Mendelian GeneticsMendelian Genetics

Pea Plants and Punnett Squares

Pea Plants and Punnett Squares

Chapter 6, sections 3-5

Gregor Mendel – “Father” of Genetics

• Austrian monk in mid-1800s• Interested in nature• Studied inheritance of traits in pea

plants• Conducted experiments and

recorded data• Developed some basic laws of

inheritance

Pea Plants because…

• fast-growing• mature quickly• easy to care for• have many easy-to-distinguish traits /

characteristics; examples: flower color – either purple or white, plant height – either tall or short, seed color – either green or yellow

Mendel’s Experiments

Step One: Mendel allowed plants to self-pollinate for several generations to make sure that they were “pure”, or true-breeding, for flower color.

Today, we would describe each of these “pure” plants as homozygous.

Video clip – 4:30

Self-Pollination

Experiments, cont’d

Step Two: Mendel took two pure flowers – one purple and one white – and cross-pollinated them. He called these two flowers the P generation (P for parental).

Results: all offspring were purple; he called these offspring the F1 generation (F1 for first filial)

Cross-Pollination

Experiments, cont’d

Step Three: Mendel allowed the F1 flowers – which were all purple - to self-pollinate. The new offspring were called the F2 generation (second filial).

Results: Roughly 3 out of every 4 flowers was purple and 1 out of every 4 was white.

Interpreting the Results

• Today, we know that…– purple flower color is dominant

to white (in pea plants)– the white flower color did not

completely disappear; it was simply hidden (masked) in the F1 generation

Results, cont’d

• Remember:– Dominant = the allele that

always shows up in the phenotype as long as at least one copy is present

– Recessive = the allele that will only show up in the phenotype if there are no dominant alleles

Results, cont’d

In the F1 generation, all of the plants were heterozygous (genotype) for flower color – they each had one purple allele and one white allele, but they all had a purple phenotype (appearance)

Results, cont’d

• In the F2 generation:– one of the purple flowers was

homozygous dominant– the other two purple flowers

were heterozygous– the white flower was

homozygous recessive

Punnett Squares

• Now, we can use Punnett squares to show what happened in Mendel’s initial experiments:

The Punnett Square at left shows the cross between two pure plants; one parent is homozygous dominant and the other parent is homozygous recessive. This is like Mendel’s P generation.

Setting up Punnett Squares

Punnett Square – F1 cross

• For Mendel’s F1 cross, he allowed plants to self-pollinate.

• The results are shown at right

• 75% (3/4) were tall and 25% (1/4) were short.

Mendel’s Conclusions

1. Inheritance is based on genes, which are passed on from parents to offspring.

2. Some versions of a gene may be dominant while others may be recessive.

3. In most sexually reproducing organisms, each individual has two alleles for each gene – one from each parent.

Conclusions, cont’d

4. When gametes (eggs and sperm) form, the two alleles for each gene separate (segregate) from each other (Law of Segregation).

5. When gametes form, different genes separate (segregate) independently of each other (Law of Independent Assortment).

Law of Segregation

Law of Independent Assortment

Factors in Mendel’s Success

1. Mendel concentrated on one trait at a time.

2. Only after he established the behavior of one trait (monohybrid cross) did he consider two traits together (dihybrid cross).

3. He conducted a large number of crosses (2000-3000) to eliminate chance and to obtain a valid and accurate explanation

Success, cont’d4. He actually counted the number of offspring

of each category and maintained accurate records for each generation in each experiment.

5. Luck -- though Mendel did not know this -- in the selection of pea plants as well as in the selection of the particular traits he studied.

• In each pair, one form of the trait is completely dominant over the other.

• It also helped that the genes for the traits Mendel studied are present on seven different chromosomes in the pea plant.