PRINCIPLES OF INHERITANCE AND VARIATION

Haemophilia : This sex linked recessive disease, which shows its

transmission from unaffected carrier female to some of the male progeny

has been widely studied. In this disease, a single protein that is a part

of the cascade of proteins involved in the clotting of blood is affected.

Due to this, in an affected individual a simple cut will result 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 extremely rare because mother of such a female has to be at least

carrier and the father should be haemophilic (unviable in the later stage

of life). The family pedigree of Queen Victoria shows a number of

haemophilic descendents as she was a carrier of the disease.

Sickle-cell anaemia : This is an autosome linked recessive trait that 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 single

pair of allele, Hb

and Hb

. Out of the three possible genotypes only

homozygous individuals for Hb

(Hb

) show the diseased phenotype.

Heterozygous (Hb

) individuals appear apparently unaffected but they

are carrier of the disease as there is 50 per cent probability of transmission

of the mutant gene to the progeny, thus exhibiting sickle-cell trait (Figure

5.15). The defect is caused by the substitution of Glutamic acid (Glu) by

Figure 5.15 Micrograph of the red blood cells and the amino acid composition of the relevant portion

-chain of haemoglobin: (a) From a normal individual; (b) From an individual with

sickle-cell anaemia

2015-16

PRINCIPLES OF INHERITANCE AND VARIATION

Haemophilia

: This sex linked recessive diseas

which shows its

transmission from unaffected carrier female to some of the male progeny

has been widely studied. In this disease, a single protein that is a part

of the cascade of

oteins involved in the clottin

of blood is affected.

Due to this, in an affected individual a simple cut will result 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 extremely rare because mother of such a female has to be at least

carrier and the father should be haemophilic (unviable in the later stage

of l

haem

Sick

be t

carr

pair

homo

Hete

are

of the

5.15

of the r

ed blood cells and the amino acid co

osition of the r

elevant

rtion

-chain of haemoglobi

: (a) From a normal individual; (b) From an individual wit

-c

anaemi

2015-1

rrier and the father should be haemophilic (unviable in the later stage

life). The family pedigree of Queen Victoria shows a number of

emophilic descendents as she was a carrier of the disease.

ckle-cell anaemi

This is an autosome linked recessive trait that can

transmitted from parents to the offspring when both the partners are

rrier for the gene (or heterozygous). The disease is controlled by a single

ir of allele, Hb

. Out of the three possible genotypes only

us individuals for Hb

(Hb

) show the diseased phenot

terozy

us (Hb

) individuals appear apparently unaffected but they

carrier of the disease as there is 50 per cent probabili

of transmission

he mutant gene to the progeny, thus exhibiting sickle-cell trait (Figure

15). The defect is caused by the substitution of Glutamic acid (Glu) by

BIOLOGY

Valine (Val) at the sixth position of the beta globin chain of the haemoglobin

molecule. The substitution of amino acid in the globin protein results

due to the single base substitution at the sixth codon of the beta globin

gene from GAG to GUG. The mutant haemoglobin molecule undergoes

polymerisation under low oxygen tension causing the change in the shape

of the RBC from biconcave disc to elongated sickle like structure (Figure

5.15).

Phenylketonuria : This inborn error of metabolism is also inherited as

the autosomal recessive trait. The affected individual lacks an enzyme

that converts the amino acid phenylalanine into tyrosine. As a result of

this phenylalanine is accumulated and converted into phenylpyruvic acid

and other derivatives. Accumulation of these in brain results in mental

retardation. These are also excreted through urine because of its poor

absorption by kidney.

5.6.3 Chromosomal disorders

The chromosomal disorders on the other hand are caused due to absence

or excess or abnormal arrangement of one or more chromosomes.

Failure of segregation of chromatids during cell division cycle results

in the gain or loss of a chromosome(s), called

aneuploidy. For example,

Down’s syndrome results in the gain of extra copy of chromosome 21.

Similarly, Turner’s syndrome results due to loss of an X chromosome in

human females. Failure of cytokinesis after telophase stage of cell division

results in an increase in a whole set of chromosomes in an organism and,

Figure 5.16 A representative figure showing an individual inflicted with Down’s

syndrome and the corresponding chromosomes of the individual

Flat back of head

Many “loops” on

finger tips

Palm crease

Broad flat face

Big and wrinkled

tongue

Congenital heart

disease

2015-16

BIOLOG

aline (V

al) at the sixth position of the beta globin chain of the haemoglobi

(V(V

molecule. The substitution of amino acid in the globin protein resul

due to the single base substitution at the sixth codon of the beta

lobi

ne from GAG to GUG. The mutant haemoglobin molecule unde

lymerisation under low o

n tension causin

the chan

in the sh

of the RBC from biconcave disc to elon

ted sickle like structure (F

5.15).

