Higher Biology Genetic Crosses Dihybrid Test Crosses Genetic

Higher Biology Genetic Crosses Dihybrid & Test Crosses

Genetic Crosses By the end of this lesson you should be able to: ØKnow what a dihybrid cross is ØKnow how many alleles for each gene there is in a diploid organism ØKnow how to use a Punnett Square for a dihybrid cross ØKnow what a testcross is ØKnow how to carry out a testcross 2

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 1900 s. Mendel carried out a large number of crosses involving pea plants. 3

Introduction ¡ Mendel investigated the following characteristics in pea plants: l l l Flower colour (purple & white) Pod shape (inflated & constricted) Seed shape (round & wrinkled) Seed colour (yellow & green) Stem length (tall & dwarf) 4

Monohybrid Crosses- a reminder 1. What do the following terms refer to? a) b) c) d) e) f) g) h) i) F 1 F 2 Genotype Phenotype Allele Homozygous Heterozygous Dominant Recessive 5

Monohybrid Crosses- a reminder 2. 3. 4. 5. 6. 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 F 1? If two of the F 1 generation from Q 3 were crossed, what would be the phenotypic ratio of the F 2? If two of the F 1 generation from Q 3 were crossed, what would be the genotypic ratio of the F 2? What is a Punnett Square? 6

Dihybrid Crosses ¡ ¡ A dihybrid cross is a cross involving two characteristics at the same time. E. g. in pea plants Purple flowers Short stem ¡ X White flowers Long stem This means that four alleles are involved, two for each gene. 7

Dihybrid Crosses ¡ ¡ Mendel carried out these type of crosses. E. g. in pea plants Round yellow seeded plant ¡ 1. 2. X Wrinkled green seeded plant In the F 1 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? 8

Dihybrid Crosses ¡ Complete the dihybrid cross from your diagram pack. Black hair Green eyes X Ginger hair Blue eyes 9

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. 10

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 11

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. 12

Back Crosses TEST CROSS 1 X ? ? bb 13

Back Crosses ? ? must be BB Why? BB x bb B B b Bb Bb All the offspring have the same phenotype as each other & the dominant parent. 14

Back Crosses TEST CROSS 2 X ? ? bb 15

Back Crosses ? ? must be Bb Why? Bb x bb B b b Bb bb Half the offspring have the same phenotype as the dominant parent & half have the same phenotype as the recessive parent. 16

Practice Questions ¡ ¡ 1. 2. 3. 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 Q 2 AYK page 89 Q 3 AYK pages 101 Q 2. 17

Genetic Crosses Can you do it? ØKnow what a dihybrid cross is ØKnow how many alleles for each gene there is in a diploid organism ØKnow how to use a Punnett Square for a dihybrid cross ØKnow what a testcross is ØKnow how to carry out a testcross 18