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MENDEL’S LAWS OF INHERITANCE

The term ‘genetics’ was coined by W. Bateson (1905) and is the science that deals with
heredity and variation. Heredity is the transmission of traits from parents to offspring, whereas
variation can be hereditary (that are transmitted from generation to generation and arising mainly due to independent assortment of chromosomes and recombination in sexual reproduction
and to some extent due to mutations) or environmental (that do not get transmitted to next
generation).

A. Weisman (1834-1914) experimented on inheritance by cutting the tails of mice and
allowing them to breed. This was repeated for 22 generations and it was observed that complete tail
character was still inherited. He then proposed that the higher organisms contain two types of
tissues:

(i) Germplasm: This tissue is meant for reproduction and any change in these cells affects

the progeny, e.g., any change in egg or sperm will be transmitted to the offspring.

(ii) Somatoplasm: Its cells do not enter the sex cells and the variations arising here are not
transmitted to next generation, e.g. the tails of mice, therefore, the cut tail character was
not transmitted to the offsprings of mice with cut tails and they were born with intact tails.

Johannsen (1909) gave the ‘genotype-phenotype’ concept, according to which:

(i) Genotype is the sum total of heredity and two or more organisms with the same genotype (e.g., identical twins) can exhibit different phenotypes under different environmental
conditions (e.g., of the two identical twins, the one is nourished properly would have good
physique in comparison to the other one who is malnourished).

(ii) Phenotype (e.g., appearance) arises due to interaction between genotype and the environment (e.g., of the two identical twins that have same genotype, if one for most of the
time plays in the ground in sunlight would have dark complexion, whereas, the other
‘mostly staying at home would have comparatively fair complexion].

Many scientists tried to understand the principle of inheritance but they could not formulate the laws of inheritance as they did not give the numerical treatment to their results, e.g., Kolreuter (1733-1806) performed hybridization experiments on tall and dwarf varieties of Tobacco and got plants of intermediate height in Fl (first filial or progeny) generation. In F2 generation, obtained by
self pollination of Fl generation, tall plants of varying heights and dwarf plants were obtained. But, he did not count the number of dwarf and tall plants thus obtained.

Gregor John Mendel (1822-1884) was a teacher of Science and Math in Vienna and took interest in plant hybridization (Fig. 6.1). He is known as ‘Father of Genetics’ and formulated the laws of inheritance of traits in 1866 while experimenting on Pisum sativum (garden pea). But, at that time his work was not recognized until 1900 when Hugo de Vries, C. Correns and E. von Tschermak independently experimentally confirmed his laws. The secret of Mendel’s success was that he considered

one trait at a time and numerically treated his results by counting the number of different types of offsprings obtained in variouscrosses.

Gregor John Mendel the 'Father of Genetics'

Mendel selected garden pea as it has bisexual flowers (with both male and female reproductive parts in the same flower) and it reproduces mainly by self fertilization method (the male
gamete of the same flower fertilizes the female gamete), so that homozygous pure lines (e.g. DD
tall and dd dwarf plants) were easily obtained due to natural self fertilization. Moreover, Mendel
could select 7 pairs of contrasting traits for hybridization which was quite easier. Fortunately,
when he considered two (dihybrid cross) or more than two traits (trihybrid cross, and so on) at a time,
many these traits were located on different chromosomes (garden pea has 7 pairs of chromosomes, i.e. 2n = 14) to successfully show the independent assortment of these traits (Fig. 6.2 see
Appendix). These are also tabulated in Table 6.1.

Table 6.1: Traits selected by Mendel in garden pea for hybridization experiments

The two different forms of a trait (e.g., D and d in case of plant height, and the alphabet is
selected on the basis of initial letter of the mutant trait) are called alleles or allelomorphs (later, it was found that, actually, these alleles are the different forms of the same gene, e.g., d allele arose due to mutation in the normal allele D for plant height). There can also be possibility of occurrence of
more than two alleles of a gene when a gene mutates in more than one form. Most of the mutants are
recessive, except for, a few e.g., bar eye character in Drosophila. Since at that time no one was
aware of the term gene, Mendel called these trait determining genes as factors).

The dominant traits that can express in both homozygous (e.g., DD tall plant, the two doses
of D indicate the diploid nature of plant, where one D comes from father and the other from mother)
as well as heterozygous (e.g., Dd tall plant) conditions. Here, DD are the normal wild type, whereas,
the recessive (dd) ones are the small population of mutants that can express only in homozygous
condition (e.g., dd dwarf plant) in the absence of its dominant allele.

The crosses where only one trait is considered at a time (e.g., plant height) are known as
monohybrid crosses, whereas, those crosses are called dihybrid, trihybrid, etc. where two,
three, or more traits are considered together.

While crossing the two contrasting characters of a trait, the flower that was used as female
was emasculated (Fig. 6.3 see Appendix) (anthers removed before reaching maturity) to avoid self
fertilization and covered with a polythene or muslin cloth bag to inhibit undesired cross-pollination.
The cross was performed by collecting the pollen grains of the flower to be treated as male and
dusting them over the stigma of emasculated flower.

