Solving Crosses Biology I Fall 2015

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 L l Ll ll

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

Incomplete Dominance

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 R W RW

Multiple Alleles Blood Types in Humans Single gene, but four phenotypes Type A  can be IA IA or IA i Type B  can be IB IB or 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 IB i IB i ii IA IB 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