NCERT Solutions Class 12 Biology Chapter 5

NCERT Solutions Class 12 Biology Chapter 5

The study of life is known as biology. Biologists investigate the structure, function, growth, origin, evolution, and distribution of living creatures. Biologists have used advances in biology to help develop better  medicines and treatments, understand how climate change affects plants and animals, produce enough food for a growing human population, and predict how eating new foods or sticking to an exercise regimen will affect our bodies.

NCERT Class 12 Biology Chapter 5:Principles of Inheritance and Variations.

The Fundamentals of Iinheritance and Vvariations provides an overview of Genetics, a field of Biology. Understanding the genetic and the structural basis of genotype and phenotype material became the focus of study. and  It describes   how genes get handed down from parents to children, and why do siblings look alike? This chapter answers all of these  questions.  This chapter also covers Mendel’s pea plant studies, the inheritance of one gene, the inheritance of two genes, sex determination, mutation, and genetic diseases.

Class 12 students are advised to go through NCERT Solutions for board examination preparation.  Extramarks has  with Chapter 5 Biology Class 12 NCERT Solutions. Prepared after thorough research, these Biology Class 12 Chapter 5 NCERT Solutions meet all the requirements of the students.

Students must go through NCERT books first.It’s not necessary to opt for coaching classes. When you study from NCERT Solutions for Class 12 Science Biology Chapter 5 diligently, you will be able to understand every concept and answer any question easily. This encourages the students to master the topic and increases their confidence in achieving a higher grade. 

Apart from Chapter 5 Class 12 Biology NCERT Solutions, students can use the Extramarks website to access  other study materials such as . NCERT books, CBSE revision notes, CBSE sample papers, CBSE  past years’ question papers, and so on. 

Key Topics Covered in NCERT Solutions Class 12 Biology Chapter 5

To make it convenient for the students, Extramarks has listed  key topics that are covered in NCERT Solutions Class 12 Biology Chapter 5:

Mendel’s Laws of Inheritance 
Inheritance of One Gene 

Inheritance of Two Genes

Sex Determination
Genetic Disorders

Let us look at Extramarks’ in-depth information on each subtopic in NCERT Solutions Class 12 Biology Chapter 5- Principles of Inheritance and Variations.

What is Hereditary?

NCERT Solutions Class 12 Biology Chapter 5 explains that Heredity is how genes are passed down from generation to generation through sexual reproduction. Various genes and inheritable characteristics are handed down to ensure that the kids produced are better suited to the changing environment. The traits are encoded in the form of genes on the chromosomes. Gregor Johann Mendel is known as “the father of genetics.”

Mendel’s Law of Inheritance 

NCERT Solutions Class 12 Biology Chapter 5 says that after researching pea plants, Gregor Mendel established the fundamental inheritance laws. First, he offered three inheritance rules, which we are now studying. Then, he picked 14 true-breeding pea plant kinds with seven opposing features of specific attributes and conducted his experiment on them.

Mendel’s Laws

NCERT Solutions Class 12 Biology Chapter 5 explains that Mendel postulated the following three laws:

  • Law of Dominance- This law states that the unit known as the factor is in charge of all qualities or characters. Alleles refer to the fact that these factors are found in pairs. If the alleles are located in the same pair, they are termed homozygous, and they can be dominant or recessive. If the alleles are found in separate pairs, they are called heterozygous, and they will always be dominant.
  • Law of Segregation of Genes- The law of segregation is based on alleles that do not blend, and both traits are recovered as such in the second filial generation, even though one of them was not observed in the first. The segregation of factors or alleles happens such that each gamete receives just one of the two factors.
  • Law of Independent Assortment asserts that while transmitted from one generation to the next, pairings of qualities in the parental generation sort independently of one another. A dihybrid cross is used to demonstrate it.

Inheritance of one Gene:

A monohybrid cross can be used to explain the inheritance of one gene using Mendel’s law. The tall and dwarf plants are crossed in this experiment, resulting in all tall hybrid plants in the F1 (First Filial or First) generation. The progenies are then self-pollinated, resulting in forming an F2 era with three tall plants and one dwarf plant. As a result, the proportion will be 3:1.