When Mendel crossed the seven pairs of contrasting characters, considering one pair at a
time (monohybrid cross), it was found that reciprocal crosses (e.g., in one cross if DD taken as
male and dd as female, in another cross DD was selected as female and dd as male) gave the same
results, showing that both male and female parents make equal genetic contribution to the progeny.

 

MENDEL’S LAWS OF INHERITANCE

By performing various crosses of the seven pairs of traits, Mendel formulated two laws of

inheritance:

1. Law of Purity of Gametes or Principle of Segregation

When the homozygous dominant (e.g., DD tall) and recessive (dd dwarf) plants for a trait
were crossed, all the FI progeny (Dd heterozygous tall), thus, obtained exhibited only the dominant
phenotype.

When such Fl plants were self-fertilized (pollens of the same flower pollinated its stigma),
about 75% of F2 progeny were tall, whereas about 25% were dwarf. This shows that the trait that
appeared in FI was dominant over the trait that did not appear in F1 but reappeared in 25% of F2 progeny (recessive).

This result would only be possible if F1 produces two types of gametes for a trait (both male as well as female), D and d or it can be said that the gametes are pure for a trait and would contain either D or d allele (Law of purity of gametes). [Actually, at the time of gamete formation, the two doses of D and d in FI plants go to different gametes due to meiosis]. Thus, the two alleles D and d
get segregated or separated at the time of gamete formation (Principle of segregation).

Again, the self feltilization of F2 dwarf (dd) gave rise to only dwarf plants in F3 generation,
whereas, among the F2 tall plants 33% gave only tall plants (TT) and rest of the 66% gave both tall
(75%) as well as dwarf (25%) plants. This confirms that all the FI plants had the genotype Dd and
among the F2 individuals, 25% had DD (homozygous tall), 50 had Dd (heterozygous all) and
25% had dd (homozygous dwarf) genotypes. Thus, in F2 generation though the phenotypic ratio
(on the basis of external appearance) of tall and dwarf plants was 3 : 1, the genotypic ratio
was 1 : 2 : 1 (1 DD:2 Dd:l dd) (Fig. 6.4).

Fig.-6.4

2. Principle of Independent Assortment

 

When two (dihybrid crosses) or more than two (trihybrid and so on) traits are considered
together (e.g. yellow and green and round and wrinkled seed traits in a dihybrid cross), one pair of
trait (e.g. yellow and green seed colour) behaves completely independent of the other trait (e.g.
round and wrinkled seeds). Such crosses of homozygous dominant (e.g. GGWW yellow-round seeds)
and recessive parents (e.g. ggww green-wrinkled seeds), like the monohybrid crosses, gave only
dominant phenotype (GgWw yellow-round seeds) in FI generation. This FI upon self fertilization
gave 9 : 3 : 3 : 1 phenotypic ratio (9 yellow-round: 3 yellow-wrinkled: 3 green-round: 1 green-
wrinckled) with gnotypic ratio of 4: 2 : 2 : 2 : 2 : 1 : 1 : 1 : 1 in F2 generation, which is possible only
when FI would give rise to all the possible four types of gametes (GW, Gw, gW and gw) due to
independent assortment of various alleles (actually, at the time of gamete formation during meiosis,
the various types of maternal and paternal chromosomes can get arranged independent of each other
in all the possible combinations to give different types of gametic cells). This is known as principle
of independent assortment
(Fig. 6.5).

 

Pl GGWW ggww P2
Yellow-round seeds Green-wrinkled seeds
GgWw FI
Yellow-round seeds
Fl:
Male →

Gametes

Female ↓

GW

Gw

gW

gw

Gametes

GW

GGWW

GGWw

GgWW

GgWw

Gw

GGWw

GGww

GgWw

Ggww

gW

GgWW

GgWw

ggWW

ggWw

GgWw

GgWw

Ggww

ggWw

ggww

 

 

 

Genotype

Genotypic ratio

Phenotypic ratio

Phenotype

GgWw

4

9

Yellow-round

GGWw

2

GgWW

2

GGWW

1

Ggww

2

3

Yellow- wrinkled

GGww

1

ggWw

2

3

Green-round

ggww

1

ggww

I

1

Green-wrinkled

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

9: 3 : 3 : I

 

Fig. 6.5 Dihybrid cross in garden pea considering two traits, seed colour and shape at a time.

 Similarly, in a trihybrid cross, the FI gives rise to eight types of possible gametes due to

independent assortment of three pairs’ of traits. These gametes after fertilization gave a 27 : 9 : 9 :
9 : 3 : 3 : 3 : 1 phenotypic ratio in F2 generation.

