Download Introduction to Genetics

Introduction to Genetics
A. Genetics:
 The scientific study of heredity
1. Heredity: passing on traits and characteristics from one generation
to the next
2. Gregor Mendal
 Russian monk who studied traits in pea plants
 Pea plants grow quickly making the inheritance of traits from
generation to generation easy to see
 He transferred male pollen to the female ovule (pollination)
 Performed a controlled experiment by studying one trait at a
time
 Hybrid: offspring produced by parents with different traits
Ex. Tall x Short
 Cross pure tall plants with pure short plants = Hybrid
offspring
 What do the hybrids look like?
Tall x Short = all tall offspring
WHY?
B. What determines your traits?
1. Genes:
 basic unit of heredity
 a piece of a chromosome that determines a trait
 chromosomes come in pairs
 require 2 matching chromosomes (chromosomes that
have the same genes) in order to produce a trait
2. Alleles:
 different forms of a trait
 2 alleles (genes) required for each trait
 Ex.
Gene = height
Allele = short, tall
Gene = eye color
Allele = blue, brown, green, hazel
Gene = hair color
Allele = blonde, brown, red, black
C. Law of Dominance
1. Some alleles are dominant and some are recessive
2. Those that are dominant express the dominant trait
3. Only one dominant allele needs to be present to have the dominant
trait
How it Works:
1. Pick a trait (gene)
2. Assign a letter for the dominant allele and capitalize it
3. Use the lower case letter to represent the recessive
allele
4. Each parent has 2 alleles/genes for their own traits
5. Each parent will only donate one of their alleles/genes to its
offspring
6. “You are half of your mom and half of your dad”
D. Punnett Square
 A method used to determine the possible offspring
based on the characteristics (traits) of the parent
 Ex.
Pure tall
x
Pure short
TT
tt
T = tall
t = short
t
t
1
T
Tt
2
Tt
3
T
Offspring # 1. Tt
Tt
2. Tt
4
Tt
3. Tt
4. Tt
What height will the offspring be?
ALL TALL
(All of the offspring have 1 dominant allele and one
recessive allele, but will still display the dominant trait)
 Ex.
Pure Yellow
x
Pure green
(Yellow = dom)
y
y
1
Yy
Y
2
Yy
3
Yy
Y
4
Yy
All Yellow offspring
E. Law of Segregation
 Had the recessive allele disappeared because the
dominant allele masked it?
 NO!!!!
 The recessive allele separates (segregates) when forming
gametes (sex cells)
 The recessive trait reappears when crossing hybrids
produced from pure parents
 Ex.
Tt
x
T
Tt
t
1
TT
2
Tt
T
3
Tt
4
tt
t
 Can 2 parents with brown eyes produce offspring with
blue?
F. Phenotypes and Genotypes
 Phenotypes: physical appearance, the way they look or
behave
 Genotype: describe the type of alleles present
a. Homozygous Dominant
 Both alleles are dominant (TT)
b. Heterozygous
 One dominant allele and one recessive
allele (Tt)
c. Homozygous Recessive
 Both alleles are recessive (tt)
1. How are recessive traits expressed?
 Need 2 recessive alleles
Ex. Height
Phenotype
Tall
Short
Genotype
TT
Homoz. Dom
or
Tt
Heterozy.
tt
Homoz. Rec.
**Perform the following crosses and determine which
combination will give you the recessive phenotype for
height.
1. TT x TT
2. TT x Tt
3. Tt x Tt
4. TT x tt
5. Tt x tt
6. tt x tt
G. Law of Independent Assortment
-Genes for different traits segregate (separate) independently during
gamete (sex cell) formation
-This ensures that there are a variety of sex cells with many different
combinations of genes so that no two sex cells are identical.
- Ex. 2 factor cross
Tt Yy x Tt Yy
(tall and yellow)
1. Perform the cross of each parent to determine the combination of
height and color alleles
T
t
T
t
Y
TY
tY
Y
TY
tY
y
Ty
ty
y
Ty
ty
2. Take each of the 8 possible offspring and put them in a square
and perform the cross
TY
tY
Ty
ty
TY
TTYY
TtYY
TTYy
TtYy
tY
TtYY
ttYY
TtYy
ttYy
Ty
TTYy
TtYy
TTyy
Ttyy
ty
TtYy
ttYy
Ttyy
ttyy