Simple Inheritance-
Polygenic traits: Polygenic traits are the inheritance of “appearance” traits influenced by multiple genes. Examples are, natural height, body shape, hair and skin color.The environment can also affect some of these traits. A Polygenic trait inherited from family is height. People are a variety of heights along a spectrum. Height can also influenced by environment. A twin born with the same tall genes as his sibling could grow to a shorter height based on malnutrition or illness. Skin color is another trait which could be influenced by environment, depending on where two twins live on the planet. Even if the twins that marry other twins and coincidentally have identical twins, then it is still possible for those children to have differences.
Intermediate Expression: Intermediate Expression is when a heterozygous genotype results in a phenotype, or detectable characteristics, that is intermediate or half-way between those resulting from homozygous characteristics. A genotype is an individual’s genetic make-up. When it is heterozygous, it consists of two different alleles of a gene for a particular trait. Alleles are varieties of a gene. When it is homozygous it means the alleles are identical. An example of this is the pitch of a male voice. Lowest and highest pitches are found in Homozygous traits, but mid range pitch would show intermediate expression.
Codominance: For some traits, two alleles can be codominant. That is to say, both are expressed in heterozygous individuals. An example of this is people who have an AB blood type for the ABO blood system. When they are tested, these individuals actually have the characteristics of both type A and type B blood. Their phenotype is not intermediate between the two, both appear as a co dominant characteristic. In the case of twins or children of twins, mutations could occur in alleles even when their genetic makeup is the same so physical characteristics may vary even if they are co dominant.
Modifying Genes: These genes can alter how another gene is expressed or developed in the phenotype of an individual. An example is a dominant cataract gene which will produce vision impairment depending on the presence of a specific allele for a companion modifying gene. However, cataracts also can be developed by environmental factors such as excessive ultraviolet radiation, and alcoholism. Therefore, a twin could develop cataracts while his sibling does not.
Regulator Genes: These genes can block the functions of other genes. Regulator genes control the timing of production of a variety of chemicals in humans bodies. They control the development of body parts after conception and make changes in our bodies as we age. These genes are also called homeotic genes. Twins may have the same regulator gene, but may age and die in different manners depending upon environment and how they care for their bodies as they age.
Regulator Genes: These genes can block the functions of other genes. Regulator genes control the timing of production of a variety of chemicals in humans bodies. They control the development of body parts after conception and make changes in our bodies as we age. These genes are also called homeotic genes. Twins may have the same regulator gene, but may age and die in different manners depending upon environment and how they care for their bodies as they age.
Incomplete Penetrance: Some genes are incompletely penetrant, which means that an allele is expressed only if certain factors are present in the environment. The triggering of genetically inherited diabetes by obesity or severe emotional stress is an example. Twins could carry the same genes for diabetes and depending on their lifestyles, one may develop it when one may not.
Sex Related Genetic Effects:
Some genes may have different effects depending on an individual's gender. These are referred to as sex-limited genes, sex-controlled genes and genome imprinting. Sex-limited genes are inherited by both genders but is usually expressed in the phenotype of one of them. The heavy male beard is an example, while women have facial hair it is most often very fine and comparatively sparse. Sex-controlled genes are expressed in both sexes but differently. An example is gout, a disease that causes painful joints, which is more likely to be more severe for men. Genome imprinting is when a gene has a different effect depending on the gender of the parent from whom it was inherited.
Some genes may have different effects depending on an individual's gender. These are referred to as sex-limited genes, sex-controlled genes and genome imprinting. Sex-limited genes are inherited by both genders but is usually expressed in the phenotype of one of them. The heavy male beard is an example, while women have facial hair it is most often very fine and comparatively sparse. Sex-controlled genes are expressed in both sexes but differently. An example is gout, a disease that causes painful joints, which is more likely to be more severe for men. Genome imprinting is when a gene has a different effect depending on the gender of the parent from whom it was inherited.
Pleiotropy: Pleiotropy is when a single gene is responsible for a variety of traits. A pleiotropic trait is albinism. People with albinism have little or no pigment in their eyes, skin, or hair. They have inherited altered genes that do not make the usual amounts of a pigment called melanin. Identical twins would either both be albino or not.
Stuttering Alleles: Stuttering or unstable alleles are defective alleles that have segments which are doubled in their transmission from generation to generation. Each generation has increasingly severe symptoms. Muscular dystrophy is a condition which weakens a person's muscles and can be worse each generation. This is an example of Stuttering Alleles.