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Heredity
Heredity refers to the transfer via genes of certain characteristics from
parent to offspring. Farmers have used understandings of heredity for
thousands of years, through a process known as selective breeding to farm
larger animals and hardier crops, giving bigger yields. Gregor Mendel was
the first to document heredity of characteristics through his studies of pea
plants.
Before Mendel’s research, a primitive theory of heredity that
proposed offspring would exhibit a mixture of characteristics from their
parents. For example, that a tall and a short variety of pea would produce a
pea specimen of medium height. Mendel disproved this theory and developed
the concept of dominant and recessive genes.
Charles Darwin’s theory of evolution, proposed in 1859, espoused ‘blending
inheritance’. Darwin believed this would lead to uniformity across entire
populations in only a few generations, removing the variations on which the
process of natural selection relies. Darwin’s model has since been reworked
to correspond with the concepts of Mendelian genetics (heredity of genes).
Mendel’s concepts of dominant and recessive genes mean that parents who both
have black hair are most likely to produce a child who also has black hair.
Once in a while, however, a child could be born with blonde hair, to two
black haired parents. This is an example of the principle of segregation.
Specific genes determine inheritable traits; all individuals carry two genes
for each trait, one from their father and one from their mother. When an
individual reproduces, these two genes separate and end up in different
gametes. Mendel observed that when he crossed purebred tall plants with
purebred short plants, that the first generation of plants were all tall,
however when these tall offspring were crossed the result was a ratio of one
short plant to three tall plants.
The study of heredity can be used to predict the chances of children
inheriting genetic disorders that are carried by one of their parents. There
are thousands of genetic disorders which can be categorised into four main
groups: autosomal dominant disorders, that are transmitted by genes passed
on from just one parent; autosomal recessive disorders which are inherited
from genes from both parents; sex linked disorders, which are only carried
by either the female X chromosome or the male Y chromosome; and genetic
disorders which can be attributed to multiple factors.
If one parent has an autosomal dominant disorder, such as Huntington's
disease, their children will have a fifty percent chance of inheriting the
disease themselves, irrespective of their sex. If both parents carry a
recessive gene for a particular disorder, their children have a 25% chance
of contracting that disorder from their parents (like Mendel’s second
offspring group).
Some examples of sex-linked genetic disorders include
colour blindness and haemophilia; both are which are passed from mothers to
sons (they are carried on one of the mother’s X chromosomes).
A person’s
likelihood to develop a multifactorial genetic disorder involves a
combination of their environment and their genetic makeup. Diseases believed
to be associated with multifactorial causes include obesity, diabetes,
alcoholism, schizophrenia, and various forms of cancer. |
