Fundamentals of
Genetics
What is genetics?
 Genetics: the science that studies how
genes are transmitted from one generation
to the next
 Gene: a segment of DNA that controls a
hereditary trait
 Chromosomes = long chains of genes
 Heredity: the transmission of characteristics
from parents to offspring
 Traits: the characteristics an organism has
(ex: eye color, skin color, hair color, height)
Gregor Mendel
 Austrian monk born in 1822
 Known as the Father of Genetics
 Discovered three laws of genetics that would
forever change biology
 Law of Dominance & Recessiveness
 Principle of Segregation
 Principle of Independent Assortment
 Conducted experiments in a quiet monastery
garden & spent 14 years growing and
experimenting with pea plants
 1851- studied science & statistics at the University
of Vienna
 Statistics proved valuable in his research on
heredity
 His greatest contribution was to demonstrate
that inherited characteristics are carried by
genes
Mendel’s Garden Peas
 Observed seven characteristics of pea plants
1. Plant height (tall vs. short)
2. Flower position along stem (axial vs. terminal)
3. Pod color (green vs. yellow)
4. Pod appearance (inflated vs. constricted)
5. Seed texture (round vs. wrinkled)
6. Seed color (yellow vs. green)
7. Flower color (purple vs. white)
 Garden peas a good choice because:
 They were readily available
 They were easy to grow
 They grew rapidly
 Collected seeds from his pea plants and
carefully recorded each plant’s traits and seeds
over a span of several years
Mendel’s Experiments
 Pea flowers are normally self-pollinating
 Resulting embryos will have the same
characteristics as the parent plant
 Mendel knew that these pea plants
were “true breeding” (pure)
 Mendel cross-pollinated pairs of plants
that were true breeding for contrasting
traits of a single characteristic
 He called the true breeding parents
the P generation
 P generation = Parental generation
 F1 generation = First generation of offspring
 F2 generation = Second generation of offspring
 Hybrids: the offspring of parents with different traits
 Following this process, Mendel performed hundreds of crosses
& documented the results of each by counting and
recording the observed traits of every cross
 In one of his experiments, Mendel crossed truebreeding tall plants with true-breeding dwarf plants
Tall x dwarf  all tall offspring
 The F1 hybrids were all tall
 All of the offspring had the appearance of only
one of the parents
 The trait of the other parent seemed to have
disappeared
 Mendel thought that the dwarf trait had been
lost
 Next, Mendel allowed the hybrid tall
offspring from the first generation to selfpollinate
F1 tall x F1 tall  offspring: ¾ tall and ¼ dwarf
 He found that ¾ of the offspring were tall
and ¼ of the offspring were dwarf
 Offspring always at a 3:1 ratio
Mendel’s Results & Conclusions
 The F1 tall offspring must have been carrying
the dwarf trait but it had been hidden
 The dwarf trait had been passed down to
the offspring and it reappeared in the F2
generation
 Mendel’s observations & records led him to
hypothesize that biological inheritance is
determined by “factors” that are passed
from one generation to the next
 Today, we know these factors as genes or
alleles
 He hypothesized that each trait was
inherited by means of a separate allele
 Reasoned that a pair of alleles must
control each trait
Recessive & Dominant Traits
 Mendel realized that some traits are dominant over other traits
 He hypothesized that the trait appearing in the F1 generation was
controlled by a dominant factor
 He thought that the trait that did not appear in the F1 generation but
appeared in the F2 generation was controlled by a recessive factor
 A trait controlled by a recessive factor had no observable effect on
an organism’s appearance
Dominant vs. Recessive?
1. Pause note taking
2. Clasp your hands together
3. Which thumb did you put on top?
 Left?
 Right?
 Dominant: Placing left on top of right
 Recessive: Placing right on top of left
 Mendel’s observations from his
experiments can be summarized in
two principles:
 The Law of Segregation
 The Law of Independent
Assortment
The Law of Segregation
 Mendel concluded that the paired
factors separate during the
formation of reproductive cells
 Law of segregation states that a pair
of factors is separated during the
formation of gametes (egg and
sperm cells)
The Law of Independent
Assortment
 Mendel also concluded that the
factors for individual
characteristics are not
connected
 The law of independent
assortment states that factors
separate independently of one
another during the formation of
gametes
Homozygous or Heterozygous
 An offspring will inherit two alleles
for a trait, one allele from each
parent
 The combination of alleles
received may either be
homozygous or heterozygous
 Homozygous = the two alleles are
the same
 Ex: TT or tt
 Heterozygous = the two alleles are
different
 Ex: Tt
Genotypes & Phenotypes
 Genotype: the genetic makeup
of an organism
 Phenotype: the physical
characteristics of an organism
(what the organism looks like)
Ex:
Genotype
TT
Tt
tt
Phenotype
Tall
Tall
dwarf
Exit Ticket
Answer the following question:
Many inherited disorders of humans appear in children of
parents who do not have the disorder. How can you explain
this?