Page 141 - 86 human physiology part-2

Chapter 18

with each pregnancy. Some X-linked dominant conditions,
such as Aicardi Syndrome, are fatal to boys, therefore only
girls have them (and boys with Klinefelter Syndrome).
X-linked recessive disorders are also caused by mutations
in genes on the X chromosome. Males are more frequently
affected than females, and the chance of passing on the
disorder differs between men and women. The sons of a Hemophilia A, Duchenne
X-linked man with an X-linked recessive disorder will not be muscular dystrophy,
recessive affected, and his daughters will carry one copy of the Color blindness, Turner
mutated gene. With each pregnancy, a woman who carries Syndrome
an X-linked recessive disorder has a 50% chance of having
sons who are affected and a 50% chance of having
daughters who carry one copy of the mutated gene.
Y-linked disorders are caused by mutations on the Y
chromosome. Only males can get them, and all of the sons
of an affected father are affected. Since the Y chromosome
Y-linked Male Infertility
is very small, Y-linked disorders only cause infertility, and
may be circumvented with the help of some fertility
treatments.
This type of inheritance, also known as maternal
inheritance, applies to genes in mitochondrial DNA. Leber's Hereditary Optic
Mitochondrial Because only egg cells contribute mitochondria to the Neuropathy (LHON)
developing embryo, only females can pass on
mitochondrial conditions to their children.

Mechanisms of inheritance

A person's cells hold the exact genes that originated from the sperm and egg of his parents at the
time of conception. The genes of a cell are formed into long strands of DNA. Most of the genes that
control characteristic are in pairs, one gene from mom and one gene from dad. Everybody has 22 pairs
of chromosomes (autosomes) and two more genes called sex-linked chromosomes. Females have two
X (XX) chromosomes and males have an X and a Y (XY) chromosome. Inherited traits and disorders
can be divided into three categories: unifactorial inheritance, sex-linked inheritance, and multifactor
inheritance.

Unifactorial Inheritance

Traits such as blood type, eye color, hair color, and taste are each thought to be controlled by a
single pair of genes. The Austrian monk Gregor Mendel was the first to discover this phenomenon, and
it is now referred to as the laws of Mendelian inheritance. The genes deciding a single trait may have
several forms (alleles). For example, the gene responsible for hair color has two main alleles: red and
brown. The four possibilities are thus

Brown/red, which would result in brown hair, Red/red, resulting in red hair, Brown/brown,
resulting in brown hair, or Red/brown, resulting in red hair.

The genetic codes for red and brown can be either dominant or recessive. In any case, the

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