somatic chromosomesrefer to the chromosomes that
produce enzymes for almost everycharacteristic and function
except for those genes on the
sex chromosomes
remember that you have chromosome pairs
1 from mom and one from dad
they have genes for the same traitsbut the genes do not have to be identicalalthough some of them can be identical
this is true for femalesbut it is NOT true for males
in males the 22 somatic chromosomes are paired
just like in femalesbut
the sex chromosomes are different
the sex chromosomes have 2 functions
- they contain genes for many functions- they determine the sex of an offspring
females are referred to as XXmales are referred to as XY
the Y chromosome is much smaller than the X
in a female the sex chromosomesXX
have a copy of each gene on each Xso females have 2 genes for each trait on
the X
males only have 1 X chromosomeand the Y chromosome is much smaller than
the Xso
males have many genes on the Xwhich do not exist on the Y
there are more than 1000 genes on the Xwhich do not exist on the Y
these 1000 genesdo not exist as a pair
in males these 1000 plus genes on the Yexist as a single gene and NOT as a pair
sex-linked diseases
refer to diseases caused by recessive genes which exist
on the Xbut
not on the Y
if the gene on the X is normala male will be normal
butthere are diseases caused by recessive
genesif a male has the recessive gene for a
diseaseon the X then the male will be sick
because he only has 1 genethere is no second dominant gene
to repress the recessive gene
femalesin order to have a sex linked disease
must have a recessive geneon both X chromosomes
both parents had to givethe child a recessive gene
in females since they have 2 X chromosomes
the normal dominant genesuppresses the expression
of the recessive gene
in order for a female to have a sex linked diseaseboth parents must
pass on the recessive gene
in a male since they have many genespresent as a single gene
if they have a dominant gene = normalif they have 1 recessive gene for a disease
= sick
these are diseases which males havemore than 10 times as often as females
these aredisease which mothers pass on to their sons
dad gave you a Y – mom gave you an Xthese genes are only on the Xthese are sex-linked diseases
there are many diseases whichare found on the somatic chromosomes
some are common others are raresome are caused by dominant genessome are caused by recessive genes
But
sex-linked diseasesare caused by recessive genes
on the chromosome
if a male has 1 recessive gene for a disease they are sick
a female must have 2 recessive genes to be sick
cystic fibrosis – 1 enzymesickle cell anemia – 1 enzyme
hemophilia – 1 enzyme X-linkedHuntington’s – dominant (45-50)
Down’s syndrome – extra chromosome (21)color blindness – x linked (different kinds)
Barr Bodies (methyl DNA inactivation)
females = XXbut
in normal females one X is inactivatedit condenses into a compact Barr Body
which X is inactivatedthe one from mom
orthe one from dad
takes place randomly at the time of inactivationthe result is that
in the embryo at the time of X inactivationsome of mom’s X are inactivated
and some of dad’s X are inactivatedso
females are mosaics (mosaicism)
gene linkage
genes which are close to each otheron a chromosome are more likely
to be passed on togetherand
genes which are far from each other on a chromosome are less likely
to be passed on together
there are many kinds of errors
point mutation ( + or -)base substitution (replace 1 base)
addition (add a base)deletion (remove a base)
inversions (invert a section)translocation (put a section into wrong
place)
all cells whether in a single celled prokaryote
or a large multicellular eukaryoteare exposed to substances called
mutagenswhich cause errors in DNA
known as mutations
some mutations cause an errorin one amino acid
butsome mutations cause the entire
protein from that point onto be incorrect