Biology Principles Of Inheritance And Variation: Mendelian Genetics, Genetic Disorders, And Sex Determination by PW

Genetics helps explain how characteristics are transmitted from parents to offspring and why variations arise among individuals. To understand inheritance, you need to be familiar with genes, alleles, chromosomes, gametes, genetic crosses and different patterns of inheritance. Mendel’s experiments on garden pea laid the foundation for studying how traits are passed from one generation to another.

In this PW Principles of Inheritance and Variation revision guide, you will find the chapter covered topic-wise, beginning with the basic terminology of genetics and Mendelism before moving to genetic crosses and extensions of Mendelian inheritance. Concepts such as incomplete dominance, codominance, pleiotropy, polygenic inheritance, linkage, sex determination, genetic disorders and pedigree analysis are also included for quick and effective revision.

Fundamental Terminology Of Genetics

Before studying genetic crosses, you should be familiar with the terms used to describe inheritance.

1. Genetics: The branch of biology concerned with heredity and variation.

2. Father of Genetics: Gregor Johann Mendel.

3. Genetics was coined as a term by William Bateson.

4. Heredity: Transmission of characters from parents to offspring.

5. Variation: Differences observed among individuals or between offspring and their parents.

6. Inheritance: Transmission of characters through genetic material carried in gametes.

7. Gene: A functional segment of DNA associated with a biological character. Mendel referred to genes as factors, while the term gene was coined by Wilhelm Johannsen.

8. Allele: An alternative form of a gene located at the same locus on homologous chromosomes.

Important Genetic Terms

A homozygous individual produces one type of gamete for the gene being considered, whereas a heterozygous individual produces two types of gametes.

Mendelism And Selection Of Garden Pea

Gregor Johann Mendel studied hybridisation in the garden pea, Pisum sativum, for seven years, from 1856 to 1863. His experiments established the basic principles of inheritance.

Mendel selected garden pea because it had several useful characteristics:

1. It was easy to cultivate.

2. It had a short generation time of about three to four months.

3. It produced many seeds.

4. It had several clearly distinguishable contrasting characters.

5. The flowers were bisexual, containing both stamens and carpels.

6. Natural self-pollination allowed Mendel to maintain pure lines.

7. Controlled cross-pollination could be performed by removing the anthers, a process called emasculation, followed by bagging.

Mendel's Seven Pairs Of Contrasting Characters

Monohybrid Cross And Mendel's Laws

A monohybrid cross examines the inheritance of one gene or one pair of contrasting traits.

Mendel crossed a homozygous tall pea plant with a homozygous dwarf pea plant:

TT × tt

The parental plants produce T and t gametes, respectively. All individuals in the F1 generation are therefore Tt and show the tall phenotype.

When F1 plants are self-crossed:

Tt × Tt

Each parent produces two types of gametes, T and t, in equal proportions.

F2 Generation

The resulting F2 generation has:

• Genotypic ratio: 1 TT : 2 Tt : 1 tt

• Phenotypic ratio: 3 Tall : 1 Dwarf

A Punnett square, associated with Reginald C. Punnett, is used to represent the possible combinations of gametes and predict offspring genotypes.

Phenotype And Genotype

• Phenotype: Observable expression of a character.

• Genotype: Genetic constitution of an organism for a particular character.

Law Of Dominance

According to the Law of Dominance, when two contrasting alleles occur together in a heterozygote, one allele expresses itself while the other remains unexpressed.

The expressed allele is called the dominant allele, while the unexpressed allele is called the recessive allele.

This explains why all F1 plants in Mendel's monohybrid cross were tall even though they carried both T and t alleles.

The law does not apply universally because inheritance patterns such as incomplete dominance and codominance show different phenotypic expressions.

Law Of Segregation

The Law of Segregation states that the two alleles of a gene separate during gamete formation. Each gamete receives only one allele of the pair.

The alleles do not blend with one another. They remain distinct and are separated during meiotic gamete formation.

This law is also called the Law of Purity of Gametes.

Law Of Independent Assortment

The Law of Independent Assortment states that the segregation of one pair of alleles is independent of another pair during gamete formation, provided the genes are independently assorting.

This principle is demonstrated through a dihybrid cross.

Test Cross

A test cross is performed to determine the unknown genotype of an individual showing a dominant phenotype.

The individual is crossed with a homozygous recessive individual.

For example:

• WW × ww → all offspring show the dominant phenotype.

• Ww × ww → offspring show a 1:1 ratio of dominant to recessive phenotypes.

Therefore, when the unknown dominant individual is heterozygous, a monohybrid test cross produces:

Phenotypic ratio = 1:1

Genotypic ratio = 1:1

Non-Mendelian Inheritance

Not all inheritance patterns follow the simple dominant-recessive relationship described by Mendel's monohybrid cross. Important variations include incomplete dominance, codominance, multiple allelism, pleiotropy, polygenic inheritance, and linkage.

