PRINCIPLES OF INHERITANCE

BYVERONICA PREMANAND

MENDALIAN LAWS

• There are three laws of mendal's:1)Law of Segregation - During gamete formation, the alleles for each gene segregate from each other so that each gamete carries only one allele for each gene.2)Law of Independent Assortment - Genes for different traits can segregate independently during the formation of gametes.3) Law of Dominance - Some alleles are dominant while others are recessive; an organism with at least one dominant allele will display the effect of the dominant allele.

CO-DOMINANCE• the phenotypes produced by both alleles are clearly expressed. For

example, in certain varieties of chicken, the allele for black feathers is codominant with the allele for white feathers. Heterozygous chickens have a color described as "erminette," speckled with black and white feathers. Unlike the blending of red and white colors in heterozygous four o'clocks, black and white colors appear separately in chickens. Many human genes, including one for a protein that controls cholesterol levels in the blood, show codominance, too. People with the heterozygous form of this gene produce two different forms of the protein, each with a different effect on cholesterol levels.

INCOMPLETE DOMINANCE• some characteristics, the F1 hybrids have an appearance in between the

phenotypes of the two parental varieties. A cross between two four o'clock (mirabilis jalapa) plants shows this common exception to Mendel's principles. Some alleles are neither dominant nor recessive. The F1 generation produced by a cross between red-flowered (RR) and white flowered (WW) Mirabilis jalapa plants consists of pink-colored flowers (RW). Which allele is dominant in this case? Neither one. This third phenotype results from flowers of the heterzygote having less red pigment than the red homozygotes. Cases in which one allele is not completely dominant over another are called incomplete dominance. In incomplete dominance, the heterozygous phenotype lies somewhere between the two homozygous phenotypes.

MULTIPLE ALLELES• In Mendelian inheritance, genes have only two alleles, such as a

and A. In nature, such genes exist in several different forms and are therefore said to have multiple alleles. A gene with more than two alleles is said to have multiple alleles. An individual, of course, usually has only two copies of each gene, but many different alleles are often found within a population. One of the best-known examples is coat color in rabbits. A rabbit's coat color is determined by a single gene that has at least four different alleles. The four known alleles display a pattern of simple dominance that can produce four coat colors. Many other genes have multiple alleles, including the human genes for ABO blood group.

Gene Interactions

• Between 1884 (the year Mendel died) and 1888 details of mitosis and meiosis were reported, the cell nucleus was identified as the location of the genetic material, and "qualities" were even proposed to be transmitted on chromosomes to daughter cells at mitosis. In 1903 Walter Sutton and Theodore Boveri formally proposed that chromosomes contain the genes. The chromosomal theory of inheritance. is one of the foundations of genetics and explains the physical reality of Mendel's principles of inheritance

Gene interaction• The location of many genes (Mendel's factors) was determined by Thomas

Hunt Morgan and his coworkers in the early 1900's. Morgan's experimental organism was the fruit fly (Drosophila melanogaster). Fruit flies are ideal organisms for genetics, having a small size, ease of care, susceptibility to mutations, and short (7-9 day) generation time. The role of chromosomes in determination of sex was deduced by Morgan from work on fruit flies.

• During Metaphase I, homologous chromosomes will line up. A karyotype can be made by cutting and arranging photomicrographs of the homologous chromosomes thus revealed at Metaphase I. Two types of chromosome pairs occur.

Gene interaction Autosomes resemble each other in size and placement of the centromere, for example pairs of chromosome 21 are the same size, while pairs of chromosome 9 are of a different size from pair 21. Sex chromosomes may differ in their size, depending on the species of the organism they are from. In humans andDrosophila, males have a smaller sex chromosome, termed the Y, and a larger one, termed the X. Males are thus XY, and are termed heterogametic. Females are XX, and are termed homogametic. In grasshoppers, which Sutton studied in discovering chromosomes, there is no Y, only the X chromosome in males. Females are XX, while males are denoted as XO. Other organisms (notably birds, moths and butterflies) have males homogametic and females heterogametic. Males (if heterogametic) contribute either an X or Y to the offspring, while females contribute either X. The male thus determines the sex of the offspring. Remember that in meiosis, each chromosome is replicated and one copy sent to each gamete.

Gene interaction• Morgan discovered a mutant eye color and attempted to use this mutant

as a recessive to duplicate Mendel's results. He failed, instead of achieving a 3:1 F2 ratio the ratio was closer to 4:1 (red to white). Most mutations are usually recessive, thus the appearance of the white mutant presented Morgan a chance to test Mendel's ratios on animals. The F1 generation also had no white eyed females. Morgan hypothesized that the gene for eye color was only on the X chromosome, specifically in that region of the X that had no corresponding region on the Y. White eyed fruit flies were also more likely to die prior to adulthood, thus explaining the altered ratios. Normally eyes are red, but a variant (white) eyed was detected and used in genetic study. Cross a homozygous white eyed male with a homozygous red eyed female, and all the offspring have red eyes. Red is dominant over white. However, cross a homozygous white eyed female with a red eyed male, and the unexpected results show all the males have white eyes and all the females red eyes. This can be explained if the eye color gene is on the X chromosome.

Gene interaction

Extra Chromosomal Inheritance

• Inheritance of traits through DNA that is not connected with the chromosomes but rather to DNA from organelles in the cell. Also called cytoplasmic inheritance.

• In prokaryotes, nonviral extrachromosomal DNA is primarily found in plasmids whereas in eukaryotes extrachromosomal DNA is primarily found in organalles. Mitochondrial DNA is a main source of this extrachromosomal DNA in eukaryotes. Extrachromosomal DNA is often used in research of replication because it is easy to identify and isolate.

Extra chromosomal inheritance

• Extrachromosomal DNA was found to be structurally different from nuclear DNA. Cytoplasmic DNA is less methylated than DNA found within the nucleus. It was also confirmed that the sequences of cytoplasmic DNA was different from nuclear DNA in the same organism, showing that cytoplasmic DNAs are not simply fragments of nuclear DNA.

• In eukaryotes it can be mitochondrial inheritance, chloroplast inheritance, extrachromosomal circular dna.

• One of the main example is kappa particle.