CHapter 9 dihybrid crosses

monohybrid cross

• A cross between two individuals that are heterozygous for one character is called a monohybrid cross.

  • Mono – one. 

  • Hybrid - a thing made by combining two different elements (in this case, heterozygous)

• Result: 

  • 1 homozygous dominant

  • 2 heterozygotes

  • 1 homozygous recessive

monohybrid cross

• For complete dominance, this leads to a phenotype ratio of 3:1 (3 dominant individuals for every 1 recessive individual)

• A - Orange flowers; a – purple flowers

• A monohybrid cross of two Aa individuals will result in 

  • 3:1 orange:purple in the offspring.

dihybrid cross

• is a cross between two organisms that are each heterozygous for two characters

  • Di – two

• Remember, the law of independent assortment applies here. Each character (gene) is independent of the other.

Think about this scenario

• If the two genes were linked (not independent) what gametes could F1 individuals make?

• RY would stay together (because they came
  from the same parent) and ry would stay together

• RY and ry

• You would then predict the
  outcome of that cross as:

• 3:1 ratio 

  • 3 yellow round for every

  • 1 green and wrinkled

  • Think about this scenario

• for independent assortment, the genes inherited from a parent don’t have to stay together in the gametes for the next generation.

• RrYy individuals can make 4 different
  types of gametes:

  • RY, Ry, rY, ry

• Every possible combination of one allele for
  each gene

  • FOIL (first, outer, inner, last)

• A dihybrid cross requires a 4x4 Punnett square!

Dihybrid Cross: RrYy x RrYy

• For a dihybrid cross showing complete dominance, you get a ratio of 9:3:3:1.

  • 9 dominant for both traits (yellow, round)

  • 3 dominant for the first and recessive for the second (yellow, wrinkled)

  • 3 recessive for the first and dominant for the second (green, round)

  • 1 recessive for both traits (green, wrinkled)

    

Test cross

• A mating between an individual of unknown genotype and a homozygous recessive individual.

• Why?

  • Because it will reveal any recessive alleles in the unknown individual.

  • Practical use: breeding

• B – black fur

• b – brown fur

Back Cross (test cross)

• A breeding technique for getting new traits into a line without losing existing traits.

• Imagine a fancy rose. It’s gorgeous and has a pleasant smell, but it’s also very frail.

• You decide to cross Fancy Rose with a hardier species: Hardy Rose.

• The offspring are stronger, but not as pretty.

• So you cross the offspring with another Fancy Rose. (original parent or an individual genetically similar/identical)

• The goal is to incorporate new traits without losing the old traits.

Test cross

• By function

  • Testcross is to determine the genotype of an individual with a dominant phenotype.

  • Backcross is to incorporate new characters into an existing genetic line with minimal loss of desired traits.

by process

• Backcross is crossing the offspring with a parent or an individual with a similar/identical genotype to the parent.

• Testcross is crossing the individual of interest with a recessive individual.