Genetics & Punnett Square Notes

Essential Question

What is Genetics and how

are punnett squares used?

History of Genetics

Father of modern genetics

Studied pea plants

Found that plants that were true-breeding (able to pollinate themselves) produced offspring identical to themselves

Found that when plants reproduced (crosspollination) with other plants that offspring showed a mix of characteristics of both parents

Gregor Mendel

Seed Shape

Flower Position

Seed Coat

ColorSeed Color

Pod Color

Plant Height

Pod

Shape

Round

Wrinkled

Round

Yellow

Green

Gray

White

Smooth

Constricted

Green

Yellow

Axial

Terminal

Tall

Short

Yellow Gray Smooth Green Axial Tall

Section 11-1

Figure 11-3 Mendel’s Seven F1

Crosses on Pea Plants

Go to

Section:

Mendel’s Principles Inheritance of biological characteristics is

determined by genes. Genes are passed from

parents to offspring.

Some forms of a gene may be dominant and

others may be recessive.

Organisms have 2 alleles for each gene

(1 from each parent). These alleles will

segregate during gamete formation.

Alleles for different genes usually

segregate independently from one another.

Mendel’s Laws of InheritanceLaw of Dominance: In heterozygous genotypes,

dominant alleles (capital letters) hide recessive alleles

(lowercase letters) when both are inherited by an

organism. (Yy means the dominant trait will show up

in the phenotype)

Mendel’s Laws of Inheritance

The Law of

Segregation

(separation):

Each organism

has 2 alleles for

a gene. Only

one allele from

each gene pair

will be passed on

to the offspring.

Polygenic Traits

Traits that are controlled by more than 1

gene.

Ex. Human eye color (at least 6 genes)

Ex. Human skin color (at least 4 genes)

Causes a wide variety of phenotypes.

In the case of skin color, this is what

causes the great variety of skin tones.

Genotype vs. Phenotype Genotype

The combination of alleles for a given trait

(the genetic make up)

Ex. TT, Tt, tt

Phenotype

The physical appearance caused by the

interaction of alleles

In general, if a dominant allele is present

then the dominant condition will be shown

Ex. Tall, Short

Genotype vs. Phenotype

Rr

Red color

Wavy hair

bb

Tall

Genotype

Phenotype

Phenotype

Genotype

Phenotype

Alleles

Homozygous = individual with 2 of the

same allele for a given trait

Ex. TT, RR (homozygous dominant)

tt, rr (homozygous recessive)

Heterozygous = individual with 2

different alleles for a given trait

Ex. Tt, Rr, Ww

Allele Practice

TT

Rr

Ww

AA

jj

Homozygous dominant

Heterozygous

Heterozygous

Homozygous dominant

Homozygous recessive

Punnett Squares

Punnett Squares

A Punnett square illustrates how the

parents’ alleles might combine in

offspring.

The top of the Punnett Square are

usually the male’s alleles, the side of the

Punnett square shows the female’s

alleles for the trait

Punnett Squares Examples

1. Cross a homozygous

dominant tall plant

with a homozygous

recessive short plant.

What letters would

you use to represent

this cross?

Dominant trait = tall

Recessive trait = short

T =

t =

tallshort

What is the genotype of a homozygous

dominant tall plant?

Genotype

% TT

% Tt

% tt

TT

TT x tt

What is the genotype of a homozygous

recessive short plant? tt

t t

T

T

Phenotype

% tall

% shortTt Tt

Tt Tt

0

100

0

0

100

2. Cross a heterozygous brown haired rabbit with a homozygous white haired rabbit.

What letters would you use to represent this

cross?

Dominant trait = brown hair

Recessive trait = white hair

B =

b =

brown

white

What is the genotype of a heterozygous

brown haired rabbit?

Genotype

% BB

% Bb

% bb

Bb

Bb x bb

What is the genotype of a homozygous

white haired rabbit? bb

b b

B

b

Phenotype

% brown

% whitebb bb

Bb Bb

0

50

50

50

50

3. Cross a homozygous dominant longhaired guinea pig with a heterozygous guinea pig.

What letters would you use to represent this

cross?

Dominant trait = long hair

Recessive trait = short hair

L =

l =

long

short

Now, you try this one!

What is the genotype of a homozygous

dominant long haired guinea pig? LL

What is the genotype of a heterozygous

long haired guinea pig? Ll

What is the phenotype of the

heterozygous guinea pig?

long hair

Genotype

% LL

% Ll

% ll

LL x Ll

L l

L

L

Phenotype

% long

% short

LL Ll

LL Ll 50

50

0

0

100

Summary

Write 2 questions on each page of notes

(relating to those notes on that page)

Write a 3 sentence summary at the end

over all of the notes.