## Diversity in Chromosome Numbers Across Species [block_0] 1. **A species is partly defined by its characteristic chromosome number, which remains stable in normal individuals.** 2. Chromosome number is **conserved across all diploid cells** within a species, ensuring accurate inheritance during reproduction. 3. Occasional **mutations or chromosomal rearrangements** may alter this in individuals, but the species norm remains stable. 4. **Across species**, chromosome number varies dramatically—from just **two chromosomes** in some nematodes to **hundreds in ferns**. ## Chromosome Numbers Are Shared Traits Within a Species [block_3] Chromosomes are **the carriers of genetic information in all living organisms**. They consist of two essential components: **deoxyribonucleic acid** (DNA) and **proteins**. 1. One of the **defining** characteristics of a species is that its members typically **shar**e the **same number of chromosomes**. 2. This uniformity is essential for successful reproduction. ## Diploid Cells and Even Chromosome Numbers [block_5] 1. Most plants and animals are **diploid**, meaning their body cells contain two sets of chromosomes, one from each parent. 2. **Humans**: Somatic cells have 46 chromosomes (23 pairs). 1. Each pair consists of **homologous chromosomes** that carry the same sequence of genes. 2. **Haploid Gametes**: During fertilization, sperm and egg cells (haploid) combine to restore the diploid number. [{"_key":"block_8","_type":"block","asset":null,"children":[{"_key":"H4lKdx2AthRpksOOBlyiLI","_type":"span","markDefs":[],"marks":[],"text":"Remember, the diploid chromosome number is always even because it represents two complete sets of chromosomes, one from each parent."}],"markDefs":[],"style":"normal"}] ## Chromosome Numbers Are Usually Even [2fc8664c773c] 1. Diploid organisms inherit **two sets of chromosomes:** one paternal, one maternal. 2. During **gamete** **formation**, chromosomes must pair during meiosis; this is only possible if the diploid number is **even**. 3. After **fertilization**, the zygote is restored to **diploid** status. ![](https://cdn.sanity.io/images/w7j7fz60/production/f1b693332193b406ea0529742cd8152ba5cad26b-2000x1800.png) [{"_key":"ea220c449ca6","_type":"block","asset":null,"children":[{"_key":"3b9c00b2c7740","_type":"span","marks":[],"text":"In IB exams, always state whether you are referring to the "},{"_key":"34ac782b6d51","_type":"span","marks":["strong"],"text":"haploid"},{"_key":"34fc41ecd1bb","_type":"span","marks":[],"text":" (n) or "},{"_key":"f6f622f49f99","_type":"span","marks":["strong"],"text":"diploid"},{"_key":"a2f3f862daa1","_type":"span","marks":[],"text":" (2n) number. "}],"level":1,"listItem":"bullet","markDefs":[],"style":"normal"},{"_key":"7c8022dc89d1","_type":"block","asset":null,"children":[{"_key":"4c5e18c49e02","_type":"span","marks":[],"text":"Saying “humans have 23 chromosomes” is incorrect. "}],"level":1,"listItem":"bullet","markDefs":[],"style":"normal"},{"_key":"7fde9da1d7c3","_type":"block","asset":null,"children":[{"_key":"fb53e8d46752","_type":"span","marks":[],"text":"It should be “"},{"_key":"1c5c0630f9e3","_type":"span","marks":["strong"],"text":"23 pairs in diploid cells"},{"_key":"7d4a504ef5d5","_type":"span","marks":[],"text":"” or “"},{"_key":"36c6e92d5d63","_type":"span","marks":["strong"],"text":"23 chromosomes in haploid gametes."},{"_key":"d81a0bbcddb2","_type":"span","marks":[],"text":"”"}],"level":1,"listItem":"bullet","markDefs":[],"style":"normal"}] ## How Chromosome Numbers Change Over Time [627ea9973d77] > Changes in chromosome numbers are **rare events**, but when they do occur, they can be caused by: ### 1. Chromosome Fusion** ** [5f6d41e8c443] 1. Two chromosomes join together end-to-end. 2. This reduces the chromosome number. [{"_key":"dab70245c426","_type":"block","asset":null,"children":[{"_key":"336a2d70d2b8","_type":"span","marks":[],"text":"Human chromosome "},{"_key":"956b7ae049bd","_type":"span","marks":["strong"],"text":"2"},{"_key":"83044d4e9e7f","_type":"span","marks":[],"text":" is the result of a fusion of two ancestral ape chromosomes. "}],"level":1,"listItem":"bullet","markDefs":[],"style":"normal"},{"_key":"11e3884890bd","_type":"block","asset":null,"children":[{"_key":"e89b1af72e3d","_type":"span","marks":[],"text":"This explains why humans have "},{"_key":"1d7f9cda4408","_type":"span","marks":["strong"],"text":"46 chromosomes"},{"_key":"03933523ef88","_type":"span","marks":[],"text":" while chimpanzees, gorillas, and orangutans have "},{"_key":"c2050e4f38a4","_type":"span","marks":["strong"],"text":"48"},{"_key":"a035bd4b1fc6","_type":"span","marks":[],"text":"."