Genetics
http://www.youtube.com/watch?fe
ature=endscreen&NR=1&v=YxKF
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Gregor Johann Mendel
• Austrian monk
• How do mothers and fathers
pass down traits to their
children?
• Mendel tested his idea of
heredity on pea plants
• http://www.youtube.com/watch?v
=EvR_Sdm1orU
• Pea plants is normally selfpollinating
• Easy to control the parental
crosses
• Easily grown and matured
quickly, producing many
seeds
• Allowed him to study many
generations over a relatively
short period of time
• Showed several pairs of
obvious, contrasting traits
Observed traits were called phenotypes
• After several years of self-pollinating the
pea plants, Mendel established purebred
plants
• When self-pollinated, these plants always create
plants that look like themselves
• Mendel also cross bred the purebred
plants that were different for only one
contrasting pair of traits
– The purebred parent plants were called the
P generation
– The offspring were called hybrids
– First generation of hybrids – F1 generation
– Second generation of hybrids – F2
generation
Results?
• The F1 generation
hybrids had the same
trait
• The traits of tallness
dominated the traits of
dwarf-ness
• Mendel called them
the dominant traits
• Trait that was not
expressed =
recessive trait
• Mendel crossed his F1
generation plants to
determine whether they
were identical to the P
generation plants
• If they were, then F1
cross would produce
only tall plants
• F2 generation yielded 3
tall plants and 1 dwarf
plant
• Phenotypic ratio of F2
was 3:1
– Proportion of individuals
in a generation that
express a certain trait
• The F1 hybrid were
different from the
purebred tall plants
• The gene for plant height has two different
forms (alleles): tall and dwarf
– Tall = dominant
– Dwarf = recessive
• He suggested that each plant’s phenotype
was determined by a PAIR of alleles that
could be identical or different
• The only way a plant to develop as a dwarf
is if both its alleles are recessive
Mendel’s Law of Segregation
• Members of a pair of alleles for a given
trait are segregated (separated) when
gametes are formed
• Once Mendel came up with the Law of
Segregation, he was able to describe the
gene makeup, or genotype
• He used letters of the alphabet to
represent the genes
• The allele for dominant trait was
represented by T
• The allele for recessive trait was
represented by t
• Purebred tall plant = TT
• Purebred dwarf plant = tt
• Hybrid tall plant = Tt
Homozygous
Heterozygous
Monohybrid Cross
Drag-and-Drop Genetics: Monohybrid
MONOHYBRID PROBLEMS
• Problems dealing with one trait only ie.
height or colour
1.Write the cross
2.Produce the gametes
3.Draw the Punnett square
4.List the phenotype and genotype ratios
• 1. In dinosaurs, the gene for sharp teeth (S) is
dominant over the gene for dull teeth (s). Cross a
heterozygous sharp toothed dinosaur with a dulltoothed dinosaur to produce the F1 (first generation)
offspring.
• 2. Cross a homozygous sharp toothed dinosaur with
a homozygous dull-toothed dinosaur to produce the
F1 offspring.
• 3. Now cross two of the F1 generation offspring from
question #2 to produce the F2 offspring.