PART 1. GENETICS

CHAPTER 1

Genes

„The genes are the atoms of heredity”

Seymour Benzer 1960

Theory

The beauty and diversity of modern World has been formed through multiple climate changes, huge

landmasses movements and continuous evolution of endless number of living creatures during

million years of the Earth history. Today, people come to walk in the forest, by the lake or in the countryside and

they can see that some plants are more similar. Pines and spruces have needles; barley and wheat have long

narrow leaves. For other people physical differences are more evident. Pines and spruces are differentiated by

shape of cones, stature, needle length and colour, to name just a few. Similarly, barley and wheat are

characterized by a number of morphological traits. This pattern of similarities and differences between

organisms is explained by the science called genetics. Before this science emerged, people had intuitively

known that pines, spruces, barley and wheat represented different types of organisms (species). They

understood that a kind of hereditary material determined appearance and function of organisms. This knowledge

was used by ancient farmers when they sown cereal grains to obtain plants alike the parent. However, the

discovery of rules governing the inheritance of traits has occurred only recently, in 1866 when Gregor Mendel, a

monk published his work about trait inheritances in peas, which led him to postulate the existence of “discrete

units of inheritance”. Today Mendel is regarded as the father of genetics and his work is one of the best examples

how to carry research in the entire history of science. Mendel's hereditary units are now called genes while

genetics is the science of heredity and variation.

What are Genes?

The hereditary material must have properties enabling from one side transmitting unchanging

information from parents to offspring, and provide variation responsible for differences between organisms. All

these features are within the scope of substances called nucleic acids. Deoxyribonucleic acid (DNA) is the

hereditary material in all life forms except of some viruses, in which ribonucleic acid (RNA) is the hereditary

material. Both acids are biopolymers made of elementary units – nucleotides (for structure see genetic,

biochemistry books and Internet sources). In plant cells, DNA molecules are found in nucleus (nDNA),

chloroplasts (cpDNA) and mitochondria (mtDNA). Traditionally, a segment of DNA encoding a single feature,

Species

Arabidopsis thaliana

Arabidopsis thaliana

Arabidopsis thaliana

Hordeum vulgare

Oryza sativa

Physcomitrella patens

Populus trichocarpa

Zea mays

Genome size

157 Mb

155 kb

367 kb

5 300 Mb

490 Mb

511 Mb

485 Mb

~2 500 Mb

Genes number

26 819

87

117

~20 000

~32 000

~30 000

45 555

~20 000

Genome

nDNA

cpDNA

mtDNA

nDNA

nDNA

nDNA

nDNA

nDNA

TABLE 1.1 Number of genes in selected plant species

UAAAUGCap poly A5’ 3’

Regulation oftranscription

Signals for terminationof transcription

FIGURE 1.1 Modern structure of the gene - Cat1 encoding catalase in Zea maysYellow boxes - exons, navy blue - introns

Based on Lang et al. 2008

with the specific sequence of nucleotides makes a gene. For instance, the classical pea R gene described by

Mendel as affecting seed forms (rounded vs. wrinkled, see Fig. 1.4) consists of 3546 bp. The whole pea genome 6is about 4 300 x 10 bp, thus roughly estimating, a DNA molecule may contain about million different genes.

However, it is not a case; genes are often only a tiny proportion of plant genomes (Table 1.1). The DNA contains

all information about structures and functions of organisms such as leaf shape, stem height, flower colors and

many others. These observable characteristics are referred as a phenotype while the genetic constitutions, e.g.,

genes controlling leaves, stems and flowers are called a genotype. In a given example of the Mendel's R gene,

the pea phenotype is “rounded” while the genotype can be RR (double R as pea is a diploid, for tetraploid would

be RRRR, for haploid R).

.– exons and non-coding parts – introns (Fig. 1.1). A set of all genes of an organism makes a genome – a kind of

a database storing data files (genes) and programmes for their execution (regulatory sequences; non-coding).

M o d e r n

understanding of a

gene, covers not only a

coding part but also

associated regulatory

regions. Plant genes,

l ike all Eukaryotic

genes, are built from

both coding sequences

How do Genes Function?

