Inheritance
Genetic inheritance is the combination of cellular processes that result in the transmission of genetic traits from parent to offspring. Genes are the basic unit of inheritance that act as blueprints for these traits and are found in pairs known as alleles. Types of inheritance vary depending on whether genes are located on sex chromosomes or autosomes and whether genes are dominant or recessive. Identifying inheritance patterns allows scientists to predict the probability that a child will inherit a certain characteristic such as brown eyes, color blindness, or cystic fibrosis.
One of the first theories to describe how characteristics are inherited was the blending theory. The blending theory postulated that offspring express blended characteristics from both parents. For example, two animals that breed, one with white fur and the other with black fur, would be expected to produce offspring with gray fur. However, this theory failed to explain why some generations of offspring do not show characteristics of parents, nor does it explain variation in organisms. The blending theory, also known as the anti-Mendelian theory, was replaced by Mendel's particulate theory.
Gregor Mendel, the father of genetics, first postulated the particulate theory of genetic inheritance with his experiments done in the mid-nineteenth century. Mendel suggested that inheritance occurs by individual units that control traits. Based on his research experimenting with garden peas, Mendel proposed two theories to explain the transmission of genetic traits that later became known as Mendel's laws. The first principle, the law of segregation, states that genes usually develop in pairs in somatic cells, but segregate in the development of gametes. The second principle, the law of independent assortment, asserts that the expression of a gene does not affect the expression of another gene for any individual trait. Through time and advancements in technology, Mendel's research remains the basis for geneticists today.
The basis for genetic inheritance includes several different types of inheritance: autosomal dominant, autosomal recessive, X-linked dominant, X-linked recessive, and Y-linked inheritance. With autosomal dominant inheritance, only one copy of the allele (A) is necessary because the allele is expressed phenotypically whenever it is present. All individuals who have a dominant allele have at least one affected parent. Because individuals carry two copies of a gene, A/a or A/A represents the genotype of an individual who has an autosomal dominant gene. On the other hand, autosomal recessive traits must have two copies of the same allele (a/a) in order to be expressed. Each parent of an affected individual must have at least one copy of the recessive allele (a).
Genes that are located on either the X or Y chromosome are said to be sex-linked. With X-linked dominant inheritance, genes are located on the X chromosome and only one copy of the allele (A) is necessary in order to be expressed. Because males possess an X and Y chromosome for their genotype, daughters of affected males always inherit the trait while sons never inherit the trait. Mothers who posses an X-linked dominant gene have a 50% chance of passing on the affected gene to their offspring because females have two X chromosomes. X-linked recessive genes are also located on the X chromosome. In order for females to express the trait of an X-linked recessive gene, they must have two copies of the allele (a/a). Males, on the other hand, only need one copy of the X-linked recessive allele (a/-) to express the trait because they only have one copy of the X chromosome. Y-linked inheritance, also known as holandric inheritance, only affects males because females do not have a Y chromosome. Males who have a Y-linked gene usually express the gene and always pass it on to their sons.
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