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