The function of genetic material is, first, to copy information from parents to offspring in a process of

replication and second, to provide information for growth and development of an individual. Beyond our

awareness, genes start to execute programmes that translate the DNA sequence into red flowers, growth, seed

development etc. This process is called gene expression (Fig. 1.2). This is described elsewhere. Here, suffice is

to say that structural proteins or enzymes are produced then. In a first step of gene expression called

transcription the DNA sequence in a gene (e.g., pea R gene) acts as a template for the synthesis of a

complementary strand of the RNA called transcript. Then, the RNA undergoes several modifications to produce

a final molecule - messenger RNA (mRNA). A collection of all different transcripts is referred as transcriptome.

Finally, nucleotides in the RNA are read as triplets calling codons in the process of translation – synthesis of a

protein using the gene's mRNA template. This way the pea R gene leads to the starch branching enzyme

(Accession N° CAA56319). The collection of all the different proteins in an organism is its proteome. Enzymes

control the synthesis of different metabolites, e.g., starch, alkaloids etc. A set of all metabolites is often referred as

metaboleome. Along with the technology developments, methods enabling analysis of many genes, transcripts,

proteins, metabolites developed and have become a foundation of a holistic approach in research referred as

“omics”with appropriate prefix.

PART 1. GENETICS14 CHAPTER 1. GENES 15

Gene Genome

mRNA Transciptome

ProteomeProtein

StructureFunction

Organism

GE

NE

TIC

S

IN

TE

GR

AT

ED

GE

NO

MIC

S

FIGURE 1.2 Key steps of gene expression - relation between genetics and “omics” sciences

How do Genes Change?Genes from one side are reasonable

stable to preserve functions; from the other they can

change in a process called mutagenesis, in which

new forms of genes are produced (Chapters 4 and

16). For many years, biologists have observed that

each trait can exist in different forms – white and red

flowers, round or wrinkle seeds, dwarf or high

plants. Mendel discovered that these different trait

stages are associated with different forms of the

same gene, which we now called alleles -

alternative forms of a gene. Alleles are denoted by

the same basic symbol as R for round pea seeds

and r for wrinkled seeds. Sequencing of the

Mendel's R gene proved that the difference in the

pea seed shapes is controlled by insertion of 800 bp

in the r allele. Another example involved a mutation

within tb1 gene (teosinte branched), responsible for

transition of a bushy grass (teosinte) into a modern

cereal – maize, thus being an important element of domestication and speciation. The pea example

demonstrates that mutations are responsible for variation among individuals within a species while the maize

example is unequivocal proof that mutations are the raw material for evolution. Without mutations alleles would

not exist, genetic analysis would not be possible and the most important, evolution would not be possible

(Chapter 4, 13).

Practical Examples

The genius of Mendel was that despite hundred years and the entire technical progress today, no

genetic analysis is possible without application of the Mendel's methodology based on hybridization and

statistics. Segregation analyses have still been the most important data about inheritance and differentiation. No

transgenic plant can be registered without confirming the Mendelian inheritance of a transgene. Comparing

segregation in different taxa informs about existence of reproductive barriers and their evolutionary divergence.

Monohybrid Crosses (Mono-factor) – Dominance and Segregation

Let us take two pea plants, the first with violet flowers and the second with white (Fig. 1.3). Both plants

are true-bred i.e., they have been stable with respect to the flower colour through several generations. Let us

assign the plants as P for parental. The resulted generation, F (First filial generation) will have only violet flowers. 1

Next fertilization will produce second filial generation, F , in which 75% of plants will have violet flowers and 25% 2

white flowers, i.e., violet to white flowers ratio 3:1. This is a consequence of alleles' segregation during production

of gametes. Each diploid plant has two copies of a gene (two alleles). The plant is homozygous if both alleles are

identical and the plant is heterozygous if the alleles are different. Parental plants are homozygous with

respective genotypes AA for violet and aa for white flowers. Each parental plant produces one kind of gamete.

They merged in F giving a heterozygous plant, Aa with violet flowers. It is obvious that the A allele encoding violet 1

flowers control the phenotype even it is in a single copy. The phenomenon is called dominance, and the

„stronger” allele is called dominant while the a allele for white flowers is called recessive. The heterozygous F 1

plants produce two kinds of gametes that can merge in four combinations giving the F generation. The first 2

Mendel principle says that alleles of a gene segregate from each other during the formation of gametes.

