3. Principles of inheritance and variation

IMPORTANT TERMS

· Genetics: Study of inheritance, heredity and variation of characters or Study of genes and chromosomes.

· Inheritance: Transmission of characters from parents to progeny.

· Heredity: Resemblance b/w offspring and their parents.

· Variation: Difference between parents and offspring.

· Clone: The group of organisms produced by asexual reproduction. (The individual of a clone is called rametes).

· Offspring: The organism derived by sexual reproduction.

· Alleles (Allelomorphs): The alternative forms of a gene. E.g. T (tall) and t (dwarf) are two alleles of a gene responsible for the character height.

· Homozygous: The condition in which chromosome carries similar alleles for a character. Also known as pure line (True breeding). E.g. TT, tt, YY, yy etc.

· Heterozygous: The condition in which chromosome carries dissimilar alleles for a character. E.g. Tt, Yy etc.

· Dominant character: The character which is expressed in heterozygous condition. It indicates with capital letter.

· Recessive character: The character which is suppressed in heterozygous condition. It indicates with small letter.

· Phenotype: Physical (Visible) expression of an individual.

· Genotype: Genetic constitution of an individual.

· Hybrid: An individual produced by the mating of genetically unlike parents.

· Haploid (Monoploid): An individual or cell containing a single complete set of chromosomes.

· Diploid: An individual or cell containing two complete haploid set of chromosomes.

· Punnett square (Checker board): A grid that enables to calculate the results of simple genetic crosses.

· Cross: Deliberate mating of 2 parental types of organism.

· Reciprocal cross: Two way cross of the same genotype in which the sexes of both parents are reversed.

· Trait: A phenotypic characteristic of an inherited character.

· Wild type: The species variety showing normal phenotype.

Father of genetics: Gregor Mendel

MENDEL’S LAWS OF INHERITANCE

Hybridization experiments on garden peas (Pisum sativum)

Mendel selected 7 pairs of true breeding pea varieties

Characters	Dominant	Recessive
1. Stem height	Tall	Dwarf
2. Flower colour	Violet	White
3. Flower position	Axial	Terminal
4. Pod shape	Inflated	Constricted
5. Pod colour	Green	Yellow
6. Seed shape	Round	Wrinkled
7. Seed colour	Yellow	Green

INHERITANCE OF ONE GENE

Monohybrid cross: A cross involving 2 plants differing in one character pair. E.g. Mendel crossed tall and dwarf pea plants to study the inheritance of one gene.

Steps in making a cross in pea:

§ Selection of 2 pea plants with contrasting characters.

§ Removal of anthers (emasculation) of one plant to avoid self pollination. This is female parent.

§ Collection of pollen grains from the other plant (male parent) and transferred to female parent (pollination).

§ Collection of seeds and production of offspring.

Monohybrid phenotypic ratio: Tall: 3 Dwarf: 1= 3:1

Monohybrid genotypic ratio:

Homozygous tall (TT): 1 Heterozygous tall (Tt): 2 &

Homozygous dwarf (tt): 1= 1:2:1

Mendel made similar observations for other pairs of traits and proposed that some factors were inherited from parent to offspring. Now it is called as genes.

Do not use T for tall and d for dwarf because it is difficult to remember whether T & d are alleles of same gene or not.

The F₁ (Tt) when self pollinated, produces gametes T and t in equal proportion. During fertilization, pollen grains of T have 50% chance to pollinate eggs of T & t. Also, pollen grains of t have 50% chance to pollinate eggs of T and t.

1/4^th of the random fertilization leads to TT (¼ TT).

1/2 (2/4) of the random fertilization leads to Tt (½ Tt).

1/4^th of the random fertilization leads to tt (¼ tt).

Tt x Tt

Binomial expression = (ax + by)²

Hence (½ T + ½ t)²= (½ T + ½ t) (½ T + ½ t)

= ¼ TT + ¼ Tt + ¼ Tt + ¼ tt

= ¼ TT + ½ Tt + ¼ tt

Mendel self-pollinated the F₂ plants. He found that dwarf F₂ plants continued to generate dwarf plants in F₃ & F₄. He concluded that genotype of the dwarfs was homozygous- tt.

Backcross and Testcross

§ Backcross: Crossing of F₁ hybrid with its any of parent.

