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) heterozygous Parents are crossed having Autosomal loci means Autosomes. A Heterozygous parent consist of half Dominant and half recessive and in case of dihybrid cross both genes would have one Dominant and one recessive pair. As in Pea plant dihybrid cross ----- RrYy.

In seeing individually both the genes Rr and Yy from both parents. Rr X Rr produces = 1/4 rr or recessive phenotye ; while Yy X Yy ------> produces 1/4 yy or recessive and in total produces 1/4 rr * 1/4 yy = 1/16 recessive Phenotype Individuals.

.Ans 2) when a fully heterozygous Parents are crossed the 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 alleles 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.