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Genetics and Inheritance
dominant gene overrides the recessive.
When two people create a child, they each supply their own set of genes. In simplistic cases, such
as the red/brown hair, each parent supplies one "code", contributing to the child's hair color. For
example, if dad has brown/red he has a 50% chance of passing brown hair to his child and a 50% of
passing red hair. When combined with a mom who has brown/brown (who would supply 100%
brown), the child has a 75% chance of having brown hair and a 25% chance of having red hair. Similar
rules apply to different traits and characteristics, though they are usually far more complex.
Multifactorial inheritance
Some traits are found to be determined by genes and environmental effects. Height for example
seems to be controlled by multiple genes, some are "tall" genes and some are "short" genes. A child
may inherit all the "tall" genes from both parents and will end up taller than both parents. Or the child
my inherit all the "short" genes and be the shortest in the family. More often than not the child inherits
both "tall" and "short" genes and ends up about the same height as the rest of the family. Good diet and
exercise can help a person with "short" genes end up attaining an average height. Babies born with drug
addiction or alcohol addiction are a sad example of environmental inheritance. When mom is doing
drugs or drinking, everything that she takes the baby takes. These babies often have developmental
problems and learning disabilities. A baby born with Fetal alcohol syndrome is usually abnormally
short, has small eyes and a small jaw, may have heart defects, a cleft lip and palate, may suck poorly,
sleep poorly, and be irritable. About one fifth of the babies born with fetal alcohol syndrome die within
the first weeks of life, those that live are often mentally and physically handicapped.
Sex-linked Inheritance
Sex-linked inheritance is quite obvious, it determines your gender. Male gender is caused by the Y
chromosome which is only found in males and is inherited from their fathers. The genes on the Y
chromosomes direct the development of the male sex organs. The x chromosome is not as closely
related to the female sex because it is contained in both males and females. Males have a single X and
females have double XX. The X chromosome is to regulate regular development and it seems that the
Y is added just for the male genitalia. When there is a default with the X chromosomes in males it is
almost always persistent because there is not the extra X chromosome that females have to counteract
the problem. Certain traits like colorblindness and hemophilia are on alleles carried on the X
chromosome. For example if a woman is colorblind all of her sons will be colorblind. Whereas all of
her daughters will be carriers for colorblindness.
Exceptions to simple inheritance
Our knowledge of the mechanisms of genetic inheritance has grown a lot since Mendel's time. It is
now understood, that if you inherit one allele, it can sometimes increase the chance of inheriting
another and can affect when or how a trait is expressed in an individuals phenotype. There are levels of
dominance and recessiveness with some traits. Mendel's simple rules of inheritance does not always
apply in these exceptions.
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