Chapter 5, Problem 14P

Write the number of different kinds of phenotypes, excluding sex, you would see among a large number of progeny from an F₁ mating between individuals of identical genotype that are heterozygous for one or two genes (that is, Aa or Aa Bb) as indicated. No gene interactions means that the phenotype determined by one gene is not influenced by the genotype of the other gene.

a.	One gene; A completely dominant to a.
b.	One gene; A and a codominant.
c.	One gene; A incompletely dominant to a.
d.	Two unlinked genes; no gene interactions; A completely dominant to a, and B completely dominant to b.
e.	Two genes, 10 m.u. apart; no gene interactions; A completely dominant to a, and B completely dominant to b.
f.	Two unlinked genes; no gene interactions; A and a codominant, and B incompletely dominant to b.
g.	Two genes, 10 m.u. apart; A completely dominant to a, and B completely dominant to b; and with recessive epistasis between aa and the alleles of gene B.
h.	Two unlinked duplicated genes (that is, A and B perform the same function); A and B completely dominant to a and b, respectively.
i.	Two genes, 0 m.u. apart; no gene interactions; A completely dominant to a, and B completely dominant to b. (Two possible answers exist.)

Summary Introduction

a.

To determine:

The phenotype if in a gene A is completely dominant to a.

Introduction:

Genetics is defined as the study of the genome of an organism. The complete genetic material of an organism is termed as genetics. The people who study genetics are termed as geneticists.

Explanation of Solution

If an allele A is completely dominant to allele a then there are three possible genotypes. These possible genotypes will be AA, Aa, and aa. So if the parents having genotype AA and aa are crossed with each other and then the genotype of the offsprings will be AA, Aa, Aa, and aa. So, the phenotypic ratio will be 3:1 because allele A will mask the effect of allele a.

Summary Introduction

b.

To determine:

The phenotype if in a gene A and a are codominant.

Introduction:

Codominance is a condition in which both the alleles in a heterozygote express together. A new phenotype is obtained when both the alleles express their traits.

Explanation of Solution

The cross of two heterozygous parents having genotype Aa is as follows:

♀/♂	A	a
A	AA	Aa
a	Aa	Aa

If allele A and a are codominant, then the expression of both alleles take place in case of the heterozygous condition. So, a new phenotype would be observed in the case of heterozygous individuals. Therefore, the ratio of individuals would be 1:2:1. Three phenotypes will appear in the generation.

Summary Introduction

c.

To determine:

The phenotype if in a gene A is incompletely dominant to a.

Introduction:

In case of incomplete dominance, neither allele in the genotype is dominant. An intermediate trait is obtained in case of heterozygotes.

Explanation of Solution

The cross of two heterozygous parents having genotype Aa is as follows:

♀/♂	A	a
A	AA	Aa
a	Aa	Aa

If allele A and a are incomplete dominant then the allele A will not be able to completely mask the effect of allele a. So, a new phenotype would be observed in the case of heterozygous individuals. Therefore, the ratio of individuals would be 1:2:1.

Summary Introduction

d.

To determine:

The phenotype if in two unlinked genes A completely dominant to a, and B completely dominant to b.

Introduction:

The set of the alleles in DNA that carries the information for the expression of a trait in an individual is known as its genotype.

Explanation of Solution

A dihybrid cross will take place when two unlinked genes A completely dominant to a, and B completely dominant to b. The dihybrid cross that will take place is as follows:

♀/♂	AB	Ab	aB	ab
AB	AABB	AABb	AaBB	AaBb
Ab	AABb	AAbb	AaBb	Aabb
aB	AaBB	AaBb	AaBB	aaBb
ab	AaBb	Aabb	aaBb	aabb

The phenotype will differ with 4 genotypes. These genotype are A-B-, A-bb, aaB-, and aabb. So the phenotypic ratio of the given cross is 9:3:3:1.

Summary Introduction

e.

To determine:

The phenotype if two genes are 10 m.u. apart and A completely dominant to a, and B completely dominant to b.

Introduction:

The traits expressed by the genes are known as phenotypes. The phenotypes are the results of genotypes. One example of the phenotype is the color of the flower.

Explanation of Solution

The distance between genes is 10 map units (m.u.). So there is no interaction between these genes. Therefore, the genes will be linked due to the presence of less distance between the genes. The segregation of genes will not take place properly, and the phenotype 9:3:3:1 will not be observed.

Summary Introduction

f.

To determine:

The phenotype if in two unlinked genes A and a codominant, and B incompletely dominant to b.

Introduction:

Chromosomes are present in the eukaryotic organisms. They are “thread-like” in appearance. A chromosome is made up of two units: DNA and proteins. The genes are located on the chromosomes.

Explanation of Solution

In the case of codominance, in the heterozygous individual, allele A and allele, a will be expressed together. The allele B is incompletely dominant to allele b so a new phenotype will be observed in case of heterozygous individuals. Then phenotypic classes would be AABB, AABb, AaBB, AaBb, AAbb, Aabb, aaBB, aaBb, and aabb. So the phenotypic ratio would be 1:2:2:4:1:2:1:2:1.

Summary Introduction

g.

To determine:

The phenotype if two genes are 10 m.u. apart in which A completely dominant to a, and B completely dominant to b and with recessive epistasis between aa and the alleles of gene B.

Introduction:

Chromosome mapping is an important method that indicates the relative location of genes on the chromosomes. The distance between two genes in a chromosome map is known as the map distance. It is also known as interlocus distance.

Explanation of Solution

When the expression of one gene pair is dependent on another gene pair then this phenomenon is known as recessive epistasis. Therefore, the type of phenotype that will be observed in the generation would be 4. This phenotype would be expressed because of genotypes A-B-, A-bb, aaB-, and aabb.

Summary Introduction

h.

To determine:

The phenotype if in two unlinked duplicated genes; A and B completely dominant to a and b, respectively.

Introduction:

Chromosome theory of inheritance states that a trait of an organism is regulated by the pair of genes that are located on a chromosome. During meiosis, chromosomes segregate from each other and inherit into the offspring to maintain genetic continuity.

Explanation of Solution

Homologous recombination is a process which allows the separation of unlinked genes. Homologous recombination will allow the separation of alleles from the genotype, and then in upcoming generation four phenotypic classes would be possible. These phenotypic classes are A-B-, A-bb, aaB-, and aabb.

Summary Introduction

i.

To determine:

The phenotype if two genes are 0 m.u. apart and A completely dominant to a, and B completely dominant to b.

Introduction:

Inheritance is a mechanism in which traits of the parents transfer into the offsprings of the next generation. Gregor Mendel was a monk, and he discovered three laws that describe the inheritance of factors from parents to offsprings.

Explanation of Solution

If the distance between to genes is 0 m.u means these genes are located on the same locus, and then the inheritance of these genes will take place together. The allele A will mask the effect of allele a and allele B will mask the effect of allele b. Therefore, four phenotypes are possible, and the phenotypic class will include A-B-, A-bb, aaB-, and aabb.