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Mendelian Genetics
CH 6
Section 6.3 – 6.5
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??
 Sometimes, certain traits seem to
disappear for a generation but then
return in the next generation.
 How does this happen?
 Is this a “law” or “theory”?
 Law since we are describing something, not
trying to explain it.
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Vocabulary
 Character – heritable feature
 Hair Color
 Height of plant
 Eye color
 Pea color
 Pea shape
 Trait – variant of a character (heritable feature)
 Brown hair color
 Tall plant height
 Blue eye color
 Yellow pea color
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More Vocab
 Dominant trait
 One that will mask the recessive trait if found together
 Recessive trait
 Trait that will be masked if found with the dominant trait
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Breeding
 True Breed (purebred)– Whatever traits the parent has
are expressed in ALL subsequent populations
 For example, self-pollinating a purple flowered plant
produces a generation of only purple flowered plants.
 Typically, need to do this for at least two (2) generations to
ensure that the parent is a true-breed (purebred)
 Hybrid – Mating (crossing) of two (2) true-breeding
varieties of true-breeds
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Vocabulary (Page 2)
 P Generation – Parent generation
 Two (2) true-breeding parents being crossed
 F1 – First filial (child or son) generation
 Hybrids
 F2 – Second filial generation
 Each member of F1 self-pollinates
 Hybrids again
 3:1 Dominant to Recessive ratio
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What Mendel found (stretched the truth about)
 Only looked at “all-or-nothing” traits
 Sometimes called binary traits – “yes” vs. “no”
 Mendel took purebreds for 1 trait, and pollinated it
with a purebred for another trait
 What is the difference between trait & character?
 Purple flowered + White flowered
 F1 = All colored purple (all expressed only one trait)
 F2 = 3:1 ratio of Dominant to Recessive trait
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___ Generation
What type of plants?
____________
___ Generation
What type of plants?
____________
___ Generation
What type of plants?
____________
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Genes
 Gene
 Portion of DNA that provides the instructions for making
a particular protein
 Each gene has a particular location on a chromosome
called its locus
 Like a house’s address
 Allele
 One of the different forms of the gene
 Mendel looked at genes that had ONLY 2 alleles
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Mendel’s Model
1. Alternate versions of genes account for variations in
inherited characteristics
 The alternative versions are called alleles
 One plant had the allele for purple flower color while
the other had the allele for white flower color
2. For each character, an organism inherits two (2) alleles,
one from each parent
 The two alleles may be the same or different
 If the 2 alleles are the same = Homozygous
 Different = Heterozygous
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Allele Dominance
 Just as there are dominant and recessive traits, there are
dominant and recessive alleles
 Dominant allele
 If present, always expressed
 Represented by a capitalized letter: A
 Recessive allele
 Only expressed when BOTH alleles are recessive
 is represented by a lowercase letter: a
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Mendel’s Model (Page 2)
3. If the alleles differ (heterozygous), then the dominant
allele determines the organism’s appearance
 Heterozygous individuals display dominant trait
 Homozygous Dominant display _________ trait
 Homozygous Recessive display _________ trait
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Mondel’s Model (Page 3)
4. Law of Segregation
 2 alleles for a heritable character segregate
(separate) during gamete formation and end up in
different gametes
 We already know that homologous chromosomes
assort independently during meiosis into gametes
 But Mendel did not know about chromosomes
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Questions
 What is the difference between a gene and a
character?
 What is the difference between a gene and an allele?
 What is the difference between a character and a trait?
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Important Vocab.
 Phenotype - appearance
 Characters
 Traits are different types of the character
 IF character is eye color, trait is brown, blue, etc
 Genotype - genetic makeup
 Genes
 Alleles are different types of genes
 Gene for eye color, alleles = brown (dominant), blue
(recessive), etc.
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Punnett Squares
 One parent at top, other on the left
 Here we are crossing homozygous dominant (HD) in the
form of [AA] with Heterozygote (Ht) as [Aa]
A
A
A
AA
AA
a
Aa
Aa
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Cross all 6 combinations
Cross
Genotypic
Ratio
Phenotypic
Ratio
HomoD x HomoD
HomoR x HomoR
Heter x Heter
HomoD x HomoR
HomoD x Heter
HomoR x Heter
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Monohybrid vs. Dihybrid
 Monohybrid Cross – Take pure breeds for 1
character and cross (AA x aa)
 You should already understand these.
 Dihybrid Cross – Take pure breeds for 2 characters
and cross (AABB x aabb)
 2 Characters like seed color + seed shape
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Law of Independent Assortment
 States that each pair of alleles segregates
independently of other pairs of alleles during gamete
formation
 This rule really only pertains to genes (allele pairs) on
different chromosomes
 If on the same chromosome = linked genes
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Repeat but use Probability
 Compute P(green & wrinkled) from the table
 Now do so from Punnett Squares for each character
 Compute P(Yellow & Round) from table
 Now do so from Punnett Squares for each character
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Problems
 In a dihybrid cross, what is the possibility of
heterozygous in one trait and homozygous recessive in
the other, given both heterozygous parents?
 In a trihybrid cross, what is the probability of getting
exactly 2 recessive phenotypes, if one parent is
heterozygous for all 3 genes and the other is
homozygous recessive for 2 genes, and hetero for the
3rd?
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Mendel’s Big Ideas
 The Law of Segregation
 The 2 alleles of a gene separate (segregate) during
gamete formation, so that a sperm or egg only carries 1
allele of each pair
 Explains 3:1 ratio found in hybrid crosses
 The Law of Independent Assortment
 Each pair of alleles separates independently (of other
allele pairs) into gametes
 Genes assort independently of one another
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