Genetics Using Punnett Squares Genetics Using Punnett Squares

Here are some traits observed by Mendel:

Review Definitions Genotype: the genetic makeup of an organism as indicated by its allelesGenotype: the genetic makeup of an organism as indicated by its alleles Each organism has two alleles per rait Ex. HH or Hh Phenotype: the physical appearance of an organism’s traits based on its genetic alleles Ex. Blue eyes, blonde hair

Review Definitions Dominant trait refers to a genetic feature that “hides” the recessive trait in the phenotype of an individual.Dominant trait refers to a genetic feature that “hides” the recessive trait in the phenotype of an individual. The term "recessive” describes a trait that is covered over (or dominated) by another form of that trait and seems to disappear.The term "recessive” describes a trait that is covered over (or dominated) by another form of that trait and seems to disappear. Homozygous= two alleles that are the same for a trait (PureBred YY or yy)Homozygous= two alleles that are the same for a trait (PureBred YY or yy) Heterozygous= two different alleles for a trait (Hybrid Yy)Heterozygous= two different alleles for a trait (Hybrid Yy)

.We use two letters to represent the genotype. A capital letter represents the dominant form of a gene (allele) and a lowercase letter is the abbreviation for the recessive form of the gene (allele).A capital letter represents the dominant form of a gene (allele) and a lowercase letter is the abbreviation for the recessive form of the gene (allele). Example below: P=dominant purpleExample below: P=dominant purple p = recessive white p = recessive white The phenotype for this flower is violet while its genotype (if homozygous) is PP. The phenotype for this flower is white while its genotype is pp (to be white the flower must have two of the recessive copies of the allele).

Identify the Phenotype or the Genotype Ho HoHoHo Ho Ho Ho Ho He He HeHe He He He He

1. Describe the genotypes given (use your book). The first two are already done. A. DD homozygous, dominant D. ss ______________________ B. Dd heterozygous E. Yy ______________________ C. dd __________________ F. WW ____________________ 2. In humans,brown eye color (B), is dominant over blue eye color (b). What are the phenotypes of the following genotypes? In other words, what color eyes will they have? A. BB ______________________ __ B. bb ________________________ C. Bb ________________________ Homozygous, recessive heterozygous Homozygous, dominant Brown eyes Blue eyes

Punnett Squares The Punnett square is the standard way of working out what the possible offspring of two parents will be. The Punnett square is the standard way of working out what the possible offspring of two parents will be. –. –It is a helpful tool to show allelic combinations and predict offspring ratios.

Before we go further lets discuss how to set up a Punnett Square… We begin by constructing a grid of two perpendicular lines.

Next, put the genotype of one parent across the top and the other along the left side. For this example lets consider a genotype of BB crossed with bb. B B bbbb Notice only one letter goes above each box It does not matter which parent’s genotype goes on either side.

Next, fill in the boxes by copying the column and row head-letters down and across into the empty spaces. the empty spaces. B B b B B B B b b b b b

Punnett Squares Now that we have learned the basics of genetics lets walk through some examples using Punnett Squares. Now that we have learned the basics of genetics lets walk through some examples using Punnett Squares.

Lets say: W- dominant purple w- recessive white W w W Parents in this cross are heterozygous or hybrid (Ww). Note: Make sure I can tell your capital letters from lowercase letters. What percentage of the offspring will have purple flowers? ANSWER: ¾ or 75% only ¼ or 25% would be white Usually write the capital letter first w W W w w

Red hair (R) is dominant over blond hair (r). Make a cross between a heterozygous red head and a blond. Rrrr Rrrr R r rrrr What percentage of the offspring will have red hair? 50%

Let’s try some more… In pea plants, tall pea plants (T) are dominant over short pea plants (t). Construct a Punnett Square for a heterozygous tall pea plant(Tt) and a short pea plant.(tt) Tt tt Tt tt T t tttt What are the percentage of phenotypes? 50% tall 50% short

Black eyes (R) is dominant over red eyes (r) in rats. Make a cross between a homozygous rat with black eyes and a rat with red eyes. Rr R rrrr What is the possibility of a red eye off springs? 0% 