1. Dihybrid crosses and gene linkage
HL Genetics Topic 10.2

2. Assessment Statements
Calculate and predict the genotypic and phenotypic ratio of offspring of dihybrid crosses involving unlinked autosomal genes.
Distinguish between autosomes and sex chromosomes.
Explain how crossing over between non-sister chromatids of a homologous pair in prophase I can result in an exchange of alleles.
Define linkage group.
Explain an example of a cross between two linked genes.
Identify which of the offspring are recombinants in a dihybrid cross involving linked genes.

3. Dihybrid crosses Mendel’s Peas
Seed shape – some round, others wrinkled (allele for round is dominant)
Seed colour – some green, others yellow (allele for yellow is dominant)
Mendel crossed true breeding plants with each other
One parent: homozygous dominant for both traits (round and yellow seeds) RRYY
Other parent: homozygous recessive for both traits (wrinkled and green) rryy

4. When both parents are homozgous – all the F1 offspring are the same genotype and phenotype
Parent phenotypes
Round yellow
Green wrinkled
Parent genotypes
RRYY
rryy
Parent gametes RY ry
F1 genotypes
RrYy
F1 phenotypes
R = allele for round peas
r = allele for wrinkled peas
Y = allele for yellow peas
y = allele for green peas

5. Cross the F1 double heterozygous
Allowing heterozygous offspring to self-pollinate
Phenotype Ratio is: 9 3 1

6. Dihybrid Phenotype Ratios
Homozygous parents AABB x aabb All F1 offspring the same
Heterozygous parents AaBb x AaBb 9:3:3:1 phenotype ratio 9AB 3Ab 3aB 1ab
A new shuffling of the alleles has created a new combination which does not match either of the parents’ genotypes. Recombinants

7. Autosomes and sex chromosomes
Sex chromosomes: X and Y (one pair)
Autosomes: any chromosome not X or Y (22 pairs)
Sex-linked gene is located on a sex chromosome.
Autosomal gene is located on one of the autosomes.
On which type of chromosome is the gene for haemophilia found?
So, the gene is known as ___.
On which type of chromosome is the gene for protein production in the pancreas found?

8. Exchange of alleles by crossing over
Two non-sister chromatids can swap segments of their DNA during prophase I of meiosis.
This increases genetic variety of chromosomes in gametes

9. There are now thought to be 20,500 human genes on 23 chromosomes Source: Broad Institute of MIT and Harvard (2008, January 15). Human Gene Count Tumbles
There must be approx 1000 genes on each chromosome.

10. Linkage group
Any two genes which are found on the same chromosome are said to be linked to each other.
Linked genes are usually passed on to the next generation together.
Linkage group - groups of genes on the same chromosome inherited together
Linked genes are the exception to Mendel’s law of independent assortment

11. Linked genes
Fruit fly gene for body color is in the same linkage group as the gene for wing length Alleles are G – grey body g – black body L – long wings l – short wings
Notation of linked genes is G L The two horizontal bars symbolize homologous chromosomes and that the locus of G is on the same chromosome as L
G L
G L

12. Offspring of a dihybrid cross
A cross between homozygous fruit flies
GGLL x ggll
Grey body & long wings x Black body, short wings
F1 flies will be all heterozygous for both of the traits
GgLl
Phenotype : Grey body & long wings
If these F1 heterozygotes were allowed to reproduce together the ratio of phenotypes produced would be expected to be
9:3:3:1

13. Recombinants After the F2 flies were identified the phenotypes were;
Grey long wing
Grey short wing
Black long wing
Black short wing
1600
1020
210
200
170
This is far from the expected 9:3:3:1 ratio
which should give
This is a sign that the genes are on the same chromosome (genes are linked)

14. Offspring of a dihybrid cross using linkage notation
A cross between homozygous fruit flies
GL gl
GL x gl
Grey body & long wings x Black body, short wings
F1 flies will be all heterozygous for both of the traits GL gl
Phenotype : Grey body & long wings
If these F1 heterozygotes were allowed to reproduce together the ratio of phenotypes produced would be expected to be
9:3:3:1

15. Offspring of a dihybrid cross using linkage notation
If these F1 heterozygotes make gametes
The gametes will be
GL or gl
unless crossing over happens – because the genes are on the same chromosome
The F1 will contain more than expected of these genotypes
GL GL gl gl
GL gl GL gl

16. Offspring of a dihybrid cross using linkage notation
If The F1 will contain more than expected of these genotypes The Grey short wing and black long wing phenotypes Are Recombinants -
Grey long wing
Grey short wing
Black long wing
Black short wing
1600
1020
210
200
170
GL GL gl gl
GL gl GL gl

17. Polygenic Inheritance
Topic 10.3

18. Assessment Statements
Define polygenic inheritance.
Explain that polygenic inheritance can contribute to continuous variation using two examples, one of which must be human skin colour.

19. Polygenic Inheritance
when two or more genes influence the expression of one trait
Eg Skin Colour, Height

20. Continuous and discontinuous variation
When an array of possible phenotypes can be produced, it is called continuous variation
Examples: skin color, height, body shape, and intelligence
These traits are also influenced by environmental conditions
When only a number of phenotypes can be produced, it is called discontinuous variation
Examples: earlobe attachment, blood group

21. Graphical representation
Continuous variation
Discontinuous variation

22. Eye Color
Iris is made up of zones, rings, streaks or speckles of different colored pigments with varying intensities
What color are your eyes, really?
Since there is so much variety, eye color must be influenced by multiple alleles and has continuous variation.

23. Skin color
How does the existence of multiple alleles controlling skin colour result in the appearance of many different shades of skin colour in humans?

24. Thoughts
How do people of varying degrees of skin color relocated to parts of the world that receive differing amounts of sunlight get vitamin D? How do others fight off the sun?
Should there be equal esteem for all humans?
Why is human diversity so often used to divide and discriminate, rather than be appreciated, respected, and celebrated?

25. Polygenic inheritance of color in wheat.
Kernal color in wheat is determined by two genes. A range of colors occur, from white to dark red, depending on the combinations of alleles. Dark red plants are homozygous AABB and white plants are homozygous aabb.

26. the F1 offspring are all double heterozygous AaBb.
Crossing individuals with the phenotype extremes yield offspring that are a 'blend' of the two parents.
When these homozygous phenotypes are crossed
AABB x aabb
Dark x white
the F1 offspring are all double heterozygous AaBb.

27. But what happens when the two double heterozygous genotypes are crossed?  
Parent Phenotypes: all brown
Genotypes: AaBb x AbBb
Punnet square:
There is no blending in the offspring. Offspring can be more extreme than either parent
There are grades of colour – evidence of polygenic inheritance. AB Ab aB ab
AABB
AABb
AaBB
AaBb
AAbb
Aabb
aaBB
aaBb
aabb

28. Skin color
How does the existence of multiple alleles controlling skin colour result in the appearance of many different shades of skin colour in humans?

29. Human skin colour is controlled by multiple alleles (and the environment)
It is known that at least three genes control skin color, let’s call them genes A, B, and C.
Someone who is AABBCC would have very dark skin color and someone who is aabbcc would have very light skin color.
If they got married and had children, their children would all be AaBbCc and have mid-brown skin.
If two of those people would get married and have children, the Punnett square would look like the one above.