XII
BIOLOGY
CH-3 Inheritance and
variation
1)Human somatic (2n) cell contains 23 pairs of
chromosomes.
2)They can be divided functionally as autosomes and
sex chromosomes,
3)A single pair of chromosomes is involved in sex determination
and remaining 22 pairs are called autosomes.
4)Autosomes control a variety of traits other than
sex.
5)These traits are called autosome linked traits.
6) Transmission of body characters other than the
sexlinked traits from parents to their offsprings through autosomes, is called
autosomal inheritance.
7)Some characters are influenced by dominant genes
while some other are by recessive genes, present on auto somes.
For example-
Autosomal
dominant traits like Widow's peak and Huntington's disease, etc.
Autosomal recessive traits like Phenyl ketonuria
(PKU), Cystic fibrosis and Sickle cell anaemia.
a) widow's peak:
1)A prominent "V" shaped hairline on
forehead is described as widow's peak.
2) It is determined by autosomal dominant gene.
3)Widow's peak occurs in homozygous dominant (WW)
and also heterozygous (W w) individuals.
4)Individuals with homozygous recessive (w w)
genotype have a straight hair line (no widows peak).
5)Both males and females have equal chance of
inheritance.
Widow’s peak and straight
hair line
b)Phenylketonuria
(PKU): -
1)It is an inborn metabolic disorder caused due to
recessive autosomal genes.
2)When recessive genes are present in homozygous condition,
phenylalanine hydroxylase enzyme is not produced.
3) This enzyme is essential for conversion of amino
acid phenylalanine into tyrosine.
4)Due to absence of this enzyme, phenylalanine is
not converted into tyrosine.
5)Hence, phenylalanine and it derivatlves are
accumulated in blood and cerebrospinal fluid (CSF).
6)It affects development of brain and causes mental retardation.
7)Excess phenylalanine is excreted in urine; hence
this disease is called phenylketonuria.
8)Autosomal recessive raits appear in both sexes
with equal frequency. These traits tend to skip generations.
1) Genes located on non-homologous region of sex
chromosomes, are called sex-linked genes
2) The traits that are determined by sex linked
genes, are called sex-linked traits.
3)The
inheritance of sex-linked genes
from parents to their
offsprings is called sex linked
inheritance.
4)There are two types of sex-linked genes -
b) Y -linked genes.
a) X-linked (sex
linked) genes:
1)The X linked genes are located on non-homologous
region of X chromosome and these gene do not have corresponding alleles on Y
chromosome.
2)Female has two X chromosomes. In female two
recessive sex-linked genes are required for expression of sex-linked traits.
3) If one X chromosome carries a recessive gene for
sex-linked trait (defect).
4)its effect is suppressed by the dominant gene
present on another X chromosome.
5)The females with one recessive gene are carriers.
The carrier female is physically normal as she does not suffer from the disease
(disorder).
6)Male has only one X-chromosome. .
7) If X chromosome carries X-linked recessive gene
for sex linked trait, then it is expressed phenotypically, .
8) Because there is no dominant gene on Y
chromosome to suppress its effect.
9)Therefore, sex-linked I X-linked traits
appear more frequently in males than in the females. .
10) Examples of X-linked traits include
hemophilia, colour blindness, night blindness, myopia, muscular dystrophy, etc.
b)Y-linked ( Holandric ) genes:
1)Genes located on non-homologous region of Y
chromosome, are called Y linked genes.
2)The Y -linked genes are inherited directly from
male to male.
3)In man, the Y -linked genes such as hypertrichosis
is responsible for excessive development of hair on pinna of ear.
4)This character is transmitted directly from father
to son.
Colour blindness:
1)Colour blindness is X-linked recessive disorder
where person is unable to distinguish between red and green colours as both the
colours appear grey.
2)It is caused due to recessive X-linked genes (XC)
which prevents formation of colour sensitive cells, the cones, in the retina of
eye.
3)The homozygous recessive females (XcXc) and
homozygous recessive male (XcY) are unable to distinguish between red and green
colours.
4)The frequency of colour blind women is much less
than colour blind men.
5)Dominant X linked gene (XC) is necessary for
formation of colour sensitive cells in the retina of eye.
Example-
1) Marriage between colour blind maIe with normal female, will
produce normal visioned male and female offspring in F1.
The sons have normal vision but daughter
will be carrier for the disease.
2) Marriage between carrier female (daughter) and
normal male will produce female offsprings with normal vision but
half of them will be carriers for the disease.
