Genetic Problem Solving

I

Genetic Problem Solving Each gamete has one gene for

each trait.

After fertilization the new organism has two genes for each trait (Genotype).

Sperm, Egg or Zygote?

What the ?Gene

Homozygous

Eye Colour Gene

HeterozygousHair Colour

Gene

Dominant vs. Recessive Only one allele is expressed in the

phenotype.

The allele which will always be expressed is called the Dominant allele.

The allele which may be hidden is called the Recessive allele.

Punnet Squares Punnet Squares are used to

determine the probability of an offspring having a particular genotype.

We can also work out what the expected phenotype will be.

It is made by comparing all the possible combinations of alleles from the mother with those from the father.

Punnet Square Practice1. If two parents are both

heterozygous for the tongue rolling gene, predict what genotype and phenotype their offspring may have.

Roller allele is dominant (R)Non-roller allele is recessive (r)

Punnet Square Practice

RR

r

rMal

eFemale

RR

r r

Rr

Rr

Genotype Offspring have a

25% change of being RR

50% chance of being Rr

25% chance of being rr

Phenotype Offspring have a

75% chance of being able to roll their tongues

And a 25% chance they can not

Genotype vs. Phenotype Genotype: The specific alleles an

organism possesses for a trait (eg. BB, Bb or bb).

Phenotype: The physical trait an organism shows due to their genotype (Brown eyes or Blue eyes).

Genetic Problem Solving If the two alleles in the genotype

are the same, then the organism is said to have a homozygous genotype and is pure breeding.

If the alleles are different then the organism is heterozygous.

Try This2. Ms. Honeypot crossed a heterozygous

pea plant and a purebred plant with green peas in an attempt to create yellow peas.

In pea plants, yellow peas are dominant over green peas. Use a Punnett square to predict the phenotypic and genotypic outcome (offspring).

Punnet Square Practice

yY

y

yHet

eroz

ygus

Purebred

Yy

yy

Yy

yy

Try This3. In pea plants, round peas are

dominant over wrinkled peas.

Use a Punnett square to predict the phenotypic and genotypic outcome of a cross between a plant homozygous for round peas and a plant homozygous for wrinkled peas.

Punnet Square Practice

rR

r

RRou

ndWrinkled

Rr

Rr

Rr

Rr

Punnet Square Practice

Complete worksheet “More Punnet Practice”

Oink Oink! (Try This!)

Farmer Ted has a curly tailed male pig and wants to know whether it is pure breeding or not. He has asked you (the genetics expert) to find out for him?

Curly tails (T) are dominant over straight tails (t).

Female

Mal

eOink (Answer)

Mate the pig in question witha female pig with a straight tail.

tT

t

TTt

Tt

Tt

Tt

Female

Mal

e

tT

t

tTt

tt

Tt

tt

Conclusions If the male was pure

breeding then 100% of F1 offspring would have curly tails.

If there were ANY F1 offspring with straight tails then the male pig would not be pure breeding.

Test Cross In genetics a test cross is used to

determine if an individual exhibiting a dominant trait is homozygous or heterozygous.

Test crosses determine the genotype of an individual.

Test Cross Test crosses involve breeding the individual in

question with another individual that expresses a recessive version of the same trait.

If all offspring display the dominant phenotype, the individual in question is homozygous dominant.

If any of the offspring displays a recessive phenotype, then the individual is heterozygous.