Chapter 5, Problem 1P

For parts a, b, and c, draw a diagram illustrating the alleleson homologous chromosomes for the following genotypes,assuming in each case that the genes reside on the samechromosome in the order written. For parts d and e, givethe information requested.

1. $\text{ AB/ab}$

2. $\text{aBc/abC}$

3. $\text{DFg/DFG}$

4. the gametes produced by an organism with the genotype $\text{Rt/rT}$

5. progeny of the cross $\text{Rt/rT}$* $\text{rt/rt}$

Summary Introduction

To analyze:

1. Diagram the alleles on homologous chromosomes for genotype AB/ab

2. Diagram the alleles on homologous chromosomes for genotype aBc/ abC

3. Diagram the alleles on homologous chromosomes for genotype DFg/ DFG

4. The gametes produced by organism with genotype Rt/rT

5. Progeny of the cross Rt/rT and rt/rt

Introduction:

Homologous chromosomes are a pair of chromosome (one from each parent, i.e., maternal and paternal) which are similar in length, gene position, and location of centromere. On each homologous chromosome, the position of genes is the same, but the genes may contain different alleles. On the same locus of two chromosomes, same or different alleles (e.g., AA or Aa) are present.

Explanation of Solution

1. In a cell, two homologous chromosomes will be present- maternal homologue and paternal homologue. Consider that alleles A and B are present at their respective loci on maternal homologue and alleles a and b are present at their respective loci on paternal homologue.

   

2. Consider that alleles a, B, and c are present at their respective loci on maternal homologue and alleles a, b, and C are present at their respective loci on paternal homologue.

   

3. Consider that alleles D, F, and g are present at their respective loci on maternal homologue and alleles D, F, and G are present at their respective loci on paternal homologue.

   

4. Genotype of organism – Rt/rT

   Gametes produced by the organism can be determined using Mendel’s law of independent assortment of alleles. It states that when there are two pairs of characters (here, R and T), the distribution of alleles of one character into the gametes is independent of the distribution of alleles of the other character.

   The total number of gametes produced by an organism can be calculated on the basis of heterozygosity. In the given condition, Rt/rT, i.e., the number of heterozygosity is $\text{2}$.

   Therefore, number of gametes produced = ${\text{2}}^{\text{2}}=\text{ 4 }$

   Gametes produced – RT, Rt, rT, rt

   A cross between Rt/rT and rt/rt

5. Parent $1$(♂): Rt/rT

   Gametes: RT, Rt, rT, rt

   Parent $\text{2}$(♀): rt/rt

   Gametes: rt, rt

   ♀/♂	RT	Rt	rT	rt
   rt	RrTt	Rrtt	rrTt	rrtt
   rt	RrTt	Rrtt	rrTt	rrtt

   Thus, the genotypes of offspring are RT/rt, rT/rt, Rt/rt, rt/rt.

Conclusion

Alleles on chromosomes for different genotypes are drawn and gametes and genotypes of progeny obtained from a cross between Rt/rT and rt/rt are determined using Mendelian law of independent assortment and Punnett square method.