Chapter 1, Problem 25CQ

Summary Introduction

To determine:

The difference in the phenotype of albinism among Hopi Native Americans and other types of albinism and the mutated gene resulting in those phenotypes.

Introduction:

Albinism is a congenital disorder, which results because of the defect in enzymes required for melanin production. Melanin is the pigment that provides dark color to hair, eyes, and skin. Melanin is either not present or produced in little amounts in albino people. It protects skin from skin cancer and dark color of skin prevents DNA (deoxyribonucleic acid) from getting damage because of ultraviolet radiation.

Explanation of Solution

In Hopis, the type of albinism is oculocutaneous albinism which affects skin and eyes only. This type of albinism is caused because of defect in $\text{OCA2}$(oculocutaneous albinism) gene present on chromosome $15$. Affected infant seems to be suffering from OCA type I albinism, however, with time they develop some amount of pigment.

OCA type II has differential phenotypes. With age, the hair of affected person might turn darker and pigmented freckles or nevi can be observed. The color of irides may be hazel or blue-gray in color in African American and African population. Brown OCA is one of the phenotypic variant. In them, the irides and hair turn darker with age and skin might develop tanning. In all the types of albinism, ocular features are common.

The tyrosinase-positive albinism is characterized by pigment nevi. In African population, the hair color is yellow and there are several pigmented spots in the skin. This kind of albinism develops because of blockage in formation of eumelanin. There are two different phenotypes in tyrosinase-positive albinism, namely- with pigmented patches and without pigmented patches. The individuals having $TYR$ gene mutation are classified under OCA1B

The individual suffering from ocular albinism may show photophobia, congenital nystagmus, translucent irides, impaired vision, strabismus, and hyperplasia of fovea.

In X-linked albinism, the heterozygous daughter did not have mosaic pattern in ocular fundus. Brown oculocutaneous albinism is linked to P locus. At this locus, OCA2 mutations are located.

Oculocutaneous albinism IA is characterized by absent or decreased melanin pigmentation in eyes, hair, and skin. The optic changes include nystagmus, misignaling of optic nerve, and decreased visual acuity. This is caused by mutation in Tyrosinase gene and is also known as tyrosinase negative OCA.

OCAIII is also known as rufous oculocutaneous albinism. In African population, it is characterized by dilution of color of iris, bright copper coloration of hair and skin. It is caused by mutation in $TYRP1$ gene.

In OCAIV, the hair color varies between yellow to brown, patients might have nystagmus and color of iris is from blue to brown. This is caused by mutation in $MATP$ gene. OCAV is characterized by photophobia, nystagmus, white skin, golden hairs. This is caused by OCA5.

OCAVI is caused by mutation in $SLC24A5$. The patients have white skin and irides, hair at the time of birth might be blond but they change color with age. They have photophobia and nystagmus.

OCAVII is caused by mutation in $C10ORF11$. Most of the individuals have light complexion, predominant eye symptoms, iris transillumination, and nystagmus.

Conclusion

There are various types of albinism, such as type IA, IB, II, III, IV, V, VI, BCOA, and VII. The types of albinism have differences in the phenotypes of the individuals.