OCT 7: BIO
CHROMOSOMES AND INHERITANCE
• It was not until 1900 that biology finally caught up with Gregor Mendel.
• Independently, Karl Correns, Erich von Tschermak, and Hugo de Vries all found
that Mendel had explained the same results 35 years before.
• Still, resistance remained about Mendel’s laws of segregation and independent
assortment until evidence had mounted that they had a physical basis in the
behavior of chromosomes.
• Mendel’s hereditary factors are the genes located
MENDELIAN INHERITANCE HAS ITS PHYSICAL BASIS IN THE
BEHAVIOR OF CHROMOSOMES DURING SEXUAL LIFE CYCLES
• Around 1900, cytologists and geneticists began to see parallels between the
behavior of chromosomes and the behavior of Mendel’s factors.
– Chromosomes and genes are both present in pairs in diploid cells.
– Homologous chromosomes separate and alleles segregate during meiosis.
– Fertilization restores the paired condition for both
chromosomes and genes.
THOMAS HUNT MORGAN
• first to associate a specific gene with a specific chromosome in the early 20th
• Like Mendel, Morgan made an insightful choice as an experimental animal,
Drosophila melanogaster, a fruit fly species that eats fungi on fruit.
• small and easily reared in the laboratory.
• short life cycle. A new generation every two weeks.
• a female lays hundreds of fertilized eggs during her brief life span. The resulting
large populations make statistical analysis easy and reliable.
• giant ("polytene") chromosomes salivary glands of the mature larvae. • Morgan spent a year looking for variant individuals among the flies he was
– He discovered a single male fly with white eyes instead of the usual red.
• The normal character phenotype is the wild type.
￼ Alternative traits are mutant phenotypes.
• When Morgan crossed his whiteeyed male with
a redeyed female, all the F1 offspring had red eyes,
– The red allele appeared dominant to the white allele.
• Crosses between the F1 offspring produced the classic 3:1 phenotypic ratio in
the F2 offspring.
• Surprisingly, the whiteeyed trait appeared only in males.
– All the females and half the males had red eyes.
• Morgan concluded that a fly’s eye color was
linked to its sex.
LINKED GENES TEND TO BE INHERITED TOGETHER BECAUSE THEY
ARE LOCATED ON THE SAME CHROMOSOME
• Each chromosome has hundreds or thousands of genes.
• Genes located on the same chromosome, linked genes, tend to be inherited
together because the chromosome is passed along as a unit.
• Results of crosses with linked genes deviate from those expected according to
• Morgan observed this linkage and its deviations when he followed the
inheritance of characters for body color and wing size.
– The wildtype body color is gray (b+) and the mutant black (b).
– The wildtype wing size is normal (vg+) and the mutant has vestigial
• According to independent assortment, this should produce 4 phenotypes in a
• Surprisingly, Morgan observed a large number of wildtype (graynormal) and
doublemutant (blackvestigial) flies among the offspring. – These phenotypes correspond to those of the parents.
INDEPENDENT ASSORTMENT OF CHROMOSOMES AND CROSSING OVER
PRODUCE GENETIC RECOMBINANTS
• The production of offspring with new combinations of traits inherited from two
parents is genetic recombination.
• Genetic recombination can result from independent assortment of genes located on
non homologous chromosomes or from crossing over of genes located on
• Mendel’s dihybrid cross experiments produced some offspring that had a combination
of traits that did not match either parent in the P generation.
– If the P generation consists of a yellowround parent (YYRR) crossed with
a greenwrinkled seed parent (yyrr), all F1 plants have yellowround seeds
– A cross between an F1 plant and a homozygous recessive plant (a test
cross) produces four phenotypes.
– Half are be parental types, with phenotypes that match
the original P parents, either with yellowround seeds or
– Half are recombinants, new combination of parental
traits, with yellowwrinkled or greenround seeds.
• A 50% frequency of recombination is observed for any two genes located on
different (non homologous) chromosomes.
• The physical basis of recombination between unlinked genes is the random
orientation of homologous chromosomes at metaphase 1.
• In contrast, linked genes, genes located on the same chromosome, tend to move
together through meiosis and fertilization.
• Under normal Mendelian genetic rules, we would not expect linked genes to
recombine into assortments of alleles not found in the parents.
– If the seed color and seed coat genes were linked, we would expect the F1
offspring to produce only two types of gametes, YR and yr when the tetrads
– One homologous chromosome from a P generation parent carries
the Y and R alleles on the same chromosome and the other
homologous chromosome from the other P pare