Solving Crosses
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Transcript of Solving Crosses
Solving Crosses
Polleverywhere.com Bell-Ringer
http://www.polleverywhere.com/multiple_choice_polls/LTExNDcxODUyMjI
Introduction to Punnett Squares
http://www.youtube.com/watch?v=Y1PCwxUDTl8
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
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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
L l
l Ll ll
l Ll ll
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 SquareHB Hb hB hb
HB HHBB HHBb HhBB HhBb
Hb HHBb HHbb HhBb Hhbb
hB HhBB HhBb hhBB hhBb
hb HhBb Hhbb hhBb hhbb
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 ssbbSb Sb sb Sb
sb Ssbb Ssbb ssbb Ssbb
sb Ssbb Ssbb ssbb Ssbb
sb Ssbb Ssbb ssbb Ssbb
sb Ssbb Ssbb ssbb Ssbb
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