* 15 Flashcards
chromosome theory of inheritance
A basic principle in biology stating that genes are located at specific positions (loci) on chromosomes and that the behavior of chromosomes during meiosis accounts for inheritance patterns.
wild type
The phenotype most commonly observed in natural populations
mutant phenotypes
alternatives to the wild type
SRY
- in humans, the anatomical signs of sex begin to emerge when the embryo is about 2mo old
- sex-determining region of Y chromosome
- required for development of testes; in its absence, gonads develop into ovaries
- codes for a protein that regulates other genes
X vs Y chromosome
- Y: (identified) 78 genes that code for 25 proteins; most help determine sex
- X: ~1,100 genes; have genes for many characters unrelated to sex
Duchenne muscular dystrophy
- X-linked
- affects 1/3,500 males born in US
- progressive muscular weakening, loss of coordination
- affected individuals rarely live past early 20s
- absence of a key muscle protein called DYSTROPHIN
hemophilia
- X-linked
- absnece of 1 or more of the proteins required for blood clotting
- treated w/ intravenous injections of the missing protein
Barr body
- most of 1 X in each cell in female mammals becomes inactive during early embryonic development
- it condenses into a compact object called a Barr body, which lies along the inside of the nuclear envelope
- in the ovaries, Barr-body chromosomes are reactivated in the cells that give rise to eggs, so every female gamete has an active X
mosaic
- females consist of a mosaic of 2 types of cells: those w/ the active X from the dad and those w/ the active X from the mom
- if a female is heterozygous for a sex-linked trait, about half her cells will express one allele, while the others will express the alternate allele
X inactivation process
- involves modification of the DNA and the histone proteins bound to it, including attachment of METHYL groups to one of the nitrogenous bases of DNA nucleotides
- a particular region of X chromosome contains several genes involved in the inactivation process. the 2 regions, one on each X chromosome, associate briefly w/ e/o in each cell at an early stage of embryonic development
- then one of the genes, called XIST (X-inactive specific transcript) becomes active only on the chromosome that will become the Barr body
- multiple copies of the RNA product of XIST attach to the X chromosome on which they’re made, eventually almost covering it
- interaction of this RNA w/ the chromosome initiates X inactivation, and the RNA products of other genes nearby on the X chromosome help to regulate the process
linked genes
- Genes located close enough together on a chromosome that they tend to be inherited together.
- a 50 percent frequency of recombination is observed for genes that AREN’T linked
cinnabar
in Drosophila, a mutant phenotype; brighter red than the wild type
cytogenic maps
A map of a chromosome that locates genes with respect to chromosomal features distinguishable in a microscope.
genetic map
An ordered list of genetic loci (genes or other genetic markers) along a particular chromosome.
linkage map
A genetic map based on the frequencies of recombination between markers during crossing over of homologous chromosomes.
map unit
A unit of measurement of the distance between genes. One of these, also known as a centimorgan, is equivalent to a 1% recombination frequency.
3 other chromosomal systems of sex determination
- X-0: grasshoppers, cockroaches, some other insects. only 1 type of sex chromosome: the X. females are XX; males are X0. sex of offspring is determined by whether the SPERM cell contains an X or no sex chromosome.
- Z-W: birds, some fishes, some insects. the sex chromosomes present in the EGG determine sex of offspring. females are ZW; males are ZZ.
- haplo-diploid: most species of bees and ants. no sex chromosomes. females develop from fertilized eggs and are thus diploid; males develop from unfertilized eggs and are haploid (they have no fathers).
nondisjunction
An error in meiosis or mitosis in which members of a pair of homologous chromosomes or a pair of sister chromatids fail to separate properly from each other.
aneuploidy
- A chromosomal aberration in which one or more chromosomes are present in extra copies or are deficient in number.
- when an aberrant gamete unites w/ a normal one at fertilization
common alterations of chromosome structure
- reciprocal translocation more common than nonreciprocal
- DELETIONS and DUPLICATIONS esp likely to occur during meiosis
- in crossing over, nonsister chromatids sometimes exchange unequal-sized DNA segments, so that one partner gives up more genes than it receives. the products of such an unequal crossover are one chromosome w/ a deletion and one chromosome w/ a duplication
Down syndrome rate
- 1 in 700 children in US
- most cases result from nondisjunction during meiosis I
- some research points to an age-dependent normality in a meiosis checkpt that normally delays anaphase until all the kinetochores are attached to the spindle (similar to M phase checkpt)
Klinefelter syndrome
- XXY
- 1/500 to 1/1,000 live male births
- male sex organs, but testes are abnormally small and the man is sterile
- breast enlargement and other female body characteristics; maybe subnormal intelligence
XYY
- 1/1,000 males
- normal sexual development; tend to be taller than avg
XXX
- 1/1,000 live female births
- healthy; tend to be slightly taller than avg
- fertile
- at risk for learning disabilities