§ Testcross: Crossing of an F₁ hybrid with its recessive parent (Test cross ratio=1:1). It is used to find out the unknown genotype. (See figure in T.B. Page: 75)

Mendel conducted test cross to determine the F₂ genotype.

Mendel’s Principles or Laws of Inheritance

1. First Law (Law of Dominance)

§ Characters are controlled by discrete units called factors.

§ Factors occur in pairs.

§ In a dissimilar pair of factors one member of the pair dominates (dominant) the other (recessive).

2. Second Law (Law of Segregation)

“During gamete formation, the factors (alleles) of a character pair present in parents segregate from each other such that a gamete receives only one of the 2 factors”.

Homozygous parent produces similar gametes.

Heterozygous parent produces two kinds of gametes each having one allele with equal proportion.

The concept of dominance

In heterozygotes, there are dominant and recessive alleles. The normal (unmodified or functioning) allele of a gene produces a normal enzyme that is needed for the transformation of a substrate. The modified allele is responsible for production of

(i) The normal/less efficient enzyme or

(ii) A non-functional enzyme or

(iii) No enzyme at all

In the first case: The modified allele will produce the same phenotype like unmodified allele. It becomes dominant.

In 2^nd and 3^rd cases: The phenotype will dependent only on the functioning of the unmodified allele. Here, the modified allele becomes recessive.

NON-MENDELIAN INHERITANCE

1. Incomplete Dominance

- It is an inheritance in which heterozygous offspring shows intermediate character b/w two parental characteristics.

E.g. Flower colour in snapdragon (dog flower or Antirrhinum sp.) and Mirabilis jalapa (4’O clock plant).

Here, phenotypic and genotypic ratios are same.

Phenotypic ratio= 1 Red: 2 Pink: 1 White

Genotypic ratio= 1 (RR):2 (Rr):1(rr)

- This means that R was not completely dominant over r.

- Pea plants also show incomplete dominance in other traits.

2. Co-dominance

It is the inheritance in which both alleles of a gene are expressed in a hybrid. E.g. ABO blood grouping in human.

- ABO blood groups are controlled by the gene I. The plasma membrane of the RBC has sugar polymers that protrude from its surface and is controlled by the gene.

- The gene (I) has three alleles I^A, I^B and i. The alleles I^A and I^B produce a slightly different form of the sugar while allele i doesn’t produce any sugar.

Alleles from parent 1	Alleles from parent 2	Genotype of offspring	Blood types (phenotype)
I^A	I^A	I^A I^A	A
I^A	I^B	I^A I^B	AB
I^A	i	I^Ai	A
I^B	I^A	I^A I^B	AB
I^B	I^B	I^B I^B	B
I^B	i	I^Bi	B
i	i	ii	O

When I^A and I^B are present together they both express their own types of sugars. This is due to co-dominance.

3. Multiple allelism

Here more than two alleles govern the same character. E.g. ABO blood grouping (3 alleles: I^A, I^B & i).

4. Pleiotropy

Here, a single gene produces more than one effect. E.g. starch synthesis in pea seeds, sickle cell anaemia etc.

Starch synthesis in pea plant:

Starch is synthesized effectively by BB and therefore, large starch grains are produced. bb have lesser efficiency in starch synthesis and produce smaller starch grains.

If starch grain size is considered as phenotype, the alleles show incomplete dominance.

INHERITANCE OF TWO GENES (Dihybrid cross)

Dihybrid cross: A cross between two parents differing in 2 pairs of contrasting characters.

Mendel made some dihybrid crosses. E.g. Cross b/w pea plant with round shaped & yellow coloured seeds (RRYY) and wrinkled shaped & green coloured seeds (rryy).

On observing the F₂, Mendel found that the yellow and green colour segregated in a 3:1 ratio. Round and wrinkled seed shape also segregated in a 3:1 ratio.

