4- X-LINKED GENES, SEX INFLUENCED INHERITANCE, SEX-LIMITED INHERITANCE

Compiled bySiti Sarah JumaliLevel 3 Room 14

Ext 2123

X-LINKED GENES

• Certain traits are governed by genes that are located on a sex chromosome

• Outcome of cross depends on the genotypes and sexes of parents and offspring

• Eg Duchenne Muscular Dystrophy (DMD)– A sign of muscular weakness

• Eg golden retriever dogs

– X-linked recessive• Allele on X-chromosome and recessive• Rare for female

Who’s having what??

X-LINKED INHERITANCE

DMD- DUCHENNE MUSCULAR DYSTROPHY

DISCUSS # 4..

CHROMOSOME-LINKED GENES

• Genes located on mammalian sex chromosomes can be transmitted in an X-limited, Y linked or pseudoautosomal pattern

• Hemizygous is a condition that describes a single copy of X-linked gene in the male

• Few genes are located on the X and some other genes are located on the Y chromosome

• The few genes on Y-chromosome are called holandric genes

• Y-linked is only transmitted from father to son

SEX-INFLUENCED INHERITANCE

• Some traits are influenced by the sex of the individual

• In sex-influenced inheritance:– Allele is dominant in one sex but recessive in

opposite sex– Genes for sex-influenced traits are autosomal;

not on either X, or Y– Heterozygotes phenomenon

• Eg baldness– When heterozygote Bb, men will be bald but women don’t.

Baldness

Male Female

BB Bald Bald

Bb Bald Nonbald

bb Nonbald nonbald

When heterozygote Bb, men will be bald but women don’t.

Transmitted from father to son but not X-linked because father does not supply X

Baldness cont’d

• Caused by sex hormone testosterone• Overexpression of an ezyme called 5 α-

dihydrotestosterone (DHT) which converts testosterone to DHT

• DHT binds to cellular receptors and affects the expression of many genes including those in the cells of the scalp

• Because mature men makes more testosterone than female, the allele has greater phenotypic impact on males

SEX-LIMITED INHERITANCE

• Trait occur only in 1 of the 2 sexes– Eg breast development in female– Beard development in male– Other examples include rooster and hen where

allele H for hen is in hen but not in rooster– Just like sex influenced, these are also

influenced by hormone

Limitation to gender because of hormone production

Female Male

hh Hen-feathered Cock-feathered

Hh Hen-feathered Hen-feathered

HH Hen-feathered Hen-feathered

Question..

• You did relative finger length experiment in the lab.

• What do you think it is?– X-linked? – Y-linked? – Sex-limited? – Sex-inherited?