Understanding Monohybrid Crosses in Genetics: Concepts and Example Problems

Solving Crosses Biology I September 2009

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 = recessive = ll • Ll x ll (heterozygote parent = Ll) • Punnett Square: • Genotype ratio: ½ Ll: ½ ll • Phenotype ratio: ½ short hair: ½ long hair

R R W RW RW W RW RW Incomplete Dominance • – BLENDING of traits in heterozygotes. - 4 O'clock flowers • RR = red • RW = pink • WW = white

Incomplete Dominance

R R W RW RW W RW RW Codominance • – BOTH alleles are expressed at the same time. • Roan coats in horses • Some white hairs, some red hairs • RR = red • RW=roan • WW = white

Multiple Alleles • Blood Types in Humans • Single gene, but four phenotypes • Type A  can be IA IAor IA i • Type B  can be IB IBor IB i • Type AB IA IB (codominant pattern here) • Type O  ii (recessive) • All 3 blood types are dominant to O

Hypothetically…….. • If John has type A blood .... and Ruby has type B blood... Could Farqhar (their love child) be type O? • What are the odds of this happening? • 1 in 4 !

IA i IB i IB i ii IAIB IA i

Sex-linked Traits • Carried on the X chromosome • Are recessive • More common in males because they have only one x • Examples: • Baldness - Muscular Dystrophy • Hemophilia - Colorblindness

Sex linked Traits Phenotype - Genotype • Female – XX • Male - XY • A female can be a carrier. XH Xh • A female can be affected. Xh Xh • A female can be unaffected. XH XH • A male can be affected. Xh Y • A male can be unaffected. XH Y

Understanding Monohybrid Crosses in Genetics: Concepts and Example Problems