Basic Genetics -

Basic Genetics -- Mendel (1866)
** Mendel developed some basic principles of heredity without any knowledge
of genes or chromosomes. His principles of dominance, segregation, and
independent assortment were established through the mathematical analysis of
large numbers of offspring produced by crossing pea plants.
** As a result of analyzing specific mathematical ratios associated with certain
characteristics in the offspring, Mendel proposed that characteristics were
inherited as the result of the transmission of hereditary factors.
** Mendel observed contrasting characters in pea plants in his work.
(Why was Mendel successful with the pea?)
1. Used pure breeding, contrasting varieties.
2. Studied characteristics one at a time for many generations.
3. Used statisitics in analyzing his results.
4. Obtained large numbers of offspring.
5. Chose pea plants which normally self-fertilize.
** Mendel had no knowledge of genes or chromosomes.
genes: factors that control organism traits -- the part of the DNA molecule
containing the genetic code
Every organism requires a set of coded instructions for specifying its traits. For
offspring to resemble their parents, their must be a reliable way to transfer
hereditary information from one generation to the next.
Each gene carries a separate piece of information. An inherited trait of an
individual can be determined by one or by many genes, and a single gene can
influence more than one cell.
Each human cell contains thousands of different genes.
alleles: different forms of a gene associated with a particular characteristic
(ex. height -- tallness -- shortness)
chromosomes: hereditary units of an organism
locus: particular point where a certain gene is found on a chromosome
homologous chromosomes: pair of associated chromosomes -- each
chromosome in the pair is called a homologue
** Alleles are located in the same position or locus on homologous
In asexually reproducing organisms, all the genes come from a single parent.
These genes are normally identical to the parent. (clone)
Sexually reproducing organisms normally receive half their genetic information
from the Mother's egg and half their genetic information from their Father's
sperm. Sexually reproduced offspring resemble but are not identical to their
** The significance of Mendel's work was not immediately recognized.
** Mendel's hereditary factors, now called genes, exist at definite loci in a
linear fashion on chromosomes. Two genes associated with a specific
characteristic are known as alleles and are located on homologous
chromosomes. The gene-chromosome theory provides the mechanism to
account for the hereditary patterns which Mendel observed.
homozygous (pure) -- an individual with two of the same alleles on homologous
heterozygous: (hybrid) -- an individual with contrasting alleles for the same trait
on homologous chromosomes
parental generation (P) -- the two original organisms being crossed
first filial generation -- the first generation of offspring from the parents
second filial generation -- generation of offspring arising from the first filial
Some Major Genetic Concepts
1. Law of Dominance (Mendel)
a pattern of heredity in which only one gene of an allelic pair is expressed
thus in the heterozygous condition, one allele of a gene may express itself
and mask the presence of the other allele
ex. red flowering X white flowering-->red flowering peas
dominant trait: the trait or allele that is expressed
recessive trait: the trait or allele that is present but that is not expressed
progeny: offspring
siblings: brothers or sisters
Punnett square: used to represent genetic crosses
genotype: the genetic makeup of an organism
phenotype: the appearance of an organism
2. Principle of Segregation and Recombination (Mendel)
-- When gametes are formed during meiosis there is a random segregation of
homologous chromosomes.
-- As a result of fertilization, alleles recombine. As a consequence, new allelic
gene combinations are likely to be produced.
** Segregation and recombination is illustrated by the cross between two
individuals heterozygous for a trait.
3. Law of Independent Assortment (Mendel)
--If the genes for two different traits are located on different chromosomes
(nonhomologous chromosomes), they segregate randomly during meiosis and,
therefore, may be inherited independently of each other.
The cross of two organisms heterozygous for a trait is known as a hybrid cross.
** Assuming large numbers of such crosses, the phenotypic ratio of dominant
offspring to recessive offspring is 3:1 and the genotypic ratio of homozygous
dominant offspring to heterozygous dominant offspring to homozygous recessive
offspring is1:2:1
[Intermediate Inheritance]
2 examples -- incomplete dominance and co-dominance
(a.) Incomplete Dominance
-- A case of contrasting alleles in which one allele is only partially dominant over
the other --the dominant allele is only partially expressed when the recessive
allele is present.
(Sometimes called blending inheritance)
examples: red X white snapdragons ----> pink snapdragons
This is also true in Japanese 4 o'clock flowers.
** A cross between long and round squashproducing oval offspring also
illustrates this concept.
