Need help on genotypical ratios(7th grade)?

Question

on genotypical ratios what do the three numbers stand for? heterozygous, homozygous and something else what is that and what order are they in?

Answer 1

The three numbers stand for the ratios of the three genotypes possible, heterozygous, homozygous dominant, and homozygous recessive, depending on the alleles involved. For example, if we crossed two individual organisms which were heterozygous for one trait, which we will denote as either A for the dominant allele and A' for the recessive allele, then the genotypic heterozygous condition is AA' and these 3 possible combinations exist:

AA, AA', A'A'.

1.) AA is homozygous dominant.

The dominant allele is the only one present, so it is expressed.

2.) AA' is heterozygous.

Both allele types are present. Usually, depending on the type of gene, the dominant allele controls the phenotypic expression. In other words, the dominant allele is expressed physically and the recessive allele is masked or not expressed.

3.) A'A' is homozygous recessive.

Since there is no dominant allele present, the recessive allele is expressed phenotypically.


The ratios are usually listed in this order: # of homozygous dominant: # of heterozygous: # of homozygous recessive. So, if we were to use a Punnet Square to assess the ratios, they would be in the ratio: 1:2:1 -- 1/4 homozygous dominant, 2/4 = 1/2 heterozygous, 1/4 homozygous recessive.