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Mode of inheritance6/30/2023 ![]() ![]() Over 100 years of theoretical and experimental genetics studies, and the more recent sequencing and annotation of the human genome, have helped scientists to develop a better understanding of how an individual’s genotype is expressed as their phenotype. For example, there are three different alleles that encode ABO blood type these are designated I A, I B, and i. ![]() This phenomenon is called multiple alleles. Moreover, although any one person can only have two alleles corresponding to a given gene, more than two alleles commonly exist in a population. For instance, eight or more genes-each with their own alleles-determine eye color in humans. ![]() However, it is quite common for multiple genes to interact to confer a feature. In the simplest scenario, a single pair of genes will determine a single heritable characteristic. In some cases, both alleles are expressed at the same time in a form of expression known as codominance. Sometimes dominance is complete at other times, it is incomplete. The expression of an allele can be dominant, for which the activity of this gene will mask the expression of a nondominant, or recessive, allele. The two alleles can interact in several different ways. (credit: National Human Genome Research Institute)Īlthough a person can have two identical alleles for a single gene (a homozygous state), it is also possible for a person to have two different alleles (a heterozygous state). As is visible in this karyotype, the only exception to this is the XY sex chromosome pair in males. The banding patterns are nearly identical for the two chromosomes within each pair, indicating the same organization of genes. Each pair of chromosomes contains hundreds to thousands of genes. A child may inherit the allele encoding for dimples on the chromosome from the father and the allele that encodes for smooth skin (no dimples) on the chromosome from the mother. Take for example an allele that encodes for dimples. Because one copy of a gene, an allele, is inherited from each parent, the alleles in these complementary pairs may vary. Homologous chromosomes-those that make up a complementary pair-have genes for the same characteristics in the same location on the chromosome. This occurs when the sperm and oocyte combine at the moment of your conception. You inherit one chromosome in each pair-a full complement of 23-from each parent. The characteristics that the genes express, whether they are physical, behavioral, or biochemical, are a person’s phenotype. An individual’s complete genetic makeup is referred to as his or her genotype. Each of these chromosomes carries hundreds or even thousands of genes, each of which codes for the assembly of a particular protein-that is, genes are “expressed” as proteins. The remaining 22 chromosome pairs are called autosomal chromosomes. Among these is one pair of chromosomes, called the sex chromosomes, that determines the sex of the individual (XX in females, XY in males). The image below shows the pairs in a systematic arrangement called a karyotype. \)Įach human body cell has a full complement of DNA stored in 23 pairs of chromosomes. ![]()
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