Page 127 - Genetics_From_Genes_to_Genomes_6th_FULL_Part1

4.7 Sex-Linked and Sexually Dimorphic Traits in Humans 119

Figure 4.22 Red-green color blindness is an X-linked recessive trait in humans. How the world looks to a person with either
normal color vision (a) or a kind of red-green color blindness known as deuteranopia (b).
(both): Color deficit simulation courtesy of Vischeck (www.vischeck.com). Source image courtesy of NASA

(a) (b)

Figure 4.23 X-linked traits may be recessive or dominant. TABLE 4.5 Pedigree Patterns Suggesting
(a) Pedigree showing inheritance of the recessive X-linked trait Sex-Linked Inheritance
hemophilia in Queen Victoria’s family. (b) Pedigree showing the
inheritance of the dominant X-linked trait hypophosphatemia, X-Linked Recessive Trait
commonly referred to as vitamin D–resistant rickets.
(a) X-linked recessive: Hemophilia 1. The trait appears in more males than females because a female
must receive two copies of the rare defective allele to display
l the phenotype, whereas a hemizygous male with only one
Queen Victoria Prince Albert Carrier
Hemophiliac copy will show it.
2. The mutation will never pass from father to son because sons
lI
Victoria Edward Alfred Louise Arthur Beatrice Helena receive only a Y chromosome from their father.
VII
3. An affected male passes the X-linked mutation to all his daughters,
Alice Louis IV Leopold Helene who are thus carriers. Each son of these carrier females has a one-
half chance to inherit the defective allele and thus the trait.
lII
4. The trait often skips a generation as the mutation passes from
Alix Nicholas ll grandfather through a carrier daughter to grandson.
5. The trait can appear in successive generations when a sister
IV
Alexis Rupert of an affected male is a carrier. If she is, each of her sons has a
one-half chance of being affected.
(b) X-linked dominant: Hypophosphatemia 6. With the rare affected (homozygous) female, all her sons will
be affected and all her daughters will be carriers.
l
X-Linked Dominant Trait
lI
1. More females than males show the aberrant trait.
2. The trait is seen in every generation as long as affected males
lII have female children.
3. All the daughters but none of the sons of an affected male will
be affected. This criterion is the most useful for distinguishing
an X-linked dominant trait from an autosomal dominant trait.
Vitamin D–resistant rickets, or hypophosphatemia, is an ex- 4. The sons and daughters of an affected female each have a
ample of an X-linked dominant trait. Figure 4.23b presents one-half chance of being affected.
the pedigree of a family affected by this disease. 5. For incompletely dominant X-linked traits, carrier females may show
the trait in less extreme form than males with the defective allele.
In XX Human Females, One X Y-Linked Trait
Chromosome Is Inactivated 1. The trait is seen only in males.
2. All male descendants of an affected man will exhibit the trait.
The XX and XY system of sex determination presents
human cells with a curious problem that requires a solution 3. Not only do females not exhibit the trait, they also cannot transmit it.