Download Basic Mendelian Genetics Lecture

Mendelian Inheritance
Punnett squares, pea plants, and a monk

Austrian monk who studied the inheritance of
pea plants.

Mendel used pea plants because:

Could be grown in small area

Produced lots of offspring

Could produce “pure” plants when selfpollinated

Could be artificially pollinated (he could
control breeding between plants himself)

His experiments led him to discover the Laws
of Inheritance

Let’s define some important terms regarding
Mendel’s work….

Alleles


Dominant Allele


The trait that is only expressed if there are 2 copies of
this allele and it is suppressed in a hybrid organism.
Represented by a lower case letter (ex. “r”)
Homozygous Genotype


The trait expressed if it is present in the genotype.
Represented by a capital letter (ex. “R”)
Recessive Allele


Alternate form of a gene. Ex:
“Purebred” or consisting of either 2 dominant alleles or
2 recessive alleles (ex. “RR” or “rr”)
Heterozygous Genotype

“Hybrid” or a combination of 1 dominate allele and 1
recessive allele (ex. “Rr”)
A parent may have two distinct alleles for a certain gene, each on one copy of a
given chromosome

The Law of Dominance


The Law of Segregation


A dominant trait is a trait whose appearance will always
be seen in offspring.
Alleles are segregated during the formation of gametes.
The Law of Interdependent Assortment

The alleles for different genes mix up independently of
each other and are shuffled during meiosis.
Purple
Flower
(P)
White
Flower
(p)
 Which
of the following is NOT one of
Gregor Mendel’s principles?
Some
forms of genes may be dominant
Alleles
are segregated when gametes are
formed
Crossing
The
over occurs during meiosis
alleles for different genes usually
segregate interdependently
 Which
of the following is NOT one of
Gregor Mendel’s principles?
Some
forms of genes may be dominant
Alleles
are segregated when gametes are
formed
Crossing
The
over occurs during meiosis
alleles for different genes usually
segregate interdependently

Genotype


Phenotype


The physical trait seen in an organism resulting from
a genotype (EX: red flowers or being tall)
Genotypic ratio


The gene combination for a trait, the allele letters
(EX: RR, Rr, rr)
The ratio (# of each possibility) of a certain genotype
seen in the offspring of a theoretical cross.
Phenotypic ratios

The ratio (# of each possibility) of a certain
phenotype seen in the offspring of a theoretical
cross.

Below are 2 different traits you can see in pea plants.
Discuss with a partner how you can tell which traits are
dominate and which are recessive.
1.) Seed shape --- Round (R) or Wrinkled (r)
2.) Seed Color ---- Yellow (Y) or Green (y)

What would be the genotype of a heterozygous round
seed?

What would be the phenotype of the genotypes “YY” and
“Yy”?

Below are 2 different traits you can see in pea plants.
Discuss with a partner how you can tell which traits are
dominate and which are recessive.
1.) Seed shape --- Round (R) or Wrinkled (r)
2.) Seed Color ---- Yellow (Y) or Green (y)

What would be the genotype of a heterozygous round
seed?
Rr

What would be the phenotype of the genotypes “YY” and
“Yy”?

Below are 2 different traits you can see in pea plants.
Discuss with a partner how you can tell which traits are
dominate and which are recessive.
1.) Seed shape --- Round (R) or Wrinkled (r)
2.) Seed Color ---- Yellow (Y) or Green (y)

What would be the genotype of a heterozygous round
seed?
Rr

What would be the phenotype of the genotypes “YY” and
“Yy”?
Homozygous yellow and Heterozygous yellow

Parental P1 Generation
 the
parental or starting generation in a
breeding experiment.

F1 Generation
 the
first-generation offspring in a breeding
experiment from breeding individuals from
the P1 generation.

F2 Generation
 the
second-generation offspring in a breeding
experiment from breeding individuals from
the F1 generation.

Trait: Seed Shape

Alleles: R – Round

Cross: Round seeds
RR x

r
copyright cmassengale
r – Wrinkled
x Wrinkled seeds
rr
r
R
Rr
Rr
R
Rr
Rr
Genotype: Rr
Phenotype: Round
Genotypic
Ratio: All alike
Phenotypic
Ratio: All alike
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1. All offspring ended up as?
Heterozygous for round seeds (Rr)
2. Which generation is the offspring?
F1 generation
3. Genotypic & Phenotypic ratios ended
up being?
ALL ALIKE
copyright cmassengale
14

Trait: Seed Shape

Alleles: R – Round

Cross: Round seeds

Rr
R
r – Wrinkled
x
x Round seeds
Rr
r
R
RR
Rr
r
Rr
rr
Genotype: RR, Rr, rr
Phenotype: Round &
wrinkled
G.Ratio: 1:2:1
P.Ratio: 3:1
15
copyright cmassengale
 Heterozygous x heterozygous
 Offspring:
25% Homozygous dominant RR
50% Heterozygous Rr
25% Homozygous Recessive rr
 Offspring called F2 generation
 Genotypic ratio is 1:2:1
 Phenotypic Ratio is 3:1
copyright cmassengale
16
 Traits:
Seed shape & Seed color
 Alleles: R round
r wrinkled
Y yellow
y green
RrYy
RY Ry rY ry
x
RrYy
RY Ry rY ry
All possible gamete combinations
copyright cmassengale
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RrYy
RY
x
Ry
RrYy
rY
ry
RY
Ry
rY
copyright cmassengale
ry
18
RrYy
RY
RY RRYY
Ry RRYy
rY RrYY
ry
copyright cmassengale
RrYy
x
RrYy
Ry
rY
ry
RRYy
RrYY
RrYy
RRyy
RrYy
Rryy
RrYy
rrYY
rrYy
Rryy
rrYy
rryy
Round/Yellow:
Round/green:
9
3
wrinkled/Yellow: 3
wrinkled/green:
1
9:3:3:1 phenotypic
ratio
19
Round/Yellow:
RrYy
RY
RY RRYY
Ry
RRYy
x
RrYy
rY
RrYY
ry
RrYy
Ry RRYy
RRyy
RrYy
Rryy
rY RrYY
RrYy
rrYY
rrYy
RrYy
Rryy
rrYy
rryy
ry
copyright cmassengale
Round/green:
9
3
wrinkled/Yellow: 3
wrinkled/green:
1
9:3:3:1 phenotypic
ratio
1:2:1:2:4:2:1:2:1
Genotypic ratio
20
RrYy
x
RrYy
Round/Yellow: 9
Round/green:
3
wrinkled/Yellow: 3
wrinkled/green: 1
9:3:3:1
copyright cmassengale
21

For Dihybrid Crosses
 https://www.youtube.com/watch?v=1p47ndHOUHs
 https://www.youtube.com/watch?v=qIGXTJLrLf8
 https://1.bp.blogspot.com/-N3t6k33qcJU/VXH-
t9L07XI/AAAAAAAABac/6amOxX5Lptw/s1600/Foil%2BMe
thod%2BGIF.gif