Incomplete Dominance

In incomplete dominance, neither allele completely dominates the other. As a result, the heterozygote shows an intermediate phenotype.

A classic example occurs in Mirabilis jalapa and Antirrhinum majus.

When a red-flowered plant (RR) is crossed with a white-flowered plant (rr):

RR × rr → Rr

The F1 plants have pink flowers.

Selfing the F1 generation gives:

Rr × Rr

The F2 generation shows:

• 1 Red : 2 Pink : 1 White

• Genotypic ratio = 1 RR : 2 Rr : 1 rr

• Phenotypic ratio = 1 : 2 : 1

Here, the phenotypic ratio is the same as the genotypic ratio.

Multiple Allelism And Codominance

Multiple Allelism

When more than two alternative forms of a gene exist in a population, the condition is called multiple allelism.

The ABO blood group system is controlled by three alleles:

Iᴬ, Iᴮ and i

Although three alleles occur in the population, an individual carries only two alleles at a time.

ABO Blood Group Inheritance

There are four phenotypes and six genotypes.

Iᴬ and Iᴮ are codominant because both are expressed together in the AB blood group. Both Iᴬ and Iᴮ are dominant over i.

Codominance

In codominance, both alleles express themselves fully in a heterozygous individual.

The AB blood group is a standard example because both Iᴬ and Iᴮ contribute to the phenotype.

Molecular Basis Of Dominance

Dominance can be understood through the relationship between genes, enzymes, and metabolic products.

A normal allele may produce a functional enzyme that converts a substrate into a required product. Depending on the nature of the allele and its product:

1. A normal or slightly less efficient enzyme may still produce the dominant phenotype.

2. A non-functional enzyme may result in the recessive phenotype.

3. Absence of the required enzyme may also result in the recessive phenotype.

Thus, the expression of a phenotype can depend on the functional product produced by a particular allele.

Pleiotropy

Pleiotropy occurs when a single gene influences more than one phenotypic character.

In pea plants, the B gene influences starch synthesis, seed size, and seed shape. Different genotypes produce differences in starch content and the resulting seed characteristics.

Phenylketonuria is another example of pleiotropy. It results from a deficiency of the enzyme phenylalanine hydroxylase, causing phenylalanine to accumulate. The condition can affect several characteristics, including intellectual development, pigmentation, hair colour, and urinary excretion.

Dihybrid Cross

A dihybrid cross studies the inheritance of two genes simultaneously.

For example, Mendel crossed:

RRYY × rryy

where R represents round seeds, r represents wrinkled seeds, Y represents yellow seeds, and y represents green seeds.

The F1 generation consists of:

RrYy

Each F1 individual can produce four types of gametes:

RY, Ry, rY and ry

When the F1 plants are self-crossed, the F2 generation shows the characteristic:

Phenotypic Ratio

9 : 3 : 3 : 1

The F2 genotypic ratio is:

1 : 2 : 1 : 2 : 4 : 2 : 1 : 2 : 1

The dihybrid cross provides the basis for understanding the Law of Independent Assortment.

Polygenic Inheritance

Polygenic inheritance occurs when a single character is controlled by several genes. These genes generally contribute additive effects to the phenotype.

Human skin colour is an example of polygenic inheritance. It is influenced by multiple gene pairs, with the total number of contributing dominant alleles affecting melanin production.

For three gene pairs:

AaBbCc × AaBbCc

each parent can produce eight types of gametes. The F2 generation contains:

8 × 8 = 64 combinations

The phenotypes form a continuous range based on the number of dominant alleles.

This pattern illustrates how several genes can collectively contribute to variation in a character.

Linkage

Linkage refers to the tendency of genes located on the same chromosome to be inherited together.

The phenomenon was studied extensively by T. H. Morgan using Drosophila.

The strength of linkage depends on the distance between genes:

• Genes located close together show stronger linkage.

• Genes farther apart have a greater chance of crossing over and producing recombinant combinations.

Complete And Incomplete Linkage

Linkage explains why genes located on the same chromosome may not assort independently.

Sex Determination Mechanisms

Sex determination varies among organisms. Different chromosome systems determine whether an individual develops as male or female.

XX-XY System

In humans and Drosophila:

• Females = XX

• Males = XY

• Females produce only X-bearing eggs.

• Males produce X-bearing and Y-bearing sperm.

The sperm therefore determines whether the offspring receives XX or XY chromosomes.

XX-XO System

In grasshoppers:

• Females = XX

• Males = XO

Males produce two types of sperm with respect to the sex chromosome: X-bearing and chromosome-free.

ZZ-ZW System

In birds:

• Males = ZZ

• Females = ZW

The female produces two types of eggs, Z-bearing and W-bearing. Therefore, the female determines the sex of the offspring.

