4- X-LINKED GENES, SEX INFLUENCED INHERITANCE, SEXLIMITED INHERITANCE Compiled by Siti Sarah Jumali Level 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 Xlimited, 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 Xlinked because father does not supply X Baldness cont’d • Caused by sex hormone testosterone • Overexpression of an ezyme called 5 αdihydrotestosterone (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?