Chapter 2, Problem 1P

Compare and contrast the following terms:

a. dominant and recessive

b. genotype and phenotype

c. homozygous and heterozygous

d. monohybrid cross and test cross

e. dihybrid cross and trihybrid cross

Summary Introduction

To review:

The similarities and differences between the following terms:

(a) Dominant and recessive (b) genotype and phenotype (c) homozygous and heterozygous (d) monohybrid cross and test cross (e) dihybrid cross and trihybrid cross

Introduction:

1. In Mendelian genetics, Dominance and Recessive are the terms used to describe the pattern of traits. It helps to determine the inheritance of certain traits and capability of a gene to express.

2. Genotype is the genetic makeup of an organism while phenotype is the observable character or trait of an organism.

3. Homozygous refers to the gene carrying two similar alleles while heterozygous refers to a gene carrying two different alleles at same locus.

4. Monohybrid cross is used to verify the single trait while test cross is carried out to decide the unknown genotype of an individual.

5. Dihybrid refers to the crossing for two traits while trihybrid refers for the crossing of three traits.

Explanation of Solution

1. Dominant vs Recessive-

   Dominant	Recessive
   Single allele is sufficient to express the character.	Two alleles are needed to express the character.
   Determines the superior character.	Determines the inferior character.
   Represented by capital letters.	Represented by small letters.

2. Genotype and phenotype-

   Genotype	Phenotype
   Genetic code or make up of an individual.	Morphology of an individual.
   Completely depends on the gene or DNA sequence.	Depend upon the genotype and its interaction with environment.
   Consist whole DNA sequence with both expressed and non-expressed genes.	Consist only expressed genes.

3. Homozygous and heterozygous-

   Homozygous	Heterozygous
   Presence of two similar alleles for a trait.	Presence of two different alleles for a trait.
   Have any of the allelic pair- dominant or recessive. (Example –BB or bb)	Have both dominant and recessive alleles. (Example- Bb)
   Phenotypes generally expressed in either dominant or recessive according to the genotype.	Phenotypes are generally expressed are dominant.

4. Monohybrid and testcross-

   Monohybrid	Testcross
   Monohybrid cross is carried out for single character.	Testcross is carried out to determine the unknown genotype of an individual.
   Generally the expected ratio is $1:2:1$	Generally the expected ratio is $1:1$

5. Dihybrid and trihybrid-

   Dihybrid	Trihybrid
   It is carried out between two different genes / traits.	It is carried out between three different genes/ traits.
   It determines the gene order and the distance between genes.	Have both dominant and recessive alleles.
   Phenotypic ratio is $9:3:3:1$.	Phenotypic ratio is $\text{27:9:9:9:3:3:3:1}$.

Conclusion

The comparison and contrast for all the given terms is explained above, and understanding of these terms play a vital role in the studies based on genetics.