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.
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
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 longhaired mouse?
Example 1: Monohybrid
Short hair = dominant = L (LL or Ll) long hair = recssive = l Ll x ll (heterozygote parent = Ll) L l Punnett Square: l Ll ll l Ll ll 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: Punnett Square
SB SB Sb
sB sb
Sb
sB
sb
SSBB SSBb SsBB SsBb SSBb SSbb SsBb
SsBB SsBb SsBb Ssbb ssBB ssBb
Ssbb
ssBb ssbb
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