Patterns of Inheritance (Mendelian Genetics). Gregor Mendel 1890’s Central European Monk Conducted...
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Transcript of Patterns of Inheritance (Mendelian Genetics). Gregor Mendel 1890’s Central European Monk Conducted...
Gregor Mendel 1890’s Central European Monk Conducted research on pea plants
Used garden peasEasy to growCan produce many generations quicklyCross-pollination
Mendel’s experimental design
Statistical analyses: Worked with large numbers of plants counted all offspring made predictions and tested them
Excellent experimentalist controlled growth conditions focused on traits that were easy to score chose to track only those characters that varied in an
“either-or” manner
Genetic Vocabulary Character: a heritable feature, such as
flower color Trait: a variant of a character, such as
purple or white flowers Each trait carries two copies of a unit of
inheritance, one inherited from the mother and the other from the father
Alternative forms of traits are called alleles
Mendel’s experimental design
Mendel also made sure that he started his experiments with varieties that were “true-breeding”
X
X
X X
X
X
Genetic Vocabulary Generations:
P = parental generation F1 = 1st filial generation, progeny of the P generation F2 = 2nd filial generation, progeny of the F1 generation
(F3 and so on) Crosses:
Monohybrid cross = cross of two different true-breeding strains (homozygotes) that differ in a single trait.
Dihybrid cross = cross of two different true-breeding strains (homozygotes) that differ in two traits.
Phenotype vs Genotype
Figure 14.6
3
1 1
2
1
Phenotype
Purple
Purple
Purple
White
Genotype
PP(homozygous)
Pp(heterozygous)
Pp(heterozygous)
pp(homozygous)
Ratio 3:1 Ratio 1:2:1
Mendel’s Experimental Design
Mendel mated 2 contrasting, true-breeding varieties, a process called hybridization
True-breeding parents are called the P generation
The hybrid offspring of the P generation are called the F1 generation
When F1 individuals self-pollinate the F2 generation is produced
Mendel’s Observations When Mendel crossed true-breeding white and purple flowered pea plants all of
the offspring were purple When Mendel crossed the F1 plants, many of the plants had purple flowers, but
some had white flowers A ratio of about three to one, purple to white flowers, in the F2 generation
EXPERIMENT True-breeding purple-flowered pea plants and white-flowered pea plants were crossed (symbolized by ). The resulting F1 hybrids were allowed to self-pollinate or were cross- pollinated with other F1 hybrids. Flower color was then observed in the F2 generation.
RESULTS Both purple-flowered plants and white-flowered plants appeared in the F2 generation. In Mendel’s experiment, 705 plants had purple flowers, and 224 had white flowers, a ratio of about 3 purple : 1 white.
HOMOZYGOUS: Having the same alleles for a gene
(PP, pp)
HETEROZYGOUS: having different alleles for a gene (Pp)
DOMINANCE An organism with a
dominant allele for a particular form of a trait will ALWAYS have that form
(P = purple)
RECESSIVE An organism with a
recessive allele for a particular trait will have that form only when the dominant allele is not present
Practice• use 2 letters to represent genotype• A capital letter represents the dominant form of a
gene (allele) and a lowercase letter represents the recessive form of the gene (allele).
• Example below: P=dominant purple and p= recessive white
The phenotype for this flower is violet while its genotype (if homozygous) is PP.
The phenotype for this flower is white while its genotype is pp (must have 2 of the recessive copies of the allele).
Punnett Squares standard way of
working out what the possible offspring of 2 parents will be helpful tool to show
allelic combinations and predict offspring ratios
Before we go further lets review how to set up a Punnett Square…
We begin by constructing a grid of two perpendicular lines.
Next, put the genotype of one parent across
the top and the other along the left side.
For this example lets consider a genotype of BB crossed with bb.
B B
b
b
• Notice only one letter goes above each box
• It does not matter which parent’s genotype goes on either side.
Fill in the boxes by copying the column & row
letters down and across into the empty spaces
B B
b B
B
B
Bb
b
b
b
b
Lets say:
W- dominant white
w- recessive violet
W wW
Parents in this cross are heterozygous (Ww).
Note: Make sure I can tell your capital letters from lowercase letters.
What percentage of the offspring will have violet flowers?
ANSWER: 25% (homozygous recessive)
Usually write the capital letter first
w
W W W w
W w w w
Red hair (R) is dominant over blond hair (r). Make a cross between a heterozygous red head and a blond.
RrRr rr rr
RrRr rr rr
R r
r
r
What percentage of the offspring will have red hair? 50%
Let’s try some more…
In pea plants, tall pea plants (T) are dominant
over short pea plants (t). Construct a Punnett
Square for a heterozygous tall pea plant and a short pea plant.
Tt tt
Tt tt
T t
t
t
What are the percentage of phenotypes?
50% tall
50% short