Inheritance is the transmission of genetic information from one generation to the next, leading to continuity of the species and variation within it.

Important terms

Genetics

• Genetics is the study of heredity and variation of inherited characteristics in organisms.
• Heredity is the transmission of genetic characteristics or traits from parent to offspring.

Gene

• A gene is a segment of DNA that contains information about a certain genetic characteristic or
  trait.
• It is a functional unit of heredity.

Trait

• A trait is a genetic characteristic of an organism.
• Some examples of traits in humans include height, eye colour, hair type and colour, sex and
  blood type.

Allele

• An allele is an alternative form of a gene.
• Alleles are located on the same locus of a chromosome.
• They determine alternative characters in inheritance such as tall or short, curly or straight hair,
  brown or blue eyes, male or female.

 

Locus

•  Locus is the location of a gene on a chromosome.

Homozygous

•  Homozygous is inheriting the same alleles for a particular gene from both parents.
• It produces a purebred. Purebred is an organism with the same alleles for a trait.
• Homozygous can be dominant or recessive.
• For example, if T is an allele for tall and t an allele for short, then homozygous is having TT;
  that is tall dominant or tt; that is short recessive.

Heterozygous

•  Heterozygous is inheriting different alleles for a particular gene from parents.
•  It produces a hybrid or carrier.
•  A hybrid is an organism with two different alleles for a trait.
• For example, if T is an allele for tall and t an allele for short, then heterozygous is having Tt;
  that is heterozygous tall.

Dominant

• Dominant is the form of trait that is shown if it is present.
• It is denoted by capital letters. For example, tall is represented by T.
• For height, if an offspring inherits an allele for tall (T) from one parent and an allele for short
  (t), the offspring will be heterozygous (Tt) for height containing both T and t. As tall (T) is
  dominant and short (t) is recessive, the offspring will be tall.
  Inheritance 191

Recessive

• Recessive is the form of trait that is only shown if the dominant allele is absent.
• It is denoted by a lowercase letter. For example, short is represented by t.
• For height, if T represents dominant allele for tall and t represents recessive allele for short, the
  offspring will only be short when no tall allele (T) is inherited. In other words, the offspring will be
  short when inherited both recessive alleles for short (t) from both parents, that is when alleles are tt.

Genotype

• Genotype is the genetic makeup of an organism.
• For height, for example, an individual’s genotype can be TT, Tt or tt.

Phenotype

• Phenotype is the observable physical characteristics of an organism
• For height, an individual’s phenotype can be tall or short

 

VARIATION

• living organism differ from each other with reference to a specific characteristic, eg height, size…
• Importance:

• increases/decreases the changes of survival
• formation of new species
• causes: They can be environmental or genetic factors.

• mutations
• sexual reproduction
• gamete formation
• random fusion of gametes

Mutations

spontaneous changes in the hereditary material can bring the changes in phenotype. They can be either harmful or beneficial.

Causes of mutations

• radiations
• chemicals
• error in copying genetic code
• damage DNA
• bring changes in chromosomes number

Ways of mutations

• gene mutation eg sickle cell anemia
• chromosomal mutations eg down’s syndrome

Forms of variation

Continuous variation

The differences between individuals are not clear-cut. There are intermediates between any two extremes, eg height, mass, weight,  .. in humans. It can be presented in form of histogram.

Discontinuous variation

These are distinct and clear-cut differences between organisms. There are no intermediates. Examples include sex (male or female), blood groups (O, A, B or AB), ability of tongue rolling or not etc