Genetics and Punnett Squares

Transcription

1 Name: Class: Date: Genetics and Punnett Squares Introduction The results of crossing two individuals can be predicted by making a Punnett square. At the top of the square, the two possible alleles from the father are listed. On the side of the square, the two possible alleles of the mother are listed. Within in the smaller squares of the grid, the possible offspring are written. Monohybrid Cross One of the characteristics of pea plants studied by Gregor Mendel was the color of the pea pods. Green is dominant, and yellow is recessive. Make a Punnett square predicting the F1 generation produced by placing pollen from a true-breeding green pod plant and onto the pistil of a true-breeding yellow pod plant. The Punnett square gives the genotypic ratio of the offspring. List the ratios of each possible phenotype, or physical appearance. Now, take two of the individuals from the F1 generation and cross them. List the ratios of each possible phenotype, or physical appearance. Written by James Dauray Page 1

2 Test Cross If the F2 generation has both homozygous dominant and heterozygous individuals, how do you tell them apart? The best way is with a test cross, where the unknown genotype individual is crossed with a homozygous recessive individual. Predict the phenotype ratios of performing test cross on a homozygous dominant green pea pod individual. Next, predict the phenotype ratios of performing attest cross on a heterozygous green pea pod individual.

3 Dihybrid Cross Mendel also wanted to see if the inheritance of one gene could affect the inheritance of another, or if they were independently assorted. Assume that he is starting with two true-breeding strains of plants: dominant purple-flowered tall plants, and recessive white-flowered dwarf plants. Predict the resulting phenotype ratios of the F1 generation. Next, predict the resulting phenotype ratios of the F2 generation formed by crossing two of the individuals from the prior F1 generation.

4 Incomplete dominance. Some alleles, when heterozygous, will result in a mixed or blended phenotype. Predict the offspring of a true-breeding red carnation with a true-breeding white carnation. Heterozygous individuals will appear pink. Predict the offspring of two pink carnations. Codominance Some alleles, when heterozygous, will both be expressed. The human blood type is an excellent example of this, as individuals with both dominant genes (A and B) will express both on their cell membranes. Predict the offspring of two individuals with type AB blood.

5 Predict the offspring of a heterozygous type A and heterozygous type B individual. Remember, O is recessive to the other two alleles. Genotype Ratios: Sex-Linked Traits Colorblindness is a recessive trait. Its allele is only found on the X-chromosome. Predict the offspring from a couple with a heterozygous (regarding colorblind in other words, a carrier ) female and a normal (not colorblind) male. Genotype Ratios:

6 Polygenic Inheritance Eye color in humans is the result of a pigment called melatonin being deposited into the iris of the eye. The trait is controlled by multiple alleles at multiple loci on different chromosomes. Brown (heavy melatonin) is dominant to blue (light or no melatonin). These are the possible phenotypes and genotypes: (Note: This is an simplification there are likely more than two genes for this trait). AABB = Dark brown/black AaBB or AABb = Medium brown AaBb, AAbb, or aabb = Hazel or green Aabb or aabb = Deep Blue aabb = Light Blue (Four dominant alleles) (Three dominant alleles) (Two dominant alleles) (One dominant allele) (No dominant alleles) Predict the offspring of a hazel-eyed father (AaBb) and a medium brown-eyed mother (AABb).