One Allele Inheritance

(1) The inheritance of the height of pea stems

According to the hypothesis of Mendel, the height of stems of pea is a hereditary trait. This trait is controlled by two factors, alleles.

Suppose :	T stands for the dominant allele giving the tall pea stem. then,
	t stands for the recessive allele of dwart stems.

So,	The genotype of the pea with tall stem should be : TT;
	The genotype of that with drawf stem is : tt

The two pea plants used in this experiment are homozygous. So, they represent the parent .
The tall pea plant (with genotype TT) can give one type of gametes (in this trait). It is : T
The drawf pea plant (with genotype tt) can also give one type of gametes. It is : t

Their gametes combine and produce a new generation. It is the first filial generation F₁.
In F₁, there is only one type of genotype. It is : Tt.
As T is dominance and t is recessive, so in F₁, all individuals show the dominant high stem phenotype.

If 2 F₂ individuals are crossed, the next generation produced is called the F₂ generation.

Owning to the fact that all individuals in F₁ have the phenotype of Tt, they can produce 2 types of gametes. The genotypes of these gametes are T and t. After making the cross, the genotypes of their offsprings would be as in the following table.

Individual 1		Tt
Individual 2	Gamete	T	t
Tt	T	TT	Tt
Tt	t	tT	tt

( The above diagram method is called the checkerboard method ĄC
or called Punnette's square. )

Another diagram method is called the forked-line method . It is as follow.

If the statistic of the presentation of F₂ individuals is made, it would be as follow.

(1)	The genotypic ratio in F₂ is : TT : Tt : tt = 1 : 2 : 1
(2)	The phenotypic frequency is : High stems : dwarf stems = 3 : 1

(2)The vestigial wing of (Drosophila).

Let	V be the dominant, long wing gene;
	v be the recessive, vestigial wing gene.

If a cross between a true-breed long wing Drosophila and a true-breed vestigial wing Drosophila is made, the result would be as follow.

Parent	True-breed Dominant, long wing Drosophila		True-breed recessive, vestigial wing Drosophila
	VV	X	vv
Gamete	V		v
F₁		Vv

Cross between 2 F₁	Vv		Vv
Gamete	V ..... v		V ..... v
F₂	1 VV	2 Vv	1 vv

Conclusion of F₂ :	genotypic ratio is VV : Vv : vv = 1 : 2 : 1
	phenotypic frequency is : long winged ĄG vestigial winged = 3 : 1

(3) Some simple human single gene inheritance.

(A)	tongue roller gene	tongue roller is dominant; non-tongue roller is recessive.
(B)	ear lobe gene	free ear lobe is dominant; attached ear lobe is recessive.

(09.09.2006)