Asexual Reproduction Vegetative propagation Binary Fission Budding Sporogenesis

Chromosomes and Inheritance Chapter 15

Meiosis Oogenisis Spermatogenisis http://highered.mcgraw-hill.com/olcweb/cgi/pluginpop.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

sex-linked gene The gene with the white-eyed mutation is on the X chromosome

linked genes Tend to be inherited together the chromosome is passed along as a unit

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 Hemophilia 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 One map unit is equivalent to a 1% recombination frequency

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

Nondisjunction Problems with the meiotic spindle cause errors in daughter cells

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

Down syndrome 3 copies of chromosome 21 affects 1 in 700 children born in the US

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

Genomic Imprinting The same alleles may have different effects on offspring, depending on whether they arrive in the zygote via the ovum or via the sperm