Biology X
Genetics-Some Basic Fundamentals
Summary:
Genetics is the study of transmission of characters from parents to offspring and the laws
relating to such transmission.
Heredity The phenomenon of passing of characters from parents to progeny through
successive generations is called heredity.
Variation The difference in the characters or traits among the individuals of a species is
called variation..
The chromosome is a thread-like structure in the nucleus of a cell. It is formed of DNA
and carries the genes. For example, humans have 46 chromosomes
The process by which the sex of a person is determined is called sex determination. There
are two types of sex chromosomes—X chromosome and Y chromosome. A male has one
X chromosome and one Y chromosome (XY). A female has both X chromosomes (XX).
Genes are specific DNA segments on a chromosome which determine hereditary
characteristics. They are responsible for the transfer of characteristics from parents to
their offspring. Genes work in pairs.
Every gene has two alternative forms for a character producing different effects; these
alternative forms are called alleles.
Sex-linked inheritance is the appearance of a trait which is due to the presence of an
allele exclusively either on X chromosomes or on the Y chromosome. X-linked
Inheritance Defects occur due to recessive genes, which are on the X chromosome. For
example, colour blindness and haemophilia.
X-linked inheritance is also called criss-cross inheritance. This is because the son may
inherit a trait from the normal carrier mother.
Q1.Mention the following:
(i) Total number of pairs of chromosomes in each body cell in humans ............... .
(ii) Number of pairs of autosomes in one cell of humans ............... .
Ans (i) 23
(ii) 22
Q2.Write the basic unit of heredity ?
Ans: Gene is the basic unt of heredity.
Q3.A certain couple got only four daughters in a row and no son. Does it mean that the
husband does not produce Y-bearing sperms? Explain.
Ans:No, it does not mean that the husband does not produce Y-bearing sperms. During
conception, half of the sperm released are X-bearing and the other half are Y-bearing. In this
case, only the X-bearing sperm fertilized the wife's eggs in each instance. This can happen due to
random chance, as the probability of having a male or female child is approximately 50% for
each pregnancy.
Q4. Distinguish between the following pairs:
(i) Monohybrid and Dihybrid cross
, (ii) Genotype and Phenotype
(iii) Character and Trait
(iv) Dominant and Recessive Allele
Ans (i) Difference between Monohybrid and Dihybrid cross:
Monohybrid Cross Dihybrid cross
Single feature is taken into Two features are taken into
consideration consideration.
Phenotypic ratio in second Phenotypic ratio in second
generation is 3:1 generation is 9:3:3:1
Genotypic ratio obtained is
Genotypic ratio is 1:2:1
very complex
(ii) Difference between Genotype and Phenotype:
Genotype Phenotype
Set of genes present in any Characters expressed or physical
organism is known as its appearance of characters is called
genotype. phenotype.
Can be determined by
Can be determined by observation.
genotyping method.
It is formed by expressed and It consists of only expressed
unexpressed genes. genes.
(iii) Difference between Character and Trait:
Character Trait
Any inheritable feature
The alternative forms of a character are
of an organism is a
called traits.
character.
For example, within the character of
For example, flower
flower colour, specific traits will be red,
colour is a character.
blue, or white flower colour.
(iv) Difference between Dominant and Recessive Allele:
Dominant Allele Recessive Allele
The gene that remains
The gene that expresses itself
unexpressed in presence of
phenotypically.
dominant allele.
Genetics-Some Basic Fundamentals
Summary:
Genetics is the study of transmission of characters from parents to offspring and the laws
relating to such transmission.
Heredity The phenomenon of passing of characters from parents to progeny through
successive generations is called heredity.
Variation The difference in the characters or traits among the individuals of a species is
called variation..
The chromosome is a thread-like structure in the nucleus of a cell. It is formed of DNA
and carries the genes. For example, humans have 46 chromosomes
The process by which the sex of a person is determined is called sex determination. There
are two types of sex chromosomes—X chromosome and Y chromosome. A male has one
X chromosome and one Y chromosome (XY). A female has both X chromosomes (XX).
Genes are specific DNA segments on a chromosome which determine hereditary
characteristics. They are responsible for the transfer of characteristics from parents to
their offspring. Genes work in pairs.
Every gene has two alternative forms for a character producing different effects; these
alternative forms are called alleles.
Sex-linked inheritance is the appearance of a trait which is due to the presence of an
allele exclusively either on X chromosomes or on the Y chromosome. X-linked
Inheritance Defects occur due to recessive genes, which are on the X chromosome. For
example, colour blindness and haemophilia.
X-linked inheritance is also called criss-cross inheritance. This is because the son may
inherit a trait from the normal carrier mother.
Q1.Mention the following:
(i) Total number of pairs of chromosomes in each body cell in humans ............... .
(ii) Number of pairs of autosomes in one cell of humans ............... .
Ans (i) 23
(ii) 22
Q2.Write the basic unit of heredity ?
Ans: Gene is the basic unt of heredity.
Q3.A certain couple got only four daughters in a row and no son. Does it mean that the
husband does not produce Y-bearing sperms? Explain.
Ans:No, it does not mean that the husband does not produce Y-bearing sperms. During
conception, half of the sperm released are X-bearing and the other half are Y-bearing. In this
case, only the X-bearing sperm fertilized the wife's eggs in each instance. This can happen due to
random chance, as the probability of having a male or female child is approximately 50% for
each pregnancy.
Q4. Distinguish between the following pairs:
(i) Monohybrid and Dihybrid cross
, (ii) Genotype and Phenotype
(iii) Character and Trait
(iv) Dominant and Recessive Allele
Ans (i) Difference between Monohybrid and Dihybrid cross:
Monohybrid Cross Dihybrid cross
Single feature is taken into Two features are taken into
consideration consideration.
Phenotypic ratio in second Phenotypic ratio in second
generation is 3:1 generation is 9:3:3:1
Genotypic ratio obtained is
Genotypic ratio is 1:2:1
very complex
(ii) Difference between Genotype and Phenotype:
Genotype Phenotype
Set of genes present in any Characters expressed or physical
organism is known as its appearance of characters is called
genotype. phenotype.
Can be determined by
Can be determined by observation.
genotyping method.
It is formed by expressed and It consists of only expressed
unexpressed genes. genes.
(iii) Difference between Character and Trait:
Character Trait
Any inheritable feature
The alternative forms of a character are
of an organism is a
called traits.
character.
For example, within the character of
For example, flower
flower colour, specific traits will be red,
colour is a character.
blue, or white flower colour.
(iv) Difference between Dominant and Recessive Allele:
Dominant Allele Recessive Allele
The gene that remains
The gene that expresses itself
unexpressed in presence of
phenotypically.
dominant allele.
