Mendelian inheritance has its physical basis

in the behavior of chromosomes

Chapter 15, Section 1

Overview

• Gregor Mendel’s “hereditary factors” were purely an abstract concept when he proposed their existence in 1860.• At the time, no cellular structures were known

that could house these imaginary units.

• Today, we know that genes- Mendel’s “factors”- are located along chromosomes.• We can see the location of a particular gene by

tagging chromosomes with a fluorescent dye that highlights that gene.

Cytology and Genetics

• Cytologists worked out the process of mitosis in 1975 and meiosis in the 1890s.

• Cytology and genetics converged when parallels could be seen between the behavior of chromosomes and the behavior of Mendel’s hereditary 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.

1. Chromosomes and genes in pairs

2. Segregation during meiosis

3. Fertilization restores pairs of genes and chromosomes

Chromosome Theory of Inheritance• Created in 1902 due to the

independent work done by Walter S. Sutton and Theodor Boveri.

• According to this theory, Mendelian genes have specific loci (positions) along chromosomes, and it is the chromosomes that undergo segregation and independent assortment.

Walter S. Sutton

Theodor Boveri

Morgan’s Experimental Evidence

• Thomas Hunt Morgan was an experimental embryologist at Columbia University.

• Morgan was initially skeptical about Mendelian genetics and the chromosomal theory, but his early experiments provided convincing evidence that chromosomes are indeed the location of Mendel’s heritable factors.

Morgan’s Choice of Experimental

Organism

• Morgan selected a species of fruit fly, Drosophila melanogaster, a common insect that feeds on the fungi growth of fruit.• Prolific breeders.

• Single mating produces hundreds of offspring.

• A new generation could be bred every two weeks.

• Only contains 4 chromosomes that are distinguishable under light microscope.

• Eye Color• The phenotype for a character most commonly in natural populations (wild type) in

Drosophila was red eyes.

• The mutant phenotype in Drosophila was white eyes.

Fruit Fly Gene Notation

• For a given character in flies, the gene takes its symbol from the first mutant (non-wild type) discovered.

• Eye Color• Wild type (red): w+

• Mutant (white): w

w+ w

Correlating Behavior of a Gene’s Alleles with

Behavior of a Chromosome Pair

• Morgan mated a white-eyed male fly with a red-eyed female. All the F1 offspring had red eyes.

All offspringhad red eyes.

PGeneration

F1

Generation

F2

Generation

RESULTS

EXPERIMENT

• When Morgan bred the F1 flies to each other, he observed the classical 3:1 phenotypic ratio among the F2 offspring.• The white-eye trait only showed up in males.

• All the F2 females had red eyes.

• Morgan concluded that a fly’s eye color was linked to its sex.

• The correlation between the trait of white eye color and the male sex of the affected F2 flies suggested to Morgan that the gene involved in his white-eyes mutant was located on the X chromosome.• For a male, a single copy of the

mutant allele would result in white eyes, since a male only has one X chromosome.

• A female could have white eyes only if both her X chromosomes carried the recessive mutant allele.

Why is Morgan’s work important?

• Morgan’s findings provided support for the chromosomal theory of inheritance: namely, that a specific gene is carried on a specific chromosome.

• Morgan’s work indicated that genes located on a sex chromosome exhibit unique inheritance patterns.

Recognizing the importance of Morgan’s early work, many bright students were

attracted to his fly room.

Sex-linked genes exhibit unique patterns

of inheritanceChapter 15, Section 2

The Chromosomal Basis of Sex

• Sex Chromosomes in Animals: X and Y (much smaller than the X chromosome).

• Sex Chromosome Inheritance• XX (female)

• XY (male)

• Short segments at either end of the Y chromosome are the only regions homologous to the X chromosome (allows for pairing during meiosis).

• Gametes• Each egg receives on X chromosome.

• Half the sperm cells a male produces receive an X chromosome, and half receive the Y chromosome.

X

Y

Sex-linked genes: a gene located on either sex chromosome.

• In 1990, a British research team identified a gene on the Y chromosome required for the development of testes.• SRY (sex-determining region of Y) gene.

• In the absence of the SRY gene, the gonads develop into ovaries.

• Out of the 78 genes on the Y chromosome, more are expressed only in the testis, for they are required for normal testicular functioning and the production of normal sperm.• Because there are so few Y-linked genes, very few disorders are transferred from

father to son on the Y chromosome.

• The human X chromosome contains approximately 1100 genes. These genes can code for characters unrelated to gender.

Inheritance of X-linked Genes

• Fathers pass X-linked alleles to all of their daughters but none to their sons.

• Mothers can pass X-linked alleles to both sons and daughters.

• Expression of X-linked Traits• If an X-linked trait is due to a recessive allele, a female will express the

phenotype only if she is homozygous for that allele.

• Any male receiving the recessive allele from his mother will express the trait.

• More males than females have X-linked recessive disorders.

Practice Problem 1

Vermilion eyes is a sex-linked recessive characteristic in fruit flies. If a female having vermilion eyes is crossed with a wild-type male, what percentage of the F1 males will have vermilion eyes?

Practice Problem 2

In birds, sex is determined by a ZW chromosome scheme. Males are ZZ and females are ZW. A lethal recessive allele that causes death of the embryo occurs on the Z chromosome in pigeons. What would be the sex ratio in the offspring of a cross between a male heterozygous for the lethal allele and a normal female?

Sex-linked Genetic DiseasesDuchenne Muscular

Dystrophy• Disease is characterized by a

progressive weakening of the muscles and loss of coordination.

• Researchers have traced the disorder to the absence of a key muscle protein that is encoded in the X chromosome.

Hemophilia • X-linked recessive disorder

defines by the absence of one or more of the proteins required for blood clotting.

• People with hemophilia are treated with intravenous injections of the missing protein.