Humes
Biology
Chapter 9
Fundamentals of Genetics
Genetics- the field of biology devoted to understanding how characteristics are transmitted
from parents to offspring
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Gregor Mendel
o Mendel was a monk who was in charge of tending to the garden
o He observed 7 characteristics in pea plants, each characteristic occurred in two
contrasting traits.
o A trait is a genetically determined variant of a characteristic, such as yellow or
white flower color
o He was able to observe how traits were passed on from one generation to the
next by controlling pollination
 Pollination occurs when pollen grains produced in the male reproductive
parts of a flower called the anthers, are transmitted to the female
reproductive part of the flower called the stigma
 Self pollination occurs when pollen is transferred from the
anthers of a flower to the stigma of either that flower or another
flower on the same plant
 Cross pollination occurs between flowers of two plants
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Mendel’s Experiments
o He studied each characteristic and its contrasting traits individually
o He grew plants that were pure, referred to as true breeding. True or pure plants
only produce one trait, so if there are two contrasting traits a plant always
produces only one of those traits
o When he was done he had 14 true breeding plants, 1 for each of the 7
contrasting characteristics
o He crossed plants that were contrasting for each of the seven traits
o He called the true breeding parents the P generation
o He called the offspring of the P generation the F1 generation
o He then crossed the F1 generation with each other and called the offspring the
F2 Generation
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Biology

Mendel’s Results and Conclusion
o When he crossed two plants with contrasting traits in the P generation only one
of the two traits appeared in the F1 generation (example: green pods and yellow
pods, only green pods were seen)
o He then allowed the F1 plants to self pollinate
o When the F2 plants grew he observed ¾ of the F2 plants had green pods and ¼
had yellow pods
o He hypothesized that something in the pea plants controlled the characteristics
observed
o He called the controls factors
o He also hypothesized that each trait was inherited by means of a separate factor
o Because the characteristics studied had alternate forms, he reasoned that a pair
of factors must control each trait

Recessive and Dominant Traits
o Whenever Mendel crossed strains, one of the P traits failed to appear in the F 1
plants
o In every case, that trait appeared in a ratio of 3:1 in the F2 generation
o Mendel hypothesized that the trait appearing in the F1 generation was controlled
by a dominant factor because it masked or dominated the factor for the other
trait
o The trait that did not appear in the F1 but in the F2 was controlled by a recessive
factor
o When the recessive trait was paired with a dominant trait it had not observable
effect on the organism

The Law of Segregation
o States that a pair of factors is segregated, or separated, during the formation of
gametes
o So when sperm fertilizes egg it gets one factor from each gamete

The Law of Independent Assortment
o States that factors separate independently of one another during the formation
of gametes
o Mendel concluded that the factors for individual characteristics are not
connected
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
Biology
Molecular Genetics is the study of the structure and function of chromosomes and
genes
o A chromosome is a thread like structure made up of DNA
o A gene is a segment of DNA on a chromosome that controls a particular
hereditary trait
o Each of two or more alternative forms of a gene is an allele
 Ex: Brown, blue, green, red are all alleles for eye color
Genetic Crosses allow you to predict the probable genetic makeup and appearance of offspring.

Genotype- consists of the alleles an organism inherits from its parents.
o Example: TT, Tt, tt each shows the alleles inherited from their parents

Phenotype- what is visibly seen as a result of the genotype.
o Example: If T is tall and t is short and Tall is dominant to short then the following
genotypes would result in specific phenotypes
 TT- Tall
 Tt- Tall
 tt- short
Homozygous- when both alleles of a pair are alike. An organism may be homozygous
dominant or homozygous recessive
o Example: Homozygous dominant- TT, homozygous recessive-tt
Heterozygous- when the two alleles in a pair are different
o Example- Tt
Probability is the likelihood that a specific event will occur.
o It can be expressed in the following formula
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Probability= the number of times an event is expected to happen divided by the
number of times an event could happen.

Punnett Squares aid in predicting the probable distribution of inherited trait in
offspring.
o Monohybrid cross is a cross in which one characteristic is tracked
o Dihybrid cross a cross in which two characteristics are tracked
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Biology
o Test Cross a cross in which an individual of unknown genotype is crossed with a
homozygous recessive individual. It allows you to determine the genotype of an
individual whose phenotype expresses the dominant trait.
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Incomplete Dominance occurs when the phenotype of a heterozygote is intermediate
between the phenotypes determined by the dominant and recessive traits.
o Example: In four o’clock flowers, both the allele for red fllowers (R) and the allele
for white flowers (r) influence the phenotype. Neither allele is completely
dominant over the other.
 RR will result in a red flower
 rr will result in a white flower
 Rr will result in a pink flower
Codominance occurs when both alleles for a gene are expressed in a heterozygous
offspring. In this case neither allele is dominant or recessive, nor do the alleles blend
o Example: In horses red and white hair are codominant
 RR will result in a red horse
 WW will result in a white horse
 WR will result in a horse with equally dispersed white and red hair
Complete dominance occurs when one allele completely dominates over another
o Example: Brown eyes are dominant over blue eyes
 BB will result in brown eyes
 Bb will result in brown eyes
 Bb will result in blue eyes