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 chromosomes refer to the chromosomes that produce enzymes for almost every characteristic 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 traits but the genes do not have to be identical although some of them can be identical

this is true for females but it is NOT true for males in males the 22 somatic chromosomes are paired just like in females but 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 XX males are referred to as XY the Y chromosome is much smaller than the X

in a female the sex chromosomes XX have a copy of each gene on each X so females have 2 genes for each trait on the X

males only have 1 X chromosome and the Y chromosome is much smaller than the X so males have many genes on the X which do not exist on the Y there are more than 1000 genes on the X which do not exist on the Y

these 1000 genes do not exist as a pair in males these 1000 plus genes on the Y exist as a single gene and NOT as a pair

sex-linked diseases refer to diseases caused by recessive genes which exist on the X but not on the Y

if the gene on the X is normal a male will be normal but there are diseases caused by recessive genes if a male has the recessive gene for a disease on the X then the male will be sick because he only has 1 gene there is no second dominant gene to repress the recessive gene

females in order to have a sex linked disease must have a recessive gene on both X chromosomes both parents had to give the child a recessive gene

in females since they have 2 X chromosomes the normal dominant gene suppresses the expression of the recessive gene in order for a female to have a sex linked disease both parents must pass on the recessive gene

in a male since they have many genes present as a single gene if they have a dominant gene = normal if they have 1 recessive gene for a disease = sick

these are diseases which males have more than 10 times as often as females these are disease which mothers pass on to their sons dad gave you a Y – mom gave you an X these genes are only on the X these are sex-linked diseases

there are many diseases which are found on the somatic chromosomes some are common others are rare some are caused by dominant genes some are caused by recessive genes

But sex-linked diseases are 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 enzyme sickle cell anemia – 1 enzyme hemophilia – 1 enzyme X-linked Huntington’s – dominant (45-50) Down’s syndrome – extra chromosome (21) color blindness – x linked (different kinds)

Barr Bodies (methyl DNA inactivation) females = XX but in normal females one X is inactivated it condenses into a compact Barr Body

which X is inactivated the one from mom or the one from dad takes place randomly at the time of inactivation the result is that in the embryo at the time of X inactivation some of mom’s X are inactivated and some of dad’s X are inactivated so females are mosaics (mosaicism)

gene linkage genes which are close to each other on a chromosome are more likely to be passed on together and 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 eukaryote are exposed to substances called mutagens which cause errors in DNA known as mutations

some mutations cause an error in one amino acid but some mutations cause the entire protein from that point on to be incorrect

genomic imprinting phenotype depends on which parent passed on the gene probably hundreds of genes (know 60)