Predicting the results of a cross

The results of genetic crosses can be predicted using a chart called a Punnett square.

In a Punnett square, the possible male and female gametes of each parent are written across the side and top of the square. 

The interior squares represent every possible combination of gametes that could combine to form a zygote.

To fill in the square, look at the alleles of the games on the outside edges and bring them across and down to fill in the center of the square. If the offspring is heterozygous, we will write the capital letter in front of the lower case letter.


In the example below T=tall plants and t= short.







The Punnett square can be used to understand the results of one of Mendel's crosses.

When true-breeding (homozygous) tall plants were crossed with true-breeding short plants, the F1 generation was all tall.

When the F1 generation self-pollinated, ¾ of the progeny were tall and ¼ were short (3:1 ratio).

Thus the F1 character for shortness was passed on by the tall F1 generation.  

Mendel's mathematical approach offered an explanation for the 3:1 pattern.

The F1 parents each contained one copy of the hereditary factor for height, one dominant and one recessive (they are heterozygous for the trait).






The Modern Genetics View

Mendel's results can be stated in more modern terms:

Traits are controlled by two alleles.

The dominant allele has the ability to mask the recessive allele.

The dominant allele is typically designated with a capital letter and the recessive a lowercase letter. 

Alleles occur on homologous chromosomes at a specific location called the gene locus.





Review: Genotype Versus Phenotype

It is possible that two organisms with different combinations of alleles can have the same outward appearance.

For example, if the dominant allele for finger length is "S" for short and the recessive allele is "s" for long, the individuals that are SS and Ss both have short fingers.

Separate terms are needed to describe each condition.

The genotype of an organism describes the two alleles that are present and the condition that this combination creates.

For example, if an organism has two S alleles, the genotype is SS, or homozygous dominant for finger length.

The phenotype of an organism refers to the physical appearance. 

For example, organisms that were either SS or Ss would have short fingers.




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When setting up a Punnett square, the possible gametes of one parent are written along the left side of the square and the possible gametes of teh other parent are written along the top of the square.