Dihybrid Phenotypic ratio= Round yellow 9: Round green 3: Wrinkled yellow 3: Wrinkled green 1, i.e. 9:3:3:1

The ratio of 9:3:3:1 can be derived as a combination series of 3 yellow: 1 green, with 3 round: 1 wrinkled.

i.e. (3: 1) (3: 1) = 9: 3: 3: 1

Dihybrid genotypic ratio: 1:2:1:2:4:2:1:2:1

RRYY =1 RRYy =2 RrYY =2

RrYy =4 RRyy =1 Rryy =2

rrYY =1 rrYy =2 rryy =1

Third Law (Mendel’s Law of Independent Assortment):

It states that ‘when more than one pair of characters are involved in a cross, factor pairs independently segregate from the other pair of characters’.

CHROMOSOMAL THEORY OF INHERITANCE

Mendel’s work remained unrecognized till 1900 because,

1. Communication was not easy.

2. His mathematical approach was new and unacceptable.

3. The concept of genes (factors) as stable and discrete units was not accepted. (Mendel could not explain the continuous variation seen in nature).

4. Mendel could not provide any physical proof for the existence of factors.

In 1900, de Vries, Correns & von Tschermak independently rediscovered Mendel’s results.

Chromosomal Theory (1902): Walter Sutton & Theodore Boveri say that the pairing and separation of a pair of chromosomes lead to segregation of a pair of factors they carried. Sutton united chromosomal segregation with Mendelian principles and called it the chromosomal theory of inheritance. It states that,

· Chromosomes are vehicles of heredity. They are transmitted from parents to offspring, i.e. they are immortal.

· Two identical chromosomes form a homologous pair.

· They segregate at the time of gamete formation.

· Independent pairs segregate independently of each other.

· Chromosomes are mutable.

Genes are present on chromosomes. Hence they show similar behaviours.

Thomas Hunt Morgan proved chromosomal theory of inheritance using fruit flies (Drosophila melanogaster).

It is the suitable material because,

a. It breeds very quickly

b. Short generation time (life cycle: 12-14 days)

c. Breeding can be done throughout the year.

d. Hundreds of progenies per mating.

e. They can grow on simple synthetic medium.

f. Male and female flies are easily distinguishable.

g. It has many types of hereditary variations that can be seen with low power microscopes.

Linkage and Recombination

· Recombination: It is the generation of non-parental gene combinations.

· Linkage: Physical association of 2 or more genes on a chromosome. They do not show independent assortment.

Morgan carried out several dihybrid crosses in Drosophila to study sex-linked genes. E.g.

Cross 1: Yellow-bodied, white-eyed females

Brown-bodied, red-eyed males (wild type)

Cross 2: White-eyed, miniature winged

Red eyed, large winged (wild type)

(See figure in T.B. Page: 84)

Morgan intercrossed their F₁ progeny. He found that

§ The two genes did not segregate independently of each other and the F₂ ratio deviated from the 9:3:3:1 ratio.

§ Genes were located on the X chromosome

§ When two genes were situated on the same chromosome, the proportion of parental gene combinations was much higher than the non-parental type. This is due linkage.

§ Genes white & yellow were very tightly linked and showed only 1.3% recombination while white & miniature wing showed 37.2% recombination (loosely linked).

§ Tightly linked genes show low recombination. Loosely linked genes show high recombination.

Alfred Sturtevant used the recombination frequency between gene pairs as a measure of the distance between genes and ‘mapped’ their position on the chromosome.

Genetic maps are used as a starting point in the sequencing of genomes as was done in Human Genome Project.

MUTATION

It is a sudden heritable change in DNA sequences resulting in changes in the genotype and the phenotype of an organism.

· Frame-shift mutation: Loss (deletions) or gain (insertion/ duplication) of a DNA segment.

· Point mutation: Mutation due to change in a single base pair of DNA. E.g. sickle cell anemia.

· Mutation results in Chromosomal abnormalities (aberrations). Chromosomal aberrations are seen in cancer cells.

· Mutagens (agents which induce mutation) include,

- Physical mutagens: UV radiation, α, β, γ rays, X-ray etc.

- Chemical mutagens: Mustard gas, phenol, formalin etc.

PEDIGREE ANALYSIS

In human, control crosses are not possible. So the study of family history about inheritance is used. Such an analysis of traits in several generations of a family is called pedigree analysis. The representation or chart showing family history is called family tree (pedigree).

Symbols used in pedigree analysis

In human genetics, pedigree study is utilized to trace the inheritance of a specific trait, abnormality or disease.

