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.
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
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
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?
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
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
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?
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
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
Other Types of Heredity Patterns • Incomplete Dominance – blending of traits in heterozygote. • Pink flowers • RR = red • Rr = pink • rr = white
Other Types of Heredity Patterns • Codominance – can see both alleles at the same time. • Roan coats in horses • Some white hairs, some red hairs
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
Continuous Variation • Multiple genes are involved • Examples • Eye color • Skin color • Hair color
Sex-linked Genes • Present on the X chromosome • More common in males • When would a female have this phenotype? • Examples: • Baldness • Hemophilia
Some Human Genetic Disorders Of Interest • Cystic Fibrosis • Sickle-cell Anemia • Tay-Sachs Disease • Phenylketonuria (PKU) • Hemophilia • Huntington’s Disease • Muscular Dystrophy