Incomplete Dominance:

Incomplete dominance is a kind of inheritance in which one allele for a particular characteristic is not dominant over the other allele, i.e., in heterozygous organisms, neither allele is dominant over the other. As a result, a mixed phenotype emerges. Mosaic or partial dominance are different terms for incomplete dominance. New phenotypic characteristics are fully manifested here.

Multiple Allelism or Codominance:

Extramarks NCERT Solutions Class 12 Biology Chapter 5 states that Multiple Allelism is a situation in which three or more alternate kinds of alleles exist for the same gene on the same chromosome, and the alleles are known as multiple alleles.

Multiple allelism, for example, is better understood in humans because of the ABO blood type system. The ABO blood group is inherited through the gene I (isohemagglutinin), which exists in three allelic manifestations in humans: IA, IB, and i. Any two of these alleles can be found in a single person. Blood type A is controlled by gene IA, which codes for glycoprotein A, while blood group B is controlled by gene IB, which codes for glycoprotein B.

Because the gene I does not create any glycoprotein, a person with these two alleles in a homozygous form will have O group blood. The genes IA and IB are dominant over I, but alleles IA and IB are equally prevalent and generate both glycoproteins A and B at the same time, giving rise to the blood type AB. These alleles are referred to as codominant alleles.

Inheritance of two Genes:

Two features of the same trait are required to inherit two genes. A dihybrid cross can be used to demonstrate this. To describe the inheritance of two genes, Mendel picked two features that affect the seed’s colour and form. R denotes the round form of the seed, and r represents the wrinkled shape of the seed. Y represents the dominant yellow colour seed colour, y represents a recessive green colour, and y represents a recessive green colour. The parents’ genotypes are then indicated as RRYY and rryy. After fertilisation, the gametes RY and ry will combine to form the F1 hybrid RrYy. The Law of Independent Assortment may also be studied using the dihybrid cross.

Chromosomal Theory of Inheritance

Walter Sutton proposed the chromosomal hypothesis of heredity in 1902. This hypothesis also explains the linear shape of chromosomes with genes in specific locations called loci, which Boveri investigated separately. The Boveri-Sutton chromosome theory is another name for this notion. According to this theory:

  • First, genes are located on chromosomes at specific sites.
  • Second, the homologous chromosomes split during meiosis.
  • Third, the number of chromosomes becomes diploid after conception.
  • Finally, chromosomes segregate and assort on their own.

Sex Determination

NCERT Solutions Class 12 Biology Chapter 5 explains the procedure of determining a child’s gender is known as sex determination. Sex chromosomes are responsible for selecting a child’s gender. In humans, females have XX chromosomes, whereas males have one X and one Y type. As a result, each egg (female gamete) will contain an identical X-chromosome, whereas (male gametes) will have one X-chromosome and one Y-chromosome. So which sperm unites with the egg is a question of luck (X or Y). In addition, females are homogametic (chromosomes of the same kind), whereas males are heterogametic (chromosomes of different types) (other types of chromosomes).

In insects, the XO type of sex determination mechanism is used. The X chromosomes are present in the females, but the males may contain one or no X chromosomes. Males are homogametic (chromosomes of the same kind), whereas females are heterogametic (chromosomes of different sorts) (different types of chromosomes).


The concept of Mutations has been explained in the following section of NCERT Solutions Class 12 Biology Chapter 5.

Mutation is defined as a change in the genetic material. . Changes in DNA may be heritable and handed down to future generations, impacting both an individual’s genotype and phenotype. Examples are frameshift mutations, insertions, deletions, duplications, substitutions, and other mutations. Mutations might be damaging or have no impact.

  • Frameshift mutations occur when added or deleted DNA bases, causes alterations in the reading frame.
  • Insertions are the addition of new DNA bases.
  • Deletions are the removal of DNA bases.
  • Duplication occurs when a portion of DNA is duplicated more than once.

As a result of these mutations, the DNA sequence will alter, leading to the creation of the incorrect protein.

Genetic Disorders

Pedigree Analysis

A pedigree chart depicts the incidence and emergence of specific phenotypes of a particular gene and organism. Consequently, the family information is presented in an easily legible chart.

