Solving Crosses

1. Solving Crosses

2. 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

3. 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

4. Example 1: Monohybrid • 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?

5. 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

6. 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

7. Example 2: Dihybrid • 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?

8. 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

9. Example 2: Punnett Square

10. 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

11. Incomplete Dominance • blending of traits in heterozygote; trait is controlled by both alleles • Pink flowers • RR = red • Rr = pink • rr = white Ex: Japanese 4 o’clock (Mirabilis) RR X WW = RW (pink) http://www.hobart.k12.in.us/jkousen/Biology/inccodom.htm

12. Codominance • can see both alleles at the same time. ex: Roan coats in horses • Some white hairs, some red hairs ex: erminette • Black & white speckeld chickens; heterozygotes ex: human protein for cholesterol level • Heterozygotes produce 2 forms of the protein

13. Multiple Alleles-still only get 2 alleles, but more possibilities of the forms of a gene for a trait • 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

14. Continuous Variation/Polygenic Traits • Multiple genes and often, environmental factors, are involved in an interaction of these genes to produce a trait • Examples: • @ least 3 genes involved in the reddish-brown pigment in the eyes of fruitflies • Skin color, wide range in humans produced by more than 4 genes • Milk Yield in cows • Height, weight, shoe size, hand span **different combos of alleles = very different phenotypes

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

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