Species

Arabidopsis thaliana

Arabidopsis thaliana

Arabidopsis thaliana

Hordeum vulgare

Oryza sativa

Physcomitrella patens

Populus trichocarpa

Zea mays

Genome size

157 Mb

155 kb

367 kb

5 300 Mb

490 Mb

511 Mb

485 Mb

~2 500 Mb

Genes number

26 819

87

117

~20 000

~32 000

~30 000

45 555

~20 000

Genome

nDNA

cpDNA

mtDNA

nDNA

nDNA

nDNA

nDNA

nDNA

TABLE 1.1 Number of genes in selected plant species

UAAAUGCap poly A5’ 3’

Regulation oftranscription

Signals for terminationof transcription

FIGURE 1.1 Modern structure of the gene - Cat1 encoding catalase in Zea maysYellow boxes - exons, navy blue - introns

Based on Lang et al. 2008

with the specific sequence of nucleotides makes a gene. For instance, the classical pea R gene described by

Mendel as affecting seed forms (rounded vs. wrinkled, see Fig. 1.4) consists of 3546 bp. The whole pea genome 6is about 4 300 x 10 bp, thus roughly estimating, a DNA molecule may contain about million different genes.

However, it is not a case; genes are often only a tiny proportion of plant genomes (Table 1.1). The DNA contains

all information about structures and functions of organisms such as leaf shape, stem height, flower colors and

many others. These observable characteristics are referred as a phenotype while the genetic constitutions, e.g.,

genes controlling leaves, stems and flowers are called a genotype. In a given example of the Mendel's R gene,

the pea phenotype is “rounded” while the genotype can be RR (double R as pea is a diploid, for tetraploid would

be RRRR, for haploid R).

.– exons and non-coding parts – introns (Fig. 1.1). A set of all genes of an organism makes a genome – a kind of

a database storing data files (genes) and programmes for their execution (regulatory sequences; non-coding).

M o d e r n

understanding of a

gene, covers not only a

coding part but also

associated regulatory

regions. Plant genes,

l ike all Eukaryotic

genes, are built from

both coding sequences

How do Genes Function?

The function of genetic material is, first, to copy information from parents to offspring in a process of

replication and second, to provide information for growth and development of an individual. Beyond our

awareness, genes start to execute programmes that translate the DNA sequence into red flowers, growth, seed

development etc. This process is called gene expression (Fig. 1.2). This is described elsewhere. Here, suffice is

to say that structural proteins or enzymes are produced then. In a first step of gene expression called

transcription the DNA sequence in a gene (e.g., pea R gene) acts as a template for the synthesis of a

complementary strand of the RNA called transcript. Then, the RNA undergoes several modifications to produce

a final molecule - messenger RNA (mRNA). A collection of all different transcripts is referred as transcriptome.

Finally, nucleotides in the RNA are read as triplets calling codons in the process of translation – synthesis of a

protein using the gene's mRNA template. This way the pea R gene leads to the starch branching enzyme

(Accession N° CAA56319). The collection of all the different proteins in an organism is its proteome. Enzymes

control the synthesis of different metabolites, e.g., starch, alkaloids etc. A set of all metabolites is often referred as

metaboleome. Along with the technology developments, methods enabling analysis of many genes, transcripts,

proteins, metabolites developed and have become a foundation of a holistic approach in research referred as

“omics”with appropriate prefix.

PART 1. GENETICS14 CHAPTER 1. GENES 15

Gene Genome

mRNA Transciptome

ProteomeProtein

StructureFunction

Organism

GE

NE

TIC

S

IN

TE

GR

AT

ED

GE

NO

MIC

S

FIGURE 1.2 Key steps of gene expression - relation between genetics and “omics” sciences

How do Genes Change?Genes from one side are reasonable

stable to preserve functions; from the other they can

change in a process called mutagenesis, in which

new forms of genes are produced (Chapters 4 and

16). For many years, biologists have observed that

each trait can exist in different forms – white and red

flowers, round or wrinkle seeds, dwarf or high

plants. Mendel discovered that these different trait

stages are associated with different forms of the

same gene, which we now called alleles -

alternative forms of a gene. Alleles are denoted by

the same basic symbol as R for round pea seeds

and r for wrinkled seeds. Sequencing of the

Mendel's R gene proved that the difference in the

pea seed shapes is controlled by insertion of 800 bp

in the r allele. Another example involved a mutation

within tb1 gene (teosinte branched), responsible for

transition of a bushy grass (teosinte) into a modern

cereal – maize, thus being an important element of domestication and speciation. The pea example

demonstrates that mutations are responsible for variation among individuals within a species while the maize

example is unequivocal proof that mutations are the raw material for evolution. Without mutations alleles would

not exist, genetic analysis would not be possible and the most important, evolution would not be possible

(Chapter 4, 13).

