Warning-These notes are provided only as a guide. They may not be exactly what is used in class. Due to formating for HTML they may also be less clear than might be desired. Use them at your own risk. Dr. G.
MODERN GENETICS
T. H. Morgan and Drosophila
Thomas H. Morgan - used fruit fly, Drosophila, to provide
evidence that genes are parts of
chromosomes
-this paved the way for studying chromosomal inheritance
Sex Determination and Chromosomes
-of the 23 pairs of human chromosomes:
-22 pairs are homologous chromosomes called autosomes
-the 1 unmatched pair of chromosomes are called sex
chromosomes
-female sex chromosomes are both rod-shaped X chromosomes
-male sex chromosomes have 1 rod-shaped X chromosome, and 1
hooked-shaped Y chromosome
Sex-Linked Traits in Humans
-a trait that is controlled by a gene found on a sex
chromosome is called a sex-linked trait
-many diseases in humans are caused by defective genes
-some defective genes are sex-linked, and include:
-hemophilia
-muscular dystrophy
-less serious conditions like night and color blindness are also
sex-linked
-color blindnessresults in the inability to see colors, mainly
red and green
-color blindness is more common in men
-females heterozygous for the X-chromosomes that carry the
recessive trait seldom display color
blindness
-males who receive one X-chromosomes from their mothers
express color blindness more often
Fruit flies also have sex-linked traits
(red eye)
Gene Linkage
-thousands of genes are present on each human chromosome
-genes on the same chromosome are said to be linked, and make
up a linkage group
-humans have 23 linkage groups
-in linkage groups, certain traits seem to stay together
-they defy the law of independent assortment
Crossing Over
-unusual ratios occurred in plant experiments dealing with
linkage groups
-phenotypic ratios for F2 plants were not near 9:3:3:1 as
expected
but closer to 3:1 showing linkage on particular chromosomes
-it was concluded that pieces of chromosomes were exchange
during meiosis in a process called
crossing over
-genes far apart on the same chromosome became separated more
often during crossing over than
genes closer together
Mapping
-crossing over is an important source of genetic variation in offspring
(remember clay?)
Multiple-Gene Inheritance
-many traits do not appear in two contrasting forms
-for example, humans are not simply short or tall, heights
vary greatly
-varying traits are not controlled by the alleles of a single
gene
-two or more independent genes control varying traits
-when two or more genes affect one characteristic, such as
height, it is called multiple-gene, or
polygenic inheritance
MULTIPLE GENES ARE NOT THE SAME AS MULTIPLE ALLELES
-multiple genes affect characteristics of the same phenotype
-multiple alleles produce different phenotypes
Applied Genetics-
Variation is essential to long term reproductive success
Therefore, change and differences between offspring and their
parents is a long term good.
Variation allows natural selection to allow species to adapt to changing conditions.
Changes in the actual genetic makeup of the genome=mutation.
Kinds of mutations:
chromosomal mutation-
change in the structure, or number of a chromosome
gene mutation- effects a gene on a chromosome
Mutations
Sudden change in the DNA (a gene).
Only those occurring in gametes are inherited
Most mutations are deleterious
Causes of mutations
1) Chemicals (mutagen)
2) X-rays
3) Ultra violet
Most mutagens also cause cancer
Genetic diseases-
Two main categories:
-mutation diseases
a flaw in a gene product leads to disease
-chromosomal abnormality
Chromosomal abnormalities 1:200 live births
many spontaneous abortions and miscarriages
Nondisjunction
Failure of chromosomes to separate
Occurs during in meiosis
Results in gametes:
a) with 1 extra chromosome
b) with 1 missing chromosome
Downs syndrome (trisomy 21)
Turners syndrome (X0)
(females with underdeveloped sex characteristics)
Kleinfelters syndrome (XXY)
(males with underdeveloped sex characteristics)
Polyploidy
1) Almost always occurs in plants
2) Having 3N or 4N or higher chromosome #.
3) Produces different varieties (ex seedless)
4) Contributes to genetic variation
Translocation
1) Piece of 1 chromosome attached to another (non-homologous)
chromosome
2) May result in trisomy in offspring
Addition
Chromosome gains an extra piece
Deletion
Chromosome loses a piece
Cri du chat syndrome
inversion
changes order of genes
P) Extrachromosomal inheritance
a) Some genes are located in cytoplasmic organelles
1) Mitochondria
2) Chloroplast
b) These organelles replicate independently
c) Inheritance is through the female (mother) only
Gene mutations-
point mutations
one nucleotide in the DNA sequence is changed
this changes the mRNA that is transcribed
which changes the protein that is translated
Changes in the 3rd nucleotide of a codon are usually less severe than in the 1st two nucleotides
Additions or deletions are usually VERY VERY BAD-
frame shift mutations
Specific Human genetic disease conditions:
{Traced with pedigree charts-boxes for males, circles for
females; children descend from joined
lines}
You can trace a disease and identify carriers etc from a
pedigree
Among mutation diseases they can be on the homologous
chromosomes or on the sex
chromosome.
{this is review}
(If they are on the sex chromosome the disease is called sex- linked.)
Sex linkage
a) Refers to all genes located on sex chromosomes
b) Almost always refers to genes on X chromosome
c) Most sex-linked genes are recessive
1) Color blindness
2) Hemophilia
3) Duchenne muscular dystrophy
d) Follow normal dominance rules in females.
e) Always expressed in males even if only one recessive gene is
present.
f) Heterozygous females are carriers.
g) Pure recessive females have the disease
h) Males pass genes to all daughters and no sons.
Autosomal Genetic diseases:
A disease gene can be recessive or dominant
Inheritance pattern depends on that.
Dominant- Huntington's Chorea
degenerative brain disorder
Recessive
1) Albinism
2) Sickle cell anemia
3) Tay Sachs
4) PKU
5) cleft lip
6) cystic fibrosis
Sickle cell disease-
1 aa different in about 300 (1 nucleotide out of 900) GAA ->
GUA
mainly in Africans and African Americans
carriers more resistant to malaria
[Evolutionary aside...]
If likelihood of dying from malaria is high, sacrificing 1 in 4
offspring to sickle cell disease in
return for 1/2 offspring resistant to malaria reasonable
Genetic counseling
Genetic screening
amniocentesis
chorionic villi sampling
1/10,000 births has PKU and in USA ALL virtually all babies are screened at birth for PKU
-30-