(b.) Codominance: a case of contrasting alleles in which neither allele is
dominant over the other (alleles have equal power)
ex. cross between red and white short horned cattle gives roan cattle
ex. cross between black and white Andulusian fowl gives blue (gray) fowl
** Examples of codominance in humans are the blood groups and the
sickle cell trait. (sickle cell anemia)
gene linkage: when genes for two different traits (nonallelic genes) are located on
the same chromosome pair (homologous chromosomes)
Linked genes are usually inherited together.
crossing over: during synapsis in the first meiotic division, the chromatids in a
homologous pair of chromosomes often twist around each other, break,
exchange segments and rejoin.
** Crossing over results in the rearrangement of linked genes and increases the
variability of offspring.
Note in the picture above the four chromosomes make up a tetrad in synapsis.
The second stage shows crossing over, and the last stage shows the genetic
variability produced in the new gene combinations.
karyotype: an enlarged photograph of the chromosomes in an organism
Above is a karyotype of a normal human male.
Human diploid cells contain 23 pairs of chromosomes.
autosomes: body chromosomes (22 pr. in humans)
Homo sapiens has one pair of sex chromosomes.
** In the male each sex chromosome is unlike and is designated XY. In the
female each sex chromosome is alike and is designated XX.
** The sex of a human is genetically determined at fertilization when a sperm cell
containing either an X or a Y chromosome unites with an egg cell containing an X
Mutations: changes in genetic material
** While an altered gene (mutation) may be passed on to every cell that
develops from the mutated cell, only mutations in sex cells may be passed on to
the offspring.
** A mutation occurring only in body cells may be perpetuated in the individual
but will not be passed on to the offspring by sexual reproduction.
-- Mutations may be classified as chromosomal alterations or gene mutations.
-- The effects of chromosomal alterations are usually quite visible in the
of the organism because many genes are usually involved.
[Types of Chromosomal Alterations]
1. Nondisjunction: homologous chromosomes fail to separate during meiosis,
producing offspring with one chromosome more or less than is normal
disjunction -- the separation of homologous chromosomes during meiosis
** If disjunction fails to occur (nondisjunction), gametes with an addition or a
missing chromosome will be produced.
The presence of one or more entire additional sets of chromosomes in an
organism (3n,4n number, etc.)
Polyploidy is also caused by nondisjunction (But of an ENTIRE set of
Mutation: a random change in the chemical nature of the genetic material (DNA)
Most mutations are harmful and recessive.
While the effects of some gene mutations, such as albinism, are obvious,
the effects of other gene mutations may not be as readily noticed.
Mutagenic Agents -- increase the random incidence of mutations
1. Radiation -- X-rays, ultraviolet, radioactive substances, and cosmic rays
2. Chemicals -- formaldehyde, benzene, asbestos fibers, THC, nicotine
** The adaptive value of a gene mutation is dependent upon the nature of the
mutation and the type of environment with which the organism interacts.
** The environment interacts with genes in the development and expression of
inherited traits.
(3 examples)
1. Many green plants contain chlorophyll for food making but if these leaves are
screened from the light they stop making chlorophyll.
2. Identical twins raised in different environments may be vastly different in
3. If some white fur is shaved from a Himalayan rabbit and the area is kept cold
with an ice pack, black fur will grow back in its place. (The gene for black fur is
active only at very low temperatures.)
** The principles of genetics appear to hold true for all organisms including
The acquisition of knowledge of human genetics has been limited because
humans are not suitable subjects for experimentation. An increased knowledge
of human genetics has resulted from the expansion of the field of genetic
Types of Selective Breeding
1. Artificial Selection
-- individuals with desirable traits are mated to produce offspring with those traits
2. Inbreeding
-- offspring produced by artificial selection are mated with one another to
reinforce those desirable traits
3. Hybridization
-- crossing two individuals with different desirable traits to produce offspring with
a combination of both desirable traits
ex. English shorthorn cattle X Brahman cattle --->Santa Gertrudis cattle (good
beef) (heat resistance)
(good beef)
(heat resistant)
4. Mutations may be preserved by vegetative propagation. (ex. seedless
5. Recombinant DNA (genetic engineering) --- new varieties of plants and
animals can be created by manipulating the genetic instructions of these
organisms to produce new characteristics
[Changes in Chromosome Structure]
1. Translocation: transfer of one section of a chromosome to a nonhomologous
2. Addition: a portion of one chromosome is attached to another chromosome
3. Deletion: a portion of a chromosome is taken away from a chromosome
4. Inversion: a portion of a chromosome breaks off and then becomes reattached
to the same chromosome in an inverted (upside down) fashion
Knowledge of genetics is making possible new fields of health care. Mapping of
genetic instructions in cells makes it possible to detect, and perhaps correct,
defective genes that may lead to poor health.
Substances from genetically engineered organisms may reduce the cost and
side effects of replacing missing body chemicals.