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Heredity and Variations

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Heredity and Variations - Lesson Summary

Trait is any characteristic that is transferred from parent to offspring. e.g. height and colour.

The process of passing traits from parent to offspring is called heredity.

Variations are the minor differences between offspring and the parent.

Mendel and his contributions
Gregor Johann Mendel was a pioneer among geneticists who put forward the concept of inheritance of characteristics or traits from parent to offspring.

Mendel proposed the principle of inheritance and is known as the “Father of Genetics”.

Mendel has chosen pea plants for his experimentation and found variations among them.

Gene is a structural and functional unit of heredity and variations.

Gene is a DNA segment on the chromosome. Genes control the expression of characteristics. Mendel called the genes to be factors.

Alleles are a pair of alternative forms of a gene.  Each gene is present in two alternative forms, each called an allele. Each allele controls a single trait.

Traits can be either dominant or recessive. Tallness in a plant is a dominant trait, controlled by a dominant allele and is represented by “T” (capital). Shortness in a plant is a recessive trait, controlled by a recessive allele and is represented by “t” (small).

         • Homozygous is a condition in which a gene possesses a pair of the same alleles (TT or tt) for a single characteristic.
         • Heterozygous is a condition in which a gene possesses a pair of different alleles (Tt) for a single characteristic.

Phenotype is a morphological expression of a single character. For example, tallness or shortness represents the phenotype of the plant.

Genotype is the genetic make-up of a cell, an organism, or an individual (i.e. the specific allele make-up of the individual), usually with reference to a specific characteristic under consideration. Alleles combine to make a genotype, such as TT or Tt or tt.

Punnett square is a statistical method that was used by Mendel to predict the possible genotypes and phenotypes of the offspring.

Monohybrid inheritance
It is the inheritance of a single characteristic controlled by different alleles of the same gene.

         • F1 generation is the first filial generation offspring produced by crossing two parental strains.  All the progeny of F1 generation were tall i.e. the traits of only one parent were visible.

         • F2 generation is the second filial generation offspring produced by crossing F1’s. The F2 progeny were not all tall. Instead, one quarter of them was short indicating both the traits – that of tallness and shortness were inherited in the F2 plants.

         • Genotypic ratio – 1:2:1, Phenotypic ratio – 3:1.

Dihybrid inheritance
It is the simultaneous inheritance of two characters.

         • Dihybrid inheritance is the experimentation of two characteristics with their four contrasting traits.
         • For instance, dihybrid inheritance involves a plant producing round and yellow seeds (RR and YY) crossing with a plant producing wrinkled green seeds (rr and yy).
         • F1 progeny produces round and yellow seeds (R and r, and Y and y) in which round and yellow are dominant traits.
         • F2 progeny were similar to their parents and produced round yellow seeds, while some of them produced wrinkled green seeds. However, some plants of the F2 progeny even showed new combinations, like round-green seeds and wrinkled-yellow seeds.

Sex determination
It is a mechanism which determines the individual to be a male or a female based on the sex chromosomes present in it.  X and Y are the sex chromosomes inherited one from each parent, determine the offspring to be a male.  X and X are the sex chromosomes inherited one from each parent, determine the offspring to be a female.  A child who inherits an X chromosome from her father will be a girl, and one who inherits a Y chromosome from him


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