Solving Crosses

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Introduction to Punnett Squares wxUDTl8 wxUDTl8

Punnett squares - probability diagram illustrating the possible offspring of a mating. Ss X Ss gametes

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

Ratios Genotypic Ratio: the probability that you will see an individual with a certain genotype –Ex. PP : Pp : pp  What is genotypic ratio? Phenotypic Ratio: the probability that you will see an individual with a certain phenotype –Ex. Purple Flowers : White Flowers  What is the phenotypic ratio?

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 Ll lLlll lLlll

Monohybrid Practice In mice, Brown hair (B) is dominant to black hair (b). Researchers crossed a homozygous dominant brown-haired mouse with a black-haired mouse. What are the genotypic and phenotypic ratios for this cross?

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

Practice Determining Genotypes SSBb x Ssbb

Let’s Solve Together In guinea pigs, the allele for short hair (H) is dominant to long hair (h), 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?

Determining Dihybrid Genotypes Short hair = dominant = HH or Hh Long Hair = recessive = hh Black coat = dominant = BB or Bb Brown coat = recessive = bb HhBb x HhBb (gametes done by the FOIL method) –HB, Hb, hB, hb and HB, Hb, hB, hb

Let’s Solve Together In guinea pigs, the allele for short hair (H) is dominant to long hair (h), 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: Punnett Square HBHbhBhb HBHHBBHHBbHhBBHhBb HbHHBbHHbbHhBbHhbb hBHhBBHhBbhhBBhhBb hbHhBbHhbbhhBbhhbb

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

Practice Problem What are the probable offspring for a cross involving a heterozygous short, brown haired guinea pig and a long, brown haired guinea pig? Show your Punnett Square and the percentage of individuals with the different phenotypes.

Ssbb x ssbb Sb sbSb sbSsbb ssbbSsbb sbSsbb ssbbSsbb sbSsbb ssbbSsbb sbSsbb ssbbSsbb

Other Types of Heredity Patterns Incomplete Dominance – blending of traits in heterozygote. –Pink flowers RR = red RW = pink WW = white RW x RW Show Punnett Square

Other Types of Heredity Patterns Codominance – can see both alleles at the same time. –Roan coats in horses RR= red hair RW= some red and some white hairs (roan) WW= white hair RW x RW

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