Below I will show the steps and the right way to do a monohybrid cross or any cross at all (you can be asked this in the SCT or exam).<\/p>\n\n\n\n
Mendel crossed plants with Round seeds with those with Wrinkled seeds. The round trait is the dominant one and the wrinkled is recessive.<\/p>\n\n\n\n
- When writing the genotypes, always use the same letter. Use the capital letter for the dominant and the small letter for the recessive.<\/strong><\/li>
- When writing the gametes, always write them in circles<\/strong> (we will see the reason for this in a bit)<\/em><\/li><\/ul>\n\n\n\n
Parental Phenotype: Round\u00d7Wrinkled<\/p>\n\n\n\n
Parental Genotype: RR\u00d7rr<\/p>\n\n\n\n
Parental Gametes: \u24c7\u24c7\u00d7\u24e1\u24e1<\/p>\n\n\n\n
<\/td> \u24c7<\/td> \u24c7<\/td><\/tr> \u24e1<\/td> Rr<\/td> Rr<\/td><\/tr> \u24e1<\/td> Rr<\/td> Rr<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n F1(1st generation) genotype: Rr<\/p>\n\n\n\n
F1 phenotype: Round<\/p>\n\n\n\n
As seen here, all the offspring will have the same genotype(heterozygous) and the same phenotype; round. Remember that the dominant trait(Round) will always be expressed once it is present. This is how Mendel knew which traits were dominant and which were recessive.<\/p>\n\n\n\n
Now we will cross the F1<\/strong> organisms with the same steps as above. The table where I make the cross is known as a Punnett Square<\/strong>.<\/p>\n\n\n\n
Parental(F1) Phenotype: Round\u00d7Round<\/p>\n\n\n\n
F2 genotype: Rr\u00d7Rr<\/p>\n\n\n\n
F2 gametes: \u24c7\u24e1\u00d7\u24c7\u24e1<\/p>\n\n\n\n
<\/td> \u24c7<\/td> \u24e1<\/td><\/tr> \u24c7<\/td> RR<\/td> Rr<\/td><\/tr> \u24e1<\/td> Rr<\/td> rr<\/td><\/tr><\/tbody><\/table><\/figure>\n\n\n\n F2 genotype: RR:Rr:rr<\/p>\n\n\n\n
Genotypic ratio: 1:2:1<\/p>\n\n\n\n
F2 phenotype: Round:Wrinkled<\/p>\n\n\n\n
Phenotypic ratio: 3:1<\/p>\n\n\n\n
As seen above, this ‘3:1’ ratio was observed by Mendel in the F2<\/strong> generation and this ‘3:1’ ratio is characteristic for similar monohybrid crosses that follow Mendelian Genetics. He repeated this experiment more than once and found the same ratios so he made explanations and theories for this that eventually lead to him writing Mendel’s first law<\/strong>, also known as The Law of Segregation.<\/strong><\/p>\n\n\n\n
- When writing the gametes, always write them in circles<\/strong> (we will see the reason for this in a bit)<\/em><\/li><\/ul>\n\n\n\n
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