Chromosomal Basis of Heredity

Through improved microscopy, scientists worked out:

mitosis in 1875
meiosis in 1890's

The Chromosomal Theory of Inheritance was proposed by Sutton in 1902

Observed that similar appearing chromosomes paired and then were pulled apart during meiosis.

Diploid cells have two copies of each heritable gene, and gametes each have one.

Segregation and independent assortment were observed.

Problems with the Chromosomal Theory

The observation that the number of characters that assort independently in an organism exceed the number of chromosome pairs the organism possesses made scientists wonder whether the chromosomal theory was correct.

T. H. Morgan was the first to associate a specific gene with a specific chromosome.

Morgan's White-Eyed Fly (Fig. 15.2)

Morgan chose fruit flies due to their short generation time, only had 4 chromosomes and they were easy to rear and harmless.
He searched for a mutant and finally found a mutant male with white eyes. The wild type was red eyes.
Morgan's 1910 studies of a white-eyed fruit fly showed that certain genes could be sex-linked. (Fig. 15.3)

Sex Linkage proves the chromosomal theory.

Male fruit flies have only one X chromosome while female fruit flies have two X chromosomes.

Eye color in this instance was sex-linked, which explained why males were white-eyed and females had red eyes.

Morgan's experiments were important because they illustrated that genes are carried on chromosomes, and Mendel's laws are true.
Humans have sex-linked traits also. Females have XX chromosomes and males have XY chromosomes. The Y chromosome carries very few genes.

Linked genes - Genes located on the same chromosome tend to be inherited together. Fig. 15.4

Genetic recombination

Recombination of unlinked genes: independent assorment of chromosomes
Recombination of linked genes: crossing over Fig. 15.5a and Fig. 15.5b

Genetic maps - linkage maps

Sturdevant hypothesized that the further apart genes are on a chromosome, the more likely that a crossover will occur between them. His work allowed him to determine relative position.
Testcross to determine recombination frequency
recombination frequency = # recombinants/total # of offspring X 100 = __%
Maximum value = 50%. Then cannot tell it from unlinked genes.Fig 15.6
Map units = 1 %
Genes are mapped by adding recombination frequencies. Fig 15.7

Some chromosomal systems of sex determination - fig 15.8

The transmission of sex-linked recessive traits - Fig 15.9

Sex-Linked Inheritance and disorders

Red-green color-blindness - Genes code for the proteins that make pigments in the eye necessary for absorbing the different-colored wavelengths in light.

Duchenne muscular dystrophy - affects 1 in 3500 men in the U.S. Rarely live past 20's.

Hemophilia, an autosomal or sex-linked recessive trait, results from mutations of genes encoding blood clotting proteins.

Inactivation of one X in female mammals

Barr bodies
mosaicism - Fig 15.10

Human Chromosomes

Humans have 23 pairs, or 46, chromosomes that vary by size, shape, and appearance. 22 pairs are called autosomes. The other pair are the sex-chromosomes.

Sometimes during meiosis, the homologous chromosomes or the sister chromatids do not separate properly, a mistake known as nondisjunction.
Fig. 15.11

This leads to aneuploidy, which means having an abnormal number of chromosomes.

Monosomics have only one of a pair of a particular set of chromosomes, and trisomics have three copies of a chromosome, rather than the normal two. Occurs in about 5% of human pregnancies, but are usually lethal.

Nondisjunction Involving Autosomes

Nondisjunction Involving the Sex Chromosomes

Alteration of chromosome structure. Fig15.13

These can cause severe physical and/or mental disorders.

Genomic imprinting Fig 15.15

In mammals, a small number of genes are expressed differently depending on whether they were inherited from the mother or the father. In the germ cells, this imprinting is "erased" and they are reimprinted according to the sex of this individual.
Prader-Willi syndrome - if inherited from the father
Angelman syndrome - if inherited from the mother
Same genetic deletion on chromosome 15, but symptoms very different.
Fragile X syndrome - where the X chromosome has one tip bent.

Extranuclear genes - inherited from mother

plastids, including chloroplasts
mutations in this DNA can also cause various disorders and are passed on to offspring.