Experiment 11: Study of Mendelian Inheritance Using Seeds of Different Colours / Sizes

AIM

Study of Mendelian inheritance using seeds of different colour / sizes of any plant.

PRINCIPLE

Mendelian inheritance is based on principles of dominance, segregation and independent assortment. Pisum sativum shows seven pairs of contrasting traits including seed form, cotyledon colour, flower colour, pod form, pod colour, flower position and stem size.

In monohybrid cross, F₁ shows dominant phenotype and F₂ shows phenotypic ratio 3:1 and genotypic ratio 1:2:1. In dihybrid cross, F₂ shows phenotypic ratio 9:3:3:1 due to independent assortment of two genes.

Round seed (R) is dominant over wrinkled (r) and yellow cotyledon (Y) over green (y). Study of seed samples helps to verify Mendel's laws by calculating phenotypic and genotypic ratios from observed data.

REQUIREMENTS

• Pea seed sample
• Enamel tray
• Petridish
• Napkin
• Notebook
• Pencil/pen

PROCEDURE

1. Collect 100 pea seeds of different shapes and sizes and those varied in colour in an enamel tray.
2. Put 50 round seeds in one petridish and 50 wrinkled seeds in the other to represent male and female gametes respectively. Let the round seed be indicated by 'R' and wrinkled seed by 'r'.
3. Take a seed from each container and place them together on the napkin.
4. Just like the previous step, continue to pick seeds and arrange them in pairs. Thus, 50 pairs of seeds are obtained representing the 50 heterozygous F₁-progeny.
5. Note that all the F₁-individuals are represented by one round and one wrinkled seed.
6. Note the genotype RR or Rr or rr of each pair and their possible phenotype.
7. Calculate the genotypic and phenotypic ratios of your pooled data.

OBSERVATION

Mendel selected seven contrasting traits in pea. Each trait shows dominant and recessive forms. Seed shape round is dominant over wrinkled, yellow cotyledon over green, white flower colour over violet, full pod over constricted, green pod over yellow, axial flower position over terminal and tall stem over short.

Monohybrid Cross - Seed Shape

P generation: Round seed RR × Wrinkled seed rr produces R and r gametes. F₁ all Rr round seeds. F₁ selfing Rr × Rr produces RR round, Rr round, Rr round and rr wrinkled. Phenotype 3:1 and genotype 1:2:1 observed.

Dihybrid Cross - Seed Shape and Colour

P generation: Round Yellow RRYY × Wrinkled Green rryy produces RY and ry gametes. F₁ all RrYy round yellow. F₁ selfing produces 16 combinations. Phenotypic ratio 9 round yellow : 3 round green : 3 wrinkled yellow : 1 wrinkled green observed in Punnett square.

Original Data Recorded:

Examples for Other Traits:

RESULT

The observed data shows phenotypic ratio approximately 3:1 for monohybrid traits like seed shape and seed colour, and 9:3:3:1 for dihybrid traits. Genotypic ratio for monohybrid cross is 1:2:1. Results confirm Mendelian inheritance and law of dominance and segregation.

CONCLUSION

Mendelian traits segregate independently and dominant traits express in F₁. Recessive traits reappear in F₂ in 3:1 ratio for single trait and 9:3:3:1 for two traits. Experiment verifies laws of segregation and independent assortment using pea seeds of different colour and size.

PRECAUTIONS

• Large sample of seeds should be collected to minimise the error.
• The contrasting form of the trait should be observed carefully.