Solving Crosses Biology I September 2009 Monohybrid Crosses

Solving Crosses Biology I September 2009

Monohybrid Crosses n Cross that involves one pair of contrasting traits n Solve using Punnett Square n Sample problems: – Rr x rr – RR x rr – Rr x RR

Let’s Solve Together n 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 n n n Short hair = dominant = L (LL or Ll) long hair = recessive = ll Ll x ll (heterozygote parent = Ll) L l Punnett Square: l Ll ll Genotype ratio: ½ Ll: ½ ll Phenotype ratio: ½ short hair: ½ long hair

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

Incomplete Dominance

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

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

IA IB i IA i i ii

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

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