Continuity of Life

Presented by:

SHWETA

INTRODUCTION

CONTINUITY OF LIFE:-

• Reproduction is the process by which new individuals are generated. It allows the continuity of life; without reproduction, life would come to an end.

HERIDITY

• Heredity or Hereditary is the process of passing the traits and characteristics from parents to offsprings through genes.
• The offspring, get their features and characteristics that is genetic information from their mother and father.
• Heredity and genetics are the reason you look so much like your parents. Genetics is a branch of science that studies the DNA, genes, genetic variation, and heredity in living organisms.

VARIATION

• It is difference among the offsprings and with their parents.
• The variation occurs due to some errors in DNA copying. Variation is important because it contributes to the evolution and forms the basis of heredity.
• Variation is caused due to gene mutations, the interaction of genes with the environment, and various combinations of genetic material.
• Remember that variation can occur through the asexual reproduction process too.

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Mendel conducted 2 main experiments to determine the laws of inheritance.

• These experiments were:
• Monohybrid Cross Experiment
• Dihybrid Cross Experiment

While experimenting, Mendel found that certain factors were always being transferred down to the offspring in a stable way. Those factors are now called genes i.e. genes can be called the units of inheritance.

Mendel’s Experiments�

• Mendel experimented on a pea plant and considered 7 main contrasting traits in the plants.
• Then, he conducted both the experiments to determine the aforementioned inheritance laws.
• A brief explanation of the two experiments is given below.

Monohybrid Cross�

• In this experiment, Mendel took two pea plants of opposite traits (one short and one tall) and crossed them.
• He found the first generation offsprings were tall and called it F1 progeny.
• Then he crossed F1 progeny and obtained both tall and short plants in the ratio 3:1.

Dihybrid Cross�

• In a dihybrid cross experiment, Mendel considered two traits, each having two alleles. He crossed wrinkled-green seed and round-yellow seeds and observed that all the first generation progeny (F1 progeny) were round-yellow.
• This meant that dominant traits were the round shape and yellow colour.
• He then self-pollinated the F1 progeny and obtained 4 different traits wrinkled-yellow, round-yellow, wrinkled-green seeds and round-green in the ratio 9:3:3:1.

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Dihybrid Cross Examples�

• Mendel took a pair of contradicting traits together for crossing, for example colour and the shape of seeds at a time. He picked the wrinkled-green seed and round-yellow seed and crossed them.
• He obtained only round-yellow seeds in the F1 generation.
• This indicated that round shape and yellow colour of seeds are dominant in nature.
• Meanwhile, the wrinkled shape and green colour of seeds are recessive traits.
• Then, F1 progeny was self-pollinated.
• This resulted in four different combinations of seeds in the F2 generation.
• They were wrinkled-yellow, round-yellow, wrinkled-green seeds and round-green in the phenotypic ratio of 9:3:3:1

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Cont..

• During monohybrid cross of these traits, he observed the same pattern of dominance and inheritance.
• The phenotypic ratio 3:1 of yellow and green colour and of round and wrinkled seed shape during monohybrid cross was retained in dihybrid cross as well.
• Consider “Y” for yellow seed colour and “y” for green seed colour, “R” for round shaped seeds and “r” for wrinkled seed shape.
• Thus, the parental genotype will be “YYRR” (yellow-round seeds) and “yyrr” (green-wrinkled seeds).

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Mendel’s laws�

• The two experiments lead to the formulation of Mendel’s laws known as laws of inheritance which are:
• Law of Dominance
• Law of Segregation
• Law of Independent Assortment

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Law of Dominance�

• This is also called Mendel’s first law of inheritance.
• According to the law of dominance, hybrid offsprings will only inherit the dominant trait in the phenotype.
• The alleles that are suppressed are called as the recessive traits while the alleles that determine the trait are known as the dormant traits.

Law of Segregation�

• The law of segregation states that during the production of gametes, two copies of each hereditary factor segregate so that offspring acquire one factor from each parent.
• In other words, allele (alternative form of the gene) pairs segregate during the formation of gamete and re-unite randomly during fertilization.
• This is also known as Mendel’s third law of inheritance

Law of Independent Assortment�

• Also known as Mendel’s second law of inheritance, the law of independent assortment states that a pair of trait segregates independently of another pair during gamete formation.
• As the individual heredity factors assort independently, different traits get equal opportunity to occur together.

SILENT FEATURES� OF �CHROMOSOMAL THEORY

OTHER PATTERNS� OF� INHERITENCE

INCOMPLETE DOMINANCE

MULTIPLE ALLELISM

QUANTIATIVE INHERITANCE�(Polygenic Inheritance)�

• It is defined as inheritance, where multiple independent genes have an additive or similar effect on a single quantitative trait.
• Quantitative inheritance is also known as multiple gene inheritance or multiple factor inheritance.

Quantitative Inheritance characteristics�

• Polygene/ Quantitative  refers to a gene that exerts a slight effect on a phenotype along with other genes
• Effect of a single gene is too small, so it is difficult to detect
• Multiple genes produce an equal effect
• Each allele has a cumulative or additive effect
• Polygenic inheritance differs from multiple alleles, as in multiple alleles, three or more alleles are present in the same locus of which any two alleles are present in an organism, e.g. ABO blood group system, which is controlled by three alleles

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• There is no epistasis involved, i.e. masking of the expression of an allele of the different locus
• There is no linkage or dominance, rather there exist contributing and non-contributing alleles, which are known as active or null alleles respectively
• Polygenic inheritance is characterized by the continuous variation of the phenotype of a trait
• The polygenic inheritance pattern is complex. It is difficult to predict phenotype
• The statistical analysis can give the estimate of population parameters.

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