WhatisGenetics?�Genetics is the scientific study of

heredity (i.e. how heritable traits are passed on from generations to generations)

WhatisaTrait?�A trait is a specific characteristic that varies

from one individual to another.� Examples: Brown hair, blue eyes, tall, curly

WhatisanAllele?�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 Eyesa = Blue EyesB = Green Eyesb = Hazel Eyes

WhatareGenes?�Genes are the

sequence of DNA that codes for a protein and thus determines a trait.

GregorMendel�Father of Genetics� 1st important studies of

heredity� Identified specific traits in the garden pea

and studied them from one generation to another

Mendel’sConclusions

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

Dominantvs.Recessive�Dominant - Masks the other trait; the trait that

shows if present� Represented by a capital letter

�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

r

Dominant&RecessivePractice

�TT - Represent offspring with straight hair�Tt - Represent offspring with straight hair� tt - Represents offspring with curly hair

T – straight hairt - curly hair

Homozygousvs.Heterozygous�Homozygous – Term used to

refer to an organism that has two identical alleles for a particular trait (TT or tt)

�Heterozygous - Term used to refer to an organism that has two different alleles for the same trait (Tt)

RR

Rr

rr

Genotypevs.Phenotype� 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

PunnettSquares�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

PunnettSquareParent

Parent Offspring

HowtoCompleteaPunnettSquare

Y-Yellowy-white

Genotype:1:2:1

(YY:Yy:yy)

Phenotype:3 Yellow1 White

YouTryItNow!� Give the genotype and phenotype for the following

cross: TT x tt (T = Tall and t = Short)

TTxttStep One: Set Up Punnett Square (put one parent on the top

and the other along the side)

T Tt

t

TTxttStep Two: Complete the Punnett Square

T Tt

t

Tt Tt

Tt Tt

TTxttStep Three: Write the genotype and phenotype

T Tt

t

Tt Tt

Tt Tt

Genotype: 4 - Tt

Phenotype:100% Tall

Remember: Each box is 25%

YouTryItNow!� Give the genotype and phenotype for the following

cross: Tt x tt

TtxttStep One: Set Up Punnett Square (put one parent on the top

and the other along the side)

T tt

t

TtxttStep Two: Complete the Punnett Square

T tt

t

Tt tt

Tt tt

TtxttStep Two: Complete the Punnett Square

T tt

t

Tt tt

Tt tt

Genotype:Tt - 2 (50%)tt - 2 (50%)

Phenotype:50% Tall50% Short

Remember: Each box is 25%

SomeTerminology�P1 – Original parents�F1 – First generation�F2 – Second generation

�P1 X P1 = F1

�F1 X F1 = F2

IncompleteDominance� 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� 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).

� +

MultipleAlleles�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.

ExamplesofMultipleAlleles1. 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.

ExamplesofMultipleAlleles2. Blood Type – 3 alleles

exist (IA, IB, and i), which results in four different possible blood types

3. Hair Color – Too many alleles exist to count� There are over 20

different shades of hair color.

MultipleAlleles� 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.

PolygenicTrait� 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.