 Having a genetic trait is more complex than simply just inheriting a gene. There's a lot more involved, and in this video, we'll have a look at this in a little bit more detail. Genetic variability within offsprings occurs due to the genes that are passed on from both parents. Patterns of inheritance describe the various ways that the traits are passed on, from one generation to the next. There are various modes of inheritance, however in this video the key modes that we'll focus on include autosomal, sex-linked, co-dominance, and polygenic inheritance. Autosomal and sex-linked inheritance can also be further divided into dominant and recessive. Autosomal dominant inheritance is when the inheritance of a single allele causes the development of a genetic condition. This is because the trait is a dominant allele, which is always expressed. If a parent has the allele, there is a 50% chance that the offspring will also inherit and express the gene. Since this inheritance pattern occurs due to the genes on the autosomes, sons and daughters have an equal chance of inheriting the gene. An example of an autosomal dominant condition is a brain disorder called Huntington's disease, which causes progressive degeneration of nerves within the brain. For the development of this condition, the offspring requires the inheritance of a single copy of the HTT gene. Autosomal recessive inheritance is when two alleles are needed to be inherited for the development of a genetic condition. This occurs for conditions that contain a recessive allele, hence in order to express the condition, two copies of the recessive allele are needed. An individual who has one copy is said to be a carrier. In other words, they have the gene but just don't express it. If both parents are carriers, there is a 25% that the offspring will develop the condition. An example of an autosomal recessive condition is cystic fibrosis. Cystic fibrosis occurs due to a faulty protein receptor, which causes issues with moving salts and water across the cell membrane. This condition occurs due to the inheritance of two copies of the CFT R gene. Sex-linked inheritance can either be X-linked or Y-linked, depending on which sex chromosome the allele is found. Y-linked only affects males, X-linked can affect females but mainly impacts males. In this video, we'll just focus on X-linked inheritance. The development of sex-linked dominant conditions is caused by the inheritance of one allele on the X chromosome. As these alleles are passed on through the germ cells, there is a difference in inheritance patterns between the mother and father. Males have a single X chromosome inherited from their mothers, while females carry two X chromosomes, one from each parent. If one parent has a trait, there is at least a 50% chance that this will be passed on to the offspring. However, this can vary depending on which parent has the condition and the sex of the offspring. An example of a sex-linked dominant condition is Fragile X syndrome. This occurs due to a mutation within the FMR1 gene, which is found on the X chromosome. This prevents the production of a specific protein needed for brain development. X-linked recessive conditions are more common in males. As you may recall, males have a single X chromosome, and because of this, only a single recessive allele on the X chromosome can cause the development of the genetic disorder. Female offspring can also inherit the condition, only if both parents carry a copy of the allele, causing the individual to inherit two copies of the recessive allele. Hemophilia is an example of a sex-linked recessive condition, caused by the inheritance of a mutated gene on the X chromosome. This inherited bleeding disorder, which occurs due to issues in blood clotting, happens most commonly in males than females, due to the recessive nature of the condition. Codominance is when two copies of an allele are expressed individually, which results in variations within the trait expressed. This leads to the offspring displaying both traits simultaneously, with neither trait being expressed dominantly. An example of codominant inheritance can be observed for AB blood types. Red blood cells can have different antigens or no antigens at all. Offsprings that inherit both the A and B alleles express both of these alleles together. This results in individuals having AB blood types, where both the A and B antigens are present in the blood cells. Polygenic inheritance refers to the inheritance of a trait being dependent on the expression of multiple different genes. Each gene has a small effect on the total outcome for the trait expression. However, each of these genes also have a specific role. Eye colour is an example of polygenic inheritance. It depends on the expression of more than one gene. This is because the different alleles work together to influence the melanin level produced within the outer layer of the iris, and impacts the eye colour expressed. In summary, we covered how genetic factors can be inherited from one generation to the next. We discussed four different modes of inheritance, which includes autosomal inheritance, which can be dominant or recessive, sex linked inheritance, which can also be due to a dominant or recessive allele, codominant inheritance, and polygenic inheritance.