Question

1. In a cross involving two different characteristics (a dihybrid cross), the genotypes of the two parents are RRSS and rrss. Capital letters represent dominant alleles and lower-case letters represent recessive alleles. The “R” allele specifies round, and the “r” allele specifies oval. The “S” allele specifies smooth, and the “s” allele specifies rough (2pts)
   1. What is/are the expected genotype(s) of the F1 generation?
   2. What is/are the expected phenotype(s) of the F1 individuals?
   3. What are the predicted phenotypes for the F2 generation that is produced by the cross RrSs x RrSs?
   4. In what rate will they occur?

Answer 1

Answer:

In a cross involving two different characteristics (a dihybrid cross), the genotypes of the two parents are RRSS and rrss. Capital letters represent dominant alleles and lower-case letters represent recessive alleles. The “R” allele specifies round, and the “r” allele specifies oval. The “S” allele specifies smooth, and the “s” allele specifies rough.

R is dominant over r and S is dominant over s. The possible genotypes for these R and r alleles will be RR (homozygous dominant), Rr (heterozygous) and rr (homozygous recessive). RR and Rr will produce round as phenotype and rr produce oval as phenotype. Similarly the possible genotypes for these S and s alleles will be SS (homozygous dominant), Ss (heterozygous) and ss (homozygous recessive). SS and Ss will produce smooth as phenotype and ss produce rough as phenotype.

Gametes of RRSS will be— RS and gametes of rrss will be— rs.

Cross between two parents RRSS and rrss will produce all RrSs offspring as genotype in F1 generation. The phenotype of RrSs will be round and smooth.

1. So, the expected genotype of the F1 generation is RrSs.

2. So, the expected phenotype of the F1 individuals is round and smooth.

Gametes for RrSs—

	R	r
S	RS	rS
s	Rs	rs

Self cross of RrSs:

	RS	Rs	rS	rs
RS	RRSS (yellow and round)	RRSs (yellow and round)	RrSS (yellow and round)	RrSs (yellow and round)
Rs	RRSs (yellow and round)	RRss (yellow and wrinkled)	RrSs (yellow and round)	Rrss (yellow and wrinkled)
rS	RrSS (yellow and round)	RrSs (yellow and round)	rrSS (green and round)	rrSs (green and round)
rs	RrSs (yellow and round)	Rrss (yellow and wrinkled)	rrSs (green and round)	rrss (green and wrinkled)

Genotypic ration in F2 generation = RRSS : RRSs : RRss : RrSS : RrSs : Rrss : rrSS : rrSs : rrss = 1:2:1:2:4:2:1:2:1

Phenotypic ratio in F2 generation = round and smooth : round and rough : oval and smooth : oval and rough = 9:3:3:1

3. The predicted phenotypes for the F2 generation that is produced by the cross RrSs x RrSs are— round and smooth, round and rough, oval and smooth and oval and rough.

4. The rate they will occur is— round and smooth : round and rough : oval and smooth : oval and rough = 9:3:3:1

Answer 2

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