MENDEL & THE ORIGINS OF GENETICS
Heredity and Genetics
What is Heredity?The passing of genetic
traits from parent to offspring
What is Genetics?
The branch of Biology that
studies heredity
Gregor Mendel
The Father
of Geneti
cs
Gregor Mendel studied genetics in the 1800’s.
He was born in Austria to peasant parents who worked as gardeners.
He studied at the University of Vienna and later became a monk.
Conducted experiments in the garden studying traits in pea plants.
Observed & recorded traits passed from parents to offspring.
Mendel Brainstorm
What were Mendel’s Experiments?
What characteristics did Mendel study?
Mendel’s ExperimentsGregor Mendel
carried out a series of experiments related to how certain characteristics are inherited in pea plants.
The characteristics that Mendel studied include…
flower color flower position plant height pod appearance pod color seed texture seed color
So why did Mendel pick pea plants?
1. Many traits exist in two clearly different forms
2. The mating or crossing of two plants can easily be controlled
3. Pea plants are small, easy to grow, mature quickly, and produce many offspring
Mendel’s Experiments
P Generation (Parental Generation) Mendel allowed two plants to self-pollinate until
all offspring displayed only one characteristic (example, all purple) – this is called true breeding.
A true breeding plant will only allow one trait to show through because it should be either 100% homozygous dominant (purple) or 100% homozygous recessive (white).
Mendel’s Experiments
He then picked two parents, each with contrasting traits (one purple and one white) and cross-pollinated them.
F1 Generation
The offspring of the Parental Generation which Mendel cross-bred; this generation only expressed one trait (the dominant trait, purple)
Mendel’s Experiments
Next, he allowed F1 generation plants to self-pollinate.
F2 Generation
The self-pollinating offspring of the F1 Generation; this generation expressed both traits again, although the dominant (purple) trait was expressed more often (3:1)
Mendel’s Experiments
Mendel’s Experiments - Hypotheses
An individual has two copies of a gene – one from each parent.
There are different versions of each gene. For example, the gene for flower color in pea plants can either be purple or white, represented by letters; P = purple and p = white.
Alleles the different versions of the gene;
represented by letters
One allele is dominant, one is recessive…
Dominant: expressed form of trait (capital allele)
Recessive: not expressed form of trait (lower case allele)
Mendel’s Experiments - Conclusions
Genotype the set of alleles that an individual has
(the actual genes)Ex: BB, Tt, or qq
Phenotype the physical appearance of a trait (how it
actually appears)Ex: Brown eyes or blue eyes; tall or
short
Homozygous Having identical alleles for a trait; either two
dominant alleles or two recessive alleles
Homozygous Dominant = BB or TT or QQ
Homozygous Recessive = bb or tt or qq
Heterozygous Having two different alleles
Bb or Tt or Qq
Mendel’s BIG Conclusions…The Laws of Heredity
Law of Segregation
Two alleles for a trait separate when gametes are formed
Law of Independent Assortment
The inheritance of one trait does not influence the inheritance of another trait
These both occur during Meiosis!
Mendel’s Contributions
On the left side of your IN, write a short paragraph about the importance of Mendel’s research.
STUDYING HEREDITY
Punnett Square
Simple diagram used to predict expected result of a genetic cross
Genetic CrossHh x hh
What is the genotype of
parent 1?
What is the genotype of
parent 2?
Heterozygous Homozygous Recessive
Hh
Hh
hh
hh
Punnett Squares consider all possible combination of gametes for that particular cross
Punnett Squares – Monohybrid Cross
A cross that considers ONE pair of contrasting traits between two individuals
Cross the following!Heterozygous Male x Heterozygous
FemaleGg x GgFrog Color
Key
G – Greeng- brown
G
G
g
g
GG
Gg
Gg
gg
What is the probability that the offspring is heterozygous?
50% What is the probability that the offspring is
homozygous recessive?
25%
What is the probability that the offspring is green?
75% What is the probability that the offspring is
brown?
25%
Punnett Squares – Monohybrid Cross
Cross the following!Heterozygous Female x Homozygous
Recessive Male
Gg x gg
Frog Color Key
G – Greeng- brown
G g
g
g
Gg
Gg
gg
gg
What is the probability that the offspring is green?
50% What is the probability that the offspring is
brown?50%
Punnett Squares – Monohybrid Cross
Cross the following!Homozygous Dominant Female x Homozygous
Recessive Male
RR x rr
Seed of Pea Plants Key
R – Roundr- Wrinkled
r r
R
R RrRr
RrRr
What is the genotype of all of the offspring?Heterozygous
What is the probability the offspring’s phenotype is Round?
100%
Punnett Squares – Monohybrid Cross
Try 1 more cross!Heterozygous Male x Heterozygous
FemaleRr x Rr
Seed of Pea Plants Key
R – Roundr- Wrinkled
R
R
r
r
RR
Rr
Rr
rr
How many offspring would you expect to have the: Genotype RR1/4 or 25% Genotype Rr2/4 or 1/2 or 50% Genotype rr1/4 or 25%
What is the genotypic ratio?
1:2:1
How many offspring would have the following phenotype? Round Seeds 3/4 or 75% Wrinkled Seeds 1/4 or 25%
What is the phenotypic ratio?
3:1
A test cross can be performed to determine whether a parent with a dominant phenotype is homozygous dominant or heterozygous.
In a test cross, an individual whose phenotype is dominant, but whose genotype is unknown, is crossed with a homozygous recessive individual. One can then look at the offspring to determine the genotype of the parent.
Test Cross
Test Cross - examples
P
p
p
Unknown withdominant phenotype
Hom
ozy
gous
rece
ssiv
e
Pp=purpl
e
Pea Plant Flower ColorP = purplep = white
Pp= purpl
e
purple
purple
?So if w
e get 4
purple
offspring,
what must
the other
allele be?
PPp=
Pp=
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