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 (C r C r ) or white (C w C w ). The hybrid individual (C r C w ) 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 X- chromosome. 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. 3’5’6’7’ Human Height

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