Half of male offsprings will be normal while remaining half will be colour blind.
From
above example, .
it is clear
that the X linked recessive gene for colour blindness is inherited from
colorblind father to his grandson through his daughter.
This type of inheritance is called as criscross
inheritance.
Hemophilia (Bleeder's
disease):
1)Hemophilia is X-linked recessive disorder in which
blood fails to clot or coagulates very slowly.
2)The genes for normal clotting are dominant over
the recessive genes for hemophilia.
3)The person having recessive gene for hemophilia is
deficient-in clotting factors (VIII or IX) in blood.
4)Even minor injuries cause continuous bleeding;
hence hemophilia is also called as bleeder's disease.
5)The recessive gene for hemophilia is located on non-homologous region of X chromosome.
6)As there is no corresponding allele on Y
chromosome to suppress its expression, so men suffer from this disease,
7) Women suffers only when both X chromosomes have
recessive genes (alleles).
The
genotype of male and female individuals can be represented as follow
-Like
colour blindness, hemophilia
also shows crisscross inheritance.
The inheritance of hemophilia can be studied with the help of following examples-
Hemophilia is also referred as "The royal
disease", because it affected the royal families of England, Germany,
Russia and Spain in the 19th and 20th centuries.
Queen Victoria of England, who ruled from 1837-1901,
was believeOo have been the carrier of hemophilia.
She passed the trait on to her three of nine
children.
1) The mechanism by which sex is expressed is termed
as sex cte ermination.
2)The term sex refers to sexual phenotype. .
3)In some species both male and female reproductive organs
are present in same organism. .
4)It is described as
bisexual or hermaphrodite
or monoecious.
5)On the other hand, some species in which the
organism has either male or female reproductive organs, is said to be dioecious
or unisexual. Humans are dioecious.
6)German biologist, Henking in 1891, while
studyrng spermatogenesis of the squash bug (Anasa tristis), found
a specific structure and noted that 50% of sperms receive this
specific structure while other 50% sperm do not receive it.
7)Henking gave a name to this structure as the x-body
but he could not explain its role in sex determination
8)Further investigations by other scientists led to
conclusion that the "x-body" of Henking was a chromosome and gave the name 'X'
chrornosorne
a)Sex Determination in
human-
1)The chromosomal mec a ism of sex determination in
human beings is XX-XY type.
2)In human beings, t e cleus of each somatic cell
contains 46 chromosomes or 23 pairs of chromosomes.
3)Out of these, 2 ,~are autosomes and one pair of
sex chromosomes.
4)Human femal nas a pair of XX, homomorphic sex
chromosomes while male has XV,heteromorphic sex chromosomes.
5)Thus, genotype of: Female = 44 Autosomes + XX 6.
Male = 44 Autosomes + XY
6)During gamete formation in male, the diploid germ
cells in testis undergo spermatogenesis to produce two types of haploid sperms,
50% sperms contain 22 autosomes and X chromosome while, 50%
sperms contain 22 autosomes and Y chromosome.
7)In Female, the diploid germ cells in ovaries
undergo oogenesis to produce only one type of egg.
8)All eggs contain 22 autosomes and X chromosome.
9)Thus, human male is heterogametic and female is
homogametic.
10)During gamete formation in male, the diploid germ
cells in testis undergo spermatogenesis to produce two types of haploid sperms,
50% sperms contain 22 autosomes and X chromosome while, 50% sperms contain 22
autosomes and Y chromosome.
11)In Female, the diploid germ cells in ovaries
undergo oogenesis to produce only one type of egg.
12)All eggs contain 22 autosomes and X chromosome. .
13)Thus, human male is heterogametic and female is
homogametic.
14)lf sperm containing X chromosome fertilizes egg
(ovum), then diploid zygote is formed, that grows into a female child.
15) If sperm containing Y chromosome fertilizes the
egg, then diploid zygote is formed that grows into a male child.
16)This indicates that the sex of a child depends on
the type of sperm fertilizing the egg and hence the father is responsible for determination of sex of child and
not the mother.
17) Due to lack of knowledge, women are often blamed for giving birth to female child.
b)Sex Determination in
birds:
1)In birds, the chromosomal mechanism of sex
determination is ZW-ZZ type.
2)In this type females are heterogametic and produce
two types of eggs; 50% eggs carry Zchromosome, while 50% eggs carry W- chromosome.
3)Males are homogametic and produce one type of
sperms. Each sperm carries a Z- chromosome.