}],"level":1,"listItem":"bullet","markDefs":[],"style":"normal"}] ### 2. Chromosome Fission [857d75251c8f] 1. A **single chromosome splits into two smaller chromosomes**. 2. This increases the chromosome number without increasing the DNA content. ### 3. Polyploidy [1a04785b3fc3] 1. Entire sets of chromosomes duplicate. 2. Common in plants (e.g., wheat is hexaploid, 6n). 3. Polyploidy can lead to **instant speciation** because polyploid individuals cannot breed with diploid members of the same species. [{"_key":"0e658e6726f7","_type":"block","asset":null,"children":[{"_key":"1a12189537ea","_type":"span","marks":[],"text":"Wheat is "},{"_key":"ad6bb699334f","_type":"span","marks":["strong"],"text":"hexaploid"},{"_key":"91367f0cf587","_type":"span","marks":[],"text":" (six sets of chromosomes)."}],"markDefs":[],"style":"normal"}] ![](https://cdn.sanity.io/images/w7j7fz60/production/18dfd046aae8fcf98befbced659ae70dbe1c4cf0-2560x1920.jpg) [{"_key":"block_12","_type":"block","asset":null,"children":[{"_key":"5d75957cdb5d","_type":"span","markDefs":[],"marks":[],"text":"A horse has "},{"_key":"f1ab401e0577","_type":"span","markDefs":[],"marks":["strong"],"text":"64"},{"_key":"709dd6048ec7","_type":"span","markDefs":[],"marks":[],"text":" chromosomes, while a donkey has "},{"_key":"058012be9fae","_type":"span","markDefs":[],"marks":["strong"],"text":"62"},{"_key":"d6beabe25aee","_type":"span","markDefs":[],"marks":[],"text":". "}],"level":1,"listItem":"bullet","markDefs":[],"style":"normal"},{"_key":"c96c37b65be9","_type":"block","asset":null,"children":[{"_key":"e7af58871c20","_type":"span","markDefs":[],"marks":[],"text":"Their offspring, a "},{"_key":"072049186c84","_type":"span","markDefs":[],"marks":["strong"],"text":"mule"},{"_key":"e4b66c8537d2","_type":"span","markDefs":[],"marks":[],"text":", inherits "},{"_key":"f647f7c42691","_type":"span","markDefs":[],"marks":["strong"],"text":"63"},{"_key":"a9ab277c21ab","_type":"span","markDefs":[],"marks":[],"text":" chromosomes. "}],"level":1,"listItem":"bullet","markDefs":[],"style":"normal"},{"_key":"196be9cbbe65","_type":"block","asset":null,"children":[{"_key":"280a85e74529","_type":"span","markDefs":[],"marks":[],"text":"This odd number "},{"_key":"49eee48632fd","_type":"span","markDefs":[],"marks":["strong"],"text":"prevents"},{"_key":"49c93a3c2635","_type":"span","markDefs":[],"marks":[],"text":" proper chromosome pairing during "},{"_key":"e08cd315e602","_type":"span","markDefs":[],"marks":["strong"],"text":"meiosis"},{"_key":"cb034b9ab538","_type":"span","markDefs":[],"marks":[],"text":", leading to "},{"_key":"97b84d65fe41","_type":"span","markDefs":[],"marks":["strong"],"text":"infertility"},{"_key":"65e228f0b92e","_type":"span","markDefs":[],"marks":[],"text":"."}],"level":1,"listItem":"bullet","markDefs":[],"style":"normal"}] ### Diversity in Chromosome Numbers Across Species [block_13] 1. Chromosome numbers vary dramatically among species, ranging from as few as 2 to over 100. 2. Humans share **close genetic similarity** with chimpanzees, yet differ in chromosome number (46 vs 48) due to a **fusion event** in human ancestry. [{"_key":"06371b3c2743","_type":"block","asset":null,"children":[{"_key":"59d845360a1b","_type":"span","marks":["strong"],"text":"Humans vs. Chimpanzees"}],"markDefs":[],"style":"normal"},{"_key":"2e7488112b14","_type":"block","asset":null,"children":[{"_key":"e09fbc4d6324","_type":"span","marks":["strong"],"text":"Humans"},{"_key":"00a84ec5a7ff","_type":"span","marks":[],"text":": 46 chromosomes."}],"level":1,"listItem":"number","markDefs":[],"style":"normal"},{"_key":"f4c7f5d1bcf2","_type":"block","asset":null,"children":[{"_key":"b75036a10c8a","_type":"span","marks":["strong"],"text":"Chimpanzees"},{"_key":"4b04d4ff524d","_type":"span","marks":[],"text":": 48 chromosomes."