ZZ-ZO System

This system occurs in some butterflies:

• Males = ZZ

• Females = ZO

Haplodiploid Sex Determination In Honeybees

Honeybees show a different mechanism of sex determination:

• Females are diploid and develop from fertilised eggs.

• Males are haploid and develop from unfertilised eggs.

• Male honeybees produce sperm by mitosis.

A useful way to remember this pattern is that a male honeybee develops without a father because it arises from an unfertilised egg.

Genetic Disorders

Genetic disorders can broadly be grouped into:

1. Mendelian disorders, which result from changes in a single gene.

2. Chromosomal disorders, which result from changes in chromosome number or structure.

Mendelian Disorders

Sickle Cell Anaemia

Sickle cell anaemia results from an abnormal form of haemoglobin associated with the HbS allele.

Under low-oxygen conditions, the abnormal haemoglobin can polymerise, causing red blood cells to become elongated and sickle-shaped.

Individuals with HbA HbS generally carry the allele without developing the full condition, whereas HbS HbS individuals are affected.

Thalassemia

Thalassemia results from reduced synthesis of globin chains of haemoglobin.

It may involve:

• Alpha-thalassemia, associated with genes on chromosome 16.

• Beta-thalassemia, associated with the HBB gene on chromosome 11.

Myotonic Dystrophy

Myotonic dystrophy is an autosomal dominant disorder. It is associated with prolonged muscle contraction, making relaxation of affected muscles difficult.

Haemophilia And Red-Green Colour Blindness

Haemophilia and red-green colour blindness are examples of X-linked recessive disorders.

Because males have only one X chromosome, the presence of a recessive disease-associated allele on their X chromosome can result in the condition.

An affected father passes his X chromosome to his daughters and his Y chromosome to his sons. Therefore, an affected father does not transmit an X-linked allele directly to his sons.

Chromosomal Disorders

Chromosomal disorders can arise from changes in chromosome number or chromosome structure.

Aneuploidy

Aneuploidy involves the gain or loss of one or more individual chromosomes.

Examples include:

• Monosomy: Loss of one chromosome.

• Trisomy: Gain of one chromosome.

Euploidy

Euploidy refers to changes involving complete sets of chromosomes.

Major Chromosomal Disorders

Pedigree Analysis

A pedigree is a diagram used to trace the inheritance of a character or genetic disorder through several generations.

Common Pedigree Symbols

Pedigree analysis helps identify whether a trait follows an autosomal or sex-linked pattern and whether it behaves as dominant or recessive.

Autosomal Recessive Inheritance

For an autosomal recessive disorder:

• Affected individuals are generally aa.

• Unaffected parents can be carriers, Aa.

• Two carrier parents can have an affected child.

Autosomal Dominant Inheritance

For an autosomal dominant disorder:

• An affected heterozygous individual is commonly Aa.

• An unaffected individual is aa.

• The trait can appear in successive generations.

X-Linked Recessive Inheritance

In X-linked recessive inheritance:

• A male receives his only X chromosome from his mother.

• An affected father passes his X chromosome to all daughters but not to his sons.

• A daughter of an affected father receives the affected X chromosome from him and may be a carrier if her mother contributes a normal X chromosome.

• An affected female generally requires an affected father and a mother who carries or expresses the relevant recessive allele.

Pedigree patterns therefore provide useful clues about the mode through which a genetic condition is transmitted.

Quick Revision Points

1. Genetics deals with heredity and variation.

2. Gregor Johann Mendel is known as the Father of Genetics.

3. Mendel performed major inheritance experiments on Pisum sativum.

4. A monohybrid cross involves one gene or one pair of contrasting traits.

5. Mendelian monohybrid F2 phenotypic ratio = 3:1.

6. Mendelian monohybrid F2 genotypic ratio = 1:2:1.

7. The Law of Segregation is also called the Law of Purity of Gametes.

8. A test cross uses a homozygous recessive individual.

9. Incomplete dominance produces an intermediate heterozygous phenotype.

10. Codominance allows both alleles to express fully.

11. ABO blood groups demonstrate multiple allelism and codominance.

12. Pleiotropy occurs when one gene influences multiple characters.

13. A dihybrid cross gives the classical F2 phenotypic ratio of 9:3:3:1.

14. Polygenic inheritance involves several genes contributing to one character.

15. Linkage occurs between genes located on the same chromosome.

16. Humans show the XX-XY sex determination system.

17. Birds show the ZZ-ZW system.

18. Honeybees show haplodiploid sex determination.

19. Sickle cell anaemia and thalassemia are Mendelian disorders.

20. Down syndrome is associated with trisomy 21.

21. Klinefelter syndrome has an XXY chromosome constitution.

22. Turner syndrome has an XO chromosome constitution.

23. Pedigree analysis is used to trace inheritance across generations.

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