Practical Examples

The genius of Mendel was that despite hundred years and the entire technical progress today, no

genetic analysis is possible without application of the Mendel's methodology based on hybridization and

statistics. Segregation analyses have still been the most important data about inheritance and differentiation. No

transgenic plant can be registered without confirming the Mendelian inheritance of a transgene. Comparing

segregation in different taxa informs about existence of reproductive barriers and their evolutionary divergence.

Monohybrid Crosses (Mono-factor) – Dominance and Segregation

Let us take two pea plants, the first with violet flowers and the second with white (Fig. 1.3). Both plants

are true-bred i.e., they have been stable with respect to the flower colour through several generations. Let us

assign the plants as P for parental. The resulted generation, F (First filial generation) will have only violet flowers. 1

Next fertilization will produce second filial generation, F , in which 75% of plants will have violet flowers and 25% 2

white flowers, i.e., violet to white flowers ratio 3:1. This is a consequence of alleles' segregation during production

of gametes. Each diploid plant has two copies of a gene (two alleles). The plant is homozygous if both alleles are

identical and the plant is heterozygous if the alleles are different. Parental plants are homozygous with

respective genotypes AA for violet and aa for white flowers. Each parental plant produces one kind of gamete.

They merged in F giving a heterozygous plant, Aa with violet flowers. It is obvious that the A allele encoding violet 1

flowers control the phenotype even it is in a single copy. The phenomenon is called dominance, and the

„stronger” allele is called dominant while the a allele for white flowers is called recessive. The heterozygous F 1

plants produce two kinds of gametes that can merge in four combinations giving the F generation. The first 2

Mendel principle says that alleles of a gene segregate from each other during the formation of gametes.

PART 1. GENETICS16 CHAPTER 1. GENES 17

XP1 (AA)

F (Aa)1

P2 (aa)

A a

Phenotypic ratio:red - 3

white - 1

Genotypic ratio:AA - 1Aa - 2aa - 1

F2

A a

A

a

AA

Aa aa

Aa

Self-fertilized

Each parental homozygoteproduces one kind of gamete

Heterozygoteproduces twokinds of gametsin equal proportions

FIGURE 1.3

Symbolic representation of a cross between peas. The first Mendel principle

Dihybrid (Two- factor) Crosses

Formulating and Testing Genetic Hypotheses

After crossing plants differed in two traits e.g., a plant with yellow, round seeds and a plant with green,

wrinkled seeds, four phenotypic classes are observed in F (Fig. 1.4). They represent all possible combinations 2

of these two traits with the ratio of 9 yellow, round, 3 green and round, 3 yellow and wrinkled, and finally 1 green

and wrinkled. In this case each trait is controlled by a different gene segregating two alleles. We denote each

gene with an uppercase for dominant allele and lowercase for the recessive. For the seed color, the two alleles

are G for yellow and g for green. For the seed texture, R for rounded and r for wrinkled. The F produces haploid 1

gametes with one copy of each gene. It is possible to write down all the gametes and combine them

systematically in a form of the Punnet Square. This is the Principle of Independent Assortment – alleles of

two different genes segregate or assort independently of each other.

In reality, the exact Mendelian ratios are rarely obtained. We rather observe numbers close to

expected ratio e.g., 1562 violet and 493 white flowers, 1000 round and 350 wrinkled seeds. How might we

explain the data? Let us to formulate a hypothesis that the traits are inherited according to the Mendel Principle

and the expected ratio is 3:1. Do our data really support this hypothesis? One procedure for testing the fit 2 2between the predictions and the actual data uses chi-square statistics (χ ). The χ statistics allows to compare

obtained data with their predicted values. If the data are not in line with the predicted values, the statistics will

exceed a critical number and the genetic hypothesis will be rejected. Chi-square statistics is calculated as a

square of a difference between observed and expected number for each phenotypic class dividing by expected

number. Then the sum is computed over all phenotypic classes and the result is compared with the critical value,

which is typically a value for 5% of probability. If our chi-square value is lower than the critical value our

hypothesis about phenotypic distribution is correct.

