ch 15 chromosomal basis of inheritance Flashcards
the chromosome theory of inheritance
mendelian genes have specific loci on chromosomes
it is the chromosomes that undergo segregation and independent assortment
law of segregation
two alleles for each gene separate during gamete formation
law of independent assortment
alleles of genes on non homologous chromosomes assort independently during gamete formation
law of segregation
two alleles for each gene separate during gamete formation
an organism has two copies of the same gene (Aa) different versions of genes are called alleles
parent gives one allele for a gene to each gamete they produce
law of independent assortment
alleles of genes on nonhomologous chromosomes assort independently during gamete formation
chromosomes with genes are randomly paired and divided into gametes in meiosis and crossing over also allows for alleles of genes to be assorted separately
thomas hunt morgan
provided first solid evidence associating a specific genes with a specific chromosome
morgan’s experiment : general
with fruit flies
provided evidence that chromosomes are the location of mendel’s heritable factors
why did morgan work with fruit flies
they breed at a high rate
a generation can breed every two weeks
they have only four pairs of chromosomes
what did morgan first obbsere
wild type (normal) phenotypes that were common in the fly populations
traits alternative to the wild type were mutant phenotypes
in this case red vs white eyes
what did morgan see when mating male flies with mutant eyes with female flies with wild type eyes
the f1 generation all had red eyes
the f2 generation showed a 3:1 ration of red: white but only males had white eyes
what did the fact that only male flies had white eyes indicate
that the white-eye mutant allele must be located on a sex chrosome, specifically the X chromosome
in fruit flies males only have 1 X so eye color is determined by alleles they inherit on x chromosome from their mother
why is it that bc only male fruit flies had white eyes that the allele for white eyes is on the x chromosome
alleles tend to be on X because the Y chromosome is so small it would be hard to create a visible phenotype
morgan’s discovery
transmission of the X chromosome in fruit flies correlates with inheritance of the eye-color trait
specific gene associated with specific chromosomes
linked genes
genes located on the same chromosome that tend to be inherited together (do not sort independently)
they tend to be inherited together because they are located near each other on the same chromosome
how did morgan experiment with fruit flies to see how linkage affects inheritance of two characters
analyzing which chromosomes genes are on
using law of independent assortment
how do u know if genes are linked
morgan crossed flies that differed in traits of two different characters to determine if traits were on the same chromosome
ex: wild type body and wings vs black body and vestigial wings
he then did a test cross with heterozygous dihybrid females with double mutant males to produce f2
he then compared results with results that were likely to occur if genes were on the same chromosome, and compared
what did morgan conclude
if the two genes were on different chromosomes the alleles from f1 dihybrid would sort into gametes independently and we would see equal numbers of the four types of offspring
if the two genes were on the same chromosomes we would expect each allele combination to stay together as gametes formed and only offspring with parental phenotypes would be formed
since most offspring had the parental phenotype he concluded that the genes were on the same chromosome, but there were occasional breaks in linkage between genes that caused the number of offspring with non parental phenotypes
recombination
some offspring having combinations of traits that do not match either parent in the P generation
crossing over
what explains the occasional breaks in physical connection between genes on the same chrosome, that Mendel and Morgan saw
linked genes exhibit recombination frequencies of how much
less than 50%
genetic map
an ordered list of the genetic loci along a particular chromosome
can be developed using recombination frequencies, which indicate relative distance of genes on chromosomes
humans and mammals sex determination
two varieties of sex chromosomes
X and Y
other sex determination systems
X-0 system (XX and X)
Z-W system (ZW and ZZ)
haploid diploid system ( diploid and haploid)
sex chromosomes genes
have many genes for characters unrelated to sex
X chronometer has more unrelated genes
gene located on either sex chromosome
sex-linked gene
patterns of inheritance in sex linked genes
homozygote mother and father with disorder
transmits the mutant allele to all daughters but to no sons
when mother is dominant homozygote, daughters will just be carriers
patterns of inheritance in sex linked genes
carrier female mates with male of normal phenotype
50% change that each daughter will be a carrier and a 50% chance that each son will have the disorder
if a carrier mates with a male who has the disorder
50% chance that each child born will have the disorder
disorders caused by the recessive alleles on the X chromosomes in humans
color blindness
duchenne muscular dystrophy
hemophilia
x inactivation in females
one of the two X chromosomes in each cell is randomly inactivated during embryonic development
the inactive X in each cell condenses into a compact object called a Barr body
what happens if a female is heterozygous for a particular gene located on the X chromosome
she will be a mosaic for that character
what can alterations of chromosome number or structure cause
some genetic disorders
miscarriages or developmental disorders
nondisjunction
pairs of homologous chromosomes do not separate normally during meiosis
as a result one gamete receives two of the same type of chrosomoen and another receives no copy
3:1
aneuploidy
results from fertilization of gametes in which nondisjunction occurs
offspring with this condition have an abnormal number of a particular chromosome
trisomic zygote
has three copies of a particular chromosome
monosomic zygote
only one copy of a particular chromosome
polypoidy
organisms have more than two complete sets of chromosomes
triploidy (3n) or tetrapoloidy(4n)
polyploidy vs anueploidy
aneuploidy refers to a numerical change in part of the chromosome set, whereas polyploidy refers to a numerical change in the whole set of chromosomes
breakage do a chromosome can lead to four types of changes in chromosome structure
deletion
duplication
inversion
translocation
deletion
removes a chromosomal segment
duplications
repeats a chromosomal segment
inversion
reverses a segment within a chromosome
ABCDEF to FEBCBA
translocation
moves a segment from one chromosome to another, nonhomolofous one
human disorders due to chromosomal alterations
down syndrome is an aneuploidy condition that results from three copies of chromosome 21
nondisjunction of sex chromosomes causes klinfelter syndrome resulting in an extra chromosome in a male producing XXY
male sex organs
turner syndrome
monosomy X
produces X0 females
some inheritance patterns that are exceptions to the normal chromosome theory
genes located in the nucleus and genes located outside nucleus
