asexual reproduction vegetative propagation binary fission budding sporogenesis
TRANSCRIPT
Meiosis Oogenisis Spermatogenisis http://highered.mcgraw-hill.com/olcweb/cgi/p
luginpop.cgi?it=swf::535::535::/sites/dl/free/0072437316/120074/bio19.swf::Stages%20of%20Meiosis
Chromosome Theory of Inheritance Mendelian genes have
specific loci along chromosomes, and it’s the chromosomes that undergo segregation and independent assortment.
Thomas Hunt Morgan First to associate a specific gene with a
specific chromosome Drosophila melanogaster = fruit fly
Fruit Flies Wild type = normal character phenotype Mutant = alternative traits
Morgan’s cross: White-eyed male with a Red-eyed female All the F1 offspring had red eyes
What would you conclude? The red allele is dominant to the white allele
But then… Crosses between F1 offspring produced 3:1
phenotypic ratio in F2 offspring The white-eyed trait appeared only in males Morgan concluded that a fly’s eye color was
linked to its sex
Genetic recombination Offspring with new combinations of traits
inherited from two parents
Result from: independent assortment of genes located on
nonhomologous chromosomes crossing over of genes located on homologous
chromosomes
SRY Gene Anatomical signs of sex first appear when the
embryo is about 2 months old Presence of SRY gene - sex determining
region of the Y chromosome) generic embryonic gonads are modified into
testes
Sex Linked Genes
Heterozygous females will be carriers
Any male receiving the recessive allele from his mother will express the trait
Examples Duchenne muscular dystrophy
Absence of an X-linked gene for a key muscle protein, called dystrophin
Characterized by a progressive weakening of the muscles and loss of coordination
Hemophilia Absence of one or more clotting factors Prolonged bleeding because clots form slowly
Barr body During female development, one X
chromosome per cell condenses into a compact barr body
This inactivates most of its genes Reactivated in ovarian cells that produce ova
Females consist of a mosaic of cells some with an active paternal X others with an active maternal X
Alfred Sturtevant Chromosome map
Constructed using crossing over of linked genes Ordered list of the genetic loci along a particular
chromosome
body color and wing shape are usually inherited together because their genes are on the same chromosome
linkage map used recombination frequencies from fruit fly
crosses to map the relative position of genes along chromosomes
frequency of recombinant offspring reflected the distances between genes on a chromosome
Genetic recombination Parental types = phenotypes that match the original
parents Recombinants = new combination of parental traits
dihybrid cross combination of traits that did not match either parent crosses between hybrid plants produces four phenotypes
map units Some genes on a chromosome are so far apart
that a crossover between them is virtually certain.
In this case, the frequency of recombination reaches is its maximum value of 50% the genes act as if found on separate
chromosomes and are inherited independently
Aneuploidy Trisomic cells three copies of a particular
chromosome type 2n + 1 total chromosomes
Monosomic cells only one copy of a particular chromosome type 2n - 1 chromosomes
Polyploidy Organisms with more than two complete sets
of chromosomes Relatively common among plants and much
less common among animals fishes and amphibians have polyploid species
Aneuploidy vs. Polypoidy One extra or missing chromosome upsets the
genetic balance during development more than does an entire extra set of chromosomes
changes in chromosome structure deletion a chromosome fragment lacking a
centromere is lost during cell division duplication a fragment becomes attached
as an extra segment to a sister chromatid
changes in chromosome structure inversion a chromosomal fragment
reattaches to the original chromosome in the reverse orientation
translocation a chromosomal fragment joins a nonhomologous chromosome
Outcome Most of these alterations are so disastrous that
the embryos are spontaneously aborted long before birth
Sex Chromosome Disorders Klinefelter’s syndrome
XXY male Occurs 1 in every 2000 live births have male sex organs, but are sterile
Turner’s syndrome XO female (monosomy) occurs 1 in every 5000 births produces phenotypic, but immature females
Genomic Imprinting A gene on one homologous chromosome is
silenced, while its allele on the homologous chromosome is expressed
Depends on whether the gene resides in a female or a male