Transcript Genetics - Liberty Union High School District

Mendelian Genetics
Chapter 6.3 & 6.4
Gregor Mendel
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Gregor Mendel: Father of genetics,
studied pea plants.
1822-1884
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Genetics: study of heredity
Heredity: transmission of genetic
characteristics from parents to offspring
Trait: a characteristic controlled by genes

Ex) tongue roller, hair color, eye color, etc.
Recessive and Dominant Traits
Dominant: A trait that
covers or masks another trait.
 Recessive: A trait that is
covered or masked when
paired with a dominant trait.
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Chromosomes and Genes
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Gene: A segment of DNA
on a chromosome that
controls a particular trait.
Allele: alternative form
of a gene, one is
dominant and one
is recessive.
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An individual
inherits 2 alleles for
every gene – 1 from
mom and 1 from dad
In genetics, alleles are
Homologous chromosomes
represented by letters.
 Dominant alleles: represented
F
f
with a capital letter
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Example: F = dominant allele
for purple flowers
Recessive alleles: represented
with a lower case letter

Example: f = recessive allele that
will yield white flowers
Different alleles for
the same gene
Mendel’s Laws
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Law of Segregation: during gamete formation
(MEIOSIS) pairs of alleles separate and are
randomly united during fertilization
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Offspring receives 1 allele from each parent
Segregation
happens
here during
Anaphase 1
Mendel’s Laws Cont’d
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Law of Independent Assortment: allele pairs
separate independently of one another during
gamete formation.
O
R
This law would be violated, for example, if the “G" allele
was always associated with the “Y" allele (and thus
eliminating the possibility of Gy and gY gametes).
Chapter 6.4 & 6.5
Genotype and Phenotype
 Genotype: the genetic makeup
of an organism
Consists of 2 alleles
inherited from parents
 Phenotype: the physical
appearance of an organism
resulting from genotype
Genotype and Phenotype
Genotype:
Phenotype:
FF
 Ff
 ff
Purple flowers
Purple flowers
White flowers

F = allele for purple flowers (dominant)
f = allele for white flowers (recessive)
Homozygous: both alleles are alike
Homozygous Dominant: has 2
dominant alleles (FF)
Homozygous Recessive: has 2
recessive alleles (ff)
 Heterozygous: each allele differs (Ff)
Heterozygous organisms usually
display the dominant phenotype
Always write capital letter first!
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Monohybrid Crosses
Monohybrid Cross: a cross
that involves 1 pair of
contrasting traits
 We use diagrams called
Punnett square to predict
offspring traits.
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Create a
box & add
parent
genotypes
Fill in potential
offspring
genotypes.
Offspring
Genotypic and Phenotypic Ratios
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Genotypic Ratio: ratio of genotypes that
will appear in the offspring
 Expressed in order from Most
Dominant to Most Recessive
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1 RR : 2 Rr : 1 rr
Phenotypic Ratio: ratio of phenotypes that
will appear in the offspring
 Also expressed in order from Most
Dominant to Most Recessive
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3 Red : 1 white