}],"level":1,"listItem":"number","markDefs":[],"style":"normal"},{"_key":"bb210d94d965","_type":"block","asset":null,"children":[{"_key":"aae19fe796b9","_type":"span","marks":["strong"],"text":"Cause of Difference"},{"_key":"fc73cc56be2c","_type":"span","marks":[],"text":": Research suggests that human chromosome 2 resulted from the "},{"_key":"706b69024528","_type":"span","marks":["strong"],"text":"fusion"},{"_key":"eb3dc3a0fe4e","_type":"span","marks":[],"text":" of two ancestral chromosomes after humans and chimpanzees diverged."}],"level":1,"listItem":"number","markDefs":[],"style":"normal"}] #### Polyploidy in Plants [93962464fa7b] 1. Unlike animals, plants frequently exhibit **polyploidy** (more than two sets of chromosomes). 2. Polyploidy increases genetic diversity and often leads to new species. [{"_key":"block_21","_type":"block","asset":null,"children":[{"_key":"0abb2c2c6457","_type":"span","markDefs":[],"marks":[],"text":"Polyploidy is "},{"_key":"4a8ff9032a68","_type":"span","markDefs":[],"marks":["strong"],"text":"rare"},{"_key":"b43400e7ffd0","_type":"span","markDefs":[],"marks":[],"text":" in animals but common in plants, as plants can "},{"_key":"df29c7c779a8","_type":"span","markDefs":[],"marks":["strong"],"text":"reproduce asexually"},{"_key":"ffe685ed4293","_type":"span","markDefs":[],"marks":[],"text":", bypassing the challenges of uneven chromosome numbers during meiosis."}],"markDefs":[],"style":"normal"}] ### Chromosome Size vs. Number [774eb10653f5] 1. The **same total DNA content** can be arranged in a **few large chromosomes** or **many small chromosomes**. 2. Laboratory experiments in yeast show that reducing chromosomes from **16 to just 2-4** still allowed survival, proving that chromosome number itself is not the limiting factor. 3. Instead, it is the **presence of all essential genes**. [{"_key":"bb4178a736be","_type":"block","asset":null,"children":[{"_key":"ebe40dcd56c2","_type":"span","marks":[],"text":"Some plants have "},{"_key":"52f3edd731cb","_type":"span","marks":["strong"],"text":"very high chromosome numbers"},{"_key":"2ff713eb1d95","_type":"span","marks":[],"text":" but are not genetically more complex than humans."}],"markDefs":[],"style":"normal"}] [{"_key":"block_42","_type":"block","asset":null,"children":[{"_key":"H4lKdx2AthRpksOOBlyifU","_type":"span","markDefs":[],"marks":[],"text":"How does the study of chromosome numbers challenge traditional definitions of species? Consider hybrids like ligers or polyploid plants that blur the boundaries."}],"markDefs":[],"style":"normal"}] [{"_key":"55168e179693","_type":"block","asset":null,"children":[{"_key":"b888eb2461b60","_type":"span","marks":[],"text":"Define "},{"_key":"b888eb2461b61","_type":"span","marks":["strong"],"text":"chromosome number"},{"_key":"b888eb2461b62","_type":"span","marks":[],"text":", "},{"_key":"b888eb2461b63","_type":"span","marks":["strong"],"text":"diploid"},{"_key":"b888eb2461b64","_type":"span","marks":[],"text":", and "},{"_key":"b888eb2461b65","_type":"span","marks":["strong"],"text":"haploid"},{"_key":"b888eb2461b66","_type":"span","marks":[],"text":", using humans as an example."}],"level":1,"listItem":"bullet","markDefs":[],"style":"normal"},{"_key":"2c074137a62b","_type":"block","asset":null,"children":[{"_key":"b7395895ded1","_type":"span","marks":[],"text":"Why are diploid chromosome numbers nearly always even?"}],"level":1,"listItem":"bullet","markDefs":[],"style":"normal"},{"_key":"87064d1602d3","_type":"block","asset":null,"children":[{"_key":"bcf1c6b02f71","_type":"span","marks":[],"text":"Describe "},{"_key":"b888eb2461b67","_type":"span","marks":["strong"],"text":"two mechanisms"},{"_key":"b888eb2461b68","_type":"span","marks":[],"text":" that can increase and "},{"_key":"b888eb2461b69","_type":"span","marks":["strong"],"text":"one mechanism"},{"_key":"b888eb2461b610","_type":"span","marks":[],"text":" that can decrease chromosome numbers."}],"level":1,"listItem":"bullet","markDefs":[],"style":"normal"},{"_key":"d9955f184315","_type":"block","asset":null,"children":[{"_key":"0db74764ec9f","_type":"span","marks":[],"text":"State the diploid chromosome numbers of "},{"_key":"b888eb2461b611","_type":"span","marks":["strong"],"text":"humans"},{"_key":"b888eb2461b612","_type":"span","marks":[],"text":" and "},{"_key":"b888eb2461b613","_type":"span","marks":["strong"],"text":"chimpanzees"},{"_key":"b888eb2461b614","_type":"span","marks":[],"text":", and explain the difference."}],"level":1,"listItem":"bullet","markDefs":[],"style":"normal"}]