Top 3 Fundamental Laws of Genetics

The following points highlight the three fundamental laws of genetics proposed by Mendel. The laws are: 1. Law of Segregation 2. Law of Dominance 3. Law of Independent Assortment and Di-Hybrid Cross.

1. Law of Segregation:

According to Altenburg, this law may be defined as “Non-mixing of alleles i.e., the allele for tallness does not mix with the allele for dwarfness in the hybrids.” Offspring’s arising from two parents receive contributions of hereditary characteristics from them through gametes. These gametes are the connecting links between successive generations.

The contrasting characters such as tall and dwarf stems of peas are determined by something that is transmitted from the parents to the offspring through the gametes are called factors or genes. The important point is that different factors such as those for tallness and dwarfness (D and d) do not blend, contaminate or mix with each other while they remain together in the hybrid.

Instead, the different factors separate or segregate pure and uncontaminated passing to two different gametes produced by the hybrid and then transmit to the different individuals or the offspring’s of the hybrid. Each gamete carries one of the two members of a pair of contrasting or alternative factors i.e., either for tallness or dwarfness (D or d) and never both.

D d (F₁ hybrid tall) → factor D and d remain together pure

The simplest conventional or custom method of denoting these Mendelian factors is to give each a letter, the dominant factor being represented by capital letter and recessive by small letter. In the cross of pure bred tall and dwarf plants let D stand for the gene for tallness and d for alternate form of this gene which results in dwarfness of the stem. D and d are called alleles or allelomorphs.

Since an individual develops from the union of two gametes produced by the male and female parents. It receives two alleles D and d. The true breeding tall plant may be represented as DD and its gamete as D and the true breeding dwarf plant as dd and its gamete as d.

When the two plants are crossed an egg (D) is fertilized by the male gamete (d) or vice- versa. The resulting hybrid zygote will have both D and d. Thus the two alleles of a gene are represented by the same gene symbol and are differentiated from each other by their first letter being in capital or small (D or d).

A gene can be represented by a symbol derived from the name of the character it governs. The gene controlling length of stem as dwarf in pea may be represented by the small letter ‘d’ and the symbol for the allele producing the dominant form of character is the same as that for the recessive allele, but the first letter of this symbol is in capital. For example, tall stem is dominant and is assigned D

Factors

D = factor for Tallness

d = factor for Dwarfness

According to the principle of segregation the alleles borne by the heterozygous tall plant (Dd) do not mix, fuse, blend or contaminate with each other, despite the fact that the phenotype of the F₁ hybrid shows only the tall character, and it fails to give any visible indication of the presence of the gene (d) in the genotype. The alleles segregate when the hybrid organism produces gametes so that approximately half of the gametes will carry D and the other half d.

In fertilization the gametes combines at random. There is an equal opportunity for the different types of gametes to unite with each other. The male gamete may unite or fuse with female gamete with either D or d. The other kind of male gamete ‘d’ may also have an equal opportunity to unite or fuse with the female gamete D or d. Hence four recombination’s occur. One fourth (1/4) of them are homozygous tall plants having only the allele for tallness (DD).

The other half of them (two out of four) are heterozygous having both the alleles D and d. Since D is dominant over d, these plants are tall. One fourth (1/4) of them are homozygous plants having only the allele for dwarfness (dd). In F₂ generation, tall and dwarf plants appear in the ratio 3 : 1 (3/4 tall and 1/4 dwarf plants).

Mendel tested the validity of factor hypothesis by applying further strict method by means of which it could be confirmed or disproved. In the F₂ of his cross of tall plants with dwarf plants there were tall and dwarf plants approximately in the ratio of 3:1. Mendel’s interpretation of these results by means of the law of segregation shows that there are two kinds of F₂ tall plants.

About 1/3 of them should be genotypically homozygous for tallness (DD). About 2/3 should be heterozygous (Dd) carrying both the dominant and recessive alleles (D and d). The validity of these predictions can be tested in actual experiments. The homozygous dwarf plants should breed true through all subsequent generations if self- fertilized or crossed with other.