Phenylketonuria

: This inborn error of metabolism is also inherited

enzy

esult of

uvic aci

menta

its poor

absenc

result

ome 21.

ome

divisi

m and,

Figure 5.16

A representative figure showing an individual inflicted with Down’s

syndrome and the corresponding chromosomes of the individual

2015-1

9900

Phenylketonuria

the autosomal recessive trait. The affected individual lacks an

that converts the amino acid

lalanine into

rosine. As a r

this phenylalanine is accumulated and converted into phenylpyr

and other derivatives. Accumulation of these in brain results in

retardation. These are also excreted through urine because of

absorption by kidney.

5.6.3 Chromosomal diso

ers

The chromosomal disorders on the other hand are caused due to

or excess or abnormal arrangement of one or more chromosomes.

Failure of s

tion of chromatids durin

cell division

cle

in the gain or loss of a chromosome(s), called

aneu

. For ex

Down’s syndrome results in the gain of extra copy of chromos

Similarly, Turner’s syndrome results due to loss of an X chromos

human females. Failure of cytokinesis after telophase stage of cell

results in an increase in a whole set of chromosomes in an organis

Flat bac

ack of head

Many

ny “loops” on

finger

er tips

Palm

lm crease

d fl

and wrinkled

tong

Congenital heart

disease

PRINCIPLES OF INHERITANCE AND VARIATION

this phenomenon is known as polyploidy. This

condition is often seen in plants.

The total number of chromosomes in a

normal human cell is 46 (23 pairs). Out of these

22 pairs are autosomes and one pair of

chromosomes are sex chromosome.

Sometimes, though rarely, either an additional

copy of a chromosome may be included in an

individual or an individual may lack one of any

one pair of chromosomes. These situations are

known as trisomy or monosomy of a

chromosome, respectively. Such a situation

leads to very serious consequences in the

individual. Down’s syndrome, Turner’s

syndrome, Klinefelter’s syndrome are common

examples of chromosomal disorders.

Down’s Syndrome : The cause of this genetic

disorder is the presence of an additional copy

of the chromosome number 21 (trisomy of 21).

This disorder was first described by Langdon

Down (1866). The affected individual is short

statured with small round head, furrowed

tongue and partially open mouth (Figure 5.16).

Palm is broad with characteristic palm crease.

Physical, psychomotor and mental

development is retarded.

Klinefelter’s Syndrome : This genetic disorder

is also caused due to the presence of an

additional copy of X-chromosome resulting into

a karyotype of 47, XXY. Such an individual has

overall masculine development , however, the

feminine development (development of breast,

i.e., Gynaecomastia) is also expressed (Figure

5.17 a). Such individuals are sterile.

Turner’s Syndrome : Such a disorder is

caused due to the absence of one of the X

chromosomes, i.e., 45 with X0, Such females

are sterile as ovaries are rudimentary besides

other features including lack of other secondary

sexual characters (Figure 5.17 b).

Figure 5.17 Diagrammatic represe-

ntation of genetic disorders due to sex

chromosome composition in humans :

(a) Klinefelter Syndrome; (b) Turner’s

Syndrome

Tall stature

with feminised

character

Short stature and

underdeveloped

feminine character

(a)

(b)

2015-16

PRINCIPLES OF INHERITANCE AND VARIATION

this phenomenon is known as

polyploidy

. This

condition is often seen in plants.

The total number of chromosomes in a

normal human cell is 46 (23 pairs). Out of these

22 pairs are autosomes and one pair of

romosomes are sex

romosome

Sometimes, though rare

, either an additional

copy of a chromosome may be included in an

indi

one

know

chro

lead

indi

synd

exam

Down

diso

of the

This

Down

stat

tong

Palm

Phys

deve

Klin

is a

addi

a ka

over

femi

i.e.,

5.17

Turn

chro

are sterile as ovaries are rudimentary besides

other features including lack of other secondary

sexual characters (Figure 5.17 b).

sex

ns :

’s

cter

2015-1

dividual or an individual may lack one of any

e pair of chromosomes. These situations are

own as trisomy or monosomy of a

romosome, respectively. Such a situation

ads to very serious consequences in the

dividual. Down’s syndrome, Turner’s

ndrome, Klinefelter’s syndrome are common

amples of chromosomal disorders.

wn’s Syndrome

: The cause of this genetic

sorder is the presence of an additional copy

the chromosome number 21 (trisomy of 21

is disorder was first described by Langdon

wn (1866). The affected individual is short

atured with small round head, furrowed

ngue and partially open mouth (Figure 5.16

lm is broad with characteristic palm creas

ysical, psychomotor and mental

velopment is retarded.

inefelter’s Syndrome

: This genetic disorder

also caused due to the presence of an

ditional copy of X-chromosome resulting into

karyotype of 47, XXY

. Such an individual has

erall masculine development , however

the

minine development (development of breas

, Gynaecomastia) is also expressed (Figure

17 a

Such individuals are sterile.

rner’s S

drome

ed d

f th

romosomes, i.e., 45 with X0, Such females

e sterile as ovaries are rudimentary besides

.17

Diagrammatic repr

ntnt

atat

ioio

of genetic disorders due to

chch

roro

mosome composition in huma

)

Klinefelter Syndrome; (b) Turn

rome

with feminised

character

underdevelop

feminine chara

(a)

(b)

BIOLOGY

SUMMARY

Genetics is a branch of biology which deals with principles of inheritance

and its practices. Progeny resembling the parents in morphological and

physiological features has attracted the attention of many biologists.

