Chapter 10
BASIC PRINCIPLES OF HEREDITY
Genetics is the branch of biology that studies the structure, expression and transmission of
hereditary traits.
Heredity is the transfer of genetic information from parent to offspring.
Gregor Mendel (1822 - 1884)
A monk who lived Brunn, now known as Brno, in the Czech Republic.
He conducted a series of experiments breeding garden peas (Pisum sativum) and published his
results in 1866.
He argued that parents pass to their offspring discrete inheritable factors.
Garden peas show a great deal of variation and are self-pollinating plants.
Mendel developed true-breeding lines for seven characteristics.
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True-breeding refers to a variety of organism, when self-fertilized, produces
offspring identical to one another and to the parent.
Flower color: purple and white.
Flower position: axial or terminal.
Seed color: yellow or green.
Seed shape: round or wrinkled
Pod shape: inflated or constricted.
Pod color: green or yellow.
Stem length: tall or dwarf.
Phenotype refers to the external appearance of the organism, e.g. Seed shape: round or
wrinkled.
Genotype refers to the genetic makeup of the organism.
Mendel's conclusions have been tested repeatedly by many scientists over the year and found
to be generally true.
The term allele refers to genes that govern variations of the same feature, e.g. yellow seed and
green seed are determined by two alleles of the same gene.
Expressed in modern terms, these conclusions are...
1. There are different forms of genes (alleles), the units that determine heritable traits.
2. For each inherited characteristic, an organism has two genes, inherited one from each
parent.
3. Alleles separate (segregate) during gamete formation.
4. When two alleles of a pair are different, one is usually expressed and the other has no
noticeable effect. The expressed allele is called dominant, and unexpressed allele is
called recessive.
5. Alleles assort independently of one another.
We know now that alleles occupy corresponding loci in homologous chromosomes.
A locus refers to the location of the gene in a chromosome (plural, loci).
Genes exist as pairs of alleles in diploid individuals.
MONOHYBRID CROSS
Monohybrid cross involves individuals with different alleles of a given locus.
Heterozygous individuals carry two different alleles of a locus.
Homozygous individuals carry identical alleles.
A Punnett square allows to predict the ration of genotypes and phenotypes of the offspring of a
cross.
A dominant allele can mask the expression of a recessive allele.
Individuals with the same appearance (phenotype) may differ in their genetic makeup
(genotype).
A test cross is performed to determine the if an individual of unknown genotype is homozygous
or heterozygous.
PROBABILITY
Events are independent if the occurrence of one does not affect the probability that the other
will occur.
Product rule: if two or more events are independent of each other, the probability of their both
occurring is the product of their individual probabilities.
In some cases there is more than one way to obtain an outcome. These different ways are
called mutually exclusive.
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If one occurs the other cannot occur.
Sum rule: if two or events are mutually exclusive, the probability of obtaining the outcome is the
sum of the individual probabilities of the events.
DIHYBRID CROSS
Dihybrid cross involves individuals who have different alleles in two different loci.
Principle of independent assortment: the alleles of different loci on nonhomologous
chromosomes are distributed into gametes.
Independent assortment occurs because of the mechanics of meiosis.
This can result in combinations that are not present in the parent generation.
Alleles found in the same chromosome are said to be linked and do not assort independently.
Recombination of linked genes can occur as a result of crossing over in meiotic prophase I.
By measuring the frequency of recombination between linked genes, it is possible to construct a
genetic map of a chromosome.
SEX CHROMOSOMES
Sex is commonly determined by special sex chromosomes.
Typically one sex is homogametic, that is it has a pair of similar chromosomes.
The other sex is heterogametic. It has two different sex chromosomes chromosomes.
In many animals, the female is homogametic and the male heterogametic.
Chromosomes other than the sex chromosomes are called autosomes.
The Y chromosome determines male sex in many mammals.
The X chromosomes has many important genes unrelated to sex determination and needed in
both males and females.
Genes found in the X chromosome are called sex-linked genes.
Y chromosome has several genes involved in determining the male sex.
Some individuals have an abnormal number of sex chromosomes:
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XXY (Klinefelter syndrome) is male with underdeveloped testes but otherwise almost
normal male appearance.
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XO (Turner syndrome) has the appearance of an immature female.
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YO embryos do not survive.
In birds and butterflies, the male is homogametic and the female heterogametic.
Hermaphroditic organisms have organs of both sexes.
Hermaphroditic animals and plants do not have sex chromosomes.
X-linked genes have an unusual pattern of inheritance.
Males are neither homozygous nor heterozygous for X-linked genes. They are called
hemizygous.
Dosage competition is a mechanism that makes the two doses of in female and the single
dose in the male equivalent.
In mammals, dosage competition is accomplished by the inactivation of one X chromosome.
The metabolically inactive X chromosomes is called a Barr body.
Inactivation of the X chromosome occurs randomly in each cell of the body resulting in
variegation.
Sex-influenced genes are autosomal genes whose expression is affected by the individual sex.
NONMENDELIAN INHERITANCE
Multiple alleles is the condition when three or more alleles that can occupy a locus exist in the
population.
Incomplete dominance occurs when the heterozygote has a phenotype intermediate between
those of its parent.
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Red and white flower bearing plants produce a pink flower plant.
In codominance, the heterozygote expresses the phenotype of both types of homozygotes.
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Roan horses have both white and red hairs mixed giving a characteristic color to the
animal.
The white and red hairs are expressed independently hair by hair.
Pleiotropy is the characteristic of genes to have multiple effects.
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The many symptom of cystic fibrosis.
A defective enzyme that affects the function of many types of cells.
In epistasis, the allele of one locus can mask the expression of the allele in another locus.
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Albinism: the enzyme required for melanin production is inactive. Melanin is the
precursor of other pigments, brown, black, etc.
Polygenes act additively to produce a phenotype: polygenic inheritance.
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Many characteristics are not inherited through alleles in a single locus but through
several pairs of alleles in different loci (polygenes), e.g. human height, skin color,
body form, etc.
Inbreeding is the mating of two closely related individuals, greatly increases the chances that a
homozygous for one or more recessive genes will result.
Outbreeding, the mating of totally unrelated organisms, increases the probability that the
individual will be heterozygous.
Hybrid vigor or heterozygous advantage is shown by heterozygotes that have an advantage
over both homozygous.
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The homozygotes show a disadvantage to two different conditions.
Each strain contain dominant genes to make up for the undesirable genes of the
other strain.
There is a debate about the causes. It may be the result of multiple causes.
Sickle cell anemia and resistance to malaria.