chromosome theory of inheritance
stating that genes are located at specific locations (loci) on chromosomes and that the behavior of chromosomes during meiosis accounts for inheritance patterns
why did Morgan choose a fruit fly
a single mating produces hundreds of offspring and a new generation can be bred every 2 weeks
it only has 4 chromosomes easily distinguishable under a light microscope
the phenotype for a character most commonly observed in natural populations
traits that are alternatives to the wild types, alleles assumed to have originated as changes or mutations, in the wild type allele
why did Morgan think the fly's eye color was related to sex?
because only males had white eyes; allele only located on the X chromosome with no corresponding locus on the Y chromosome
Could the f2 females have white eyes?
to have white eyes both of her X chromosomes had to carry the recessive allele but is impossible cause all the f1 fathers had red eyes
What provided support for the chromosome theory of inheritance?
Morgan finding the correlation between a particular trait and an individual's sex
Are there corresponding regions on the XY chromosome?
yes- short segments at either end of the Y chromosome are the only regions that are homologous with the corresponding regions of X
What allows XY to pair and behave like homologous chromosomes during meiosis in the testes?
the homologous regions
What are some other chromosomal systems for determining sex?
X-0 system: (grasshoppers and cockroaches) female XX-24 and males X-23
Z-W system: (birds, some fishes, and some insects) female ZW-78 males ZZ-78
Haplo-diploid system: (bees and ants) females diploid-32 fertilized and males haploid-16 unfertilized
What does the gene SRY determine?
if present it determines the sex is a male and is required for the development of the testes but if not present it determines the sex is a female
sex linked gene
a gene located on either sex chromosome
Y linked genes
a gene located only on the Y chromosome
Why are there few disorders transferred from father to son on the Y chromosome?
because there are so few Y linked genes
X linked genes
a gene located only on the X chromosome
What is it that leads to a pattern of inheritance different from that produced by genes located on autosomes between males and females?
that that males and females inherit a different number of X chromosomes
What's the difference between passing X linked genes to kids from the mother and father?
mother can pass genes on X chromosome to both daughter and son but father can't pass X linked genes to son only to daughter
Why are males not considered homozygous or heterozygous in terms of X linked genes?
because they only have one X chromosome with one locus the term hemizygous is used
Why do more males than females have X linked disorders?
because any male receiving the recessive allele from his mother will express the trait since the male only has one X chromosome
Duchenne muscular dystrophy
a genetic disease caused by a sex linked recessive allele that codes for the absence of dystrophin a key muscle protein; characterized by progressive weakening and a loss of muscle tissue
a genetic disease caused by a sex linked recessive allele resulting in the absence of one or more blood clotting proteins; characterized by progressive bleeding following injury; most common among the kids of Queen Victoria of England for constant intermarriage with royal family
the inactive X chromosome in each cell of a female condenses into a compact object and lies along the inside of the nuclear envelope
When does most of one X chromosome in a female become inactivated?
during early embryonic development
How do the cells of females and males have the same effective dose (one copy) of most X linked genes?
because the one X chromosome in a female becomes inactivated
What is involved in the inactivation of an X chromosome?
modification of the DNA and the histone proteins bount to it, including attachment of methy groups (--CH3) to one of the nitrogenous bases of DNA nucleotides
What does XIST (X-inactive specific transcript) do?
a gene that becomes active only on the chromosome that will become the barr body; multiple copies of RNA product of this gene attach to X chromosome eventually almost covering it
genes located near each other on the same chromosome tend to be inherited together
What's the difference between sex linked genes and linked genes?
sex linked genes are referring to a single gene on a sex chromosome and linked genes is referring to two or more genes on the same chromosome that are inherited together
the production of offspring with combinations of traits that differ from those found in either parent
an offspring with a phenotype that matches one of the true breeding parental (P generation) phenotypes
Recombinant/ recombinant type
an offspring whose phenotype differs from that of the true breeding P generation parents
What is said when 50% of all offspring are recombinants?
that there is a 50% frequency of recombination
A 50% frequency of recombination in such testcrosses is observed for what?
for any two genes that are located on different chromosomes and thus can't be linked
What are the physical basis of recombination between unlinked genes?
the random orientation of homologous chromosomes at metaphase I of meiosis, which leads to the independent assortment of the two unlinked genes
What was the main concept Morgan saw after doing the fruit fly test cross?
that the genes were on the same chromosome since the occurrence of parental types with a frequency greater than 50% indicates that the genes are linked
the recipricol exchange of genetic material between non-sister chromatids during prophase I of meiosis; accounts for the recombination of linked genes
Recombinant chromosomes are the result from what?
an ordered list of the genetic loci along a particular chromosome
What did Sturtevant hypothesize?
that the recombination frequency calculated from experiments like the fruit fly depends on the distance between genes on a chormosome
What assumptions did Sturtevant make?
assumed that crossing over is random, with the chance of crossing over approximately equal at all points along a chromosome
What did Sturtevant predict about the recombination frequency in regards to two genes?
