1. Predicting Results of Monohybrid Crosses

2. Incomplete Dominance a condition in which both
alleles contribute to the
phenotype of a heterozygous
individual to produce a trait
that is not like either parent.
**heterozygous form = A BLEND**
(Example) Snapdragons
Petal color determined by
by two alleles
R = red
R’= white
Phenotypic ratio is
1 red: 2 pink: 1 white
Genotypic ratio
1 RR: 2 R’R: 1 R’R’
Incomplete Dominance

4. Incomplete Dominance Rules
Andalusion Chickens
Pure black x Pure white produces offspring that appear blue
Capital letters with superscripts are often used to represent genotypes in codominance.
Write alleles in alphabetical order when writing gametes or genotypes
F1 generation has a phenotype that is different from that of every parent
Incomplete Dominance Rules

5. 1. Davey Jones from Pirates of the Caribbean
has strange facial growth; tentacles instead
of whiskers. There are two alleles for facial
growth, one that codes for curly tentacles
and one that codes for straight tentacles.
The heterozygous condition produces wavy
tentacles.
Davey Jones has wavy tentacles
and his wife has curly ones, what are the
possible genotypic and phenotypic ratios of
their offspring? 1.

6. 2. Lop-eared rabbits have 2 alleles for ear
length, one codes for long ears (10 inches)
and the other for short ears (4 inches).
The heterozygous condition produces a
rabbit with medium ears (7 inches).
What is the genotypic ratio of a cross
between a long eared rabbit and a medium
eared rabbit? What is the phenotypic ratio? 2.

7. 3. In cats the absence of a long tail allele
results in a manx cat. The presence of a
tail allele results in a long tail cat. A cat
with both alleles is short tailed.
What would be the genotypic & phenotypic
ratios of the possible offspring if you cross
two short tail cats? 3.

9. Codominance- a condition in which two dominant alleles are expressed at the same time.
Neither allele is dominant or recessive.
The alleles do not blend in the phenotype.
Example: Homozygous red cows crossed with homozygous white cows will generate cows with a roan coat. Roan coats consist of a mixture of all red hairs and all white hairs.
Each hair is either all red or
all white therefore it is
codominant!
Codominance

10. How to Show Codominance…
Use capital initial letters for each allele (Since both alleles influence phenotype)
Genotype for homozygous red coat:
Genotype for homozygous white coat:
Genotype for heterozygous animal with roan coat:
How to Show Codominance…

12. 1. The alleles for hair texture in Koala bears
will produce straight hair if homozygous for
the trait, curly hair if homozygous for the
trait or straight & curly if heterozygous for
the trait.
Cross a straight haired koala with a curly
haired Koala. What are the genotypic and
phenotypic ratios of their offspring? 1.

13. 2. Two lop-eared rabbits are bred. One of the
rabbits is black and the other has both
black & grey fur.
What would be the genotypic ratios and
phenotypic ratios of their offspring? 2.

14. 3. Alec has a white mouse & a black mouse.
Black coloration is dominant to white. Over
a period of 2 years their offspring number
90 babies. If 48 of the babies were white,
what are the genotypes of the parents. 3.

15. 4. In Koalas brown fur is dominant over grey
fur. If the pair in the San Diego Zoo
produced 25 offspring in their lifetime and
19 were brown 6 were grey. What did the
zookeepers determine as the genotype
of the parents? 4.

16. Incomplete Dominance Or Codominance???
1. Yellow pigs are crossed with white pigs and produce cream offspring.
2. Radishes can be long, round, or oval.
3. Birds can be black, white , or black with white feathers.
4. A grasshopper can have red stripes, yellow stripes, or both red and yellow.
5. A mouse can be black, white or gray.
Incomplete Dominance Or Codominance???

