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.

• Red and green pigments are made by proteins on the X chromosome.
• Mutations in the genes for these pigments result in inability to see those colors.
• These mutations are recessive because one good copy of the gene is sufficient for color vision.
• Effect men much more frequently than women. Because women have two X chromosomes, they can be heterozygous (have one mutant allele) but still have normal color vision (0.5% of women are color blind).
• Men have only one X chromosome, so if they have one mutant allele, they will be color blind (8% are color-blind).

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.

• Affected the royal families of England and Russia prior to World War I. image
• The sex-linked forms are much more common because men only have one gene on the X chromosome and there is no allele on the Y chromosome.

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

• Down syndrome is an example of a trisomic condition in which the individual is born with an extra copy of chromosome 21. Occurs in about 0.1% of all live births, most caused by trisomy 21 in the egg, but about 10% are in sperm. Fig 15.14
• This condition results in small oval head, lower than normal IQ, short stature, reduced life span, and infertility in males.
• Frequency of non-disjunction (and Down syndrome) increases with age of the mother. graph

Nondisjunction Involving the Sex Chromosomes

• Aneuploidy can occur with the sex chromosomes, resulting in a number of abnormal conditions.
• Turner's (monosomy X) (sterile females with XO) and Klinefelter's (sterile males with XXY) syndromes. Usually have a lower than normal IQ and abnormal sexual characteristics.
• Can also have nondisjunction of the Y chromosomes which will result in XYY which are normal males, but the number of males with XYY is 20 times higher in prisons and mental institutions than in the normal populace. image

Alteration of chromosome structure. Fig15.13

deletion

 

duplication

 

inversion

 

translocation

 

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.

 

Disorders:

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

mitochondria

plastids, including chloroplasts

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