GENETIC DISORDERS

2 types: Mendelian disorders and Chromosomal disorders.

1. Mendelian Disorders

· Caused by alteration or mutation in the single gene.

· The pattern of inheritance of Mendelian disorders can be traced in a family by the pedigree analysis.

· E.g. Haemophilia, Cystic fibrosis, Sickle-cell anaemia, Colour blindness, Phenylketonuria, Thalesemia, etc.

· Mendelian disorders may be dominant or recessive.

· By pedigree analysis one can easily understand whether the trait is dominant or recessive.

Pedigree analysis of (a) Autosomal dominant trait (E.g. Myotonic dystrophy) (b) Autosomal recessive trait (E.g. Sickle-cell anaemia).

Haemophilia (Royal disease):

· Sex linked recessive disease.

· In this, a protein involved in the blood clotting is affected.

· A simple cut results in non-stop bleeding.

· The heterozygous female (carrier) for haemophilia may transmit the disease to sons.

· The possibility of a female becoming a haemophilic is very rare because mother has to be at least carrier and father should be haemophilic (unviable in the later stage of life).

· Queen Victoria was a carrier of the disease. So her family pedigree shows a number of haemophilic descendents.

Sickle-cell anaemia:

· This is an autosome linked recessive trait.

· It can be transmitted from parents to the offspring when both the partners are carrier for the gene (or heterozygous).

· The disease is controlled by a pair of allele, Hb^A and Hb^S.

Homozygous dominant (Hb^AHb^A): normal

Heterozygous (Hb^AHb^S): carrier; sickle cell trait

Homozygous recessive (Hb^SHb^S): affected

· The defect is caused by the substitution of Glutamic acid (Glu) by Valine (Val) at the sixth position of the β-globin chain of the haemoglobin (Hb).

· This is due to the single base substitution at the sixth codon of the β-globin gene from GAG to GUG.

· The mutant Hb molecule undergoes polymerization under low oxygen tension causing the change in shape of the RBC from biconcave disc to elongated sickle like structure.

Phenylketonuria:

· An inborn error of metabolism.

· Autosomal recessive trait.

· The affected individual lacks an enzyme (phenylalanine hydroxylase) that converts the amino acid phenylalanine into tyrosine. As a result, phenylalanine accumulates and converts into phenyl pyruvic acid and other derivatives.

· They accumulate in brain resulting in mental retardation. These are also excreted through urine because of poor absorption by kidney.

2. Chromosomal disorders

They are caused due to absence or excess or abnormal arrangement of one or more chromosomes. 2 types:

a. Aneuploidy: The gain or loss of chromosomes due to failure of segregation of chromatids during cell division. It includes,

· Nullysomy (2n-2): A chromosome pair is lost from diploid set.

· Monosomy (2n-1): One chromosome is lost from diploid set.

· Trisomy (2n+1): One chromosome is added to diploid set.

· Tetrasomy (2n+2): 2 chromosomes are added to diploid set.

b.Polyploidy (Euploidy): It is an increase in a whole set of chromosomes due to failure of cytokinesis after telophase stage of cell division. This is often seen in plants.

Examples for chromosomal disorders

§ Down’s syndrome (Mongolism): It is the presence of an additional copy of chromosome number 21 (trisomy of 21).

- Genetic constitution: 45 A + XX or 45 A + XY (i.e. 47 chromosomes).

- Features:

o They are short statured with small round head.

o Broad flat face.

o Furrowed big tongue and partially open mouth.

o Many “loops” on finger tips.

o Palm is broad with characteristic palm crease.

o Retarded physical, psychomotor &mental development.

o Congenital heart disease.

§ Klinefelter’s Syndrome: It is the presence of an additional copy of X-chromosome in male.

- Genetic constitution: 44 A + XXY (i.e. 47 chromosomes).

- Features:

o Overall masculine development, however, the feminine development is also expressed. E.g. Development of breast (Gynaecomastia).

o Sterile.

o Mentally retarded.

§ Turner’s syndrome: This is due to the absence of one of the X chromosomes in female.

- Genetic constitution: 44 A + X0 (i.e. 45 chromosomes).

- Features:

o Sterile, Ovaries are rudimentary.

o Lack of other secondary sexual characters.

o Dwarf.

o Mentally retarded