All the plants although they look alike would not behave in the same way. About 1/3 of them homozygous with the genetic formula (DD) should breed true. But 2/3 of the F₂ tall plants, the heterozygotes (Dd) should breed exactly like the F₁ hybrid plants. They should produce tall and dwarf plants in the phenotypic ratio 3: 1 and the genotypic ratio 1:2:1. This is what Mendel obtained in his experiments. Thus the law of segregation has been confirmed in actual experiments.

Summary:

Characters become separated or segregated in the second filial (F₂) generation. Thus the factors responsible for hereditary characters are independent units, which although enter the crosses together but segregate out again as distinct characters. This law is by far the most important of Mendel’s discoveries. This law is some times called as the law of purity of gametes or law of the splitting of hybrids.

Or

(Law of segregation means that when a pair of allelomorphs are brought together in the hybrid (F₁), they remain together in the hybrid without blending and in F₂ generation they separate complete and pure during gamete formation. This law is also known as law of the purity of gametes).

Or

(The two alleles present in the F₁ are able to separate and pass in to separate gametes in their original form producing two different types of gametes in equal frequencies; this is known as segregation).

Main facts about segregations:

To summarize Mendel’s monohybrid cross experiment, following cardinal points are notable:

1. Existence of genes:

The hereditary differences among the individuals depend upon the difference in cellular units of genes or factors. These genes are hereditary units, control a particular character and are present at a fixed place in the chromosomes called loci. Thus genes for tall character in the peas shown by ‘D’ in chromosome is at a fixed locus and genes for dwarf character ‘d’ is at the same locus in the other chromosome.

2. Purity of gametes:

Law of segregation itself shows the purity of gametes and their freedom from mixing or blending with each other. The gametes contain only one factor or gene and are pure for a particular trait or character governed by the same factor or gene of gamete.

3. Non-mixing of alleles in hybrids:

These genes or factors of heredity, whatever the nature may be, unite when derived from different parental sources in the hybrids from which they may be separated out during successive or subsequent generation and unmodified with the presence of other alleles in hybrids.

In summary, the cross between tall and dwarf pea is as follows:

The original tall and dwarf variety of pea constitute the first parental generation (P₁). The hybrids produced by their cross constitute first filial generation (F₁) and offspring of the hybrids constitute second filial or F₂ generation.

Johansen (1911) proposed the following four terms to distinguish individuals among themselves:

1. Homozygous

2. Heterozygous

3. Phenotype and Genotype.

1. Homozygous:

An organism or hybrid or zygote in which both members of a pair of genes are alike (DD or dd) are referred to as homozygous (Greek: Homos = alike = zygos, yoke (bond or under bondage of another).

Individuals having identical genes (DD or dd) are called homozygous. Homozygous are always pure.

2. Heterozygous:

An organism or hybrid or zygote in which both members of a pair of genes are unlike (Dd) are termed as heterozygous (heteros = dissimilar). Heterozygous individuals are always hybrid. In the F₂ generation, there is a ratio of 3 tall and 1 dwarf plant apparently but genetically, this ratio is 1 DD tall: 2 Dd tall: 1 dd dwarf.

3. Genotype and Phenotype:

Genotype is the term used to denote genetic constitution of an organism. It represents the total hereditary possibilities within the individual. In the monohybrid cross experiments, the hybrid plant of F₁ generation is phenotypically tall but genetically it is a hybrid (Dd).

The external morphological feature of an organism constitute its phenotype or it is the term used to denote the visible characteristics of an organism or individual. It represents the sum total of all apparent characteristics of an organism regardless of it genetic make up or genotype.

In the F₂ generation, 3 out of 4 (3/4) are phenotypically tall but genotypically one third (1/3) of them is pure tall and two third (2/3) hybrid tall with two contrasting allele.

What we observe or which is visible or otherwise measurable are called phenotypes. While the genetic factors responsible for creating the phenotype are called genotype. Phenotype is determined by the dominant alleles.

