GENETICS AND ADAPTATION Dihybrid Cross Mr G Davidson

Genetics Genetics is the study of inherited characteristics and of those factors that determine or affect inheritance. Inherited characteristics are passed on from generation to generation. August 27, 2019 G Davidson

Genetics In sexual reproduction, gametes (sex cells) are formed by a special kind of cell division called meiosis. A normal cell has two sets of homologous (similar) chromosomes and is referred to as diploid Meiosis produces cells with a single set of chromosomes and is referred to as haploid . August 27, 2019 G Davidson

Genetics Human chromosomes before meiosis August 27, 2019 G Davidson

Genetics When a male gamete combines with a female gamete, a zygote is formed. The single set of chromosomes in each gamete combine to form the normal double set. This cell contains the normal complete chromosome set. August 27, 2019 G Davidson

Genetics Male Gamete with 23 chromosomes 23 Female Gamete with 23 chromosomes Zygote with 46 chromosomes Genetics August 27, 2019 G Davidson

Genetics Sexual reproduction results in new combinations of genes from parents which results in increased variation. This in turn allows a species to evolve and adapt to a changing environment. August 27, 2019 G Davidson

Meiosis and Independent Assortment A human zygote has 46 chromosomes, 23 from each parent, so human cells normally carry 23 pairs of similar homologous chromosomes. Gametes are produced by meiotic cell division. Meiosis occurs in two stages, the first and second meiotic divisions: Meiosis Animation August 27, 2019 G Davidson

Meiosis 1 First meiotic division Early mitosis with 4 chromosomes (2 homologous sets of 2) First meiotic division Homologous chromosomes divide and pair First division of cell leaves one pair in each cell August 27, 2019 G Davidson

Meiosis 2 Second meiotic division Chromosomes separate a second time Creating four cells with a haploid (single) set of chromosomes Second meiotic division August 27, 2019 G Davidson

Meiosis and Independent Assortment In human males, for example, this results in a cell containing 46 chromosomes (called diploid or 2n number) These divide by meiosis to produce 4 sperm cells each containing 23 chromosomes, (the haploid or n number). August 27, 2019 G Davidson

Meiosis and Independent Assortment Homologous chromosomes normally repel each other At meiosis however they pair up The points at which they join are called chiasmata and at this point, exchange of genetic material may occur This is called crossing over. August 27, 2019 G Davidson

Crossing over Original chromatids in early first meiotic division. The letters represent genes. Chiasma cause the formation of new, hybrid chromatids New combinations of genes on one chromosome are created August 27, 2019 G Davidson

Meiosis and Independent Assortment This provides variation among the chromosomes. Because there are 23 pairs of homologous chromosomes, the way they line up on the equator during division is random, making about 8½ million possible alternatives. Formation of chiasmata increase these combinations enormously. August 27, 2019 G Davidson

Dihybrid Cross The dihybrid cross usually involves a cross between parents with a difference in two characteristics. The cross involves two genes in this case and all the possible offspring phenotypes have to be worked out. August 27, 2019 G Davidson

Dihybrid Cross This is done using a 16 square Punnett Square. e.g. In pea plants, Tall is dominant (T) over dwarf (t) and round seeds are dominant (R) over wrinkled seeds (r). August 27, 2019 G Davidson

Dihybrid Cross If we cross a homozygous tall plant with round seeds and a dwarf plant with wrinkled seeds, we get:- Parental Genotypes: TTRR x ttrr Gametes: all TR all tr Therefore all the F1 are: TtRr This gives them the phenotype tall with round seeds. August 27, 2019 G Davidson

Dihybrid Cross If the F1 are now crossed or self pollinated, we get:- F1 Genotypes: TtRr x TtRr Gametes: TR, Tr, tR, tr TR, Tr, tR, tr This is now entered into a Punnett Square August 27, 2019 G Davidson

Dihybrid Cross Gametes TR Tr tR tr TTRR TTRr TtRR TtRr TTrr Ttrr ttRR Phenotypic Ratio 9 Tall Round :3 Short Round : 3 Tall Wrinkled :1 Short Wrinkled August 27, 2019 G Davidson

Dihybrid Cross 9 Tall Round :3 Short Round : 3 Tall Wrinkled :1 Short Wrinkled 10 of these have the same phenotype as the original parents, but the other six are called recombinants because they are new phenotypes. August 27, 2019 G Davidson

Dihybrid Cross If we carry out the test backcross we get:- Parental Genotypes: TtRr x ttrr Gametes: TR Tr tR tr x all tr Draw the Punnett Square … August 27, 2019 G Davidson

Dihybrid Cross Gametes TR Tr tR tr TtRr Ttrr ttRr ttrr Phenotypic Ratio 1 Tall Round : 1 Short Round : 1 Tall Wrinkled : 1 Short Wrinkled August 27, 2019 G Davidson