At most gene locuses you have a version from your mom and a version from your dad. Usually both versions are not expressed and only one of the genes affects the phenotype (Observable characteristic). The gene that is expressed over another allele of the same gene is called Dominant. Individuals appear to have the phenotype of the dominant gene whether they are homozygous (have 2 copies of the dominant allele) or heterozygous (1 copy of the dominant allele). The allele that does not affect the phenotype when a dominant allele is present is called Recessive. Recessive alleles only change the phenotype when there is no dominant allele present. Heterozygous individuals do not show the phenotype of the recessive allele, but can pass this allele on to their offspring. These heterozygous individuals are called carriers. An exception to this system is blood type where codominace is present and 2 genes (blood type A & B) can simultaneously be dominant.

Dominant vs. recessive is usually represented in a Punnet square. The dominant allele is often given the capital letter while the recessive allele is given the lower case. Therefore, a heterozygous individual who is a carrier for the recessive gene would be represented as Aa. Consider Brown eye (dominant) and blue eyes (recessive). If a heterozygous brown eyed person has a child with homozygous blue eyed person there is a 50% chance the child will have blue eyes and a 50% chance the child will have brown eyes. If two heterozygous brown eyed individuals have a child there is a 75% chance the child will have brown eyes. If a homozygous brown eyed individual has has a child with a homozygous blue eyed individual 100% of there children will have brown eyes.

Autosomal Dominant Inheritance is when one allele, on any chromosome other than X or Y, is expressed over another allele of the same gene. This allele determines the phenotype (observable characteristics) and is referred to as dominant. The allele that is does not affect the phenotype is referred to as recessive. The dominant allele is often given the capital letter for a character while the recessive allele is given the lower case. Therefore, a heterozygous individual who is a carrier for the recessive gene would be represented as Aa. Usually on a pedigree nearly every generation has an affected individual. Here are a few examples:

• Huntington’s
• Familial Hypercholesterolemia
• Marfans Syndrome
• Heriditary Spherocytosis
• Polycystic Kidney Disease

Autosomal Recessive Inheritance is basically the opposite of autosomal dominant.Recessive alleles only change the phenotype when there is no dominant allele present. Heterozygous individuals do not show the phenotype of the recessive allele, but can pass this allele on to their offspring. These heterozygous individuals are called carriers. Usually on a pedigree few individuals are affected. Here are a few examples of diseases with Autosomal Recessive inheritance:

• Cystic Fibrosis
• Most lysosomal storage diseases
• Phenylketonuria
• Thalasemia
• Most enzyme deficiencies

X Linked Recessive Inheritance is a type of recessive inheritance for genes on the X chromosome. Males express the phenotype when they inherit 1 effected allele, while females need to inherit 2 effected alleles. This is because the gene lies on the X chromosome, and males only receive a single X while females receive 2. Males cannot pass the effected X allele onto sons, because a son must receive a Y from the father to be male. Males are affected far more often than females. Women are very rarely affected by these disorders, and are primarily heterozygous carriers when they have the gene. Here are a few examples of diseases with X-Linked Recessive Inheritance:

• Muscular Dystrophy
• Hemophilia
• G6PD Deficiency
• Bruton’s Agammaglobinemia

Mitochondria have DNA (mtDNA) that is circular and separate from the chromosomes in the nucleus. Mitochondrial Inheritance is only through the mothers and the fathers mitochondrial DNA is not passed onto children. Heterosplasmy is when a single individual has more than 1 type of mitochondrial DNA in their body due to mutations. The most common disease with this type of inheritance is Mitochondrial Myopathy which presents with “Ragged Red” muscle fibers on biopsy.

Polygenic or Multifactorial Inheritance is when the phenotype is not dictated by a single gene locus. These types of diseases are determined by an interaction between many contributing genetic and environmental factors. Here are a few examples of diseases with Polygenic Inheritance:

• Cleft lip and palate
• Schizophrenia
• Epilepsy
• Baldness
• Diabetes
• Hypertension

Other terms related to Inheritance:

• Variable Expressivity = same genetic defect presents differently in different patients. Neurofibromatosis is an example
• Mosaicism = when populations of cells within a single individual have different genotypes due to post-fertilization changes. Often in reference to chromosomal abnormalities caused by improper mitosis. Germline Mosacism is when only gametes (sperm and eggs) are affected by the genetic defect. Therefore, the individual would not show signs of the disease, but they could pass it on to their offspring
• Pleiotrophy = a single genetic defect has multiple effects (same gene is expressed in many different tissues)
• Incomplete Penetrance = not everyone with genetic defect gets the disease. Low penetrance means many people with the genotype do not show the phenotype

Now that you have finished this video you should check out the next video in the Genetics section which covers Pedigrees. 

Pictures Used (In order of appearance)

• “Autosomal Dominant” by Domaina available at ttp://en.wikipedia.org/wiki/File:Autosomal_dominant_-_en.svg via Creative Commons 3.0 Attribution Share Alike
• “XlinkedRecessive” by US National Library of Medicine available at http://en.wikipedia.org/wiki/File:XlinkRecessive.jpg via Public Domain
• “Mitochondrial” By US National Library of Medicine available at http://en.wikipedia.org/wiki/File:Mitochondrial.jpg by Public Domain