he predicted that the farther apart two genes are, the higher the probability that a crossover will occur between them and therefore the higher the recombination frequency
a genetic map based on recombination frequencies between markers during crossing over of homologous chromosomes
a unit of measurement of the distance between genes; 1 map unit is equivalent to a recombination frequency of 1%
locate genes with respect to chromosomal features that can be seen in the microscope
The phenotype of an organism can be affected by small-scale or large-scale chromosomal changes?
small scale and large scale both have an affect on the organisms phenotype
What do large scale alterations in humans and other mammals often lead to?
spontaneous abortion of a fetus and individuals born with these types of genetic defects commonly exhibit various developmental disorders
What can damage chromsomes in major ways or alter their number in a cell?
physical and chemical disturbances as well as errors during meiosis
What can tolerate such genetic defects better than animals?
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
a chromosomal aberration in which one or more chromosomes are present in extra copies or are deficient in number
referring to a diploid cell that has only one copy of a particular chromosome instead of the normal two
referring to a diploid cell that has three copies of a particular chromosome instead of the normal two
What occurs with the cell that has wrong number of chromosomes?
mitosis will transmit that cell to all embryonic cells and if the organism survives, it usually has a set of traits caused by the abnormal dose of the genes associated with the extra or missing chromosome
a chromosomal alteration in which the organism possesses more than two complete chromosome sets; it's the result of an accident of cell division
indicates 3 sets of chromosomes
indicates 4 sets of chromosomes
How can a triploid cell occur?
by the fertilization of an abnormal diploid egg produced by nondisjunction of all it's chromosome
How can tetraploidy occur?
from the failure of 2n zygote to divide after replicating its chromosomes then normal mitotic division would then produce a 4n embryo
Polyploidy is common amongst what species?
fairly common amongst the plant kingdom; it plays an important role in the evolution of plants (ex: triploid- bananas, hexaploid-wheat, and octoploid-berries)
Why are polyploids mroe nearly normal in appearance than aneuploids?
because one extra or missing chromosome apparently disrupts genetic balance more than does an entire extra set of chromosomes
Errors in meiosis or damaging agents like radiation can cause breakage of a chromosome that can lead to what four types of changes in chromosome structure?
deletion, duplication, inversion, and translocation
a deficiency in a chromosome resulting from the loss of a fragment through breakage
In terms of deletion, what happens if the centromere is deleted?
the entire chromosome will be lost
a deleted fragment can attach to a sister chromatid or a nonsister chromatid of a homologous pair
a chromosomal fragment may reattach to the original chromosome but in the reverse orientation
the fragment of a chromosomal breakage joins to a nonhomologous chromosome
Which to types of chromosomal breakage are likely to occur during meiosis?
deletion and duplication
Is there an effect on an organism with reciprocal and translocation?
yes- the balance of genes between the two chromosomes may be normal but because they're switched up it can alter a phenotype since the gene's expression can be influenced by its location among neighboring genes
a human genetic disease aka trisomy 21 known for having a total of 3 chromosomes at 21 instead of 2; characteristic features such as facial features, short stature, correctable heart defects and developmental delays
What mostly leads to down syndrome?
mostly results from nondisjunction during meiosis I and some research think it has to do with an age-dependent abnormality in a meiosis checkpoint that normally delays anaphase until all the kinetochores are attached to the spindle
Why does aneuploid conditions seem to cause less upset to the genetic balance involving autosomes?
because the Y chromosome carries few genes and cause extra copies of the X chormosome become inactivated as Barr bodies in somatic cells
when a male has an extra X chromosome (XXY); the male has sex organs but the testes are abnormally small and is sterile; even though extra X is inactivated some breast enlargement and other female body characteristics are common
female has XXX but is healthy and have no unusual physical effects other than slightly taller than usual andn learning disabilities
Monosomy X aka turner syndrome
only known viable monosomy in humans; females are sterile cause their sex organs do not mature
What causes cri du chat (cry of the cat) syndrome?
results from a deletion in chromosome 5 causing the child to usually die in infancy or early childhood and is severely intellectually disabled, has a small head with unusual facial features, and has a cry that sounds like a cat meow
What causes chronic myelogenous leukemia (CML)?
occurs when a reciprocal translocation happens during mitosis of cells that will become WBCs
What causes philadelphia chromosome?
the exchange of a large portion of chromosome 22 with a small fragment of chromosome 9 produces a shortened 22; causes cancer by activating a gene that leads to uncontrolled cell cycle progression
a variation in phenotype depending on whether an allele is inherited from the male or female parent
Where does genomic imprinting occur?
during gamete formation
A zygote can express how many alleles of an imprinted gene?
can only express one imprinted allele that was inherited from either mom or dad
What occurs in each generation in regards to imprinting?
the old imprints are erased in gamete producing cells and chromosomes of the developing gametes are newly imprinted according to the sex of the individual forming the gametes
Methylation can do what to a given gene?
it can activate or inactivate the gene
Normal development requires how many active copies of genes?
one active copy