Inheritance or heredity: characteristic which is transfered from one generation to another generation.
A Characteristic refers to the variant of a trait which is a distinctive feature that is inheritable from one generation to another.
Gene: a heredity unit consisting of a sequence of DNA
Homologous chromosomes pair up during meiosis. You may be confused about the word gene loci. A gene is found on a chromosome. A chromosome contains many genes, and they occupy specific places. Genes are said to have the same locus if they occupy the same position on the homologous chromosomes
Allele: is the alternative form of gene ( on the same locus)
Since they occupy the same loci, the genes must be referring to the same trait. For example, if a gene on position 9 on homologous A codes for eye color, then the gene on position 9 of homologous B also codes for eye color.
But one gene may code for black eye while the other gene may code for blue eye.
In this case, the two genes are called alleles.
After reading this, you may be thinking what color will the eye be when two alleles are different.
In fact, only one of the alleles will be expressed in this case.
The gene of the unexpressed allele is not transcribed and translated.
The expressed gene is said to be dominant over the unexpressed gene, which is described as recessive.
We usually assume that there are only two distinctive traits, in this case, the eye can either be black or blue, not any other color.
If blue eyes is the dominant trait, the person will have blue eyes when
both or either one of his alleles for eye color is blue,
because the dominant allele will always be expressed instead of the recessive alleles. The person can only have black eyes when both of the alleles codes for black eye.
You can thus calculate that the chance of this person having blue eye to black eye is 2:1.
Genotype describes the actual set (complement) of genes carried by an organism. In contrast, phenotype refers to the observable expression of characters and traits coded for by those genes.
Two possible states of a gene for a diploid organism. Each gene is made up of two representative alleles - one inherited from the maternal source (mother) and the other inherited from the paternal source (father).
When a gene is homozygous, both alleles for that gene are the same (ie both the mother and father contributed the same version of the gene).
When a gene is heterozygous, each allele is different.
Monohybrid inheritance
This is when we cross two organisms together, but we are simply interested in what happens to one character or trait. This could be say crossing a tall pea plant with a short pea plant and then counting the number of short and tall pea plants produced by this crossing.
In Mendel's Experiment, F2 generation is formed with the phenotypic ratio 3: 1
Mendel's First Law (Law of Segregation) states that: during gamete formation each member of the allelic pair separates from the other member to form the genetic constitution of the gamete
Mendel's Conclusions
The hereditary determinants are of a particulate nature. These determinants are called genes.
Each parent has a gene pair in each cell for each trait studied.
The F1 from a cross of two pure lines contains one allele for the dominant phenotype and one for the recessive phenotype. These two alleles comprise the gene pair.
One member of the gene pair segregates into a gamete, thus each gamete only carries one member of the gene pair.
Gametes unite at random and irrespective of the other gene pairs involved.
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