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Chapter 8

Page history last edited by Shelly Turner 10 years, 5 months ago

Chapter 8:  Mendel and Heredity




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Genetics Project Links


Genetic Disorders List


I.  The Origins of Genetics

A.  heredity – the passing of traits from parents to offspring

B.  trait – genetically determined characteristic, condition, or behavior

C.  genetics – the branch of biology that focuses on heredity

D.  Gregor Mendel – “father” of genetics, using garden pea plants to experiment he formed the basic framework of knowledge for modern genetics

E.  Mendel used the following ideas in producing pea plants to observe changes in traits over generations…

1.    Mono-hybrid cross – used to study the results of combining two contrasting traits

2.    True-breeding – self-pollinating several generations of a plant until the offspring consistently express only one form of a trait

3.    P-generation – (parent generation) the first pair of individuals to be crossed in an experiment

4.    F1 –generation – offspring of the P-generation

5.    F2 –generation – offspring from the self-pollination of the F1 –generation

F.   Mendel discovered…

1.    All the F1 –generation showed the same form of the observed trait

2.    The F2 –generation however showed a combination of traits in the ratio of 3:1

G.  Mendel’s Theory

1.    An individual organism has 2 copies of the gene for each trait

2.    There are different versions of these genes (called alleles)

3.    When 2 different alleles occur together one of the versions may be completely expressed (dominant allele); while the other has no observable effect (recessive allele)

4.    When gametes are formed (by meiosis) the alleles for each gene in the individual separate and only 1 version goes into each gamete.  A new homologous pair forms during fertilization




II.   Describing Traits

A.  Genotype

1.    The set of alleles that an organism has for a trait is called the genotype

2.    Dominant alleles are represented by capital letters

3.    Recessive alleles are represented by lower case letters

4.    Homozygous genotype – contains two alleles with the same version of the trait (ex. 2 dominant or 2 recessive)

5.    Heterozygous genotype – contain two different versions of the allele for a trait (ex. 1 dominant and 1 recessive)

B.  Phenotype

1.    The physical appearance of a trait is called the phenotype

2.    Homozygous genotypes show the common phenotype (ex.  2 recessive alleles show the recessive trait)

3.    Heterozygous genotype show the dominant allele’s phenotype

III.  Laws of Heredity based on Mendel’s research

A.  Law of Segregation – the two alleles for a trait separate during meiosis so that only one is passed down during fertilization

B.  Law of Independent Assortment – the inheritance of one trait does not affect the inheritance of another trait 




IV.   Predicting Traits

A.  Punnett Square – diagram that predicts the expected outcomes of a genetic cross

1.    Monohybrid Cross – involves one pair of contrasting traits

2.    Dihybrid Cross – involves two pairs of contrasting traits

3.    Polyhybrid Cross – involves multiple pairs of contrasting traits

4.    Test Cross – cross between an individual showing a recessive trait and one showing the dominant trait whose genotype is unknown; the results of the cross will indicate if the dominant genotype is heterozygous or homozygous

B.  Probability – the mathematical likelihood an event or outcome will occur

1.    If the event defiantly WILL occur then we say the probability is 1, or 100%, or 1/1, or 1:1

2.    If the event defiantly WILL NOT occur then we say the probability is 0, or 0%, or 0/1, or 0:1

3.    In between 100% and 0% probability is a continuum of possible ratios (ex. 3:1, or 75%, or .25)

4.    Formula for probability

P  = # of a specific outcome        .

       # of total possible outcomes

C.  Pedigree

1.    Chart or record that traces when a trait is expressed over many generations

2.    May be used to trace the occurrence of a specific trait, and predict how and when it will occur in future generations

D.  Sex-linked traits

1.    Trait whose gene(s) are found on the X chromosome

2.    Ex. Baldness in humans, calico coloring in cats

V.   Complex Patterns of Heredity

A.  Poly-genetic trait – trait influenced by a combination of many genes (ex. hair, eye or skin color, weight, height)

B.  Incomplete dominance – a trait has two or more dominant alleles resulting in the display of an intermediate version of the trait in the offspring (ex. pink flowers from white and purple parent flowers)

C.  Codominance – two dominant traits may be displayed (ex. red and white hairs on a horse with a red parent and a white parent)

D.  Multiple alleles – trait with three or more variations (ex. blood type) 

VI. Phenotypes are sometime affected outside factors

A.  Environmental factors                    

1.    Hydranga – flower color is determined by soil composition

2.    Siamese cats – fur color controlled by heat (darker in cooler areas like the ears, lighter in warmer areas like the sides and belly)

3.    Artic Fox – fur color affected by exposure to sunlight (brown in summer, white in winter) 

II.   Mutations may be passed through many generations

A.  Sickle-cell anemia – recessive mutation that causes poor circulation

B.  Hemophilia sex-linked trait found in males that causes failure of blood clotting

C. Huntington’s disease – dominant mutation that causes a lack of muscle control as the person ages 




Dihybrid Practice:




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