notes 5-1 & 5-2

Mendel and Punnett Squares

Mendel was a geneticist who studied pea plants

He began his experiments by crossing 2 purebred organisms

Gregor Mendel crossed pea plants that had different traits. The illustrations show how he did this.

In all of Mendel’s crosses, only one form of the trait appeared in the F1 generation. However, in the F2 generation, the “lost” form of the trait always reappeared in about one fourth of the plants.

Mendel studied several traits in pea plants.

Genes are factors that control a trait Each gene has 2 contrasting forms called

alleles Some alleles are dominant (represented by

a capital letter) Some alleles are recessive (represented by

a lower case letter) If a gene has both dominant alleles, it is

homozygous dominant (TT) If a gene has both recessive alleles, it is

homozygous recessive (tt) If a gene has 1 dominant and 1 recessive

allele, it is heterozygous (Tt)

In this cross, both parents are heterozygous for the trait of seed shape. R represents the dominant round allele, and r represents the recessive wrinkled allele.

In a genetic cross, the allele that each parent will pass on to its offspring is based on probability.

An organism’s phenotype is its physical appearance, or visible traits. An organism’s genotype is its genetic makeup, or allele combinations.

Mendel allowed several F1 pea plants with yellow seeds to self-pollinate. The graph shows the approximate numbers of the F2 offspring with yellow seeds and with green seeds.

Yellow–6,000; green–2,000

Reading Graphs:

How many F2 offspring had yellow seeds? How many had green seeds?

8,000; 75% have yellow peas and 25% have green peas.

Calculating:

Use the information in the graph to calculate the total number of offspring that resulted from this cross. Then calculate the percentage of the offspring with yellow peas, and the percentage with green peas.

Both parents probably had the genotype Bb.

Inferring:

Use the answers to Question 2 to infer the probable genotypes of the parent plants. (Hint: Construct Punnett squares with the possible genotypes of the parents.)

In codominance, the alleles are neither dominant nor recessive. As a result, both alleles are expressed in the offspring.