Thus, when one pair of trait is considered, FI produces two types of gametes (n = 1, 21 = 2),
when two pairs of traits are considered FI gives four types of gametes (n. = 2,22 = 4), when three
pairs of traits are considered, FI gives 8 types of gametes (n. = 3, 23 = 8), and so on or it can be said
that FI can give 2n possible types of gametes, where n. is the number of pairs of traits considered at
a time. (Independent assortment is not shown when the genes for two or more traits are located on
the same chromosome (linked) as they would always move together at the time of meiosis)

Back Cross and Test Cross

When Fl (e.g., Dd) is crossed either with homozygous dominant (e.g. DD) or with homozygous
recessive (e.g., dd) parent, it is called back cross.

When FI is crossed with homozygous dominant parent, all the resulting progeny exhibit only
dominant phenotype (e.g., tall plants with 50% DD and 50% Dd genotype).

But, when FI is crossed with homozygous recessive parent, 50% of progeny show dominant
phenotype (e.g., Dd) and rest of the 50% recessive (e.g., dd genotype) phenotype. Since this cross
is performed to know whether an individual is homozygous dominant or heterozygous dominant, it is
known as test cross. Thus, when a homozygous dominant (e.g., DD) individual is back crossed with
homozygous recessive individual, 100% of the resulting progeny would be of dominant phenotype (e.g., 50% DD and 50% Dd). But when a heterozygous dominant (e.g., Dd) individual is back crossed with a homozygous recessive individual, 50% of progeny shows dominant phenotype (e.g., Dd) whereas 50% recessive (e.g.,dd) phenotype (Fig.6.6)

a)                              Dd                                                                      DD

F1                                      ↓                                P1

 

                                                      Gametes

Male →

D

D

Female ↓

Gametes

D

DD

DD

d

Dd

Dd

 

50% DD + 50% Dd →7 100% tall

Back cross with homozygous dominant parent

b)                              Dd                                                                      dd

F1                                      ↓                                P2

 

Gametes

Male →

d

d

Female ↓

Gametes

D

Dd

Dd

d

dd

dd

50% Dd + 50% dd

Tall           Dwarf

Back cross (test cross) with homozygous recessive parent.

Fig. 6.6 Monohybrid back crosses.

Similarly in dihybrid crosses, where two traits are considered together, the back cross of FI
(e.g., GgWw yellow-round seeds) with homozygous dominant parent (e.g., GGWW yellow-round
seeds) would give 100% dominant phenotype.

Whereas, test crossing of PI with homozygous recessive parent (e.g., ggww wrinkled seeds) gives 1 : 1 : 1 : 1 ratio in the progeny (Fig. 6.7).

 

a)      GgWw                                                                GGWW

F1                                      ↓                                P1

 

                                                      Gametes

Male →

GW

GW

Female ↓

Gametes

GW

GGWW

GGWW

Gw

GGWw

GGWw

gW

GgWW

GgWW

gw

GgWw

GgWw

100% Yellow-round seeds

Back cross with homozygous dominant parent.

b)      GgWw                                                                ggww

F1                                      ↓                                P1

 

                                                      Gametes

Male →

gw

gw

Female ↓

Gametes

GW

GgWw

GgWw

Gw

Ggww

GgWw

gW

ggWw

ggWw

Gw

ggww

ggww

 

Phenotypic ratio = 1 : 1 : 1 : 1

Back cross (test cross) with homozygous recessive parent.

Fig. 6.7 Dihybrid back crosses.

 Deviations from Mendel’s Laws

The results of Mendel, as found in garden pea, are not universal, except for the law of purity of gametes, and various traits in different organisms exhibit different behaviors e.g.,

1.Incomplete Dominance: In many cases the dominant allele for a trait is not completely dominant over its recessive allele, e.g., in Mirabilis jalapa (4’O clock plant) the red flower colour-imparting allele R does not exhibit its complete dominance over its recessive allele r that does not characterize any colour to the flower. Thus, the plants with Rr genotype bear pink flowers that is an intermediate phenotype between red and pink (Fig. 6.8).

 

RR                               ×                                  rr

Red Flower                                                      White flower

Rr

Pink flower                                                              F1

F2

 

RR                               Rr                           rr

25%                             50%                       25%

 

Fig. 6.8 Incomplete dominance in Mirabilis jalapa.

 

  1. Interaction of Genes: Sometimes, many genes interact in different ways to control a
    trait, e.g. epistasis, where one gene marks the effect of other gene.

    1. Linkage: Independent assortment is exhibited only when the two or more pairs of traits
      are located on different chromosomes and is not shown when they are located on the
      same chromosome (linked) as they would always move together at the time of meiosis.
      For example, the gene for yellow/green seed trait was located on one pair of chromosomes
      whereas, the gene for round/wrinkled trait on different chromosome pair, therefore, they
      exhibited independent assortment. But, it they were located on the same chromosome
      pair they would not show any independent assortment of traits (Fig. 6.9).

 


 

G|         |g                                  G|         |g

W|        |w

F1

Gig and W/w located on different             Gig and W/w located on
homologous chromosome pairs                             same homologoues

                            ↓                                                ↓

G,g,W and w would exhibit                      G,g,W and w would not exhibit
independent assortment                                         independent assortment a

GW would move together

and gw again move together

Fig 6.9 Inheritance of linked (right) non-linked (left) genes.