Sex Linkage

• sex linkage is when a given trait is on a sex chromosome

• humans: there are few Y-linked traits and several X-linked traits. Y-linked traits affect males only and are always expressed. X-linked traits are always expressed in the male because they only have 1 X chromosome. While females with 1 affected X chromosome are silent carriers for the recessive trait.

• birds: there are no known W-linked traits, but there are several Z-linked traits in birds. W-linked traits would affect females only and are always expressed. Z-linked traits are always expressed in the female because they only have 1 Z chromosome. Males with 1 affected Z chromosome are silent carriers for a recessive trait or are affected individuals for a dominant trait.

• clues to recognize sex linkage: (1) when reciprocal crosses are done (eg) purple male X pink female and pink male X purple female, they give different results (2) male and female offspring are different (males and females are NOT equally likely to inherit a given trait. (3) males show the trait more often than females.

BIRDS - SEX-LINKED RECESSIVE TRAIT

ZZ = MALE

ZW = FEMALE

• The male has 2 Z chromosomes while the female has 1 Z chromosome and 1 W chromosome.

• Only the Z chromosome can carry genes like "A", "a", "B", or "b". The W chromosome is too small to carry many genes.

• Males can be normal, carriers, or affected while females can only be normal or affected. Try all possible combinations for practice.

• Females can never be carriers or heterozygotes for a certain condition !!!!

• Example: purple colour in birds

ZBZB or ZBW = Green Bird

ZBZb = Green Bird (Carrier/Heterozygote for Purple gene)

ZbZb or ZbW = Purple Bird

(1) ZBZB (green male) x ZBW (green female) = all green birds (50% green males ZBZB & 50% green females ZBW )

	ZB	ZB
ZB	ZBZB	ZBZB
W	ZBW	ZBW

(2) ZBZB (green male) x ZbW (purple female) = all green birds (50% heterozygote males ZBZb & 50% green females ZBW )

	ZB	ZB
Zb	ZBZb	ZBZb
W	ZBW	ZBW

(3) ZbZb (purple male) x ZBW (green female) = 50% heterozygote males ZBZb & 50% purple females ZbW )

	Zb	Zb
ZB	ZBZb	ZBZb
W	ZbW	ZbW

(4) ZBZb (heterozygote male) x ZBW (green female) = 25% green males ZBZB, 25% green females ZBW, 25% heterozygote males ZBZb, & 25% purple females ZbW

	ZB	Zb
ZB	ZBZB	ZBZb
W	ZBW	ZbW

(5) ZBZb (heterozygote male) x ZbW (purple female) = 25% heterozygote males ZBZb 25% green females ZBW, 25% purple males ZbZb & 25% purple females ZbW

	ZB	Zb
Zb	ZBZb	ZbZb
W	ZBW	ZbW

(6) ZbZb (purple male) x ZbW (purple female) = all purple offspring

	Zb	Zb
Zb	ZbZb	ZbZb
W	ZbW	ZbW

• An example of a sex linked recessive trait in birds is the Lutino mutation in Lineolated Parakeets!

 

BIRDS - SEX-LINKED DOMINANT TRAIT

ZZ = MALE

ZW = FEMALE

• The male has 2 Z chromosomes while the female has 1 Z chromosome and 1 W chromosome.

• Only the Z chromosome can carry genes like "A", "a", "B", or "b". The W chromosome is too small to carry many genes.

• With sex-linked dominant traits, males can be normal or affected and females can be normal or affected. Neither sex can be carriers.

*Example: orange colour in birds

ZBZB or ZBW = Green Bird

ZBZb = Orange Bird (This bird only has 1 copy of the orange gene, but since that gene, the bird is orange) To differentiate this form of orange bird from an orange bird with 2 copies of the orange gene, it can be termed a orange heterozygote

ZbZb or ZbW = Orange Bird

(1) ZBZB (green male) x ZBW (green female) = all green birds (50% green males ZBZB & 50% green females ZBW )

	ZB	ZB
ZB	ZBZB	ZBZB
W	ZBW	ZBW

(2) ZBZB (green male) x ZbW (orange female) = 50% orange heterozygote males ZBZb & 50% green females ZBW

	ZB	ZB
Zb	ZBZb	ZBZb
W	ZBW	ZBW

(3) ZbZb (orange male) x ZBW (green female) = 50% orange heterozygote males ZBZb & 50% orange females ZbW 

	Zb	Zb
ZB	ZBZb	ZBZb
W	ZbW	ZbW

(4) ZBZb (orange heterozygote male) x ZBW (green female) = 25% green males ZBZB, 25% green females ZBW, 25% orange heterozygote males ZBZb, & 25% orange females ZbW

	ZB	Zb
ZB	ZBZB	ZBZb
W	ZBW	ZbW

(5) ZBZb (orange heterozygote male) x ZbW (orange female) = 25% orange heterozygote males ZBZb, 25% green females ZBW, 25% orange males ZbZb, & 25% orange females ZbW

	ZB	Zb
Zb	ZBZb	ZbZb
W	ZBW	ZbW

(6) ZbZb (orange male) x ZbW (orange female) = 100% orange offspring

	Zb	Zb
Zb	ZbZb	ZbZb
W	ZbW	ZbW