Question

You have considered single locus and two locus (di) hybrid crosses for loci with alleles that are completely dominant/recessive.

I. Come up with a general rule for the proportion of offspring expected to have the phenotype of all the recessive traits associated with X autosomal loci when fully heterozygous parents are crossed.

II. Now, use that logic and provide the expected proportion of offspring that show recessive phenotype for 4 loci and the dominant phenotype the 5th locus when two fully heterozygous parents are crossed.

Answer 1

Ans 1) when Parents are fully heterozygous then proportion of offsprings expected to have phenotype of all recessive traits associated is :

Example --- consider individual Aa Bb Cc it is Heterozygous of all the genes and it is allowed to self or a genetic similar individual is allowed to cross with it. So, Aa Bb Cc X Aa Bb Cc then we will use a general rule according to which each gene is crossed with that gene of other parent by using monohybrid cross that is Aa from Parent 1 is crossed with Aa from Parent 2. And similar for other genes too. The progenies obtained are as : 1/4 AA ; 2/4 Aa; 1/4 aa. And the same result we will get for each gene in this Cross. So, we will get 1/4 aa from Aa X Aa ; 1/4 bb from Bb X Bb ; and 1/4 cc from Cc X Cc.

Proportion of offspring to have recessive Phenotype for all traits = 1/4 aa * 1/4 bb * 1/4 cc.= 1/64

.Ans 2) when a fully heterozygous Parents are crossed what proportion of offspring are expected to show recessive Phenotype for 4 loci and Dominant Phenotype for 5th loci.

The parent is heterozygous so they must have gene with allelea as Aa ; one Dominant and recessive for all genes. When Aa from one parent is crossed with Aa of another Parent the proportion of offsprings expected are : 1 AA, 2 Aa and 1 aa genotypically and 3:1 where 3 = AA and Aa and 1 = aa phenotypically.

Ratio of Dominant phenotye = 3/4 and recessive Phenotype = 1/4.

If 4 loci of offspring are expected to show recessive Phenotype so becomes = 1/4 * 1/4 * 1/4 * 1/4. 1/4 is individual proportion of a gene to have recessive phenotye.

If 5th loci shows Dominant Phenotype the proportion becomes = 3/4 ; where 3 represents Dominant phenotye on crossing a heterozygote Individual and 4 is total expected progenies.

Total Probability of having offsprings with 4 loci recessive and 5 loci as Dominant is =

1/4 * 1/4 * 1/4 * 1/4 * 3/4

= 3/ 1024.