Understanding Genetics: Homozygous, Heterozygous, and Punnett Squares, Slides of Ecology and Environment

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Inheritance of single traits

GENETIC TERMS

• Genotype
• Phenotype
  • The particular set of genes an organism contains (represented by letters, ex. Bb)
  • The physical expression of the genes (ex. Brown hair)

Punnett Square

• A Punnett square helps geneticists organize the results of a cross involving the sex cells of two individuals.

Now try to complete these crosses…

RR X Rr Rr X rr Rr X Rr

RR X Rr Sex cells formed R, R X R, r Fertilization R r R RR Rr R RR Rr

To complete a Punnett square there are five steps. Here is an example of a question and the right answer.

Q. Two heterozygous green pod plants are crossed with each other. What ratios would you expect in the F1 generation?

Step 3. Give the gametes (sex cells) of the parents.

G , g X G, g

Step 4. Draw the Punnett Square G g G GG Gg g Gg gg

Step 5. Write out the phenotypic and genotypic ratios.

Genotypic ratio: 1GG:2Gg:1gg

Phenotypic Ratio: 3 green pod plants: 1 yellow pod plant