Monohybrid Back cross or Test Cross:

The cross between the F₁ hybrid (Dd) to one of its parents (DD or dd) is called back cross while cross between F₁ hybrid (Dd) and homozygous recessive parent (dd) is called test cross since it confirms the purity of gametes.

(i) The above cross between homozygous dominant (DD) and hybrid (Dd) is called dominant back cross and (ii) Cross between homozygous recessive (dd) and hybrid (Dd) is called recessive back cross. This recessive back cross has great importance in experimentation because phenotypic and genotypic ratios are identical. Hence recessive back cross is termed test cross to identify or test gamete nature or whether an individual is homozygous or heterozygous as shown below.

In case of Back cross:

Diagram showing Monohybrid back cross between F₁ hybrid and dominant homozygous parent

Phenotype – 2 Tail: 2 dwarf (50% tall and 50% dwarf)

Genotype – 2 Tall: 2 dwarf (50% tall and 50% dwarf)

Diagram showing Monohybrid test cross between F₁ hybrid and recessive homozygous parent (1 : 1).

2. Law of Dominance:

Mendel’s first experiments were crosses between pea varieties differing in only one visible character. These are monohybrid cross experiments.

A heterozygote (F₁ hybrid) contains two contrasting genes, but only one of the two is able to express itself, while the other remain hidden. The gene which is able to express itself in F₁ hybrid is known as dominant gene, while the other gene which is unable to express itself in presence of the dominant gene is the recessive gene. No doubt recessive gene is unable to express itself, but is transmitted to the next generation without change.

When Mendel crossed true breeding tall peas, with true breeding dwarf peas the first offspring’s formed were all tall plants.

The dwarf character appears to have been suppressed and tallness seems to dominate. Such characters like tallness, redness, roundness of seeds, yellow coloured cotyledons, inflated seed pods, green unripe pods and axial flowers, were called dominants and their respective alleles as dwarfness, whiteness, wrinkledness of seeds, green coloured cotyledons, constricted seed pods, yellow unripe pods and terminal flowers were called recessives.

The law of dominance, thus states that out of a pair of a allelomorphic characters (= alternative or contrasting characters) one is dominant and other recessive. Mendel found this fact to be true between all the seven pairs of characters studied by him. The pair of contrasting or alternative characters are called allelic pair or allelomorphic pair and each member of the pair may be regarded the allele of the other.

Thus the tallness and dwarfness are alleles of each other. The hereditary units which are responsible for the appearance of character in the offsprings or progenies have been called factors or determiners. Now these are called genes.

Four types of dominance are seen:

1. Co-dominance:

The phenomenon in which both alleles are expressed in the hybrid (F₁) is called co-dominance. Blood group antigens of man is one of the best example of Co-dominance. It produces 1:2:1 ratio in F₂.

2. Complete dominance or simple dominance:

It is the ability of one allele to mask or inhibit the presence of another allele at the same locus in the heterozygote or F₁ hybrid.

3. Incomplete dominance:

If the F₁ hybrids or heterozygotes are phenotypically intermediate between both homozygous type.

4. Over dominance:

The superiority of heterozygote or hybrid over its both homozygotes or parents (DD and dd) is termed as over dominance. Unlike complete, partial and co-dominance, over-dominance is not the characteristics of an allele but is the consequence of the heterozygous condition of the related gene.

3. Law of Independent Assortment and Di-Hybrid Cross:

Mendel discovered not only crosses in which the parent differed in single pair or characters, but also others in which the parent differed in two pairs. Such a cross, which includes two pair of contrasting characters at a time is called di-hybrid cross. The law of independent assortment is applicable to the inheritance of two or more pair of characters.

For a di-hybrid experiment, Mendel crossed two pea plants, one of which was homozygous for yellow and round seeds and the other for green and wrinkled seeds. Genes for yellow and round characters were dominant over the green and wrinkled characters described by the Mendel. The F₁ hybrids produced as a result of this cross were yellow round which were heterozygous for both the alleles known as Di-hybrid.

Genotypes and Phenotypes of F₂ offsprings:

The above phenotypic ratio, which Mendel obtained may be thought of as a monohybrid phenotypic ratio 3 : 1 multiplied algebraically by 3 : 1 that means (3: 1) x (3: 1) = 9: 3: 3: 1.

