Genetic Crosses How to predict the probable genetic makeup and appearance of offspring resulting from specified crosses.

Genotype The genetic makeup of an organism. Consists of the alleles that the organism obtains from its parents. Ex. Blue eyes is bb and brown eyes could be BB or Bb.

Phenotype The appearance of an organism as a result of its genotype. What the characteristic of the organism looks like. i.e. does it have blue eyes or brown eyes.

Homozygous When both alleles of a pair are alike. An organism may be homozygous dominant (BB) or homozygous recessive (bb).

Heterozygous When the two alleles in the pair are different. Ex. Bb Ex. Bb

Predicting Results Monohybrid cross: A cross between individuals that involves 1 pair of contrasting traits. Ex. A cross between a blue eyed person and a brown eyed person. A Punnett square is used.

Homozygous X Homozygous A monohybrid cross between a person homozygous for brown eyes and a person homozygous for blue eyes.

Homozygous X Heterozygous A monohybrid cross between a person homozygous for blue eyes and a person heterozygous for brown eyes.

Heterozygous X Heterozygous A monohybrid cross between a person heterozygous for brown eyes and a person heterozygous for brown eyes.

Genotypic Ratio The ratio of the genotypes that appear in offspring. Homozygous X homozygous: 4 Bb Homozygous X Heterozygous: 2 Bb: 2bb 2 Bb: 2bb Heterozygous X heterozygous: 1BB:2Bb:1bb

Phenotypic Ratio The ratio of the offspring’s phenotypes or what they look like. Homozygous X homozygous: 4 brown Homozygous X Heterozygous: 2 brown and 2 blue Heterozygous X heterozygous: 3 brown and 1 blue

Incomplete Dominance When offspring have a phenotype that is in between that of the parents. Ex. Four o’clock flowers. RR is red, Rr is pink and rr is white.

Four O’Clock Flowers

Codominance Occurs when both alleles for a gene are expressed in a heterozygous offspring. Neither allele is dominant or recessive nor do they blend in the phenotype. Ex. The color roan coat color. The genes for both white coat and red coat are expressed.

Roan Coat Color

Dihybrid Crosses A cross between individuals that involves two pairs of contrasting traits.

Homozygous X Homozygous This Punnett square shows a dihybrid cross between a pea plant that is homoozygous for green pods and yellow seeds (GGYY) and a pea plant that is homozygous for yellow pods and green seeds (ggyy). Pod color G (green) and g (yellow) Seed color Y (yellow) and y (green )

Dihybrid Cross All plants will be have the same genotype. Heterozygous for both traits. (GgYy). All plants will have the same phenotype. All will be green pods and yellow seeds.

Heterozygous X Heterozygous This Punnett square shows a dihybrid cross between a pea plant that is heterozygous for green pods and yellow seeds (GgYy) and a pea plant that is heterozygous for green pods and yellow seeds (GgYy). Pod color G (green) and g (yellow) Seed color Y (yellow) and y (green )

Dihybrid Cross 9 different genotypes: 9/16 green pods yellow seeds. 3/16 green pods, green seeds. 3/16 yellow pods, yellow seeds. 1/16 yellow pods green seeds. 9/16 green pods yellow seeds. 3/16 green pods, green seeds. 3/16 yellow pods, yellow seeds. 1/16 yellow pods green seeds. 4 different phenotypes. 9 Green pods, yellow seeds 3 Green pods, green seeds 3 Yellow pods,yellow seeds 1 Yellow pods, green seeds