Chapter 15

Chromosomes and Inheritance

Dr. Joseph Silver

we will study the following topics

- sex linked diseases- human diseases

- mutations

there are 2 types of chromosomes

- somatic chromosomes- sex chromosomes

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

genomic imprinting

phenotype depends on which parent passed on the gene

probably hundreds of genes (know 60)