Mendel was the first to study this phenomenon systematically. While

studying the pattern of inheritance in pea plants of contrasting

characters, Mendel proposed the principles of inheritance, which are

today referred to as ‘Mendel’s Laws of Inheritance’. He proposed that

the ‘factors’ (later named as genes) regulating the characters are found

in pairs known as alleles. He observed that the expression of the

characters in the offspring follow a definite pattern in different–first

generations (F

), second (F

) and so on. Some characters are dominant

over others. The dominant characters are expressed when factors are

in heterozygous condition (Law of Dominance). The recessive characters

are only expressed in homozygous conditions. The characters never

blend in heterozygous condition. A recessive character that was not

expressed in heterozygous conditon may be expressed again when it

becomes homozygous. Hence, characters segregate while formation of

gametes (Law of Segregation).

Not all characters show true dominance. Some characters show

incomplete, and some show co-dominance. When Mendel studied the

inheritance of two characters together, it was found that the factors

independently assort and combine in all permutations and

combinations (Law of Independent Assortment). Different combinations

of gametes are theoretically represented in a square tabular form known

as ‘Punnett Square’. The factors (now known as gene) on chromosomes

regulating the characters are called the genotype and the physical

expression of the chraracters is called phenotype.

After knowing that the genes are located on the chromosomes, a

good correlation was drawn between Mendel’s laws : segregation and

assortment of chromosomes during meiosis. The Mendel’s laws were

extended in the form of ‘Chromosomal Theory of Inheritance’. Later, it

was found that Mendel’s law of independent assortment does not hold

true for the genes that were located on the same chromosomes. These

genes were called as ‘linked genes’. Closely located genes assorted

together, and distantly located genes, due to recombination, assorted

independently. Linkage maps, therefore, corresponded to arrangement

of genes on a chromosome.

Many genes were linked to sexes also, and called as sex-linked

genes. The two sexes (male and female) were found to have a set of

chromosomes which were common, and another set which was

different. The chromosomes which were different in two sexes were

named as sex chromosomes. The remaining set was named as

autosomes. In humans, a normal female has 22 pairs of autosomes

2015-16

BIOLOG

SUMMAR

Genetics is a branch of biology which deals with principles of inheritance

and its practices. Pro

ny resemblin

the parents in morphol

ical and

physiolog

cal features has attracted the attention of many biologists.

Mendel was the first to study this phenomenon systematically. While

studying the pattern of inheritance in pea plants of contrasting

ch are

that

ound

f the

first

inant

s are

acters

never

not

en i

ion of

d the

ctors

and

ations

known

omes

ysical

mes, a

and

were

hold

These

orted

sorted

emen

nked

genes. The two sexes (male and female) were found to have a set of

chromosomes which were common, and another set which was

different. The chromosomes which were different in two sexes were

named as sex chromosomes. The remaining set was named as

autosomes. In humans, a normal female has 22

airs of autosomes

2015-1

9922

characters, Mendel proposed the principles of inheritance, whi

today referred to as ‘Mendel’s Laws of Inheritance’. He proposed

the ‘factors’ (later named as genes) regulating the characters are f

in pairs known as alleles. He observed that the expression o

characters in the offspring follow a definite pattern in differe

t–

generations (F

), second (F

) and so on. Some characters are dom

over others. The dominant characters are expressed when factor

in heterozygous condition (Law of Dominance). The recessive char

are only expressed in homozygous conditions. The characters

blend in heteroz

ous condition. A recessive character that was

expressed in heterozygous conditon may be expressed again wh

becomes homo

us. Hence, characters se

ate while format

gametes (Law of Segregation).

incomplete, and some show co-dominance. When Mendel studie

inheritance of two characters together

, it was found that the fa

independently assort and combine in all permutations

combinations (Law of Independent Assortment). Different combin

of gametes are theoretically represented in a square tabular form

as ‘Punnett Square’. The factors (now known as gene) on chromos

regulating the characters are called the genotype and the ph

expression of the chraracters is called phenotyp

After knowing that the genes are located on the chromoso

good correlation was drawn between Mendel’s laws : segregation

assortment of chromosomes during meiosis. The Mendel’s laws

exte

m of ‘Chr

omosomal Theory of Inheritance’. Lat

was found that Mendel’s law of ind

endent assortment does not

true for the genes that were located on the same chromosomes.

nes were called as ‘linked

enes’. Close

located

enes ass

together

, and distantly located genes, due to

ecombination, as

independently. Linkage maps, therefore, corresponded to arrang

of genes on a chromosome

Many genes were linked to sexes also, and called as sex-li

genes. The two sexes (male and female) were found to have a