NCERT Solutions Class 12 Biology Chapter 5 describes the two types of genetic disorders :

Mendelian Disorders- Mendelian diseases are caused by a single gene mutation or change. Haemophilia, Sickle-cell anaemia, Cystic fibrosis, Colour blindness, Thalassemia, Phenylketonuria, and other disorders are among the most frequent. Mendelian diseases are classified as dominant or recessive. The attribute has also been connected to conditions caused by sex chromosomes, such as haemophilia and colour blindness.

Chromosomal Disorders- Chromosomal disorders are caused by adding or removing one or more chromosomes or by their aberrant organisation. The faulty segregation of chromatids during the cell division cycle results in aneuploidy, characterised by the addition or deletion of chromosomes. For example, an additional copy of chromosome 21 is found in Down’s syndrome patients. The omission of one X chromosome characterises Turner’s syndrome in human females. The other circumstance is polyploidy, which occurs when the cytokinesis process is absent after the telophase stage of cell division, resulting in an increase in a complete set of chromosomes in an organism, most commonly seen in plants.

NCERT Solutions Class 12 Biology Chapter 5 Exercise and Solutions

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By getting access to NCERT Solutions Class 12 Biology Chapter 5, students can easily understand all the concepts related  to Inheritance and Variation.

Key Features of NCERT Solutions Class 12 Biology Chapter 5 

NCERT Solutions are a golden feature of Extramarks. These Solutions provide students with a better understanding of the chapter and help them achieve their goals. NCERT Solutions are often recommended to students, and hence, there are plenty available on google. But why choose Extramarks? Here’s why:

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  • These Solutions have been prepared to keep in mind the NCERT guidelines.

Q.1 Mention the advantages of selecting pea plant for experiment by Mendel.


Mendel chose the pea plants to carry out his genetics experiment because of the following features:

  1. Pea plants have naturally occurring visible contrasting traits such as purple/white flowers, tall/dwarf plants, yellow/green pods etc.
  2. Peas are self-pollinating plants i.e. pollen from a flower fertilises the eggs of the same flower giving rise to purebred lines having the same trait generations after generations.
  3. Pea plants can be easily cross-pollinated by emasculation whereby, the stamen of the plant is removed and the pistil is dusted with the stamen of the desired parent.
  4. The life cycle of pea plants is short and they produce many seeds in one generation enabling the statistical analysis of the result.

Q.2 Differentiate between the following –
(a) Dominance and Recessive
(b) Homozygous and Heterozygous
(c) Monohybrid and Dihybrid


(a) A dominant factor or allele expresses itself in the presence or absence of the recessive allele. For example in a pea plant tall, violet flowers, green pod etc. are dominant factors. A recessive factor or allele cannot express itself in the presence of the dominant character. For example in a pea plant dwarf, white flowers, yellow pods are recessive factors.

(b) A homozygous genotype is the one in which both the alleles for a trait are similar. The genotype can be homozygous for a dominant (TT) or a recessive (tt) trait. All gametes produced by the homozygous organism will carry the same allele. A heterozygous genotype is one in which the two alleles for a trait are different. This type of genotype will have both dominant and recessive allele for example Tt. The gametes produced will be of two kinds one will only carry the dominant allele and one will carry the recessive allele.

(c) A monohybrid cross is a cross between two pure breed parents that differ in only one pair of contrasting character. For example a cross between tall (TT) and dwarf (tt) pea plants. A dihybrid cross is a cross between two parents that differ in two pairs of contrasting characters. For example a cross between pea plants having round yellow (RRYY) and wrinkled green (rryy) seeds.

Q.3 A diploid organism is heterozygous for 4 loci, how many types of gametes can be produced?


Loci means the position on the chromosome at which a gene lies. If an organism is heterozygous for 4 loci it means that for four genes the organism has dissimilar alleles. During meiosis the alleles will segregate in 16 possible ways to form 16 different types of gametes given that the genes are not linked. If the genes are linked then the outcome could vary as two or more alleles could co-segregate.

Explanation: To calculate the number of possible gametes when an organism is heterozygous for a locus the formula is as follows:

Number of heterozygous genes in the organism Example Number of types of gametes
1 heterozygous gene Aa 2^1 = 2
2 heterozygous genes AaBb 2^2 = 4
3 heterozygous genes AaBbCc 2^3 = 8
4 heterozygous genes AaBbCcDd 2^4 = 16

*The above holds true for genes that are not linked and segregate independently.

Q.4 Explain the Law of Dominance using a monohybrid cross.


