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INHERITANCE IN LIFE
HEREDITY
• Heredity is the transmission of particular
characteristics from parent to offspring.
Gregor Johann Mendel
1822-1884
• Mendel presented completely new theory of
inheritance in the journal ‘Transactions of the
Natural History society of Brunn’.
• His work was rediscovered in 1900,
simultaneously by Hugo de Vries in Holland,
Carl Correns in Germany and Erich
Tschermak in Austria.
• Mendel is often referred to as Father of
Genetics,
• His experiments and principles collectively
form “Mendelian genetics”.
Gregor Mendel (1822 – 1884)
• Studied garden peas
• 1st to use mathematics to examine outcomes of crosses
• Number of pea varieties with at least 7 easily distinguished traits
• Peas are small, easy to grow, short generation time
• Peas can self-fertilize; bisexual
Dominance and recessive traits
• Principal of dominance is only one of the two
contrasting character will appear or expressed
• F1 generation showed only purple flowers (dominant)
• White is a recessive trait in F1 generation
Terminologies
1. Mendelian factors are now called genes
2. Alleles are different versions of the same gene
3. An individual with two identical alleles is termed homozygous
4. An individual with two different alleles, is termed heterozygous
5. Genotype refers to the specific allelic composition of an individual
6. Phenotype refers to the outward appearance of an individual
Genotype and Phenotype
• Hereditary traits are under the control of genes (Mendel called them factors).
• Genotype is the genetic makeup of an organism, a description of the genes it
contains.
• Phenotype is the characteristics that can be observed in an organism.
• Phenotype is determined by interaction of genes and environment
• Genes provide potential, but environment determines whether that potential
is realized
Terminologies
• Dominant - stronger of two genes expressed in the hybrid; represented by a
capital letter (R)
• Recessive - gene that shows up less often in a cross; represented by a
lowercase letter (r)
• Self-fertilization can occur in the same flower
• Cross-fertilization can occur between flowers
• P/F1/F2 generations
Keys to Mendel’s success
• The garden pea was an ideal organism for study because:
–Vigorous growth
–Self fertilization
–Easy to cross fertilize
– Produces large number of offspring each generation
Keys to Mendel’s success
• Mendel analysed traits with discrete alternative forms (one of two options)
– purple vs. white flowers
– yellow vs. green peas
– round vs. wrinkled seeds
– long vs. short stem length
• Mendel established pure/true breeding lines to conduct Controlled
fertilization experiments
– (traits remain constant from generation to generation)
Monohybrid cross
• In monohybrid cross Mendel selected one character for his experiment.
• Crosses were made between white flowered and purple flowered plants.
• Pollens from the purple flowers were placed onto the stigma of white
flower.
• Allowed it to cross fertilization
• All the seeds in the pod resulted from this pollination were hybrids.
LAW OF SEGREGATION
• Whenever a pair of ‘factors’ for character brought together in a hybrid, they
segregate during the formation of gametes. Hence each gamete is pure with
reference to this character.
Test cross and Back cross
• A cross involving the F1 individuals with either of the two parents is called
back cross.
• The cross between F1 and recessive parent is called test cross (Pp X pp).
• All the test crosses are back cross but all the back crosses are not a test
cross
• Test cross helps to test whether the individual is homozygous or
heterozygous.
Back cross - Testcross
Dihybrid cross
• Mendel crossed a pea plant producing Round yellow seeds with one
producing green and wrinkled seeds of pure breed variety.
• In F1 generation plants obtained producing only round yellow seeds
• F1 were allowed for self-pollination to get F2 generation.
• In F2 generation, 4 different types of plants were produced that is a) Round
yellow b) Round green c) wrinkled yellow d) wrinkled green.
• Phenotype ratio of 4 types of plants were 9:3:3:1
Principle of Independent Assortment (Mendel’s 2nd
Law)
• The factors for two or more pairs of contrasting characters are distributed
independently of one another at the time of gamete formation.
• In a dihybrid cross, the alleles of each gene assort independently
Dihybrid test cross
RY Ry rY ry
ry RrYy
Round
Yellow
Rryy
Round
Green
rrYy
Wrinkled
Yellow
rryy
Wrinkled
Green
A dihybrid test cross involves crossing of the F1 dihybrid with a double
recessive parental type.
RrYy (F1) X rryy (P)
Genotype and Phenotype ratio is 1:1:1:1
Chromosomal theory of inheritance
• The chromosome theory of inheritance allows us to see the relationship
between Mendel’s laws and chromosome transmission
• Mendel’s law of segregation and independent assortment can be explained
by the homologous pairing and segregation of chromosomes during meiosis
Cell division
Mitosis
Meiosis
Meiosis
Meiosis
Chromosomal theory of inheritance
• The chromosome theory of inheritance describes how the transmission of
chromosomes account for the Mendelian patterns of inheritance
• This theory was independently proposed in 1902-03 by
• Theodore Boveri, a German Scientist
• Walter Sutton, an American Scientist
Chromosomal theory
• The hereditary material must reside within the nuclei of the gametes.
• Chromosomes segregate during meiosis.
• Gametes have a copy of one member of each pair of homologous
chromosomes
• diploid individuals have a copy of both members of the pair.
• During meiosis, each pair of homologous chromosomes orients on the
metaphase plate independently of any other pair.
• It is the chromosomes that undergo segregation and independent
assortment.
• Humans have 46 chromosomes
44 autosomes
2 sex chromosomes
• Males contain one X and one Y chromosome
They are termed as heterogametic
• Females have two X chromosomes
They are homogametic
• The Y chromosome determines the gender
Morgan’s experiment
• In 1910 the American geneticist Thomas Hunt Morgan, studying the fly
Drosophila melanogaster,
• He detected a mutant fly, a male fly that differed strikingly from normal
flies of the same species.
• The eyes were white instead of the normal red.
• Morgan’s discovery of a white eyed trait that correlated with the sex of flies
was a key episode in the development of the chromosome theory of
inheritance.
Morgan’s Experimental evidence
Recombination of linked genes crossing over
Alterations in Chromosomal number
Down syndrome
Genetic disorder
Alterations in chromosomal structure
Alterations in chromosomal structure
Chromosomal translocations
Pedigree analysis
• Collecting the information about a family’s history for a particular trait and
assembling this information into a family tree describing the traits of
parents and children across the generations is the family pedigree.
Pedigree of dominant trait
Widow’s peak is a dominant or recessive trait?
Pedigree of recessive trait
Is attached earlobe a dominant or recessive trait?
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