UNIT 4: GENETICS, IMMUNITY AND BIOTECHNOLOGY
BASICS OF GENETICS
Geneticist William bateson coined the word genetics in 1906. It is
derived from the greek word ‘gen’, meaning ‘to become’ or ‘to grow
into’. Characters that pass from the parents to their offsprings are called
‘trait’ or hereditary characters. In 1865, Gregor Mendal, the father of
genetics. Medel’s laws hold true and form the basics of getics.
Important vocabulary terms to know before studying the Mendel's
Laws:-
Genotype-The genes present in the DNA of an organism. A pair of letters
(eg. Tt or Yy or ss etc.) are used to represent genotypes for one
particular trait.
Alleles - Alternative forms of the same gene. Alleles for a trait are
located at corresponding positions on homologous chromosomes, eg,
there is a gene for curly hair with code C, the same gene with code c for
straight hair will be present on same location on homologous
chromosomes. So the gene for hair texture exists as two alleles - one
curly code C and one straight code c.
There are 3 possible genotypes:-
two capital letters (TT), one of each (Tt) or two lowercase letters (tt) .
Homozygous or PURE - when we have two capital letters or two lower
case letters. eg. Tt or tt
Heterozygous or Hybrid - When genotype is made up of one capital
letter and one lower case letter eg. Tt.
Phenotype-How the trait physically shows up in an organism. To
determine an organism's phenotype-Look at It. eg. Blue eyes, curly hair,
brown fur, red flowers etc.
, LAWS OF HEREDITY
Mendel’s laws are as follows:
Mendel's First Law - The Law of Dominance
In a cross of parents that are pure for contrasting traits, only one form of
the trait will appear in the next generation. Offspring will be hybrid for
the trait (genotypically) and only the dominant trait will be expressed in
the phenotype.
While Mendel was crossing pure Tall pea plants (Genotype TT) with pure
short pea plants (Genotype tt); all the new pea plants (called F1
generation) were tall (Genotype Tt).
Same was true for other pure traits in pea plant experiments. Therefore
it is inferred that Hybrids always show the dominant trait in their
phenotype.
Anytime two parents look different for a trait and all their offsprings
resemble only one of the parents, you are dealing with Mendel's law of
Dominance.
, The Punnett Square or P-square
Punnett square is a diagram that is used to predict an outcome of a
particular cross or breeding experiment. It is a tabular summary of every
possible combination of one maternal allele with one paternal allele for
each gene being studied in the cross. It is used by biologists to
determine the probability of an offspring's having a particular genotype.
The Punnett square is a visual representation of Mandelian Inheritance.
Monohybrid Cross
In this example both organisms have the genotype Bb. They can produce
gametes that contain either the B or the b allele. The dominant allele of
color black indicated by allele B; recessive allele of color white indicate
by allele b.
The probability of an individual offspring's having the genotype BB (Pure
black) is 25% Bb (Hybrid black) is 50% and bb (Pure white) is 25%.
Punnette squares give probabilities only for genotypes not phenotypes.
The way in which the B and b alleles interact with each other affect the
phenotype of the offspring. However as per the Mendel's first law of
Dominance, in the above example, 75% of the offsprings will be black
(BB and Bb) while only 25% will be white (bb).
BASICS OF GENETICS
Geneticist William bateson coined the word genetics in 1906. It is
derived from the greek word ‘gen’, meaning ‘to become’ or ‘to grow
into’. Characters that pass from the parents to their offsprings are called
‘trait’ or hereditary characters. In 1865, Gregor Mendal, the father of
genetics. Medel’s laws hold true and form the basics of getics.
Important vocabulary terms to know before studying the Mendel's
Laws:-
Genotype-The genes present in the DNA of an organism. A pair of letters
(eg. Tt or Yy or ss etc.) are used to represent genotypes for one
particular trait.
Alleles - Alternative forms of the same gene. Alleles for a trait are
located at corresponding positions on homologous chromosomes, eg,
there is a gene for curly hair with code C, the same gene with code c for
straight hair will be present on same location on homologous
chromosomes. So the gene for hair texture exists as two alleles - one
curly code C and one straight code c.
There are 3 possible genotypes:-
two capital letters (TT), one of each (Tt) or two lowercase letters (tt) .
Homozygous or PURE - when we have two capital letters or two lower
case letters. eg. Tt or tt
Heterozygous or Hybrid - When genotype is made up of one capital
letter and one lower case letter eg. Tt.
Phenotype-How the trait physically shows up in an organism. To
determine an organism's phenotype-Look at It. eg. Blue eyes, curly hair,
brown fur, red flowers etc.
, LAWS OF HEREDITY
Mendel’s laws are as follows:
Mendel's First Law - The Law of Dominance
In a cross of parents that are pure for contrasting traits, only one form of
the trait will appear in the next generation. Offspring will be hybrid for
the trait (genotypically) and only the dominant trait will be expressed in
the phenotype.
While Mendel was crossing pure Tall pea plants (Genotype TT) with pure
short pea plants (Genotype tt); all the new pea plants (called F1
generation) were tall (Genotype Tt).
Same was true for other pure traits in pea plant experiments. Therefore
it is inferred that Hybrids always show the dominant trait in their
phenotype.
Anytime two parents look different for a trait and all their offsprings
resemble only one of the parents, you are dealing with Mendel's law of
Dominance.
, The Punnett Square or P-square
Punnett square is a diagram that is used to predict an outcome of a
particular cross or breeding experiment. It is a tabular summary of every
possible combination of one maternal allele with one paternal allele for
each gene being studied in the cross. It is used by biologists to
determine the probability of an offspring's having a particular genotype.
The Punnett square is a visual representation of Mandelian Inheritance.
Monohybrid Cross
In this example both organisms have the genotype Bb. They can produce
gametes that contain either the B or the b allele. The dominant allele of
color black indicated by allele B; recessive allele of color white indicate
by allele b.
The probability of an individual offspring's having the genotype BB (Pure
black) is 25% Bb (Hybrid black) is 50% and bb (Pure white) is 25%.
Punnette squares give probabilities only for genotypes not phenotypes.
The way in which the B and b alleles interact with each other affect the
phenotype of the offspring. However as per the Mendel's first law of
Dominance, in the above example, 75% of the offsprings will be black
(BB and Bb) while only 25% will be white (bb).
