2.8 Flashcards
Human cells contain how many chromosomes?
46
There are 23 different chromosomes, but we have one set of chromosomes from our mother and one from our
father
Chromosomes are arranged in
homologous pairs
The chromosomes of a homologous pair look identical to each other, except for one pair:
sex chromosomes
The X chromosome is much larger than the
Y chromosome
Females have
XX
Males have
XY
The DNA in the human Y chromosome is 58 million base pairs long, but represents only about…
0.4% of the total DNA in a human cell
Y chromosome contains very few genes, one it does contain is known as
SRY
SRY is the gene that triggers an embryo to
develop as a male
Other genes on the Y chromosome are needed for
normal sperm production
Because females have two X chromosomes, every gamete they produce will contain a
single X chromosome
The gametes produced by a male will differ from each other. Half will contain X and half will contain a
Y chromosome
The sex of the offspring will be determined by whether the sperm that fertilised the oocyte contains an
X or Y chromosome
Genes found on the sex chromosome are said to be
sex linked
There are very few genes on the human Y chromosome, so sex linked genes are usually found on the
X chromosome
An example of a sex-linked characteristic is Duchenne Muscular Dystrophy (DMD) this is a genetic disorder caused by an error in the
dystrophin gene
(SEX-LINKED GENES) Dystrophin is an important protein in muscle tissue. The condition causes
-Progressive muscle weakness as the muscle cells break down and die
(SEX-LINKED GENES) Symptoms usually appear in boys under the age of 6. The first signs are a
loss of muscle tissue and weakness of the arms and pelvis
(SEX-LINKED GENES) Children with the condition DMD have have progressive difficulties with mobility, and usually need a wheelchair from the age of about 11. By the time the child has reached his late teens or early twenties, the breathing muscles become affected and the conditions become
life-threatening
(SEX-LINKED GENES) DMD usually only affects
boys
(SEX-LINKED GENES) DMD is caused by a
recessive allele on the X chromosome
(SEX-LINKED GENES) A woman who carries the allele for DMD does not usually show any symptoms of the condition, since she has a second X chromosome with a
normal allele on it
(SEX-LINKED GENES) We can use the symbol XD for the normal (dominant) allele and Xd for the
recessive DMD allele
