A character is determined by a gene. Different versions of the gene are alleles. 

 

Identical alleles =  homozygous (can be dominant AA or recessive aa)

Non-identical alleles = heterozygous Aa

 

Genotype =  the make-up of the genes in the nucleus (AA, Aa, aa …)

Phenotype =  what can be seen or measured (red, tall, 2mg/ml, …)

 

In the heterozygous condition, the phenotype is determined by the dominant allele. One dominant allele is sufficient to show the dominant phenotype, therefore, the genotype AA and Aa cannot be distinguished by their appearance.  The genotype may be determined by a test cross.  A? x aa.  If a recessive phenotype is observed in the next generation, then the dominant parent has to be heterozygous. If many offspring show nothing but dominant phenotype then the dominant parent is most likely homozygous (See cases 3 and 5)

 

Two recessive alleles are necessary in order to produce a recessive phenotype, therefore, a recessive phenotype is a known genotype , e.g. aa

 

A monohybrid cross looks at the results of breeding two parents that are both heterozygous at one gene

Aa  x  Aa 

The expected genotypic ratio among offspring is 1 AA: 2 Aa: 1 aa, or 25% homozygous dominant, 50% heterozygous and 25% homozygous dominant.  The expected phenotypic ratio is 3 dominant: 1 recessive, or 75% dominant and 25% recessive.

 

These figures are expectations and are not obligated to be found in Nature.

 

Not all crosses at one locus (gene location) will be monohybrid. There are six variations when we consider a dominant allele A and a recessive allele a

 

1.        AA x  AA    all offspring will be AA and exhibit the dominant phenotype

2.        aa  x  aa        all offspring will be aa and exhibit the recessive phenotype

3.        AA  x   aa     all offspring will be Aa and exhibit the dominant phenotype  (compare test cross)

4.        Aa    x  AA   half the offspring will be AA and exhibit the dominant phenotype

half the offspring will be Aa and exhibit the dominant phenotype  

5.     Aa   x   aa     half the offspring will be Aa and exhibit the dominant phenotype   (see test cross)

                                half the offspring will be aa and exhibit the recessive phenotype

6.     Aa  x   Aa    a quarter of the offspring will be AA and exhibit the dominant phenotype

half the offspring will be Aa and exhibit the dominant phenotype

a quarter of the offspring will be aa and exhibit the recessive phenotype

 

Looking ONLY at the F1 generation, are there any cases in which you can be absolutely certain about the genotypes of the parents?

 

Sometimes the heterozygote displays a phenotype that is intermediate between the homozygous phenotypes. This type of inheritance is known as incomplete dominance.

 

RR = red   RR’ = pink    R’R’ = white                                                 Note the use of same letter  and ‘prime’

 

Is there any conditions possible that an F1 generation could consist entirely of pink flowers? Red flowers?

White flowers? How does this fit in with our magic six? What if an F1 generation consisting entirely of pink flowers was bred together? How does the genotypic/phenotypic ratios compare to a standard monohybrid cross?

 

When two phenotypes are present together in the heterozygote, the condition is known as co-dominance.

 

CC = red    CW = red and white (no blending!) and WW = white.               Note the use of different capital

letters throughout.