This document discusses several principles of inheritance: 1) Mendel's laws of segregation, independent assortment, and dominance. 2) Codominance and incomplete dominance where both alleles are expressed in heterozygotes. 3) Multiple alleles where a single gene can have more than two forms. 4) Gene interactions and how Morgan's work with fruit flies demonstrated chromosomes contain genes and determine sex inheritance. 5) Extrachromosomal inheritance where traits are inherited through organelle DNA rather than chromosomes.

1. PRINCIPLES OF
INHERITANCE
BY
VERONICA PREMANAND

2. 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.

3. 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.

4. 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.

5. 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.

6. 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

7. 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.

8. 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.

9. 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.

10. Gene interaction

11. 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.

12. 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.