GR

GR

Gr

Gr

gR

gR

gr

gr

XP1 (GGRR)

F (GgRr)1

F2

P2 (ggrr)

GR gwG - yellowg - greenR - roundedr - wrinkled

Phenotypic ratio:yellow, rounded - 9yellow, wrinkled - 3green, rounded - 3green, wrinkled - 1

Genotypic ratio:GGRR - 1GgRR - 2GGRr - 2GgRr - 4

GGrr - 1Ggrr - 2

ggRR - 1ggRr - 2

ggrr - 1

Self-fertilized

GGRR

GGRr

GgRR

GgRr

GGRr

GGrr

GgRr

Ggrr

GgRR

GgRr

ggRR

ggRr

GgRr

Ggrr

ggRr

ggrr

FIGURE 1.4

Symbolic representation of a cross between peas. The second Mendel principle

Yellow, Round

Yellow, Wrinkled

Green, Wrinkled

PART 1. GENETICS16 CHAPTER 1. GENES 17

XP1 (AA)

F (Aa)1

P2 (aa)

A a

Phenotypic ratio:red - 3

white - 1

Genotypic ratio:AA - 1Aa - 2aa - 1

F2

A a

A

a

AA

Aa aa

Aa

Self-fertilized

Each parental homozygoteproduces one kind of gamete

Heterozygoteproduces twokinds of gametsin equal proportions

FIGURE 1.3

Symbolic representation of a cross between peas. The first Mendel principle

Dihybrid (Two- factor) Crosses

Formulating and Testing Genetic Hypotheses

After crossing plants differed in two traits e.g., a plant with yellow, round seeds and a plant with green,

wrinkled seeds, four phenotypic classes are observed in F (Fig. 1.4). They represent all possible combinations 2

of these two traits with the ratio of 9 yellow, round, 3 green and round, 3 yellow and wrinkled, and finally 1 green

and wrinkled. In this case each trait is controlled by a different gene segregating two alleles. We denote each

gene with an uppercase for dominant allele and lowercase for the recessive. For the seed color, the two alleles

are G for yellow and g for green. For the seed texture, R for rounded and r for wrinkled. The F produces haploid 1

gametes with one copy of each gene. It is possible to write down all the gametes and combine them

systematically in a form of the Punnet Square. This is the Principle of Independent Assortment – alleles of

two different genes segregate or assort independently of each other.

In reality, the exact Mendelian ratios are rarely obtained. We rather observe numbers close to

expected ratio e.g., 1562 violet and 493 white flowers, 1000 round and 350 wrinkled seeds. How might we

explain the data? Let us to formulate a hypothesis that the traits are inherited according to the Mendel Principle

and the expected ratio is 3:1. Do our data really support this hypothesis? One procedure for testing the fit 2 2between the predictions and the actual data uses chi-square statistics (χ ). The χ statistics allows to compare

obtained data with their predicted values. If the data are not in line with the predicted values, the statistics will

exceed a critical number and the genetic hypothesis will be rejected. Chi-square statistics is calculated as a

square of a difference between observed and expected number for each phenotypic class dividing by expected

number. Then the sum is computed over all phenotypic classes and the result is compared with the critical value,

which is typically a value for 5% of probability. If our chi-square value is lower than the critical value our

hypothesis about phenotypic distribution is correct.

GR

GR

Gr

Gr

gR

gR

gr

gr

XP1 (GGRR)

F (GgRr)1

F2

P2 (ggrr)

GR gwG - yellowg - greenR - roundedr - wrinkled

Phenotypic ratio:yellow, rounded - 9yellow, wrinkled - 3green, rounded - 3green, wrinkled - 1

Genotypic ratio:GGRR - 1GgRR - 2GGRr - 2GgRr - 4

GGrr - 1Ggrr - 2

ggRR - 1ggRr - 2

ggrr - 1

Self-fertilized

GGRR

GGRr

GgRR

GgRr

GGRr

GGrr

GgRr

Ggrr

GgRR

GgRr

ggRR

ggRr

GgRr

Ggrr

ggRr

ggrr

FIGURE 1.4

Symbolic representation of a cross between peas. The second Mendel principle

Yellow, Round

Yellow, Wrinkled

Green, Wrinkled

PART 1. GENETICS18 CHAPTER 1. GENES 14

http://

ww

w

Technological Links

http://www.ncbi.nlm.nih.govThe NCBI database. Probable the biggest and the most used data base of DNA, protein and genomic sequences. For instance,

the Mendel's R gene sequence has the X8009 accession number. Except of huge scientific resources there are educational resources including on line Mendelian inheritance in Animals (OMIA) and in Man (OMIN) – NCBI database of genes, inherited disorders and traits with references and links to PubMed and Gene.

