Genetics and Punnett Squares Genetics and Punnett Squares

Early Genetics The study of geneticsThe study of genetics began with observations made by Gregor Mendel. began with observations made by Gregor Mendel. After noticing that the flowers on his pea plants were either violet or white, Mendel began to study how traits were inherited.After noticing that the flowers on his pea plants were either violet or white, Mendel began to study how traits were inherited. Between 1856 and 1863 he cultivated and tested at least 28,000 pea plants. Remember that Mendel worked almost 150 years ago when nobody knew about genes or even the structures (chromosomes) that carry genes.

Here are some traits observed by Mendel:

Let’s consider a single gene… A gene carries information that determines your traits. Traits are characteristics you inherit from your parents.A gene carries information that determines your traits. Traits are characteristics you inherit from your parents. Genes are located on chromosomes.Genes are located on chromosomes. Chromosomes come in pairs and there are thousands of genes on one chromosome.Chromosomes come in pairs and there are thousands of genes on one chromosome.

Genetic Terminology Trait – any characteristic that can be passed from parent to offspring Trait – any characteristic that can be passed from parent to offspring Heredity – passing of traits from parent to offspring Heredity – passing of traits from parent to offspring Genetics – the study of heredity Genetics – the study of heredity Gene – basic unit of genetic information; sections of a chromosome that code for a trait Gene – basic unit of genetic information; sections of a chromosome that code for a trait Chromosomes – storage units of genes. Chromosomes – storage units of genes. DNA - the genetic instructions to build an organism, is made of genes and winds up to form a chromosome DNA - the genetic instructions to build an organism, is made of genes and winds up to form a chromosome

Continued… In humans, a cell’s nucleus contains 46 individualIn humans, a cell’s nucleus contains 46 individual chromosomes or 23 pairs of chromosomes. Half of the chromosomesHalf of the chromosomes come from one parent and half come from the other parent. This is a human karyotype representing the 23 pairs of chromosomes in a male Here is the detailed structure of a chromosome identical copies holds 2 chromatids together segment of DNA that codes for a trait gene

Human Genome Most human cells contain 46 chromosomes: 22 pairs of chromosomes named autosomes. 22 pairs of chromosomes named autosomes. 2 sex chromosomes (X,Y): 2 sex chromosomes (X,Y): XY – in males. XX – in females.

Definitions Alleles – possible forms of a geneAlleles – possible forms of a gene Example: blue, brown and green are all alleles for eye colorExample: blue, brown and green are all alleles for eye color Genotype- the gene’s allele combination of an organism for one specific traitGenotype- the gene’s allele combination of an organism for one specific trait Example: BB, Bb, bbExample: BB, Bb, bb Phenotype- the physical appearance of a trait in an organismPhenotype- the physical appearance of a trait in an organism What you see when you look at the organismWhat you see when you look at the organism Example: blue eyesExample: blue eyes

Dominant vs. Recessive A dominant allele is the stronger of the two alleles and is expressed even if it is paired with a recessive allele; written as a capital letter A recessive allele is the weaker of two alleles and is only visible when paired with another recessive allele because it is masked by the dominant allele; written as a lower case letter The genotype of cat #1 is BB and its phenotype is brown.

Definitions Homozygous - two alleles that are the same for a trait (pure trait)Homozygous - two alleles that are the same for a trait (pure trait) Homozygous dominant pair = AAHomozygous dominant pair = AA Homozygous recessive pair = aaHomozygous recessive pair = aa Heterozygous - two different alleles for a trait (hybrid trait)Heterozygous - two different alleles for a trait (hybrid trait) Heterozygous pair = AaHeterozygous pair = Aa Homo = same Homo = same Hetero = different Hetero = different

Practice 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).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). Example below: P=dominant purple and p= recessive whiteExample below: P=dominant purple and 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).

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 allele combinations and predict offspring ratios.

Before we go further let’s learn 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 let’s walk through some examples using Punnett Squares. Now that we have learned the basics of genetics let’s walk through some examples using Punnett Squares.

Lets say: W- dominant white w- recessive violet W w W Parents in this cross are heterozygous (Ww). Note: Make sure I can tell your capital letters from lowercase letters. What percentage of the offspring will have violet flowers? ANSWER: 25% (homozygous recessive) 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 and a short pea plant. 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% 