Summary
Highlights
Inheritance is the transmission of traits from one generation to the next, mediated by homologous chromosomes which are pairs of chromosomes with the same set of genes. Each parent contributes one homologous chromosome, with one from the mother and one from the father. Genes are DNA segments carrying genetic information for specific traits, and different versions of a gene are called alleles. Alleles can be dominant (uppercase) or recessive (lowercase). A dominant trait only needs one dominant allele to manifest, while a recessive trait requires two recessive alleles. Human cells have 23 pairs of chromosomes (22 somatic, 1 sex chromosome), totaling 46. Gametes (sperm and egg cells) contain half of this genetic information (23 chromosomes) and combine during fertilization to form one of three genotypes: homozygous dominant, heterozygous, or homozygous recessive, which then determines an individual's phenotype.
Genetic diseases occur when a gene malfunctions due to a mutation. These mutations can be inherited by offspring. Pedigree charts are used to visualize inheritance patterns, with circles representing females and squares representing males. Dominant inheritance occurs when a mutation in a dominant allele is sufficient to cause the disease. Recessive inheritance requires two mutant recessive alleles. Autosomal inheritance involves mutations on somatic chromosomes, while sex-linked inheritance involves mutations on sex chromosomes (X or Y). Mitochondrial inheritance is also discussed, where mitochondrial DNA mutations are passed from mother to all offspring but not from father to offspring.
Autosomal dominant diseases, such as Huntington's disease, result from a mutation in a dominant allele. Individuals with homozygous dominant (DD) or heterozygous (Dd) genotypes develop the disease. Most affected individuals who reproduce are heterozygous due to the severity of the homozygous dominant form. Using a Punnett square, if an affected heterozygous parent (Dd) reproduces with an unaffected parent (dd), there is a 50% chance of offspring inheriting the disease (Dd) and a 50% chance of being unaffected (dd). In pedigrees, affected individuals appear in every generation, with affected grandparents, aunts, uncles, and siblings. Spontaneous mutations can also occur.
Autosomal recessive diseases, like cystic fibrosis, manifest in individuals with two recessive alleles (dd). Heterozygous individuals (Dd) are carriers but do not show symptoms. If two carrier parents (Dd x Dd) reproduce, there is a 25% chance of an unaffected non-carrier child (DD), a 50% chance of a carrier child (Dd), and a 25% chance of an affected child (dd). If one parent has the disease (dd) and the other is healthy (DD), all children will be carriers (Dd). Pedigrees for recessive diseases often show skipped generations, indicating unaffected parents having affected children. Consanguineous relationships increase the risk of autosomal recessive diseases.
Sex-linked inheritance patterns differ between males and females because males have only one copy of X and Y chromosomes (hemizygous), while females have two X chromosomes. X-linked dominant diseases, like Fragile X syndrome, arise from a dominant mutation on the X chromosome. Both heterozygous and homozygous females, as well as males, can be affected. If an affected male (XDY) reproduces with an unaffected female (XdXd), all daughters will be affected heterozygotes (XdXD), and all sons will be unaffected (XdY). If a heterozygous female (XdXD) reproduces with an unaffected male (XdY), there's a 50% chance for sons and daughters to be affected. Pedigrees show affected individuals in every generation, with more affected females than males. Affected males pass the trait only to their daughters.
X-linked recessive diseases, such as hemophilia, require two recessive mutations on the X chromosome for females to be affected, while males are affected with just one such X chromosome (XdY). Females with one recessive allele are carriers (XdXD). If an affected male (XdY) reproduces with a healthy female (XDXD), all daughters will be carriers (XdXD), and all sons will be unaffected (XDY). If a carrier mother (XdXD) reproduces with a healthy father (XDY), there is a 50% chance sons will be affected (XdY) and a 50% chance daughters will be carriers (XdXD), and a 50% chance daughters will be healthy (XDXD). Pedigrees show more affected males than females, and the disease is never passed from father to son. Daughters of an affected male are always carriers.
Y-linked inheritance involves mutations on the Y chromosome, affecting only males, such as in cases of alopecia. The disease is always transmitted from father to son and never from father to daughter. Pedigrees for Y-linked traits show only affected males in every generation. This video provides a comprehensive overview of various inheritance patterns, emphasizing the distinctions between dominant, recessive, autosomal, and sex-linked forms, and providing real-world examples.