Lect 2 - Test Crosses

Monohybrid inheritance only involves one gene, one trait Plant height

Dihybrid inheritance involves TWO genes, TWO traits

Page 16 Test Crosses

WINTER Template 01What I know … • 15(c) Predict the results of simple crosses with expected ratios of 3:1 and 1:1, using the terms homozygous, heterozygous, F1 generation and F2 generation

WINTER Template 01What I need to know … • 2(o) Use genetic diagrams to solve problems involving test crosses

Test Cross • An individual that shows the effect of a dominant allele can have two possible genotypes. • Example: A tall pea plant could either be homozygous dominant (TT) or heterozygous (Tt) for the gene => Identical phenotype Genotype Phenotype Environment + TT Tt Pg 16 Pg 16

What can be done to determine the genotype of a tall pea plant? • A TEST CROSS can be carried out. • In a test cross, the individual of unknown genotype (TT or Tt) showing the effect of a dominant allele is crossed with one who is homozygous recessive e.g. tt, at the locus in question. Pg 16 Pg 16

Worked Example • Carry out a genetic cross to determine the genotype of a tall plant • Ans : test-cross Key: Let T represent the allele for tall offspring Let t represents the allele for short offspring The allele for tall offspring (T) is dominant to the allele for short offspring (t) Pg 16

Parental phenotype : Tall plant Short plant Parental genotype : TT X tt Parental gametes : T t Genotype of offspring : Tt Phenotype of offspring : All tall plants A test cross on a tall pea plant whose genotype is TT will produce only tall offspring Pg 16

Parental phenotype : Tall plant Short plant Parental genotype : Tt X tt Parental gametes : T t t Genotype of offspring : Tt tt Phenotype of offspring : Tall plant Short plant Phenotypic ratio of offspring : 1 : 1 A test cross on a tall pea plant whose genotype is Tt will produce tall and short offspring in the ratio of 1 : 1 Pg 16