Genetics
The study of heredity
• Genes are found on chromosomes.
Chromosomes are made up of DNA.
Alleles: Different forms of a gene.
Dominant: If the dominant allele (gene) is present, the
organism will show that trait.
Recessive: Trait will be exhibited only if organism does not
have dominant allele or is pure for recessive trait.
Gregor Mendel (1822-1884)
An Austrian monk who cultivated and studied garden pea
plants. His studies produced the basic concepts of heredity.
Genotype: The Allele combination, heterozygous
(hybrid) or homozygous (gg, GG, or Gg).
Phenotype: The physical characteristic displayed (green
or yellow).
Trait
Dominant
Allele
Recessive
Allele
Possible
Genotype
Possible
Phenotypes
Seed
Color
G (green)
g (yellow)
GG
Homozygous
Gg
Heterozygous
gg
Green
Flower
Color
P (purple)
p (white)
PP
Homozygous
Pp
Heterozygous
pp
Green
Yellow
Purple
Purple
white
Why Pea Plants?
•Many observable contrasting
traits.
•Easy to grow.
•Short time between one
generation and the next.
Structure of flower
Cross Pollination
Punnett Square
In Pea plants Green pods (G) are dominant over yellow (g) pods.
Genotype
GG homozygous
Gg heterozygous
gg
Phenotype
Green Pod
Green Pod
Yellow Pod
P Cross : GG x gg
g
Possible Gametes : ____ ____ X ____ ____
g
G
Gg
Gg
Green Pod
Green Pod
G
Gg
Gg
Green Pod
Green Pod
F1 Genotypic ratio:
F1 Phenotypic ratio:
F1 Cross: Gg x Gg
Possible Gametes: ____ ____ X ____ ____
F2 Genotypic ratio:
F2 Phenotypic ratio:
In mice black is dominant over white; B – black, b- white, BB
homozygous black, Bb heterozygous black, bb recessive white
Cross two heterozygous black mice, Bb x Bb
Cross : Bb x Bb
Possible Gametes : ____ ____ X ____ ____
Genotypic ratio:
Phenotypic ratio:
Test Cross
Is this back mouse homozygous black BB or heterozygous black Bb?
One way to find out is to do a Test Cross.
Cross this black mouse with the recessive white bb.
BB x bb
Bb x bb
If any of the offspring produced are white or show the recessive trait, then the
black mouse had to be heterozygous Bb.
If all of the offspring produced are black, you cannot be 100% positive that the
black mouse is homozygous BB because the inheritance of a an allele is a
50/50 chance.
Monohybrid Cross: Rr x Rr
Dihybrid Cross: RrYy x RrYy
Dihybrid Cross
2 traits; plant height and seed color
P Cross: TTRR ( pure tall & round) x ttrr (pure short & wrikled)
_____ _____ ______ _____
_____ _____ _____ _____
F1 Cross TtRr (hybrid tall & round) x TtRr (hybrid tall & round)
_____ _____ _____ _____
_____ _____ _____ _____
F2 Genotype
F2 Phenotype
Incomplete Dominance
Genotype
RR
WW
RW
Phenotype
Red flower
White flower
Pink flower
P Cross: RR x WW
W
R
R
W
RW
RW
Pink
Pink
RW
RW
Pink
Pink
F1 Genotype: 100% RW
F1 Phenotype: 100% Pink flower
Co dominance
Genotype
RR
WW
RW
Phenotype
Red cow
White cow
Roan cow
P Cross: RR x WW
R
R
W
RW
W
RW
Roan
Roan
RW
RW
Roan
Roan
Genotype of F1 offspring: 100% RW
Phenotype of F1 offspring: 100% Roan
Multiple Alleles
Blood Types
Allele Antigen on RBC
IA
A
IB
B
i
none
Genotype
IA IA homozygous
IA i heterozygous
IB IB homozygous
IB i heterozygous
IA IB
ii
Phenotype
Type A
Type A
Type B
Type B
Type AB
Type O
Crosses Involving Multiple Alleles
Cross: a man who is homozygous
for blood type A with a woman
who is homozygous blood type B.
IA IA x IB IB
IB
IA
IA
IB
IA IB IA IB
Type AB
Type AB
IA IB IA IB
Type AB
Cross: a man who is heterozygous
for blood type A with a woman
who is blood type O.
IA i x i i
Type AB
i
i
IA
IA i
IA i
Type A
Type A
i
ii
ii
Type O
Type O
Sex Determination
Sex Linked Traits
Traits located on the X chromosome
Genotype
XX
XcX
XcXc
XY
XcY
Cross: A normal male with a carrier
female. XY x XcX
X
Xc
X
Xc
X
Phenotype
Normal Female
Carrier Female
Colorblind Female
Normal Male
Colorblind Male
Cross: A colorblind male with a carrier
female. XcY x XcX
Y
X cY
Carrier
Female
Colorblind
Male
XX
XY
Normal
Female
Normal Male
Xc
X
Xc
Y
Xc Xc
Xc Y
Colorblind
Female
Colorblind
male
Xc X
XY
Carrier
Female
Normal
Male
Human Pedigree Key
Human Pedigree Autosomal Recessive
Human Pedigree Autosomal Dominant
Human Pedigree Sex linked Recessive
http://www.zerobio.com/drag_gr11/pedigree/pedigree4.htm
Chromosomes
Karyotype
Female XX
Male XY