17. Multiple alleles - for some traits there are more than two alleles in the species
Single individual cannot have more then two alleles for each trait (1 on each homologous chromosome)
Multiple alleles explains why there are four different blood types
(A, B, AB, and O)
Multiple Alleles

18. Multiple Alleles Three alleles code for blood type (A, B, O)
O is the recessive allele
A & B are both dominant over O, but neither one is dominant over the other.
Which genetic relationship does this sound like?
Multiple Alleles

19. Multiple Alleles Use capital letter “I” to show a dominant allele
Use a lower case letter “i” to show the recessive allele
Superscript letters stand for each dominant allele
Six possible genotypes!
Multiple Alleles

20. Blood Antigens
On the surface of red blood cells are chemical structures (proteins) called antigens
Blood plasma contains antibodies- proteins that bind to/attack specific antigens

21. Blood Typing
Blood also contains antibodies – proteins that recognize and bind to certain, specific antigens
Blood Type Antigens on RBC Antibodies Can Receive
A A anti-B A, O
B B anti-A B, O
AB AB none A, B, AB, O
O none both A & B O
(Rh + blood) contains Rh antigen
(Rh – blood) no Rh antigen

22. Why is it bad if you receive the wrong blood type?
If blood containing a specific antigen is mixed with blood containing the corresponding antibodies, the blood clots
Universal donor- (type O blood) no antigens (RBC) to be attacked
Universal recipient- (type AB) no antibodies to attack received blood
Why is it bad if you receive the wrong blood type?

23. Blood Typing
Human serum containing specific antibodies can be purchased
Separately mix a drop of unknown blood sample with drop of each anti-serum
Reaction between blood and anti-serum
No reaction between blood and anti-serum

24. Blood Typing Example
A sample of unknown blood is mixed with three anti-sera samples:
Tube 1 (Anti-A): No reaction
Tube 2 (Anti B): No reaction
Tube 3 (Anti Rh): Cloudy reaction
In terms of the A-B-O and Rh systems, what type blood is the sample?
O positive

25. Sex Chromosomes & Sex Determination
1905 Wilson studied insects
In male, 1 homologous pair or chromosomes did not match
In female, all homologous chromosomes matched
Called the female sex chromosome the X chromosome & the unmatched male chromosome the Y (determine sex
All the other body chromosomes are called autosomes
Male Gametes (X & Y) Female (X & X)
Sex Chromosomes & Sex Determination

26. Sex-linked traits- a trait that is controlled by a gene found on the sex chromosome.
Chance of showing trait is affected by the sex of the individual
Most determined by genes found on the X chromosome
Sex-linked Traits

27. Sex-linked Genes Morgan studied Drosophila melanogaster (fruitflies)
Surprised to see a white-eyed fly (it was male)
Mated white-eyed male with normal red-eyed female resulting in an F1 generation: all red eyes
Conclusion: red eyes is _________ over white
Mated members of F1 generation:
Results: ¾ red eyes, ¼ white eyes
BUT all white eyes were male!!
Concluded that this trait is carried on a sex chromosome! (sex-linked)
Gene for red & white eye carried on the X chromosome… no corresponding allele on the Y chromosome
Sex-linked Genes

28. Therefore, males get eye color from 1 gene…on the X chromosome from mom
For a female to have white eyes, must get two white alleles
Determine genotypes of crossing two individuals from the F1 fruitfly generation:
Crossed male & heterozygous female with red eyes…
Resulted in 3 red eye: 1 white eye (male=white)
= red allele on X chromosome
= white allele on X chromosome
Y = male sex chromosome with no allele for
eye color
Sex-Linked Genes

29. Genotypes if F1 fruitfly generation

30. Human Sex-linked Traits
Color-blindness
Hemophilia
Duschenne muscular dystrophy
Human Sex-linked Traits

32. Testcross Testcross- the crossing of an individual of
unknown genotype with
an individual with a
homozygous recessive
individual to determine
the unknown genotype.
Can determine the
genotype of any individual
Whose phenotype
expresses the dominant
trait.
Testcross