Lesson Outline

Lesson Media Objects

Backcross Breeding

(Take Passel Quiz)

Backcross Breeding 1 - Basic Gene Inheritance

Rate Me

Punnett Squares

Components of a Punnett Square.

The diagram to the right is called a Punnett square, and is used to predict the possible genotypes of offspring from a specific cross. In the scenario below, a corn plant that is hemizygous for the Bt gene is pollinated by itself or by another hemizygous Bt corn plant. There are two possible female gametes (silk/egg), one with the Bt gene, and one without. There are also two possible male gametes (pollen), one with the Bt gene, and one without. When fertilization occurs, the male and female gametes randomly join. There are four possible combinations of chromosome pairs in the offspring.

The next diagram shows that one in four progeny will have 2 copies of the Bt gene, two in four progeny will have one copy of the Bt gene, and one in four will have no copies of the Bt gene and be non-transgenic. If one copy of the Bt gene per cell makes enough of the Bt protein for complete toxicity to the European corn borer (ECB), the Bt gene is considered dominant.

Therefore, 3/4 of the plants would be resistant to ECB. If the Bt gene is recessive, only plants with two copies of the Bt gene will express a high enough level of the Bt protein to be toxic. Therefore in this case, only the 1/4 of the plants with two copies would have resistance.


When two hemizygous transgenic plants are mated, (or one hemizygous plant is selfed), the offspring will have a 3:1 ratio of transgenic to non-transgenic plants with 1/4 plants having two copies of the transgene and 1/2 having only one copy.	The ratio of possible offspring when a hemizygous plant is selfed or mated with another hemizygous plant.

A typical Punnett square used to determine offspring genotypes uses just letters to represent alleles and looks like the example below.

Typically Punnett squares use letters to represent the alleles.

Comments

Be the first to write a comment...

Add Comment

Control Panel

Community Blogs Community Media

My Joined Communities

more Register for another course

Class Blogs Class Media

This project was supported in part by the National Research Initiative Competitive Grants CAP project 2011-68002-30029 from the USDA National Institute of Food and Agriculture, adminstered by the University of California-Davis and by the National Science Foundation (NSF), Division of Undergraduate Education, National SMETE Digital Library Program, Award #0938034, administered by the University of Nebraska. Any opinions, findings, conclusions or recommendations expressed in this publication are those of the author(s) and do not necessarily reflect the views of the USDA or NSF.