11.2 – Applying Mendel’s Principles

Mendel realized that the principles of probability could be used to explain the results of his genetic crosses. Probability is the likelihood that a particular event will occur. Probabilities predict the average outcome of a large number of events. The larger the number of offspring, the closer the results will be to the predicted values.

In the F2 cross, both the TT and Tt allele combinations resulted in tall pea plants. The tt allele combination produced a short pea plant.

If the two alleles of a particular gene present in an individual are the same, the individual is said to be homozygous. example: tall is dominant over short (T = tall & t = short) TT = homozygous dominant tt = homozygous recessive

If the two alleles of a particular gene present in an individual are different, the individual is heterozygous. example: tall is dominant over short Tt = heterozygous In heterozygous individuals, only the dominant allele is expressed – the recessive allele is present but unexpressed (not seen).

The set of alleles that an individual has is called its genotype The set of alleles that an individual has is called its genotype. (written as letter combination) The physical appearance of a trait is called a phenotype. (written as a word) Phenotype is determined by which alleles are present.

F2 genotypes (2 letters) TT Tt tt F2 phenotypes (word) Tall Short

Punnett square is a diagram that predicts the outcome of a genetic cross by considering all possible combinations of gametes in the cross. The possible gametes that one parent can produce are written along the top of the square. The possible gametes that the other parent can produce are written along the left side of the square.

Each box inside the square is filled in with two letters obtained by combining the allele along the top of the box with the allele along the side of the box. Punnett squares can be used to predict the outcome of a monohybrid cross (a cross that considers one pair of contrasting traits between two individuals).

Monohybrid Cross: Homozygous Plants

Monohybrid Cross: Heterozygous Plants

Crossing two factors at a time is called a dihybrid cross. Mendel did this to see if traits would segregate together.

Mendel observed that 315 of the F2 seeds were round and yellow, while another 32 seeds were wrinkled and green—the two parental phenotypes.   But 209 seeds had combinations of phenotypes (which means combinations of alleles) that were not found in either parent.

The alleles for seed shape segregated independently of those for seed color.   Genes that segregate independently do not influence each other’s inheritance. The principle of independent assortment states that genes for different traits can segregate independently during gamete formation.