Unit 4: Heredity and Development
4-2 Mendelian Genetics
Important Vocabulary
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True-breedingHybridizationP generationF1 generationF2 generationHomozygousHeterozygousGenotypePhenotype-
Mendel’s Conclusions
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Alternative versions of genes (alleles) account for
variations in inherited characters
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Gene
Allele
Dominant and Recessive Alleles
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If the two alleles at a locus differ, then one, the dominant
allele determines the organism’s appearance, the other the
recessive allele has no noticeable effect
The Law of Segregation
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Alleles segregated during gamete formation and end up in
different gametes
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Homozygous
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True breeding
Two of the same alleles
100% of gametes will get the same allele
Heterozygous
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One dominant and one recessive allele
50% of offspring will get the dominant allele
50% will get the recessive allele
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For each trait, an organism inherits two alleles, one from
each parent
The Law of Independent Assortment
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Each pair of alleles segregates independently of other
alleles during gamete formation
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NOTE: only applies to alleles located on separate,
nonhomologous chromosomes.
Three Rules of Probability
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Past Events never effect future outcomes.
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A woman has 13 children-all of them boys! She is
pregnant with a 14th child. What is the probability that she
will have a girl?
The Multiplication Rule
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The probability of two separate events occurring
simultaneously is the multiple of their separate
probabilities.
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A pair of dice is rolled. What is the probability of rolling
double sixes?
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Two cards are drawn from a deck, what is the probability
of drawing a king?
The Addition Rule
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The probability that either of two events will occur is the
sum of their separate probabilities.
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A single die is rolled. What is the probability of rolling a 5
or a 6?
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A single card is drawn from a deck. What is the
probability of drawing a kind or a queen?
Complete Dominance
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Occurs when one form of a trait is completely dominant
over the other
The Monohybrid Cross
Example 1
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Brown eyes are dominant over blue eyes. Cross two
parents who are heterozygous for eye color.
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What is the genotypic ratio?
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What is the phenotypic ratio?
Example 2
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A brown-eyed woman whose father was blue eyed
married a blue-eyed man whose mother was brown-eyed.
What are the possible phenotypes of their children?
The Dihybrid Cross
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Brown hair is dominant over blond. Curly hair is
dominant over straight. Cross a heterozygous brown
curly haired woman with a man who has brown curly hair.
The man’s mother had straight blond hair.
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Normal skin pigmentation is dominant over albino skin.
Brown eyes are dominant over blue eyes. Cross a
heterozygous, brown eyed, normal skinned female with an
albino, blue-eyed man.
Incomplete Dominance
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Offspring between a homozygous dominant individual and
a homozygous recessive individual display a blending of
the two phenotypes
WW
RR
RW
Incomplete Dominance
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Example: Snapdragons
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Homozygous red flower x homozygous recessive flower=
100% pink flowers
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What happens when you cross two pink flowers?
Codominance
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Occurs when both dominant alleles are apparent in the
phenotype of heterozygous offspring
Example 2: Blood Type
Multiple Alleles
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Occurs when there are
multiple alleles (more than
2) for a specific gene
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Example: ABO Blood
Group’
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A woman is heterozygous for type A blood. Her husband
is heterozygous for type B blood. What is the probability
that they will have a child with type O blood?
Pleiotrophy
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Occurs when a gene has an effect on more than one
phenotypic character
Epastasis
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Occurs when a gene at one locus effects a gene at
another locus
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If gene A masks the effects of gene B, then gene A is said
to be epistatic to B.
In sweet peas, two pairs of genes are
at work in determining the color of
the flower.
C= colored flowers possible
c= white flowers
P= purple flowers
p= white flowers
What color flowers would be
produced by the following genotypes:
Which gene is epistatic to the other?
CCPPCCPpCCppCcPPCcPpCcppccPP
ccPpccpp-
Epistasis
Polygenic Inheritance
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Characteristics are controlled by multiple genes
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Quantitative Characters- those that vary in populations
along a continuum
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Skin color
Height
Recessively Inherited Disorders
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Carriers- heterozygotes that carry the recessive allele,
but do to having a dominant allele appear normal
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Typically occur in specific groups or populations- why?
Tay-Sachs Disease
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Occurs among Jewish people with ancestors from central
Europe
This population is 100 times more likely to be effected
Brain cells are unable to metabolize lipids
Lipids accumulate in the brain and the child will eventually
die
Cystic Fibrosis
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Occurs most frequently among people with ancestors
from Europe
4% of population are carriers
Defect causes a lack of chlorine ion channels, chlorine
builds up outside the cells and forms a thick mucus
Lungs, pancreas, and digestive tract are most affected
Sickle-Cell Disease
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Most common among those from African descent; effects
1 in 400 African Americans
1 in 10 African-Americans carries the trait
Causes a single amino acid substitution in the hemoglobin
protein
Causes RBCs to become sickle shaped; cannot carry
oxygen as well and can easily cause blockages
Carriers are said to have the sickle cell trait- which may
be evolutionarily advantageous
Sickle Cell Disease
Dominantly Inherited Disorders
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Recessive alleles for some traits are much more common
than the dominant version
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Achondroplasia- dwarfism
Polydactyly- extra fingers and toes
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Lethal Recessive Alleles
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Usually cause the death of the individual before they
mature and reproduce
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Huntington’s Disease
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Does not cause symptoms until 35-45 years of age
By this point the affected individual may have already had
children
The child will then have a 50% chance of having the disorder