Page 142 - 86 human physiology part-2
P. 142

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.







                                                                                                Wikibooks | 357
   137   138   139   140   141   142   143   144   145   146   147