Chapter 15: Chromosomes Flashcards
Wild type
The phenotype for a character that is most commonly observed in natural populations
Morgan’s gene notation
A gene for a certain character takes its symbol from the first mutant type discovered
A superscript + identifies the allele for the wild type trait
E.g. the allele for white eyes in Drosophila is symbolized by w while the red eye allele is symbolized as w+
Sex-linked gene
A gene located on either sex chromosome
X-linked genes are located on the X chromosome; approx. 1,100
- X chromosomes have genes for many characters unrelated to sex
- X-linked genes follow specific patterns of inheritance
Y-linked genes are located on the Y chromosome; 78 genes
- most Y-linked genes are related to sex determination
X-linked gene expression
A female must be homozygous; needs two copies of the allele
A male only needs to be hemizygous- one copy of the allele
X-linked disorders caused by recessive alleles on the X chromosome in humans:
- Color blindness (mostly X-linked)
- Duchenne muscular dystrophy- affects males resulting in progressive muscle weakening and loss of coordination
- Hemophilia- absence of one or more of the proteins required for blood clotting
X inactivation in mammals
Only applies to females due to the fact that they have two X chromosomes
One of the two X chromosomes in each cell is randomly inactivated during embryonic development and all mitotic decendents of that cell will have the same inactive X
- Inactivation involves modification of DNA and histones including attachment of methyl groups to DNA nucleotides
The inactive X condenses into a Barr body; most of its genes are not expressed
- Barr body chromosomes are reactivated in the cells that give rise to eggs resulting in every female gamete having an active X during meiosis
As a consequence females consist of a mosaic of cells with different inactive X chromosomes
- Sweat glands in humans
- Fur coat of some cats
Linked genes
Linked genes located on the same chromosome that tend to be inherited together
Genetic recombination
The production of offspring with combinations of traits that differ from either P generation parent due to the independent assortment of chromosomes and crossing over in meiosis I and random fertilization
Parental types- offspring with a phenotype matching either one of the parental phenotypes are called
Recombinant types/recombinants- offspring with a nonparental phenotypes; a new combinations of traits
Recombination of unlinked genes
A 50% frequency of recombination is observed for any two genes that are located on different chromosomes
Physical basis is the random orientation of homologous chromosomes at meaphase I which leads to the independent assortment of two unlinked genes
Recombination of linked genes
A greater than 50% frequency is observed for genes that are close together and on the same chromosome
Physical basis results from the crossing over of homologs that occurs during prophase I
Linkage map
A genetic map of a chromosome based on recombination frequencies
Gives an ordered list of the genetic loci along a particular chromosome, NOT the percise locations of those genes
Distances are expressed in map units (centimorgan) which are defined as equivalent ot a 1% recombination frequency
The farther apart two genes are, the higher the probability that a crossover will occur between them; therefore, the higher the recombination frequency
Nondisjunction
Members of a pair of homologous chromosomes do not properly move apart during meiosis I or sister chromatids fail to separate during meiosis II
One gamete receives two copies of the same type of chromosome while another gamete receives no copy
If either aberant gamete forms a zygote it will have an abnormal number of chromosomes; known as aneuploidy
- Monosomic zygote- fertilization involving a gamete that has no copy of a particular chromosome; cell has 2n - 1 chromosomes
- Trisomic zygote- fertilization involving a gamete with an extra (triplicate) of a particular chromosome; cell has 2n + 1 chromosomes
Polyploidy
A condition in which an organism has more than two complete sets of chromosomes
- Triploidy- three sets of chromosomes; 3n
- Tetraploidy- four sets of chromosomes; 4n
Polyploidy is common in plants but not animals
Polyploids are more normal in appearance than aneuploids; one extra or missing chromosome apears to disrupt genetic balance more so than an entire extra set of cromosomes
Alterations of chromosome structure
Breakage of a chromosome can lead to four types of changes in chromosome structure
- Deletion- removal of a chromosomal segment
- Duplication- repetition of a chromosomal segment
- Inversion- reversal of orientation of a segment within a chromosome
- Translocation- transfer of a segment from one chromosome to another
Deletions and duplication are expecially likely during crossing over in meiosis I if non-sister chromatids exchange unequal-sized segments of DNA; one chromosome ends up with a deletion while the other with a duplication
Trisomy 21
Down syndrome is an aneuploid condition that results from three copies of chromosome 21
Aneuploidy of sex chromosomes
Klinefelter syndrome- result of an extra X chromosome in males producing XXY individuals
- Individuals have male sex organs however testes are small and individuals are sterile
Monosomy X- Turner syndrome; result of a missing X chromosome in females producing X0 individuals
- Individuals are phenotypically female but are usually sterile
- Only known viable monosomy in humans
Trisomy X- result of an extra X chromosome in females
- Individuals are generally healthy with normal physical features but are taller than average
Disorders caused by structurally altered chromosomes
Cri du chat- results from specific deletion of chromosome 5
- A child born with this syndrome is severely intellectually disabled and has a catlike cry; individuals usually die in infancy or early childhood
Chronic myelogenous leukemia (CML)- cancer caused by reciprocal translocation that occurs during mitosis in cells that are precursors of white blood cells
- A large portion of chromosome 22 is exchanged with a small fragment from the tip of chromosome 9
- Produces a much shortened chromosome 22 called a Philadelphia chromosome
Genomic imprinting
Phenotype depends on which parent passed along the alleles for those traits
Occurs during gamete formation and results in the silencing of a particular allele of cetain genes; offspring expresses only one allele of an imprinted gene
Can involve either silencing of an allele in one type of gamete or activating it in another; commonly consists of a methyl group added to cytosine nucleotides of one of the alleles
Normal development seems to require that embryonic cells have exactly one active copy of certain genes
