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Sexual reproduction and genetic variation

In sexually reproducing organisms, each parent contributes half of the genes acquired (at random) by the offspring. Individuals have two of each chromosome and hence two alleles of each gene, one acquired from each parent. These versions may be identical or may differ from each other. Created by Khan Academy.

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Video transcript

- [Narrator] Have you ever wondered why children often look a little similar but also very different from their biological parents, or even how biological siblings tend to share some common features but still have different traits from each other? To answer this question, we have to go beyond the physical traits that we see in these family portraits and dive into genetic inheritance. In this video, we're going to see that it's sexual reproduction, a mechanism used by many organisms to produce offspring, that creates the diversity of traits that exist in biological families and in animal and plant populations all around the world. Let's start from the beginning. All life comes from other life through the process of reproduction. Parents reproduce to form offspring, and during this process, they pass on their genetic information to their offspring. During sexual reproduction, two parents produce offspring. So each offspring gets a mixture of genetic information from two parents. Parents pass this genetic information to their offspring via chromosomes, the coiled up DNA molecules found inside your cells that contain genes. Sexually reproducing organisms often have many different chromosomes, each containing specific genes. For example, this diagram represents a complete set of human chromosomes. As we can see, there are 23 different chromosomes assigned numbers one through 23. However, there are two copies of each chromosome, so that there are 23 chromosome pairs instead of 23 single chromosomes. Each chromosome pair is a homologous pair, which means that the two chromosomes are the same size and contain the same genes in the same order. However, the alleles on the two homologous chromosomes may be different, meaning that the chromosomes may not exactly have the same genetic information. Also, in case you're wondering, the last chromosome set is a little different, because that chromosome 23 is the human sex chromosome, which influences the biological sex of the individual, but we don't have to get into that just yet. What's important to know for our purposes is that sexually reproducing organisms with two sets of chromosomes in each of their cells are called diploid. Diploid organisms, the D-I, di indicating two, have cells with two sets of chromosomes that are organized into homologous pairs. Sexual reproduction occurs through a process called fertilization, and during fertilization, cells called gametes, which are egg and sperm cells, fuse to form a new organism. Each parent contributes one gamete. So you might be wondering, if each of the parents' organism cells are diploid, and offspring result from the fusion of cells from two parents, how do the offspring of sexual reproduction maintain the same number of chromosomes? Well, diploid organisms form gait that are haploid, meaning that they only contain one set of chromosomes. When you hear the word haploid, you can think of half, because haploid cells have half the amount of genetic information than diploid cells have. A human haploid gamete, for example, contains 23 single chromosomes, one of each homologous pair. When gametes fuse during fertilization, that brings the total number of chromosomes back to 46, or 23 homologous pairs. So why is sexual reproduction so important? Well, not only does it allow organisms to produce offspring, but it also creates genetic variation and diversity. The reason that offspring have different traits compared to their parents, and that one sibling looks different from another, can be attributed to sexual reproduction. This diagram here helps illustrate how sexual reproduction creates genetic variation. The diagram shows a cross between two hypothetical parents. It shows the chromosomes and the possible gametes that the parents can form, and the possible chromosome combinations in the offspring. So in the diagram, we can see that each possible parent gamete contains one chromosome from a homologous pair, and during fertilization, gametes from each parent fuse together, resulting in offspring that have a combination of chromosomes from both parents, and this is where the genetic variability between parents and offspring comes from. Offspring are not genetically identical to either parent because they contain a mixture of genes from both. The diagram also shows us that, because each parent passes on only one chromosome from each homologous pair, there are multiple combinations of chromosomes that can occur in the offspring. For example, the pink chromosome from parent one can be paired with the dark chromosome from parent two in one offspring, and the light blue chromosome from parent two in another offspring. Keep in mind that this diagram only shows the inheritance of a single chromosome, but in humans, this occurs for all 23 of our chromosomes, and as a result, there are millions of different chromosome combinations that an offspring can inherit. This is why siblings can look alike, but aren't identical. Even more mind blowing, there are other genetic processes that occur during fertilization that increase variation even more, resulting in trillions of possible allele combinations for each offspring. This is why no two people except monozygotic twins are genetically alike. To summarize, we learned that sexual reproduction occurs when two haploid gametes fuse together in fertilization, creating a diploid offspring with homologous chromosome pairs. We also learned that the patterns of chromosome inheritance during sexual reproduction lead to genetic variation in families and populations. It's why children look different from their biological parents, brothers, or sisters. We've all inherited different sets of chromosomes because of sexual reproduction, which in turn makes each and every one of us one of a kind.