 Each gamete has one gene for each trait.  After fertilization the new organism has two genes for each trait (Genotype).

Gene Homozygous Eye Colour Gene Heterozygous Hair Colour Gene

 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 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.

1. 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)

R R r r Male Female RR r Rr

 Offspring have a 25% change of being RR  50% chance of being Rr  25% chance of being rr  Offspring have a 75% chance of being able to roll their tongues  And a 25% chance they can not

 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).

 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.

2. 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).

y Y y y Heterozygus Purebred Yy yy Yy yy

3. 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.

r R r R Round Wrinkled Rr

 Complete worksheet “More Punnet Practice”

 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 Male  Mate the pig in question with a female pig with a straight tail. t T t T Tt Female Male t T t t Tt tt Tt tt

 If the male was pure breeding then 100% of F 1 offspring would have curly tails.  If there were ANY F 1 offspring with straight tails then the male pig would not be pure breeding.

 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 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.