October 3, 2012
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 Mendelʼs laws of segregation and independent
assortment until evidence had mounted that they had a physical basis in the behavior
•Mendelʼs hereditary factors are the genes located on chromosomes.
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 Mendelʼs 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 century.
• 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 breeding
- 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 white-eyed male with a red-eyed female, all of 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 ration in the F2
- Surprisingly, the white-eyed trait only appeared in males.
- All of the females and half the males had red eyes.
- Morgan concluded that a ﬂyʼs eye color was linked to its sex. Linkage:
- Linkage genes tend to be inherited together because they are located on the same
- 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 too
- Morgan observed this linkage and its deviations when he followed the inheritance of
characters for body colour and wing size
- The wild-type body colour is gray (b+) and the mutant black is (b)
- The wild-type wing size is normal (vg+) and the mutant has vestigial wings (vg)
- According to independent assortment, this should produce 4 phenotypes in a 1:1:1:1
- Surprisingly, Morgan observed a large number of wild-type and double mutant ﬂies
among the offspring.
- These phenotypes correspond to those of the parents.
- Independent assortment of chromosomes and crossing over produce genetic
- 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
- Mendelʼs 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 yellow-round parent (YYRR) crossed with a
green wrinkled seed parent (yyrr), all plants in the F1 have yellow rounded
- A cross between an F1 plants and a homozygous recessive plant produces four
- Half are be parental types, with phenotypes that match the original P parents,
either with yellow-round seeds or with green-wrinkled seeds.
- Half are recombinant, new combination of parental traits, with yellow-wrinkled or
- A 50% frequency of recombination is observed for any two genes located on different
- 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 colour 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 parent carries the y and r alleles.
- The results of Morganʼs test cross for body colour and wing shape did not
conform to either independent assortment or complete linkage.
- Under independent assortment, the test cross should produce a 1:1:1:1
- If completely linked we should expect to see a 1:1:0:0 ratio with only
parental phenotypes among offspring.
- Most of the offspring had parental phenotypes, suggesting linkage between the genes.
- However, 10% of the ﬂies were recombinants suggesting incomplete linkage.
- Geneticists can use recombination data to map a chromosomes genetic loci (location)
- One of Morganʼs students, Alfred Sturtevant, used crossing over of linked genes to
develop a method for constructing a chromosome map.
- This map is an ordered list of the genetic loci along a particular chromosome.
- Sturtevant hypothesized that the frequency of recombinant offspring reﬂected the
distances between genes on a chromosome.
- The farther apart the two genes are, the higher the probability that a crossover will
occur between them and therefore, a higher recombination frequency.
- The greater the distance between two genes, the more point between them
where crossing over can occur.
- Sturtevant used recombination frequencies from fruit ﬂy crosses to map the relative
position of genes along chromosomes, a linkage map.