Tips for solving Genetics Problems Complete Dominance Incomplete

Tips for solving Genetics Problems • Complete Dominance • Incomplete Dominance • Sex-Linked

C. Probability: 1. Determining the chance of a trait appearing in an individual. 2. Use the Punnett Square to determine the chance of a particular trait appearing. Some practice problems follow.

A plant that produces only round seeds (RR), and a plant that produces only wrinkled seeds (rr), were crossed. Find the following: The percent hybrid? The percent purebred? The percent having round seeds? The percent having wrinkled seeds?

R R r Rr Rr

A plant that produces only round seeds (RR), and a plant that produces only wrinkled seeds (rr), were crossed. Find the following: 100% The percent hybrid? The percent purebred? 0% The percent having round seeds? 100% The percent having wrinkled seeds? 0% R r Rr Rr

Two hybrid plants with round seeds (Rr), are crossed. The dominant allele R, produces round seeds. The recessive allele r, produces wrinkled seeds. Find the following: The percent hybrid? The percent purebred? The percent having round seeds? The percent having wrinkled seeds?

R r R RR Rr rr

Two hybrid plants with round seeds, Rr are crossed. The dominant allele R, produces round seeds. The recessive allele r, produces wrinkled seeds. Find the following: R R The percent hybrid? 50% The percent purebred? 50% r r RR Rr Rr rr The percent having round seeds? 75% The percent having wrinkled seeds? 25%

General Rules for filling in Punnett Squares: 1) Each letter fills in the two boxes immediately below or to the right of them. 2) lower case letters are written after UPPER CASE letters no matter which was on top or the side. (e. g. Rr )

Four steps to solving probability problems: STEP 1: Choose letters to represent the alleles of the parents being crossed. STEP 2: Write the genotypes of the parents being crossed. STEP 3: Construct a Punnett Square. STEP 4: Answer the questions.

R R r Rr Rr

D. Complete Dominance vs. Incomplete Dominance genes:

Complete Dominance: a) condition in which dominant genes completely hide or mask recessive genes. b) the same letter is used to represent both the dominant and recessive forms of a gene. c) but, capital letters are used to represent dominant genes and lower case letters represent recessive genes.

Incomplete Dominance: a) condition in which dominant genes incompletely hide other genes and results in a “blending” of traits. b) different capital letters are used to represent different forms of a gene (e. g. R = red, W = white). c) a hybrid is represented by a mixture of capital letters (e. g. RW).

Sex-linked, Complete Dominance Problems: a) STEP 1 – Choose upper & lower case letters to represent the dominant and recessive alleles (e. g. , C= Normal Vision & c= colorblind. b) STEP 2 – Use XX to represent female sex chromosomes and XY male. Write letters identified in STEP 1 above in superscript form on sex chromosomes (see examples

Sex-linked continued: (example: C X Xc x C X Y) b) STEP 3 – Construct Punnett square and complete. c) STEP 4 – Use the Punnett square results to document percent outcomes.

References: Holt, Rinehart & Wilson Science and Technology textbook, 2001, California Life Science, pp. 130 -137. Prentice Hall textbook, 1993, Heredity.