Biology RegentsPeriods 2, 4 and 7

What is Genetics?Genetics is the scientific study of heredity

What is a Trait?A trait is a specific characteristic

that varies from one individual to another.Examples: Brown hair, blue eyes, tall,

curly

What is an Allele?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

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

Gregor MendelFather of Genetics1st important studies of

heredityIdentified specific traits in the

garden pea and studied them from one generation to another

Mendel’s Conclusions

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 vs. RecessiveDominant - Masks the other trait; the trait

that shows if presentRepresented 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 presentRepresented by a lower case letter

R

r

Dominant & Recessive Practice

TT - Represent offspring with straight hair

Tt - Represent offspring with straight hair

tt - Represents offspring with curly hair

T – straight hairt - curly hair

Genotype vs. PhenotypeGenotype – 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 looksExample: Curly hair,

straight hair, blue eyes, tall, green

Homozygous vs. HeterozygousHomozygous – 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

Punnett SquaresPunnett Square – Diagram

showing the gene combinations that might result from a genetic cross

Used to calculate the probability of inheriting a particular traitProbability – The chance

that a given event will occur

Punnett SquareParent

Parent Offspring

How to Complete a Punnett Square

Y-Yellowy-white

Genotype:1:2:1

(YY:Yy:yy)

Phenotype:3 Yellow1 White

You Try It Now!Give the genotype and phenotype for the

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

TT x ttStep One: Set Up Punnett Square (put one parent

on the top and the other along the side)

T T t t

TT x ttStep Two: Complete the Punnett Square

T T t t

Tt Tt

Tt Tt

TT x ttStep Three: Write the genotype and phenotype

T T t t

Tt Tt

Tt Tt

Genotype: 4 - Tt

Phenotype:100% Tall

Remember: Each box is 25%

You Try It Now!Give the genotype and phenotype for the

following cross: Tt x tt

Tt x ttStep One: Set Up Punnett Square (put one parent

on the top and the other along the side)

T t t t

Tt x ttStep Two: Complete the Punnett Square

T t t t

Tt tt

Tt tt

Tt x ttStep Two: Complete the Punnett Square

T t t t

Tt tt

Tt tt

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

Phenotype:50% Tall50% Short

Remember: Each box is 25%

Some Terminology P1 – Original parents

F1 – First generation

F2 – Second generation

P1 X P1 = F1

F1 X F1 = F2

Incomplete DominanceIncomplete Dominance - Situation

in which one allele is not completely dominant over another.Example – Red and

white flowers are crossed and pink flowers are produced.

CodominanceCodominance - 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).

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Multiple AllelesMultiple 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.

Examples of Multiple Alleles1. 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.

Examples of Multiple Alleles2. 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 countThere are over 20

different shades of hair color.

Multiple AllelesThere 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 TraitPolygenic 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.