Transcript The Alternate Modes of Heredity

Alternate Modes of Heredity
Just When You Think You Had It All
Figured Out…Now We’re Gonna
Change It!!!
Not So Simple Mendel!
• The first mode of heredity (we have already
addressed it) is simple Mendelian inheritance.
• Characteristics of this mode of heredity include:
– Only two forms for each trait/characteristic.
– Genetically, the previous statement means there are
only two alleles for each gene.
– There is a clear dominant and recessive form of the
trait. The dominant form that is seen in a heterozygous
individual is the same as that of a homozygous
dominant one. (There is no blending – it’s either
dominant or recessive – a “this or that” situation.)
Incomplete Dominance
• Incomplete dominance occurs when the two
homozygous forms of the trait give different
physical traits – BUT – the heterozygous
individual, or hybrid, is a blend of the two forms of
the trait.
• An example would be snapdragon flowers where
the genotype “RR” is red and “rr” is white.
• These two parents when crossed (RR x rr) would
yield all “Rr” individuals which are pink!
Incomplete Dominance
Codominance
• A trait that is governed by codominance
will have separate phenotypes for the
homozygous forms of the trait while the
heterozygous individual will show both
forms of the trait.
• Horses can be red (CrCr) or white (CwCw).
The hybrid individual (CrCw) that would
result from these two parents would be a
“roan” horse. A roan horse has hairs that
are fully white and hairs that are fully
red.
• Please note: The trait does not blend –
both forms are there sharing the trait.
Codominance
Multiple Alleles
• Some traits have more than two forms.
(Some genes can have more than two
alleles.)
• When this is the case, we say the trait is
governed by multiple alleles.
• Human blood-types are governed by
multiple alleles. There are three forms of
the gene (rather than just two).
• The forms of the gene for blood-type are
A, B & O.
• A and B are dominant over O.
• If A & B are together – the trait exhibits
codominance and the blood type is AB.
Multiple Alleles
Sex–Linked Traits
• Sex-linked traits are found on the Xchromosome.
• The male sex chromosome – the Y
chromosome – is genetically inactive – it
plays no role in the expression of your
traits.
• This means that recessive sex-linked
traits (regardless of how detrimental
they may be) are more easily passed on
to males than females because the
males only need one copy of the
recessive gene to express the trait while
females need two.
Sex-Linked Traits
Multifactorial Traits
• Some traits are under the control of several
pairs of genes acting together. These traits
that are influenced by more than one pair of
genes are said to be multifactorial traits.
• A clear indicator of a multifactorial trait is that
you see a continuous distribution expressed
within the trait.
• A continuous distribution means that there
many forms of the trait that range from one
extreme to the other with most of the forms
being around the average. You will see a bell
curve when you graph the trait in a population.
• Height (in humans) is a good example of a
multifactorial trait. This is why you can have
two parents of average height having a child
that is extremely tall or short.
• To illustrate this point…let’s play with money!
Human Height Bell Curve
% of
Pop.
3’
5’
6’
Human Height
7’
That’s All I Got…