Mendel’s Explanations:

Although Mendel was not aware with the behaviour of chromosomes during meiosis even then he assumed that the members of each two pairs of factors (WW, ww) for the two pairs of contrasting characters (round/wrinkled) are separated independently or freely of the members of the other pair.

In brief, according to Mendel at the time of reduction division during gamete formation, the members of each chromosome (= genes or factors) pair segregate (or separate) from one another.

They do not dilute or affect the other pair and behave independently. The separation of chromosomes or genes belonging to one pair without reference to those belonging to the other pair at reduction division is known as independent assortment (or separation) of genes.

The dihybrid (GgWw) produces four kinds of gametes (parental or non-parental types or crossover or non-crossover types) namely GW, Gw, gW, gw which by self fertilization produced F₂ generation in 16 possible ways. Since G (Yellow) and W (round) are dominant characters so whatever genes (G or W) will be, the seeds will show dominant characters.

Genotypically, typical di-hybrid will show following ratio:

1GGWW : 2 GgWW : 2 GGWw : 4 GgWw : 1 ggWW : 2 ggWw : 1 GGww : 2 Ggww : 1 ggww. Their phenotypic ratio will be 9 Yellow round: 3 Yellow wrinkled: 3 Green round: 1 Green wrinkled.

Fractional Method of Calculated Ratio:

The checker board method of determining Mendelian ratio given by Punnet is useful in certain aspects. It represents graphically all the essential steps like formation of gametes, their union to form zygotes and resulting phenotypes. But its disadvantage is that it is time consuming and many other errors may come in it. Therefore, M.D. Jones (1947) described fractional method to determine ratios which is algebraic in nature.

(i) GgWw – F₁ hybrid

(ii) F₂ di-hybrid phenotypes:

(a) 3/4 G x 3/4 W – 9/16 GW

(b) 3/4 G x 1/4 w – 3/16 GW

(c) 1/4 g x 3/4 W – 3/16 gW

(d) 1/4 g x 1/4 w – 1/16 gW

The genotypic ratios may be obtained by dividing the dominants in to homo and heterozygotes i.e.,

Di-hybrid test cross:

If we cross the di-hybrid (GgWw) with the homozygous recessive parent (ggww) then di-hybrid will produce four types of gametes (GW, Gw, gW, gw) while green wrinkled seeds will form only one type of gamete (gw).

This gamete becomes fused with four types of gametes thus producing four classes of offsprings as follows:

1 Yellow round: 1 Yellow wrinkled: 1 Green round: 1 Green wrinkled

Thus, a dihybrid test cross will give a genotypic and phenotypic ratio of 1: 1: 1: 1 because four different types of gametes will be produced by the F₁ hybrid in equal numbers.

Poly-hybrid cross:

In case of di-hybrid cross, Mendel demonstrated the independent assortment (or segregation) of factors or genes. Likewise, tri-hybrid experiments were carried out by Mendel involving three pairs of characters.

For instance, he took yellow round grey seeds and crossed them with green wrinkled white seeds, the F₁ progeny will be heterozygous for three genes and will phenotypically resemble the dominant parent. Each of these F₁ progeny will produce 8 types of gametes and therefore 64 combinations of F₂ progeny.

Results of tri-hybrid cross worked out by the forked line method:

Genotypes of F₂ and their relative proportions:

Phenotypes of F₂ and their relative proportions:

A tri-hybrid test cross will give a phenotypic and genotypic ratio of 1: 1: 1: 1: 1: 1: 1: 1, because 8 different types of gametes and in equal numbers will be produced by the F₁ hybrid. Test crosses are of great importance since they yield or produce same genotypic and phenotypic ratios.

It is obvious from foregoing descriptions that the number of heterozygous genes involved in a cross increases the number of types of gametes and the number of types of F₂ progeny.

Phenotypes; GgWwCc, GgWwcc, GgwwCc, Ggwwcc, ggWwCc, ggWwcc, ggwwCc, ggwwcc.