According to Mendel’s Law of Dominance, a physical trait is controlled by a pair of factors (also called alleles). When the factors are dissimilar one factor dominates (dominant factor) the other (recessive factor). But the recessive factor is not lost and appears in the second generation.

This law is explained by a monohybrid cross which is a cross of two parents that are pure for one contrasting trait, for example, tall (TT) and dwarf (tt). The F1 generation yields all the progeny with a heterozygous genotype Tt that express the dominant trait – Tall. The F2 generation which is a self cross (TtxTt) yields progeny with a phenotypic ratio of 3:1 ::: tall:short and a genotypic ratio of TT:Tt:tt::1:2:1. The recessive trait (dwarf plant) appears in the F2 generation which was masked by the dominant trait (tall plant) in the F1 generation.

Parents: TT (tall) X tt (dwarf)

F1 generation

Gametes T T
t Tt Tt
t Tt Tt


Genotype – Tt

Phenotype – Tall

Parents: Tt (tall) X Tt (tall)

F2 Generation

Gametes T t
t Tt tt

Genotype – TT:Tt:tt::1:2:1

Phenotype – 3:1:: tall:short

Q.5 Define and design a test-cross.


A test cross is when an organism exhibiting the dominant phenotype is crossed with the homozygous recessive parent to determine the genotype of the former by analyzing proportions of offspring displaying the recessive phenotype. If all offspring display the dominant phenotype, the individual in question is homozygous dominant; if the offspring displays both dominant and recessive phenotypes, then the individual is heterozygous.

Test cross design:

Plants with violet flowers (dominant trait; unknown genotype WW/Ww) were crossed with a purebred line yielding white flowers. The outcome of the experiment determines the genotype of the parent with the dominant trait. If all offspring yield violet flowers the parent with the dominant trait would be homozygous (WW) whereas, if the offspring yield both violet and white flowers 50% of the times the parent with the dominant trait would be heterozygous (Ww).

Parents: Violet (unknown parent could be WW/Ww) X White (ww)










Phenotype – 100% flowers Purple

Inferred parental genotype – WW

Offspring Genotype – Ww

Parents: Violet (unknown parent could be WW/Ww) X White (ww)










Phenotype – 50% flowers purple, 50% flowers white

Inferred parental genotype – Ww

Offspring Genotype – 50% Ww, 50% ww

Q.6 Using a Punnett Square, workout the distribution of phenotypic features in the first filial generation after a cross between a homozygous female and a heterozygous male for a single locus.


Let us assume that the man is heterozygous for the blood group antigen AB and the female is homozygous BB. The first filial generation of the two parents will have the following distribution:

Parents: Female (BB) X Male (AB)











Progeny Genotype – AB:BB::1:1

Progeny Phenotype – AB:B::1:1

Therefore in this situation, the phenotypic and genotypic ratio of the progeny will be the same. Also, the genotype of the female and male will be equally expressed.

Q.7 When a cross is made between tall plant with yellow seeds (TtYy) and tall plant with green seed (Ttyy), what proportions of phenotype in the offspring could be expected to be
(a) Tall and green
(b) Dwarf and green


A dihybrid cross between two parents differing in two pairs of contrasting traits: Plant height and seed colour was made using a Punnet square.

Parent: TtYy X Ttyy

Gametes TY Ty tY ty

(tall, yellow)


(tall, green)


(tall, yellow)


(tall, green)

ty TtYy

(tall, yellow)


(tall, green)


(dwarf, yellow)


(dwarf, green)

Number of offspring with phenotype

(a) Tall and Green – 3

(b) Dwarf and Green – 1

Explanation: This phenomenon is based on Law of Independent assortment coined by Mendel in which he states that when two pairs of traits are combined in a hybrid, segregation of one pair of characters is independent of the other pair of characters.

Q.8 Two heterozygous parents are crossed. If the two loci are linked what would be the distribution of phenotypic features in F1 generation for a dibybrid cross?


When two genes are linked they do not follow the expected ratio for a dihybrid cross between heterozygous parents as seen in Mendel’s crosses (9:3:3:1). Instead, the phenotype ratio will be like that of a monohybrid cross if the two genes are very tightly linked because they will be inherited together. Recombinant phenotypes may also appear in low numbers or varying numbers depending on the distance/extent of linkage between the two loci.

