Let's answer each question step by step: --- ### **Q1. For a species with a diploid number of 18, indicate how many chromosomes will be present in the somatic nuclei of individuals that are: haploid, tetraploid, trisomic, and monosomic.** #### **Step 1: Understanding Diploid Number** - **Diploid number (2n) = 18** - This means the basic set (haploid number) is: **n = 18 / 2 = 9** #### **Step 2: Chromosome Number for Each Condition** 1. **Haploid (n):** - Haploid means having only one set of chromosomes (n). - **Chromosome number = 9** 2. **Tetraploid (4n):** - Tetraploid means having four sets of chromosomes. - **Chromosome number = 4 × n = 4 × 9 = 36** 3. **Trisomic (2n + 1):** - Trisomic means there is one extra chromosome (one chromosome is present in three copies instead of two). - **Chromosome number = 2n + 1 = 18 + 1 = 19** 4. **Monosomic (2n – 1):** - Monosomic means one chromosome is missing (one chromosome is present in only one copy instead of two). - **Chromosome number = 2n – 1 = 18 – 1 = 17** #### **Final Answer to Q1:** - **Haploid:** 9 chromosomes - **Tetraploid:** 36 chromosomes - **Trisomic:** 19 chromosomes - **Monosomic:** 17 chromosomes --- ### **Q2. Certain varieties of chrysanthemums contain 18, 36, 54, 72, and 90 chromosomes; all are multiples of a basic set of nine chromosomes. How would you describe these varieties genetically? What feature do the karyotypes of each variety share? A variety with 27 chromosomes has been discovered, but it is sterile. Why?** #### **Step 1: Identify the Basic Set (x)** - The smallest number is 18, which is 2×9. - So the **basic chromosome set (x)** is **9**. #### **Step 2: Describe the Varieties Genetically** The numbers of chromosomes are: - 18 = 2 × 9 → **Diploid (2x)** - 36 = 4 × 9 → **Tetraploid (4x)** - 54 = 6 × 9 → **Hexaploid (6x)** - 72 = 8 × 9 → **Octoploid (8x)** - 90 = 10 × 9 → **Decaploid (10x)** So, genetically, these varieties are **polyploids**—they have chromosome numbers that are exact multiples of the basic set (x = 9). #### **Step 3: Shared Feature of the Karyotypes** - All karyotypes are **euploid**: they have whole multiples of the basic chromosome set. - **Common feature:** Each variety has complete sets of 9 chromosomes. #### **Step 4: The Sterility of the 27-Chromosome Variety** - 27 is **not a whole multiple** of 9. - 27 / 9 = 3, so this is **triploid (3x)** - **Triploids** are generally **sterile** because during meiosis, homologous chromosomes cannot pair evenly (there are three of each type), leading to the formation of unbalanced gametes. - Thus, the variety with 27 chromosomes is sterile because of **impaired meiosis** and production of nonviable gametes. --- ### **Final Answers:** #### **Q1** - **Haploid:** 9 chromosomes - **Tetraploid:** 36 chromosomes - **Trisomic:** 19 chromosomes - **Monosomic:** 17 chromosomes #### **Q2** - The varieties are **polyploid** forms with chromosome numbers that are exact multiples of the basic set of 9 (i.e., diploid, tetraploid, hexaploid, octoploid, decaploid). - **Shared feature:** Each has complete sets of 9 chromosomes (euploidy). - The variety with **27 chromosomes (triploid)** is **sterile** because triploids cannot undergo normal meiosis, leading to unbalanced, nonviable gametes.