1. Solving Crosses
Biology I
Fall 2015

2. Monohybrid Crosses Cross that involves one pair of contrasting traits
Solve using Punnett Square
Sample problems:
Rr x rr
RR x rr
Rr x Rr
Rr x RR

3. Let’s Solve Together
Short hair (L) is dominant to long hair (l) in mice. What is the genotype and phenotype ratio of a heterozygous short-haired mouse crossed with a long-haired mouse?

4. Example 1: Monohybrid
Short hair = dominant = L (LL or Ll) long hair = recessive = ll
Ll x ll (heterozygote parent = Ll)
Punnett Square:
Genotype ratio: ½ Ll: ½ ll
Phenotype ratio: ½ short hair: ½ long hair L l Ll ll

5. Incomplete Dominance – BLENDING of traits in heterozygotes.
- 4 O'clock flowers
RR = red
RW = pink
WW = white R W RW

6. Incomplete Dominance

7. Codominance – BOTH alleles are expressed at the same time.
Roan coats in horses
Some white hairs, some red hairs
RR = red
RW = roan
WW = white R W RW

8. Multiple Alleles Blood Types in Humans
Single gene, but four phenotypes
Type A  can be IA IA or IA i
Type B  can be IB IB or IB i
Type AB IA IB (codominant pattern here)
Type O  ii (recessive)
All 3 blood types are dominant to O

9. Hypothetically…….. If John has type A blood ....
and Ruby has type B blood...
Could Farqhar (their love child) be type O?
What are the odds of this happening?
1 in 4 !

10. IA IB i
IB i ii
IA IB
IA i

11. Sex-linked Traits Carried on the X chromosome Are recessive
More common in males because they have only one x
Examples:
Baldness Muscular Dystrophy
Hemophilia Colorblindness

12. Sex linked Traits Phenotype - Genotype Female – XX Male - XY
A female can be a carrier. XH Xh
A female can be affected. Xh Xh
A female can be unaffected. XH XH
A male can be affected. Xh Y
A male can be unaffected. XH Y