Q.9 Briefly mention the contribution of T.H. Morgan in genetics.


Morgan formulated the theory of inheritance of chromosomes. He performed several dihybrid crosses in Drosophila to demonstrate linkage and recombination (the two terms coined by him) of genes on the X chromosome. He saw that some pair of contrasting characters did not segregate in the ratio of 9:3:3:1 (the expected outcome when the two genes are independent). Morgan attributed this to the genes being physically linked in such a way that they didn’t segregate independently of each other. Recombination was the event that was used to describe the generation of non-parental gene combinations. He demonstrated that tightly linked genes showed very low recombination while others that were loosely linked showed a higher rate of recombination. Morgan won the Nobel Prize in Physiology or Medicine in 1933 for his contribution to genetics.

Q.10 What is pedigree analysis? Suggest how such an analysis can be useful.


Pedigree analysis is a type of genetic analysis that analyses the pattern of inheritance of a particular trait, disease or abnormality. This type of analysis is made in several generations of a family using special symbols and lines.

Uses of pedigree analysis:

  1. Can help to determine if the trait is dominant or recessive
  2. Whether the trait is linked to the sex chromosome

Predict and trace the pattern of inheritance of single mutation disorder like Haemophilia, sickle cell anaemia etc so that parents who are carrying the mutated genes or belong to families afflicted by these disorders can make an informed decision about their progeny.

Q.11 How is sex determined in human beings?


Sex determination in humans is associated with sex chromosomes that are different between male and female individuals. Normal human females have two sex chromosomes – XX. The normal human male has 2 sex chromosomes – XY. The males produce two different types of gametes one having the X chromosome and one with the Y chromosome, while all-female gametes have an X chromosome. The sex of the baby is determined by whether the fertilising sperm contains the X or the Y chromosome. There is an equal probability of having a girl or a boy for each fertilisation event.

Parent: Male (XY) X Female (XX)










Progeny Genotype – XX:XY::1:1

Q.12 A child has blood group O. If the father has blood group A and mother blood group B, work out the genotypes of the parents and the possible genotypes of the other offspring.


The child of a set of parents that have blood group A and B can only be of blood group O if the genotype of the parents is heterozygous having one allele as the gene “i” that expresses neither antigen A or B. Therefore the genotypes of the parents and the offspring will be:

Genotype Phenotype
Father IAi A
Mother IBi B
Offspring IA IB , IA I, IB I, ii AB, A, B, O

The following Punnet cross explains how the various genotypes and phenotypes are derived:

Parent: Father (IAi) X Mother (IBi)

Gametes IA i
i IAi ii

Progeny Genotype – IA IB: IA i: IB i:ii::1:1:1:1

Progeny Phenotype – AB:A:B:O:: 1:1:1:1


Human blood type is determined by co-dominant alleles. It means that in a heterozygous situation, both alleles are expressed. In the case of blood type, there are three different alleles that result in variable expression of antigens on the RBCs.

Allele Type Blood Group Antigen expressed on RBC
IA A Antigen A
IB B Antigen B
i O No Antigen (neither A nor B)

Each of us has two ABO alleles, one from each parent. A pair of allele is called the genotype for that trait. Since there are three alleles 6 different genotypes are possible at the ABO genetic locus.

Allele from
Parent 1
Allele from
Parent 2
Genotype of
Blood types of

*Two recombinations result in AB genotype resulting in 6 different genotypes.

Q.13 Explain the following terms with example
(a) Co-dominance
(b) Incomplete dominance


(a) Co-dominance is a genetic phenomenon where the F1 generation resembles both parents i.e. the two alleles of a gene are equally dominant and both are expressed in a heterozygous condition. ABO blood group is an example of co-dominance. The ABO blood group is controlled by gene I which has three alleles IA, IB and i. IA and IB produce two different types of antigens on the RBC while the I allele doesn’t produce any antigen. In an individual, with the genotype IAIB both the A and B antigens are expressed on the RBC because of co-dominance.

