Solving Crosses

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

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?

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 Ll lLlll lLlll

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

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?

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: Punnett Square SBSbsBsb SBSSBBSSBBSSBbSsBBSsBbSsBb SbSSBbSSBbSSbbSsBbSsbbSsbb sBSsBBSsBBSsBbssBBssBbssBb sbSsBbSsBbSsbbssBbssbbssbb

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

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)

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

Multiple Alleles -still only 2 alleles, but more possibilities 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/Polygenic Traits Multiple genes 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 **different combos of alleles = very different phenotypes

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