1. Today’s Objectives
TSW employ the four primary rules for solving genetics problems.
TSW successfully solve genetics crosses involving one and two alleles.
TSW successfully solve monohybrid and dihybrid crosses using the Punnett Square.

2. Solving Crosses

3. Steps for Solving a Genetics Problem:
Trait – dominant = A (AA or Aa) Trait – recessive = a (aa)
___________ x ___________
Punnett Square
Answer questions based on results from Punnett Square
____

4. 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

5. 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?

6. Example 1: Monohybrid
Short hair = dominant = L (LL or Ll) long hair = recssive = l
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

7. Dihybrid Crosses Involves two pairs of contrasting traits
Pea shape and pea color
Coat length and coat color in rodents
Plant height and flower color

8. Let’s Solve Together
In guinea pigs, the allele for short hair (S) is dominant to long hair (s), and the allele for black hair (B) is dominant over the allele for brown hair (b). What is the probable offspring phenotype ratio for a cross involving two parents that are heterozygotes for both traits?

9. Example 2: Dihybrid
Short hair = dominant = SS or Ss Long Hair = recessive = ss Black coat = dominant = BB or Bb Brown coat = recessive = bb
SsBb x SsBb (gametes done by the FOIL method)
SB, Sb, sB, sb and SB, Sb, sB, sb

10. Example 2: Punnett SquareSB Sb sB sb
SSBB
SSBb
SsBB
SsBb
SSbb
Ssbb
ssBB
ssBb
ssbb

11. Example 2: Answer the Question
What is the probable offspring phenotype ratio for a cross involving two parents that are heterozygotes for both traits?
9/16 Black, short coats
3/16 Black, long coats
3/16 Brown, short coats
1/16 Brown, long coats

12. Other Types of Heredity Patterns
Incomplete Dominance – blending of traits in heterozygote.
Pink flowers
RR = red
Rr = pink
rr = white

13. Other Types of Heredity Patterns
Codominance – can see both alleles at the same time.
Roan coats in horses
Some white hairs, some red hairs

14. Multiple Alleles Blood Types in Humans
Single gene, but four phenotypes
Type A  can be AA or Ao
Type B  can be BB or Bo
Type AB  only AB (codominant pattern here)
Type O  only oo (both recessive)
All 3 blood types are dominant to O

15. Continuous Variation Multiple genes are involved Examples Eye color
Skin color
Hair color

16. Sex-linked Genes Present on the X chromosome More common in males
When would a female have this phenotype?
Examples:
Baldness
Hemophilia

17. Some Human Genetic Disorders Of Interest
Cystic Fibrosis
Sickle-cell Anemia
Tay-Sachs Disease
Phenylketonuria (PKU)
Hemophilia
Huntington’s Disease
Muscular Dystrophy