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Chapter 10 Rules of Inheritance: Classical Genetics
CHAPTER OUTLINE
10.1 The science of genetics brings together DNA, cell division and gene expression.
Genetics involves all levels of inheritance from molecules to genes to the physical
expression of traits. Humans have manipulated organisms' genomes through selective
breeding for thousands of years. Scientific methods are now used to cross individuals to
study patterns of inheritance.
10.2 Gregor Mendel was the first systematic researcher in the field of genetics.
Gregor Mendel studied inheritance of traits in the mid-1800s using the garden pea.
Mendel used thorough and diligent methods to uncover the laws of inheritance.
Mendel first developed plots of true-breeding parents before crossing individuals through
careful, controlled pollination.
10.3 Mendel discovered three rules of inheritance.
Mendel's laws have been supported by over 100 years of evidence.
Mendel discovered dominant and recessive traits.
Working with true breeding pea plants as the parental or P generation, Mendel crossed
these plants to produce an F1 generation followed by crossing F1 individuals for an F2
generation. Looking at the traits in each generation led Mendel to label differing forms
of a trait as dominant or recessive. We now know these traits are controlled by dominant
or recessive alleles. Mendel's Law of Dominance states that an individual must have two
recessive alleles to express the recessive form of the trait but could have two or only one
of the dominant alleles to express the dominant form of the trait. The genotype of a trait
is the alleles contained within the organism; the phenotype is the physical expression of
those alleles.
Mendel understood that alleles segregate during the formation of gametes.
An organism is homozygous if it contains two identical alleles for a trait, heterozygous if
it contains two different alleles for a trait. These alleles are separated during gamete
formation so that only one allele is passed to an offspring from each parent during sexual
reproduction. A Punnett square can be used to reveal all the possible genotypes of an
offspring from two parents with known alleles.
Mendel found that the alleles for different traits usually act independently to influence
traits.
Following an experiment using a dihybrid cross, Mendel discovered that alleles do not
influence each other's inheritance but act independently. This is Mendel's Law of
Independent Assortment.
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Mendelian inheritance governs some human traits.
Some human traits are passed from parent to offspring through the dominant/recessive
inheritance pattern.
10.4 More complex patterns of inheritance are an extension of Mendel's basic rules.
Other types of inheritance patterns have been discovered since Mendel's initial work.
Some alleles show incomplete dominance or co-dominance.
Traits which show incomplete dominance have three phenotypes, the heterozygous
condition is typically intermediate between the two homozygous conditions. In
codominance, both alleles are dominant and express some aspect of the trait. The human
ABO blood type system is an example of codominance.
One phenotype can be the result of the action of multiple genes, and one gene can affect
multiple phenotypes.
Some traits are polygenic, that is, many genes are involved in one phenotypic expression.
Human skin color is an example of polygenic inheritance. Alternatively, one gene can
affect many phenotypes; this is called pleiotropy. Albinism is one example of pleiotropy.
Recessive alleles can be expressed differently if they are carried on the X chromosome.
In mammals, females have two X chromosomes which determine their sex while males
have an X and a Y chromosome, also determining sex. Since males only have one X
chromosome, alleles on this chromosome are expressed as they have no match on another
X chromosome. This results in the sex-linked inheritance pattern.
Genes interact with the environment to produce a phenotype.
The environment in which the allele, protein and organism function affects the physical
expression of the allele.
Human traits are influenced by both nature and nurture.
Numerous studies of human genetics have shown that both genes and environment affect
human development.
10.5 The modern understanding of genetics combines knowledge of inheritance with
knowledge of how DNA works.
Our knowledge of genetics at the molecular level has increased our understanding of
inheritance and expression of traits.
Mutations in DNA produce different alleles.
The occurrence of mutations in DNA is a normal process which can give rise to new
alleles, both beneficial and detrimental. Genetic variation depends on mutations.
The modern understanding of sickle cell anemia brings together mechanisms of
inheritance with DNA and protein functions.
Sickle cell anemia is an example of a human disease understood at the molecular, cellular
and organismal level. The allele for sickled hemoglobin is pleiotropic and is affected by
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environmental factors. Environmental factors may also drive the maintenance of the
mutated allele within the human population.
10.6 How do you know? The genetics of eugenics.
The eugenics movement of the early twentieth century was not based on science. Today,
knowledge of a fetus's genotype may result in parents choosing offspring for their traits.
Now you can understand.
Looking at incest from the genetic perspective, closely related parents can more easily
have offspring with genetic defects than parents who are not closely related.
What do you think? Does heredity or environment exert more influence upon human
intelligence?
Current knowledge of the genetics of human intelligence and the effects the environment
has on the development of intelligence is presented to stimulate critical thinking.
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