# Today’s Objectives TSW employ the four primary rules for solving genetics problems. TSW successfully solve genetics crosses involving one and two alleles.

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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 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 Ll ll

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 SSBB SSBb SsBB SsBb SSbb Ssbb ssBB ssBb ssbb

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

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