Chromosomal Patterns of Inheritance The Next Step – Relating Mendel to Genes

Chromosomal Chromosomal Patterns of Patterns of InheritanceInheritance

The Next Step – Relating The Next Step – Relating Mendel to GenesMendel to Genes

Biology finally caught up with Gregor Mendel.Biology finally caught up with Gregor Mendel. Not until 1900Not until 1900 Independently, Karl Correns, Erich von Independently, Karl Correns, Erich von

Tschermak, and Hugo de Vries all found that Tschermak, and Hugo de Vries all found that Mendel had explained the same results 35 years Mendel had explained the same results 35 years before.before.

Resistance remained about Mendel’s laws of Resistance remained about Mendel’s laws of segregation and independent assortment segregation and independent assortment until evidence had mounted that they had a until evidence had mounted that they had a physical basis in the behavior of physical basis in the behavior of chromosomes.chromosomes.

Mendel’s hereditary factors are the genes Mendel’s hereditary factors are the genes located on chromosomes.located on chromosomes.

First Steps after MendelFirst Steps after Mendel

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Parallels between the behavior of Parallels between the behavior of chromosomes and the behavior of chromosomes and the behavior of Mendel’s factors.Mendel’s factors. Chromosomes present in pairs in diploid Chromosomes present in pairs in diploid

cells.cells. Homologous chromosomes separate and Homologous chromosomes separate and

alleles segregate during meiosis.alleles segregate during meiosis. Fertilization restores the paired condition Fertilization restores the paired condition

for both chromosomes and genes.for both chromosomes and genes.

The behavior of chromosomes The behavior of chromosomes during sexual life cyclesduring sexual life cycles


Around 1902, Walter Sutton, Theodor Around 1902, Walter Sutton, Theodor Boveri, and others noted these parallels Boveri, and others noted these parallels and a and a chromosome theory of chromosome theory of inheritanceinheritance began to take form. began to take form.


Fig. 15.1

Thomas Hunt Morgan was the first to Thomas Hunt Morgan was the first to associate a specific gene with a specific associate a specific gene with a specific chromosome.chromosome.

Experimental animal, Experimental animal, Drosophila Drosophila melanogastermelanogaster, a fruit fly species that eats , a fruit fly species that eats fungi on fruit.fungi on fruit. prolific breeders prolific breeders generation time of two weeks.generation time of two weeks. Fruit flies have three pairs of autosomes and a Fruit flies have three pairs of autosomes and a

pair of sex chromosomes (XX in females, XY in pair of sex chromosomes (XX in females, XY in males).males).

Morgan traced a gene to a Morgan traced a gene to a specific chromosomespecific chromosome


Morgan spent a year looking for Morgan spent a year looking for variant individuals among the flies he variant individuals among the flies he was breeding.was breeding. He discovered a single male fly with He discovered a single male fly with

white eyes instead of the usual red.white eyes instead of the usual red. The normal character phenotype is The normal character phenotype is

the the wild typewild type.. Alternative Alternative

traits are traits are mutant mutant phenotypesphenotypes. .


Fig. 15.2

When Morgan crossed his white-eyed When Morgan crossed his white-eyed male with a red-eyed female, all the male with a red-eyed female, all the FF11 offspring had red eyes, offspring had red eyes, The red allele appeared dominant to the The red allele appeared dominant to the

white allele.white allele. Crosses between the FCrosses between the F11 offspring offspring

produced the classic 3:1 phenotypic produced the classic 3:1 phenotypic ratio in the Fratio in the F22 offspring. offspring.

Surprisingly, the white-eyed trait Surprisingly, the white-eyed trait appeared only in males.appeared only in males. All the females and half the males had All the females and half the males had

red eyes.red eyes. Morgan concluded that a fly’s eye Morgan concluded that a fly’s eye

color was linked to its sex. color was linked to its sex. Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Morgan deduced that Morgan deduced that the gene with the the gene with the white-eyed mutation is white-eyed mutation is on the X chromosome on the X chromosome alone, a sex-linked alone, a sex-linked gene.gene. Females (XX) may have Females (XX) may have

two red-eyed alleles and two red-eyed alleles and have red eyes or may be have red eyes or may be heterozygous and have heterozygous and have red eyes.red eyes.

Males (XY) have only a Males (XY) have only a single allele and will be single allele and will be red eyed if they have a red eyed if they have a red-eyed allele or white-red-eyed allele or white-eyed if they have a eyed if they have a white-eyed allele.white-eyed allele.


Fig. 15.3

Each chromosome has hundreds or Each chromosome has hundreds or thousands of genes.thousands of genes.

Genes located on the same chromosome, Genes located on the same chromosome, linked geneslinked genes, tend to be inherited together , tend to be inherited together because the chromosome is passed along as because the chromosome is passed along as a unit.a unit.

Results of crosses with linked genes deviate Results of crosses with linked genes deviate from those expected according to from those expected according to independentindependent assortment.assortment.

Linked genes tend to be inherited Linked genes tend to be inherited together because they are located together because they are located

on the same chromosomeon the same chromosome


Morgan observed this linkage and its Morgan observed this linkage and its deviations size.deviations size. The wild-type body color is gray (bThe wild-type body color is gray (b++) and ) and

the mutant black (b).the mutant black (b). The wild-type wing size is normal (vgThe wild-type wing size is normal (vg++) )

and the mutant has vestigial wings (vg).and the mutant has vestigial wings (vg).

Morgan crossed FMorgan crossed F11 heterozygous heterozygous females (bfemales (b++bvgbvg++vg) with homozygous vg) with homozygous recessive males (bbvgvg).recessive males (bbvgvg).



Fig. 15.4

Morgan reasoned that body color and wing shape Morgan reasoned that body color and wing shape are usually inherited together because their genes are usually inherited together because their genes are on the same chromosome.are on the same chromosome.


• The other two phenotypes (gray-vestigial and black-normal) were fewer than expected from independent assortment (and totally unexpected from dependent assortment).

• These new phenotypic variations must be the result of crossing over.

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Chromosomal Patterns of Inheritance The Next Step – Relating Mendel to Genes