4.3 Flashcards
(21 cards)
dihybrid cross
a cross between two parents that differ by two pairs of alleles
heterozygous for two different traits
ex: RrYy x RrYy
dependent assortment
the transmission of one alleles depends on the transmission of another
the alleles could maintain the association they had in the parental generation (linked genes)
independent assortment
alleles of different genes are transmitted independently of each other
the segregation of R from r could be independent of the segregation of Y from y (unlinked genes)
dependent assortment results
F1 RrYy would produce 2 types of gametes (RY, ry)
F2 offspring would have 2 phenotypes: round yellow and wrinkled green in a 3:1 ratio
independent assortment results
F1 RrYy would produce 4 types of gametes (RY, rY, Ry, ry)
in the F2 offspring, there should be 9 genotypes and 4 phenotypes in a 9:3:3:1 ratio
Mendel’s Second Law
independent assortment
alleles of different genes are transmitted independently of one another during gamete formation
the segregation of the R and r alleles is independent of the segregation of the Y and y alleles
the nine F2 genotypes result in 4 phenotypes in a ratio of 9:3:3:1
independent assortment + meiosis
when homologs line up on either side of the metaphase plate during metaphase I, where R and r go does not determine where Y and y go
four different genotypes in equal proportions
dihybrid test cross
parents of known dominant phenotype but unknown genotype
cross with homozygous recessive genotype
fruit flies
small size
ease of breeding
short generation time
abundant offspring
test cross of Drosophila (fruit flies)
test cross for the heterozygote Drosophila for body color and wing size did not yield expected ratio according to the law of independent assortment
gene linkage, Morgan’s first proposal
the tendency of genes to be inherited together because they are located on the same chromosome
if linkage were absolute:
-linked genes do not assort independently
-all gametes will show parental types
Morgan’s second proposal
first proposal rejected due to presence of parental types
Morgan proposed that genes on the same chromosome may recombine to produce recombinant gametes
Morgan referred to novel phenotypes as recombinant phenotypes
when crossing over takes place between two linked genes
-not all gametes will be parental phenotypes
recombination
accounts for why linked alleles are not always inherited together
by crossing over:
genes on the same chromosome can recombine
linked alleles can be separated from one another
two recombinant chromosomes and two non-recombinant chromosomes can be formed
crossing over
the exchange of genetic material by crossing-over results in new combinations of genetic information on the recombinant chromosomes
chiasmata: regions of attachment that form between nonsister chromatids
one exchange event involves only two non-sister chromatids, one from each homologous chromosomes
increases genetic variability of the offspring
recombinant frequency
number of recombinant offspring divided by total number of offspring
greater for genes that are farther apart on the chromosome than genes that are closer together
genetic map
shows the relative positions of genes along a particular chromosome
the recombination frequencies in crosses involving two genes are converted to map units (1 cM = 0.01%)
the recombination frequency depends on the distance between genes
sex-linked inheritance
inheritance of a genes carried on a sex chromosome
in mammals, the X and Y chromosomes are not complete homologs of one another
the X chromosome is larger and carries more genes than the Y chromosome
-X-linked genes have genes for many characters unrelated to sex
-most Y-linked genes help determine sex
wild type
allele that is present in most individuals in nature
gives rise to an expected trait
mutation
are considered inherited changes in genes
different phenotypic effects
sex-linked
the gene is on the sex chromosome
hemizygous
gene is present as a single copy in a diploid organism
