CE Biology

CE Biology-A Summary

Part 5-Genetics

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Section 5 Genetics

DNA carries the genetic code controlling the synthesis of proteins. By controlling the type of proteins produced in a cell, DNA controls the development ,structure and function of a cell.

Genes are short lengths of DNA on chromosome. They are basic units determining inherited characters. (A gene is a sequence of codons codes for one polypeptide chain.)

Allele is one of a group of alternative forms of a gene that may occur at a given site on a chromosome.

Homozygote is an organism which contains two identical alleles.

Heterozygote is an organism which contain two different alleles.

Dominant character is character which shows up in heterozygous state.

Recessive character is character which is masked by dominant character in the heterozygous state.

Genotype is the genetic make up of an organism.

Phenotype is the external appearance of an organism usually resulting from the interaction between the genotype and the environment in which development occurs.

Mendel’s first law (the principal of segregation)

The alleles of a gene exist in pairs but during gamete formation, the alleles will separate and enters in different gametes. Thus each gamete contains only one alleles of each gene.

Monohybrid inheritance

It is the cross between two individuals with respect to one contrasting character only.

e.g. The shape of peas

Let S be the dominant allele for round peas

Let s be the recessive allele for wrinkled peas

Genotypic ratio: SS : Ss : ss = 1 : 2 : 1

Phenotypic ratio: round pea : wrinkle pea = 3 : 1

Living organisms differ from one another. Each individuals within a species have slightly different sets of characteristics. Some of these differences are inherited from their parents , and others are the result of the environment.

Type of variation

Continuous variation: the variation in a characteristic between individuals of the same species where there is a gradual transition between the two extremes of the quality. The expression of a continuous variation by an individual is affected by the environment.

e.g. weight, height…

Discontinuous variation: there is no range of these characteristics between extremes. Discontinuous variations are not affected by the environment.

e.g. tongue rolling ability

Genetic variation is caused by independent assortment of chromosomes at meiosis, random fertilization and mutation.

[Mutation is a change in genetic material caused by influence of radiation, chemicals…]

Variation is necessary for survival. Variation in a population has great survival value because it may allow differences in adaptability of organisms when environment conditions change.