4)Thus, sex of individual depends on the kind
of egg (ova) fertilized by the sperm.
c)Sex Determination in
honey bees:
1) In honey bees, chromosomal mechanism of sex
determination is haplo-diploid type.
2)In this type, sex of individual is determined by
the number of set of chromosomes received.
3)Females are diploid (2n=32) and males
are haploid (n=16).
4)The female produces haploid eggs (n=16) by
meiosis and male produces haploid sperms (n=16) by mitosis.
5)If the egg is fertilized by sperm, the zygote develops
into a diploid female (2n=32) (queen and worker) and unfertilized egg
develops into haploid male (n=16) (Drone) by way of parthenogenesis.
6)The diploid female gets differentiated into either
worker or queen depending on the food they consume during their development.
7)Diploid larvae which get royal jelly as food
develops into queen (fertile female) and other develops into workers
(sterile females).
Genetic Disorders:
1)Genetic Disorders are broadly g ouped into two
categories as, Mendelian disorders and chromosomal disorders,
2)Mendelian disorders are
mainly caused due to alteration or mutation in the gene. e.g. thalassemia,
sickle-cell anaemia, colorbliness, haemophilia, phenylketonuria, etc.
3)chromosomal disorders are caused due to absence or
excess of one or more chromosomes or their abnormal arrange ent. E.g., Down's
syndrome, Turner's syndrome, Klinefelter's syndrome etc.
1)Thalassemia:
1)Thalassemia is an autosomal, inherited recessive
disease.
2)Haemoglobin molecule is made of four
polypeptide chains- 2 alpha (a) and 2 beta (b) chains.
3)The synthesis of alpha chains is controlled by two
closely linked genes (HBA 1 and HBA2) on chromosome 16.
4)The synthesis of beta chain is controlled by a
single gene (HBB) on chromosome 11.
5)Depending upon which chain of haemoglobin is
affected, thalassemia is classified as alphathalassemia and
beta-thalassemia.
6)It is caused due to deletion or mutation of gene
which codes for alpha (a) and beta (b) globin chains that result in abnormal
synthesis of haemoglobin.
7) In Thalassemia, person shows symptoms like
anaemia, pale yellow skin, change in size and shape of RBCs, slow growth and
development, dark urine, etc.
8)Massive blood transfusion is needed to these
patients.
9)Thalassemia differs from sickle-cell anaemia.
10)Thalassemia is a qualitative problem of
synthesizing few globin molecule
11) while the sickle cell anaemia is a qualitative
problem of synthesizing an incorrectly functional globin.
2)Down’s Syndrome (21st
trisomy) :
1)Down's syndrome is
named after the physician John Langdon Down who first described this autosomal chromosomal
disorder in 1866. .
2)This Syndrome is caused due to an extra copy of
chromosome number 21st chromosome hence it is also known as trisomy
of 21st chromosome.
3)It shows presence of three copies of 21st
chromosome instead of homologous pair.
4)These individuals will have 47 chromosomes
instead of the normal number 46.
5)21st Trisomy occurs due to non-disjunction or
failure of separation of chromosomes (autosomes) during gamete formation
(during anaphase).
6)The incidence of non-disjunction is distinctly
higher in mothers who are over 45 years old.
7)These patients show mild or moderate mental
retardation and poor skeletal development.
8)Distinct facial features like small
head, ears and mouth,
9)face is typically flat and rounded with flat nose.
10)open mouth and protruding tongue,
11)eyes slant up and out with internal epicanthal
folds.
12)flat hands and stubby fingers and palm is broad with single palmer crease
Turner’s Syndrome : (X monosomy / XO females)
1) It was first describe by H.H. Turner.
2)It is sex chromosomal disorder caused due to
non-disjunction of chromosome during gamete formation.
3)Individual born with Turner's syndrome has 44
autosomes with XO.
4)They are phenotypically female.
5)They have a short stature (height) and webbed
neck, lower posterior hair line, broad shield-shaped chest, poorly developed
ovaries and breast, and low intelligence.
Klinefelter’s syndrome
(XXY males) :
1)It is chromosomal disorder caused due to extra X
chromosome in males.
2)Thus, genotype of individuals is 44 + XXY.
3)They are described as feminized males.
4)Extra chromosome is a result of non-disjunction of
X-chromosome during meiosis.
5)Individual is male and has over all masculine
development.
6)Voice pitch is harsh and have under developed
testis.
7)They are tall with long arms, feminine development
(development of breast i.e. Gynecomastia) and no spermatogenesis,therefore,
individuals are sterile.