Monohybrid crosses:
A cross is the mating or pollination of two organisms to produce offspring. A monohybrid cross is conducted with the aim of studying only one pair of contrasting characteristics. For example, a monohybrid cross may be conducted to predict the colouring of the next generation of peas produced from a homozygous yellow pea and a homozygous green pea. Other characteristics such as seed shape, plant height or flower colour would be ignored. The characteristic in question is referred to as an allele - one of the alternative forms of a gene/characteristic. These are represented using letters in diagrammatical representations of crosses: a letter of the alphabet will be chosen to represent the characteristic in question. The dominant characteristic will be the capitalized version of this letter, while the recessive gene will be the lowercase version. For example, in peas, yellow colour is dominant to green colour. A diagrammatical representation of a cross between a homozygous yellow pea and a homozygous green pea would be: YY x yy, with "Y" representing pea colour.
Representing alleles in this way also helps one determine the phenotype and genotype of the offspring. Phenotype is the physical characteristics/traits of an organism. It is observable and is influenced by the genotype. For example, using green and yellow peas, the phenotype of the parents would be green (green pea) and yellow (yellow pea). The genotype is the set of characteristics that an organism has inherited and is represented using allele notation. For example, the genotype for a homozygous yellow pea would be YY; for a homozygous green pea it would be yy; and for a heterozygous yellow pea it would be Yy. A popular way to diagrammatically represent a cross is known as a punnett square. These were created by English geneticist Reginald Punnett in the early 20th century. They allow one to determine all the possible genotypes from a cross and thus work out phenotypes, and proportions.
When dealing with homozygous monohybrid crosses, certain patterns emerge:
- If both parents are homozygous, the F1 generation will all be heterozygous and thus display the dominant phenotype.
- If the F1 generation of homozygous parents breed, the offspring will always appear in a 3:1 ratio, with 75% of offspring exhibiting the dominant phenotype, and the other 25% exhibiting the recessive characteristic.
- If the F1 generation of homozygous parents breed, the offspring, in terms of genotypes, will always appear in a 1:2:1 ratio, with 25% being homozygous with the dominant trait; 50% being heterozygous with the recessive trait; and 25% being homozygous with the recessive trait.
Representing alleles in this way also helps one determine the phenotype and genotype of the offspring. Phenotype is the physical characteristics/traits of an organism. It is observable and is influenced by the genotype. For example, using green and yellow peas, the phenotype of the parents would be green (green pea) and yellow (yellow pea). The genotype is the set of characteristics that an organism has inherited and is represented using allele notation. For example, the genotype for a homozygous yellow pea would be YY; for a homozygous green pea it would be yy; and for a heterozygous yellow pea it would be Yy. A popular way to diagrammatically represent a cross is known as a punnett square. These were created by English geneticist Reginald Punnett in the early 20th century. They allow one to determine all the possible genotypes from a cross and thus work out phenotypes, and proportions.
When dealing with homozygous monohybrid crosses, certain patterns emerge:
- If both parents are homozygous, the F1 generation will all be heterozygous and thus display the dominant phenotype.
- If the F1 generation of homozygous parents breed, the offspring will always appear in a 3:1 ratio, with 75% of offspring exhibiting the dominant phenotype, and the other 25% exhibiting the recessive characteristic.
- If the F1 generation of homozygous parents breed, the offspring, in terms of genotypes, will always appear in a 1:2:1 ratio, with 25% being homozygous with the dominant trait; 50% being heterozygous with the recessive trait; and 25% being homozygous with the recessive trait.