Genetic Crosses Dihybrid & Test Crosses Higher Biology Genetic Crosses Dihybrid & Test Crosses

Genetic Crosses By the end of this lesson you should be able to: Know what a dihybrid cross is Know how many alleles for each gene there is in a diploid organism Know how to use a Punnett Square for a dihybrid cross Know what a testcross is Know how to carry out a testcross

Introduction The foundations of genetics were laid by an Austrian monk called Gregor Mendel. He published a paper in 1866, called “Experiments with Plant Hybrids”- but his work was largely ignored until the early 1900s. Mendel carried out a large number of crosses involving pea plants.

Introduction Mendel investigated the following characteristics in pea plants: Flower colour (purple & white) Pod shape (inflated & constricted) Seed shape (round & wrinkled) Seed colour (yellow & green) Stem length (tall & dwarf)

Monohybrid Crosses- a reminder What do the following terms refer to? F1 F2 Genotype Phenotype Allele Homozygous Heterozygous Dominant Recessive

Monohybrid Crosses- a reminder How many traits are involved in a monohybrid cross? If both parents are true-breeding, one dominant and one recessive, what would be the phenotype of the F1? If two of the F1 generation from Q3 were crossed, what would be the phenotypic ratio of the F2? If two of the F1 generation from Q3 were crossed, what would be the genotypic ratio of the F2? What is a Punnett Square?

Dihybrid Crosses A dihybrid cross is a cross involving two characteristics at the same time. E.g. in pea plants X This means that four alleles are involved, two for each gene. Purple flowers Short stem White flowers Long stem

Dihybrid Crosses Mendel carried out these type of crosses. E.g. in pea plants X In the F1 he found that all the plants had round yellow seeds. Which two characteristics do you think are dominant? What term would be used to describe the other two characteristics? Round yellow seeded plant Wrinkled green seeded plant

Dihybrid Crosses Complete the dihybrid cross from your diagram pack. X Black hair Green eyes Ginger hair Blue eyes

Back Crosses Sometimes called a testcross This procedure is used to determine the genotype of an individual with the dominant phenotype. When an organism exhibits a dominant trait (characteristic), it is not obvious whether its genotype is homozygous (true breeding) or heterozygous for that trait.

Back Crosses Example: We have a black mouse (shown below), it could be Bb or BB. How do we know which genotype it has? B=Black b=white

Back Crosses B=Black b=white Is this mouse BB (homozygous) or Bb (heterozygous)? Cross it with a homozygous recessive (bb) and check out the offspring produced.

Back Crosses TEST CROSS 1 X ?? bb

Back Crosses ?? must be BB B Why? b BB x bb All the offspring have the same phenotype as each other & the dominant parent. Bb Bb Bb Bb

Back Crosses TEST CROSS 2 X ?? bb

Back Crosses ?? must be Bb Why? Bb x bb B b Half the offspring have the same phenotype as the dominant parent & half have the same phenotype as the recessive parent. B b Bb bb Bb bb

Practice Questions Complete the “Unlinked Backcross” sheet from your diagram pack. Don’t fill in the boxes in the middle of the page at this stage! Torrance TYK page 93 Q2 AYK page 89 Q3 AYK pages 101 Q2.

Genetic Crosses Can you do it? Know what a dihybrid cross is Know how many alleles for each gene there is in a diploid organism Know how to use a Punnett Square for a dihybrid cross Know what a testcross is Know how to carry out a testcross