Polygenic (multiple-gene) inheritance (fig. 4.18) is a phenomenon in which genes at two or more loci, or even on different chromosomes, contribute to a single phenotypic trait.

Human eye, hair, and skin colors are normal polygenic traits, for example. They each result from the combined expression of more than 100 genes, making it impossible to reliably predict traits of the offspring from those of their parents

Several diseases are also thought to stem from polygenic inheritance, including some forms of alcoholism,

mental illness, cancer, and heart disease.

Pleiotropy is a phenomenon in which one gene produces multiple phenotypic effects.

For example, about 1 in 200,000 people has a genetic disorder called alkaptonuria, caused by a mutation on chromosome 3.

The mutation blocks the normal breakdown of the amino acid tyrosine, resulting in the accumulation of an intermediate breakdown product, homogentisic acid, in the body fluids and connective tissues.

When homogentisic acid oxidizes, it binds to collagen and turns the tissues gray to bluish black, and for unknown reasons, it causes degeneration of cartilages and other connective tissues.

Among the multiple phenotypic effects of this disorder (fig. 4.19) are darkening of the skin;

darkening and degeneration of the cartilages in places such as the ears, knees, and intervertebral discs; darkening of the urine

when it stands long enough for the homogentisic acid to oxidize; discoloration of the teeth and the whites of the eyes; arthritis of the shoulders, hips, and knees;

and damage to the heart valves, prostate gland, and other internal organs-numerous effects from just one gene!

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Question	Answer
Polygenic (multiple-gene) inheritance (fig. 4.18) is a phenomenon in which genes at two or more loci, or even on different chromosomes, contribute to a single phenotypic trait.	Human eye, hair, and skin colors are normal polygenic traits, for example. They each result from the combined expression of more than 100 genes, making it impossible to reliably predict traits of the offspring from those of their parents
Several diseases are also thought to stem from polygenic inheritance, including some forms of alcoholism,	mental illness, cancer, and heart disease.
Pleiotropy is a phenomenon in which one gene produces multiple phenotypic effects.	For example, about 1 in 200,000 people has a genetic disorder called alkaptonuria, caused by a mutation on chromosome 3.
The mutation blocks the normal breakdown of the amino acid tyrosine, resulting in the accumulation of an intermediate breakdown product, homogentisic acid, in the body fluids and connective tissues.	When homogentisic acid oxidizes, it binds to collagen and turns the tissues gray to bluish black, and for unknown reasons, it causes degeneration of cartilages and other connective tissues.
Among the multiple phenotypic effects of this disorder (fig. 4.19) are darkening of the skin;	darkening and degeneration of the cartilages in places such as the ears, knees, and intervertebral discs; darkening of the urine
when it stands long enough for the homogentisic acid to oxidize; discoloration of the teeth and the whites of the eyes; arthritis of the shoulders, hips, and knees;	and damage to the heart valves, prostate gland, and other internal organs-numerous effects from just one gene!

Created by: Russells3709

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