Sex-Influenced TraitAutosomal gene with traits expressed in one of the sexes, either because of internal hormones environment or anatomical dissimilarities.

e.g., Bulls have genes for milk production that may be transmitted to daughters, but their sons are unable to express this trait.

Another example of sex influenced trait: in manPattern Baldness b+ = bald b = non-bald

PHENOTYPEGENOTYPES MALES FEMALES

b+b+ bald baldb+b bald non-baldb b non-bald non-bald

Sex-Influenced TraitQuestion: Let’s consider two sex-influenced traits simultaneously, pattern baldness and short index finger, both of which are dominant in men and recessive in women.

A heterozygous bald man with long index finger marries a heterozygous long-fingered, “bald-prone” woman.

Determine the phenotypic expectation of their children.

Long-fingered

Long-fingered

F2F2Non-baldNon-baldB2B2

Long-fingered

Short-fingered

F1F2Non-baldBaldB1B2

Short-fingered

Short-fingered

F1F1BaldBaldB1B1

FemaleMaleGenotypeFemaleMaleGenotype

Sex-Influenced TraitP: B1B2, F2F2 x B1B1, F1F2

bald, long-fingered man bald-prone, long fingered women

F1: 1/2 F1F2 = 1/4 B1B1, F1F2

1/2 B1B1

1/2 F2F2 = 1/4 B1B1, F2F2

1/2 F1F2 = 1/4 B1B2, F1F2

1/2 B1B2

1/2 F2F2 = 1/4 B1B2, F2F2

Dichotomous branching

system

bald, short (men) / bald, long (women)

bald, long (men) / bald, long (women)

bald, short (men) / non-bald, long (women)

bald, long (men) / non-bald, long (women)

F1 summary: Men: 1/2 bald, short-fingered : 1/2 bald, long fingered

Women: 1/2 bald, long-fingered : 1/2 non-bald, long fingered

Sex-limited Traits• Traits that occur in only one of the two sexes

• For example in humans– Breast development is normally limited to females– Beard growth is normally limited to males

• Another (textbook) example: Feather plumage in chicken– Caused by an autosomal gene– Hen-feathering is controlled by a dominant allele

expressed in both sexes– Cock-feathering is controlled by a recessive allele only

expressed in males

Sex-limited Traits

hen-featheredhen-featheredHH

hen-featheredhen-featheredHh

cock-featheredhen-featheredhh

Phenotype in Males

Phenotype in Females

Genotype

• The pattern of hen-feathering depends on the production of sex hormones

• If the single ovary is surgically removed from a newly hatched hh female– She will develop cock-feathering and look

indistinguishable from a male