Monohybrid Crosses -Mendelian Genetics

I. Introduction

Heritable variations among individuals in a population…

“Blending model”?

“Gene control model”?  and How?

II. Abbey Garden:  The beginning…

Gregor Mendel…1843  ~ 1857, Augustinian monastery…   temporary teacher… U of Vienna… Brünn Modern School and Abbey Garden…

A. Mendel’s Experiments with garden peas

1. Monohybrid crosses:




True breeding:


Monohybrid cross:

P generation:

F1 generation:

F2 generation:

2. Mendels’ First Law:  Principles of Segregation

Allele:  An alternative form of the same

Dominant allele:

Recessive allele:

Four Components in Mendel’s First Law:

i. Alternative versions of genes (different alleles) account for variations in inherited characters.

ii. For each character, an organism inherits two alleles, one from each parent.

iii. If the two alleles differ, then the dominant allele is fully expressed, while the recessive allele has no noticeable effect on the organisms’ appearance.

*iv. The two alleles (of the same gene) for each character segregate during gamete production.

3. Modern Explanation of Monohybrid Cross

Some useful genetic vocabulary:





III. Test a Monohybrid Cross

Back cross with the recessive parent…

IV. Dominance allele, Recessive allele and their Relationships:

1. Complete dominance:

2. Codominance:

Both alleles are separately manifest in the phenotype:

Three different human blood groups: M, N, MN.

MN phenotypes are individually expressed by the presence of two types of molecules (M, N) on red blood cells.

3. A dominant allele doesn’t always subdue a recessive allele ---- incomplete dominance in
    snapdragon color.

The dominant allele codes for a red color pigment. The recessive allele codes for a white color pigment. When the number of the red and white pigments is equal, it gives out a pink color.

4. Dominant allele for a character is not necessarily more common than the recessive allele for the character:
Human:  extra finger is dominant, only 1/400, while 399/400 is recessive with no extra finger.

5. Biochemical explanation of dominance:
A dominant allele coexists with a recessive allele in a heterozygous genotype, and they don’t interact at all.  It is in the pathway from genotype to phenotype (gene expression) that dominance and recessiveness come into play.

Smooth round shape seed:  dominant allele codes for the synthesis of an enzyme, which convert sugar to starch.

Wrinkled shape seed:  controlled by the recessive allele, which codes for a defective form of this enzyme.  Sugar in the seed cannot be converted into starch, when the seeds dry.  They show a wrinkled shape.