(b) Incomplete dominance refers to a phenomenon whereby one allele does not completely dominate another allele, and therefore the progeny resembles neither of the parents resulting in a new phenotype which is a mixture of parental phenotypes. For example, a cross between the purebred red (RR) and white (rr) flowered plants of snapdragon (Antirrhinum sp.). The F1 generation (Rr) of such a cross yields 100% pink flowers as opposed to the expected red colour. This is because the RR genotype is partially dominant in the recessive trait of white flowers. Therefore, the white pigment of the flowers is also expressed resulting in pink colour flowers.

Parents: Red (RR) X White (rr)

Gametes R R
r Rr Rr
r Rr Rr

Phenotype – 100% flowers Pink

Genotype – 100% Rr (Red trait shows incomplete dominance)

Parents: Pink (Rr) X Pink (Rr)

Gametes R r
r Rr rr

Phenotype – 1:2:1::Red:Pink:White

Genotype – 1:2:1::RR:Rr:rr

Q.14 What is point mutation? Give one example.


A gene mutation involving the substitution, addition, or deletion of a single nucleotide base is called a point mutation.

Sickle-cell anaemia is caused by the substitution of glutamic acid by valine at the sixth position of the beta-globin chain of the haemoglobin molecule. This substitution is caused due to a point mutation that changes the nucleotide A to T in the coding strand of the DNA.

Sequence for Normal Haemoglobin (HbA gene)

START Val His Leu Thr Pro Glu Glu Lys Ser Ala Val Thr

Sequence for Sickle Cell Haemoglobin (HbS gene)

START Val His Leu Thr Pro Val Glu Lys Ser Ala Val Thr


Point mutations can have one of three outcomes.

Type of point mutation Outcome
Silent Mutation Altered codon corresponds to the same amino acid
Missense Mutation Altered codon corresponds to a different amino acid
Nonsense Mutation Altered codon corresponds to a stop signal

Point mutations may arise from spontaneous mutations that occur during DNA replication. The rate of mutation may be increased by mutagens which are physical, chemical or biological agents that increase the frequency of mutations above the normal level. Mutagens associated with cancers are often studied to learn about cancer and its prevention.

Sickle-cell anaemia is an autosome linked recessive trait that is transmitted from parents to offspring when both parents are carriers of the point mutated gene. The disease is controlled by a single pair of allele, HbA and HbS. Double recessive individuals of genotype HbSHbS show the disease phenotype. The heterozygous individuals are only carriers of the disease and are unaffected.

Q.15 Who had proposed the chromosomal theory of the inheritance?


The chromosomal theory of inheritance was proposed by Walter Sutton and Theodore Bovery in 1902. The theory linked the inheritance of traits described in Mendelian Laws to the inheritance of chromosomes.

Explanation: The theory explains that chromosomes carried the factors described in Mendelian inheritance. It also stated that the chromosomes are linear structures with genes located at specific sites called loci along their length.

Q.16 Mention any two autosomal genetic disorders with their symptoms.


  1. Sickle cell anaemia: This autosomal recessive genetic disorder is caused by the substitution of glutamic acid by valine at the sixth position of the beta-globin chain of the haemoglobin molecule. This point mutation causes a physiological change in the haemoglobin, which in turn changes the shape of the RBC from biconcave disc to sickle-like structure.

Symptoms: Shortness of breath, dizziness, headaches, cold hands and feet and pale jaundiced skin.

  1. Phenylketonurea: This autosomal recessive genetic disorder leads to the absence of an enzyme that converts the amino acid phenylalanine to tyrosine. This causes the accumulation of phenylalanine, which in turn leads to the associated symptoms.

Symptoms: Mental retardation, seizures, delayed development, behavioural problems, and psychiatric disorders.

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FAQs (Frequently Asked Questions)

1. What topics are covered in Chapter 5 of Class 12 Biology?

The fundamentals of heredity and variation are discussed in Chapter 5, Biology Class 12. It starts with Gregor Mendel’s pea plant experiment and ends with the Punnett square. It also gives a detailed explanation of the inheritance of one and two genes. The principles of recessive and dominant genes are explained in this chapter. It continues to address genetic mutations and concludes with genetic disorders at the end of the chapter.

2. Class 12 Biology, Chapter 5 What is meant by Alleles?

A variable version of a gene is known as an allele. Alleles are found in pairs on each chromosome and occupy a defined position. Different versions of these genes are in charge of managing a single specific attribute in the organism and have a dominant or recessive influence on it. An allele for the human blood group AB is a typical example.