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Introduction to Mendelian Genetics
Packet #19
Chapters 6 & 7
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Mendelian GeneticsWho is Mr.
Gregor Mendel?
What is he famous for?
Describe the experiments of Gregor Mendel
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Introduction I
Heredity The biological similarity
of offspring and parents
Gene Region of DNA, found
on the chromosome, that controls a discrete hereditary characteristic of an organism
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Introduction II
Allele One of several alternate
forms of a particular gene
Locus Particular place along
the length of a chromosome where a given gene is located
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Introduction III
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Genotype The specific allele
composition of a cell The combination of
alleles located on homologous chromosomes that determines a specific characteristic or trait.
Introduction IV
Phenotype The observable physical
or biochemical characteristics of an organism, as determined by organism’s genetic makeup (genotype).
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Introduction V
Dominant AlleleAn allele that expresses its phenotype effect even
when combined with a recessive allele.
Recessive AlleleAn allele whose phenotype effect is not expressed
unless it is combined with another recessive allele.
However, there are exceptions to this rule in specific genetic disorders.More to come in future packets…Please hold those
questions until then.
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Introduction VI
A = dominant allele
A = recessive allele
AA = homozygous dominant genotype
Aa = heterozygous genotype
aa = homozygous recessive genotype
Genotype Variations Homozygous dominant
Two dominant alleles Heterozygous
One recessive allele One dominant allele
Homozygous recessive Two recessive alleles
Incomplete Dominance
Occurs when hybrids, with a heterozygous genotype, have an appearance between the phenotypes of the parental varieties.
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Incomplete Dominance II
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Codominance
Situation in which the phenotypes of both alleles are exhibited in a heterozygote
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Epistasis
Phenomenon in which one gene alters the expression of another gene that is independently inherited.
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Epistasis II
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Sex-Linked Genes
Sex-linked genes Genes that are found on
the sex chromosomes.
Multiple Alleles
On some occasions, there is more than two alleles (forms) of a particular gene. Example: - Alleles for blood
group.
When discussing genotypes for blood groups, there are three alleles that one must consider i iA
iB
More to come on blood types later and how blood types are determined in a couple’s offspring and how blood groups impact the blood transfusions.
Punnett Squares
Punnett Square A diagram used in the
study of inheritance Shows the result of
random fertilization in genetic crosses. Shows the probable
results of crossing over. More to come in
the next packet.
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Mendel’s Laws
Mendel’s Laws
When Mendel carried out his research, the processes of mitosis and meiosis had not yet been discovered.
However, Mendel knew, through his experiments, that genes (alleles) existed.
From Mendel’s research, he devised two laws. Principle of Segregation Principle (Law) of
Independent Assortment
Principle of Segregation
Principle of Segregation The principle states that
in diploid organisms genes come in pairs and that when sex cells get produced each gamete gets one gene at random.
Principle of Segregation II
When developing this idea Gregor Mendel conducted a series on monohybrid (test) crosses using pea plants. A monohybrid is when
only one allele is investigated.
Principle of Independent Assortment
The Law of Independent Assortment states that the alleles (or separate members of a gene pair) separate independently to form the gamete. To do this, one must be comparing
at least TWO traits. Dihybrid cross
By doing so, the traits are transferred independent from one another. This allows for much more
variation in the offspring since the alleles are randomly matched with the gamete from the other parent to form the zygote. According to how many traits
are in question, the number of possible variations can become quite high.
Dihybrid Cross
A dihybrid cross involves an investigation of two alleles at the same time.
Principle of Independent Assortment II
Mendel concluded that alleles (traits) are transmitted to offspring independently of one another.
If the genes are transmitted independently, then the genes are determined as being unlinked.
If the genes are transmitted together, the majority of the time, then the genes are determined as linked genes. Hence, the principle
of independent assortment does not apply.
Principle of Independent Assortment III
This law holds true as long as the two genes (traits) in question are: - Located on separate
chromosomes Not linked together if they
are located on the same chromosome. Unlinked genes
More to come later in AP Biology
The principle of independent assortment allows/results in recombination The presence of new gene
combinations not present in the parental (P) generation.
Principle of Independent Assortment IV
The principle of independent assortment allows/results in recombination The presence of new
gene combinations not present in the parental (P) generation.
Genetic Crosses
Monohybrid (Test) Cross
Examples
Dihybrid Crosses
Examples
Parent #1 Genotype FfEe
Parent #2 Genotype FfEe
Review