In chickens creeper X creeper 2/3
deformed chicks
dominance" as far as the lethal aspect of the C' allele is concerned.
lethal alleles behave just as Mendel's rules predicted.
cattle and even humans. Many"little people" are heterozygous for achondroplasia.
by all indications, homoygosity for the defect is lethal.
are rather common; each of us is estimated to carry recessive alleles
for about 5 genes that if homozygous would be lethal, but the recessive lethals
generallygo unnoticed in the heterozygous condition.
20 white's is heterozygous for the "cftr"gene. Homozygous individuals have salty
sweat. They also have severe problems with digestion and congestion of bronchi and
lungs leads to many secondary infections. Although the life expectancy has grown
from 2 year to over 30 since 1940, both males and females are sterile, so the
condition is still a recessive lethal. There are some indications that heterozygotes
may have had an advantage in surviving diseases such as typhoid fever and cholera
to account for the high frequency of the disease. Extra information can be found
online at
detail later in the course.
Another gene that can be lethal in the heterozygous condition
will help us describe another problem that can make genetic analysis difficult.
control, with death occurring in many cases 10-15 years after the initial diagnosis.
Although anyone who inherits a defective Huntington allele can expect to eventually
be affected, the age of onset varies a great deal. Less than 2% of Hhindividuals show
symptoms by age 12, the average age of onset is 38-40, but some "carriers" may not
show symptoms before age 60 so may easily die from other causes. Most affected
individuals survive long enough to reproduce, so in family pedigrees where the
condition is present, half the children where one parent is Hhcan expect to develop
the disease.
were at risk, would you want to know? Would your insurance company want to
know?
Just as the timing of symptoms can vary, so can the severity.
In some cases, the phenotypic effects can be so great that it is difficult to realize the
same gene is defective. (In fact, in many of these cases, different changes in the
same gene, that is different alleles, can account for some differences in the final
phenotype).
individuals the only symptom may be blue sclera, while in others, the bones are so
brittle that even walking is too much impact and leads to broken bones.
: Based on the previous examples, it is not surprising that in some
cases, individuals known to have a particular gene from pedigree or molecular
analysis do not show the expected phenotype. One common example is a dominant
form of polydactyly, (extra fingers and/or toes). In addition to differences from one
limb to another, some persons who pass the trait on to their offspring do not have
extra fingers. In some cases, x-rays may show the extra bones were formed, but no
extra digits are present.
Pleiotrophy:Many genetic defects cause multiple phenotypic changes. Examples
include syndromes such as that caused by a simple recessive disease called
galactosemia. Individuals with galactosemia cannot digest the sugar galactose,
which if untreated leads to cirrhosis and enlargement of the liver, cataracts and
mental retardation.
Environmentally induced phenotypes can mimic genetic defects. For
example, in low temperatures, flies with a gene for curly wing will have normal
straight wings, and plants treated with the herbicide fluridone mimic a recessive
gene that causes albinism after exposure to bright sunlight.
is a teratogen.
These defects are not the result of mutations, so will not be passed to the
next generation.
babies born with missing limbs.
medicine.
Genetic Heterogeneity : Different genes may cause the same phenotype. For
example, several dominant and recessive forms of polydactyly have been described,
as have many genes that cause cataracts in animals or albinism in plants.
Epistasis:When just 2 genes affect the same trait we often see examples of epistasis,
which is defined as one gene masking the expression of a different gene.
d1 d1 Deaf (lacks anvils)
D2_ normal
d2 d2deaf (lacks stirrups)
(deaf male)
all the F1 progeny are D1 d1, D2 d2 and can hear.
males and females should produce the classical 9:3:3:1 ratio:
d1 d1, D2 _
9:3:3:1 phenotypic ratio. Here d1d1 masks the expression of D2, and vice
versa, so we see a 9 hearing to 7 deaf ratio.
Modified 9:3:3:1 ratios are a great clue that epistasis
is involved!If 2 genes are involved the F2 dihybrid
ratios will add up to 16; for example 15:1, 9:3:4, 13:3
etc. Coat colors in animals and flower colors often
involve epistasis.
on the old exams.
:In rabbits:
_
_
and paws, white body)
a heterozygote (sometimes called a compound) will have the
phenotype of the highest "ranking" allele.
classes I the progeny, but still give 1-gene ratios.
For examplecchc, X chc will produce 2 chinchilla: 1
himalayan : 1 albino
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