 Genetics is the scientific study of heredity

 A trait is a specific characteristic that varies from one individual to another.
 Examples: Brown hair, blue eyes, tall, curly

 Alleles are the different possibilities for a given trait.
 Every trait has at least two alleles (one from the mother and one from the father)
 Example: Eye color –
Brown, blue, green, hazel
Examples of Alleles:
A = Brown Eyes a = Blue Eyes
B = Green Eyes b = Hazel Eyes

 Genes are the sequence of DNA that codes for a protein and thus determines a trait.

 Father of Genetics
 1 st important studies of heredity
 Identified specific traits in the garden pea and studied them from one generation to another

1.
Law of Segregation – Two alleles for each trait separate when gametes form; Parents pass only one allele for each trait to each offspring
2.
Law of Independent Assortment – Genes for different traits are inherited independently of each other

 Dominant - Masks the other trait; the trait that shows if present
 Represented by a capital letter
R
 Recessive – An organism with a recessive allele for a particular trait will only exhibit that trait when the dominant allele is not present; Will only show if both alleles are present
 Represented by a lower case letter r

T – straight hair t - curly hair
 TT - Represent offspring with straight hair
 Tt - Represent offspring with straight hair
 tt - Represents offspring with curly hair

 Genotype – The genetic makeup of an organism;
The gene (or allele) combination an organism has.
 Example: Tt, ss, GG, Ww
 Phenotype – The physical characteristics of an organism; The way an organism looks
 Example: Curly hair, straight hair, blue eyes, tall, green

 Homozygous – Term used to refer to an organism that has two identical alleles for a particular trait ( TT or tt )
RR rr
 Heterozygous - Term used to refer to an organism that has two different alleles for the same trait
( Tt )
Rr

 Punnett Square – Diagram showing the gene combinations that might result from a genetic cross
 Used to calculate the probability of inheriting a particular trait
 Probability – The chance
that a given event will occur

Parent
Offspring
Parent

How to Complete a Punnett Square

Y-Yellow y-white
Genotype:
1:2:1
(YY:Yy:yy)
Phenotype:
3 Yellow
1 White

 Give the genotype and phenotype for the following cross: TT x tt (T = Tall and t = Short)

Step One: Set Up Punnett Square (put one parent on the top and the other along the side)
T T
t
t

Step Two: Complete the Punnett Square
T T
t Tt Tt
t
Tt Tt

Step Three: Write the genotype and phenotype
T T
t Tt Tt
t
Tt Tt
Genotype:
4 - Tt
Phenotype:
100% Tall
Remember: Each box is 25%

 Give the genotype and phenotype for the following cross: Tt x tt

Step One: Set Up Punnett Square (put one parent on the top and the other along the side)
T t
t
t

Step Two: Complete the Punnett Square
T t
t Tt tt
t
Tt tt

Step Two: Complete the Punnett Square
T t
t Tt tt
t
Tt tt
Remember: Each box is 25%
Genotype:
Tt - 2 (50%) tt - 2 (50%)
Phenotype:
50% Tall
50% Short

 P
1
– Original parents
 F
1
– First generation
 F
2
– Second generation
 P
1
X P
1
= F
1
 F
1
X F
1
= F
2

 Incomplete Dominance - Situation in which one allele is not completely dominant over another.
 Example – Red and white flowers are crossed and pink flowers are produced.

 Codominance - Situation in which both alleles of a gene contribute to the phenotype of the organism.
 Example – A solid white cow is crossed with a solid brown cow and the resulting offspring are spotted brown and white (called roan).

+

 Multiple Alleles- Three or more alleles of the same gene.
 Even though three or more alleles exist for a particular trait, an individual can only have two alleles - one from the mother and one from the father.

1.
Coat color in rabbits is determined by a single gene that has at least four different alleles. Different combinations of alleles result in the four colors you see here.

2.
3.
Blood Type – 3 alleles exist (I A , I B , and i), which results in four different possible blood types
Hair Color – Too many alleles exist to count
 There are over 20 different shades of hair color.

 There Are Always Multiple Alleles!
 Genetic inheritance is often presented with straightforward examples involving only two alleles with clear-cut dominance. This makes inheritance patterns easy to see.
 But very few traits actually only have two alleles with clear-cut dominance. As we learn more about genetics, we have found that there are often hundreds of alleles for any particular gene.
 We probably know this already - as we look around at other people, we see infinite variation.

 Polygenic Trait - Trait controlled by two or more genes .
 Polygenic traits often show a wide range of phenotypes.
 Example: The wide range of skin color in humans comes about partly because more than four different genes probably control this trait.