Download Recessive and dominant heredity in humans

Recessive and dominant heredity in humans
Most genetic disorders are caused by recessive alleles
Why? Since the disease is recessive both parents must carry the allele in
order to actually produce a child with the disorder. But individuals that get
both recessive alleles die before passing on the allele, reducing the number of
that allele that exists in the gene pool. The result is the majority of the
general population is homozygous dominant for the normal condition and
therefore only passes on a normal allele.
D
d
D
d
DD
Dd
normal
normal
Dd
dd
normal
disorder
If a deadly mutation occurred that was dominant, people with either Dd or two DD would die out. The only
surviving people would be recessive dd and would only have other dd people to have children with. The
population would be disease free as the D allele would quickly go extinct.
Many of these alleles (and the disorders) are rare, but can be more common in ethnic groups where the
custom is to marry someone from that specific ethnic or religious group instead of from the general
population. (It is more likely for two carriers to marry and have children.)
Simple recessive
heredity
Disorder/condition
Cystic Fibrosis
Tay-Sachs disease
Phenylketonuria
What Happens
Thick mucus in lungs and
digestive tract. Fatal by
age 20-40.
Dysfunction of central
nervous system (CNS).
Fatal by age 5.
Phenylalanine builds up,
damages CNS. Fatal when
untreated.
Why it Happens
Who Gets It Most
defective protein in
the cell membrane
1/2000 white
Americans
missing a normal
enzyme
Amish, E. European
Jews
missing a normal
enzyme
Originally Swedish &
Norwegian people
Simple dominant
heredity
Unattached earlobes
Earlobes not attached to
head
no skin grew
anyone
polydactyly
six fingers on a hand
extra bone grows
from other fingers
mostly Amish people
Huntington disease
No symptoms prior to age
30. Brain breaks down
after age 30-50, fatal soon
after.
Single defective
gene; thought to alter
normal cholesterol
distribution in brain,
disrupting networking
certain families; and a
village in Venezuela