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?