1. GENETICS AND ADAPTATION
Dihybrid Cross
Mr G Davidson

2. 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

3. 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

4. Genetics
Human chromosomes before meiosis
August 27, 2019
G Davidson

5. 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

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

7. 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

8. 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

9. 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

10. 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

11. 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

12. 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

13. 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

14. 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

15. 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

16. 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

17. 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

18. 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

19. 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

20. 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

21. 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

22. 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