The Web dedicated to Mendel and his work

Basic concepts of Mendelian genetics with examples

•Photos of mutants: pea, barley, black oat – examples of allelic variation and characters controlling by a single gene. Also a good demonstration of the efficiency of chemical mutagenesis. The serve as a warning to some botanists against rash delimitation of species based on just a few characters.

•Genetic problem book with sample solutions.

http://www.mendelweb.org/

http://www.ndsu.edu/pubweb/~mcclean/plsc431/mendel/mendel1.htm

http://www.uwm.edu.pl/katgenbiol

collection can

Challenging Questions?1. Explain how advances in genetics

influence life quality, social behavior and politics.

2. Based on the NCBI database find a DNA sequence of a plant gene, transcribe and translate it. How many amino acids are in a putative protein?. How many possibilities of sequence reading can you predict?

3. Compare DNA and RNA chemical structure. How the DNA structure is connected with its function.

4. Compare plant and prokaryotic genes.

5. How can you explain Mendel laws by meiosis? Prepare models of two non-homologous chromosomes (e.g., one blue, one red); locate one gene on each chromosome (e.g., for height, and flower color), use your desktop as a dividing cell of a diploid organism that is heterozygous for each gene, simulate both meiotic divisions.

6. Scientific papers contain four main sections of text – Introduction, Material and Methods, Results and Discussion. Imagine that you are Mendel and use the scientific paper format to describe results of experiments in pea and discovered laws.

7. Mendel seems to have done his research without any government grant. Likely, he had no knowledge about project management. Nevertheless, his experiments enabled to formulate general laws. What do these circumstances say about Mendel and his personality? Compare his manner of work with scientists working on research projects today.

8. In the chapter simple examples of interactions among two genes were presented. Find examples of other interactions that influence the distribution of phenotypes in F . Do these examples mean that 2

Mendel principles are not universal?

1. After crossing of two peas, a total of 125 plants with red flowers and 130 plants with white flowers were obtained. What are genotypes and phenotypes of crossed plants?

2. What would be the frequency of six rowed barley plants with long awns in F generation after crossing of six 2

rowed, awnless plant with two rowed plant with long awns if F has two rows and long awns? 1

3. In the F of pea, a total 1 761 plants with tendrils and 2

round seeds were obtained, 628 plants with tendrils and wrinkled seeds, 596 plants without tendrils and with round seeds and 203 plants without tendrils and with wrinkled seeds. Which characters are dominant? What is the phenotype of F ?Describe all possible genotypes 1

and phenotypes of parents. How do both characters inherit? Check the hypothesis using chi- square test.

4. A breeder crossed two snapdragon plants with pink flowers. He obtained 53 pink plants, 25 white plants and 27 red plants. What is the inheritance of flower color in snapdragons?

5. A geneticist crossed two true bred pea lines with white flowers and he received only F plants with red flowers. 1

After self-fertilizing F he obtained 320 seeds that were 1

sown in a field. During flowering 180 plants appeared to have red flowers and 140 white flowers. How to explain results. Did a geneticist make a mistake? Might he mix the seeds?

6. The colour of a grape skin can be dark red, white and pink. A single gene control these differences and the allele for dark red skin is partially dominant over the allele for white skin. Find in literature available in Internet what the basis of differences in skin color is .

Problems to Solve

Further Reading

Gregor Mendel. 1909. Experiments in plant hybridisation. 2008 edition. New York: Cosimo, Inc. 54 p.

Gregor Mendel's experiments on plant hybrids: a guided study. Corcos AF, Monagham F editors. 1993.The Mendel's original work as well as the work with editors explanations. It is the description of one of the best experiments in the science history. It is very rare for one individual to carry out the whole process of science so completely and elegantly. It is a pioneering work not only in genetics but also in integrating scientific disciplines – botany, physics and mathematics.

Snustad DP, Simmons MJ. 2006. Principles of genetics. Fourth Edition. John Willey & Sons, Inc. 866 p. One of the best books in genetics, covers all topics, excellent pedagogy of the book.