Predicting Probabilities of Outcomes from Monohybrid Crosses
- Monohybrid inheritance is the inheritance of characteristics controlled by a single gene
- This can be determined using a genetic diagram known as a Punnett square
- A Punnett square diagram shows the possible combinations of alleles that could be produced in the offspring
- From this, the ratio of these combinations can be worked out
- Remember the dominant allele is shown using a capital letter and the recessive allele is shown using the same letter but lower case
Pea plants
- The height of pea plants is controlled by a single gene that has two alleles: tall and short
- The tall allele is dominant and is shown as T
- The small allele is recessive and is shown as t
Crossing two heterozygous plants
A genetic cross diagram (F2 generation). It shows a ratio of 3 tall : 1 short for any offspring.
- All of the offspring of the first cross have the same genotype, Tt (heterozygous), so the possible combinations of offspring bred from these are: TT (tall), Tt (tall), tt (short)
- There is more variation in the second cross, with a 3:1 ratio of tall : short
- The F2 generation is produced when the offspring of the F1 generation (pure-breeding parents) are allowed to interbreed
Calculating probabilites from Punnett squares
- A Punnett square diagram shows the possible combinations of alleles that could be produced in the offspring
- From this, the ratio of these combinations can be worked out
- However, you can also make predictions of the offsprings’ characteristics by calculating the probabilities of the different phenotypes that could occur
- For example, in the genetic cross above, two plants with the genotype Tt (heterozygous) were bred together
- The possible combinations of offspring bred from these two parent plants are: TT (tall), Tt (tall), tt (short
- The offspring genotypes showed a 3:1 ratio of tall : short
- Using this ratio, we can calculate the probabilities of the offspring phenotypes
- The probability of an offspring being tall is 75%
- The probability of an offspring being short is 25%
