A. In snapdragon, the F1 of red × white is pink.
B. In snapdragon, the F2 phenotypic ratio is 3 : 1.
C. A recessive trait can be due to a non-functional enzyme.
D. Equivalent allele pairs are rare.
Choose the correct answer: (A) A and C only (B) A, B and C only (C) B and D only (D) A, C and D onlyAnswer: (A). B is wrong (it is 1 : 2 : 1); D is wrong (equivalent allele pairs are very common). - Which of the following shows pleiotropy? (A) ABO blood groups (B) Human skin colour (C) Phenylketonuria (D) Snapdragon flower colourAnswer: (C). A single gene mutation causes mental retardation and reduced hair and skin pigmentation.
- Which of the following is NOT a polygenic trait in humans? (A) Height (B) Skin colour (C) ABO blood group (D) All of these are polygenicAnswer: (C). ABO blood group is controlled by one gene with multiple alleles.
- In the three-gene model of skin colour, which genotype is darker: AaBbCc or AABbCC?Answer: AABbCC, because it has five dominant alleles against three, and the effects are additive.
- Statement I: Dominance is an autonomous feature of a gene.
Statement II: The pea starch gene is dominant for seed shape but incompletely dominant for starch grain size.
(A) Both correct (B) I correct, II incorrect (C) I incorrect, II correct (D) Both incorrectAnswer: (C). Dominance is not autonomous; Statement II is the example that shows this. - A child has blood group O. If the father has blood group A and the mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offspring.Answer: Father IA i, mother IB i. Possible offspring: IA IB (AB), IA i (A), IB i (B) and ii (O), each with a probability of 1/4.
- Explain the following terms with an example: (a) co-dominance, (b) incomplete dominance.Answer: (a) Co-dominance: both alleles of the heterozygote express, so the F1 resembles both parents; IA IB red blood cells carry both A and B sugars (blood group AB). (b) Incomplete dominance: the F1 is intermediate; snapdragon red RR × white rr gives pink Rr, and the F2 is 1 red : 2 pink : 1 white.
Common Mistakes to Avoid
Watch out
- Writing 3 : 1 for the snapdragon F2. Correct: incomplete dominance gives 1 red : 2 pink : 1 white.
- Calling blood group AB an example of incomplete dominance. Correct: AB shows co-dominance; both sugars are present.
- Saying i makes the “O sugar”. Correct: i makes no sugar at all.
- Saying IA is dominant over IB. Correct: they are co-dominant; each is dominant only over i.
- Counting four genotypes for four blood groups. Correct: there are six genotypes and four phenotypes.
- Confusing multiple alleles with polygenic inheritance. Correct: multiple alleles are many forms of one gene; polygenic traits involve several genes.
- Calling phenylketonuria polygenic. Correct: it is a single gene mutation with several effects, an example of pleiotropy.
- Saying Bb pea seeds are wrinkled. Correct: they are round; only their starch grains are of intermediate size.
Frequently Asked Questions
What is the difference between incomplete dominance and co-dominance?
In incomplete dominance the heterozygote is in between the parents, as pink Rr snapdragons from red and white parents. In co-dominance the heterozygote shows both parental traits fully, as IA IB red blood cells that carry both A and B sugars.
Why is the F2 ratio 1 : 2 : 1 in snapdragon?
The red allele R is not completely dominant over r. So the heterozygote Rr is pink and can be told apart from red RR and white rr. Each genotype has its own phenotype, and the phenotypic ratio equals the genotypic ratio, 1 : 2 : 1.
Why are some alleles dominant and others recessive?
A gene often codes for an enzyme. A modified allele may make a non-functional enzyme or no enzyme, so the phenotype depends only on the working allele. The working (unmodified) allele is then dominant and the modified allele recessive. If the modified allele still makes a working enzyme, both alleles are equivalent.
How many genotypes and phenotypes are there in the ABO blood group system?
The gene I has three alleles, IA, IB and i. They form six genotypes: IA IA, IA i, IB IB, IB i, IA IB and ii. These give four phenotypes, the blood groups A, B, AB and O.
Why is the ABO blood group system an example of multiple alleles?
More than two alleles of the same gene, IA, IB and i, govern the same character. A diploid person carries only two of them, so the three alleles are seen only when a population is studied, not in one individual.
How can one gene be dominant for one trait and incompletely dominant for another?
In pea, the starch gene B makes Bb seeds round, so it looks dominant for seed shape. But Bb seeds have starch grains of intermediate size, so for grain size the alleles show incomplete dominance. Dominance depends on which phenotype is examined.
What is polygenic inheritance?
It is the inheritance of traits controlled by three or more genes, such as human height and skin colour. The effect of each allele is additive, and the environment also acts, so these traits vary in small steps across a range instead of falling into two distinct classes.
What is pleiotropy? Give an example.
Pleiotropy is the expression of several phenotypes by a single gene, usually because the gene affects a metabolic pathway. In phenylketonuria, a mutation in the gene for phenylalanine hydroxylase causes mental retardation as well as reduced hair and skin pigmentation.
Previous year questions on Interactions of Genes
6 questions from past papers, each with a step-by-step solution.
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