IGCSE Biology 0610: Interpreting a Pedigree Diagram for Albinism

Albinism is an inherited condition in which the skin and hair lack pigment. A family's pedigree is described below.

Generation I: Individual 1 (unaffected male) and Individual 2 (unaffected female) are married. They have three children, who make up Generation II: Individual 3 (unaffected female), Individual 4 (male, has albinism), and Individual 5 (unaffected male).

Individual 3 marries an unrelated, unaffected male, Individual 6 (also Generation II). Individual 3 and Individual 6 have one child, Individual 7 (Generation III), who has albinism.

(a) Using evidence from Individuals 1, 2 and 4, state whether the allele for albinism is dominant or recessive, and explain your reasoning. [2]

(b) Using N to represent the dominant (normal pigmentation) allele and n to represent the recessive (albinism) allele, state the genotypes of Individual 1 and Individual 2. [2]

(c) Individual 3 and Individual 6 are both unaffected, yet their child, Individual 7, has albinism. State the genotypes of Individual 3, Individual 6 and Individual 7, and explain how two unaffected parents can have a child with albinism. [4]

Part (a): Dominant or recessive?

Individual 1 and Individual 2 are both unaffected, but their son, Individual 4, has albinism. If the albinism allele were dominant, it would always be expressed whenever present, so an affected child could only appear if at least one parent showed the dominant phenotype, but neither parent is affected. The only way two unaffected parents can produce an affected child is if the allele is recessive: each parent can carry a “hidden” copy of the recessive allele without showing the trait, and a child who inherits a recessive allele from both parents will be affected. This evidence shows the allele for albinism is recessive.

Part (b): Genotypes of Individual 1 and Individual 2

Since Individual 4 has albinism, his genotype must be nn. He must have received one n allele from each parent. This means both Individual 1 and Individual 2 must carry at least one n allele. However, both are phenotypically unaffected, so each must also carry a dominant N allele to mask the recessive one. The genotype of Individual 1 is Nn, and the genotype of Individual 2 is Nn.

Part (c): Explaining Individual 7’s albinism

Individual 7 has albinism, so their genotype is nn. Individual 3 and Individual 6 are both unaffected, so each must carry at least one N allele, but since their child is nn, each parent must also carry a hidden n allele. This makes both Individual 3 (Nn) and Individual 6 (Nn) heterozygous carriers.

Neither Individual 3 nor Individual 6 shows albinism themselves, because in each of them the dominant N allele masks the effect of the recessive n allele. However, during the production of gametes, each parent can pass on either their N allele or their n allele. In this family, both parents happened to pass on their n allele to Individual 7. With no dominant N allele present, Individual 7 is homozygous recessive (nn) and shows the recessive albinism phenotype. Even though both parents are themselves unaffected carriers.

Final answers

(a) The allele is recessive, since two unaffected parents (1 and 2) produced an affected child (4).
(b) Individual 1: Nn. Individual 2: Nn.
(c) Individual 7: nn. Individual 3: Nn. Individual 6: Nn. Both parents are unaffected heterozygous carriers who each passed their hidden recessive n allele to Individual 7.