Genetics Using Genetics Using Punnett Squares Punnett Squares

Early GeneticsEarly Genetics

• The study of geneticsThe study of genetics

began with began with observations made by observations made by Gregor Mendel.Gregor Mendel.

• After noticing that the After noticing that the flowers his pea plants flowers his pea plants were either violet or were either violet or white, Mendel began white, Mendel began to study the to study the segregation of segregation of heritable traits. heritable traits.

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:

Lets consider a single Lets consider a single gene…gene…

• A gene carries A gene carries information that information that determines your determines your traits. traits. Traits are Traits are characteristics you characteristics you inherit from your inherit from your parents.parents.

• Genes are located in Genes are located in chromosomes.chromosomes.

• Chromosomes come Chromosomes come in pairs and there are in pairs and there are thousands, of genes thousands, of genes in one chromosome. in one chromosome.

Continued…Continued…• In humans, a cell’s In humans, a cell’s

nucleus contains 46 nucleus contains 46 individualindividual

chromosomes or 23 pairs chromosomes or 23 pairs of chromosomes.of chromosomes.

• Half of the chromosomes Half of the chromosomes

come from one parent come from one parent and half come from the and half come from the other parent. other parent.

This is a human karyotype representing the 23 pairs of chromosomes in a male Here is the

detailed structure of a chromosome

DefinitionsDefinitions• Allele-Allele- discrete version of the same gene discrete version of the same gene • Genotype-Genotype- the the ggenes of an organism for enes of an organism for

one specific trait one specific trait • Phenotype-Phenotype- the the pphysical appearance of a hysical appearance of a

trait in an organismtrait in an organism

DefinitionsDefinitions• Dominant Dominant trait refers to a genetic trait refers to a genetic

feature that “hides” the recessive trait feature that “hides” the recessive trait in the phenotype of an individual. in the phenotype of an individual.

• The term "The term "recessiverecessive” describes a trait ” describes a trait that is covered over (or dominated) by that is covered over (or dominated) by another form of that trait and seems to another form of that trait and seems to disappear.disappear.

• Homozygous= two alleles that are the Homozygous= two alleles that are the same for a trait (Pure)same for a trait (Pure)

• Heterozygous= two different alleles for Heterozygous= two different alleles for a trait (Hybrid)a trait (Hybrid)

PracticePractice• We use two letters to represent the genotype. We use two letters to represent the genotype.

A capital letter represents the dominant form of A capital letter represents the dominant form of a gene (allele) and a lowercase letter is the a gene (allele) and a lowercase letter is the abbreviation for the recessive form of the gene abbreviation for the recessive form of the gene (allele). (allele).

• Example below: P=dominant purple and p= Example below: P=dominant purple and p= recessive whiterecessive 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 SquaresPunnett Squares The Punnett square The Punnett square

is the standard way is the standard way of working out what of working out what the possible the possible offspring of two offspring of two parents will be. parents will be. – It is a helpful tool to It is a helpful tool to

show allelic show allelic combinations and combinations and predict offspring predict offspring ratios. ratios.

Before we go further lets review how Before we go further lets review how

to set up a Punnett Square…to set up a Punnett Square…

We begin by constructing a grid of two We begin by constructing a grid of two perpendicular lines.perpendicular lines.

Next, put the genotype of one parent across Next, put the genotype of one parent across

the top and the other along the left side.the top and the other along the left side.

For this example lets consider a genotype of BB crossed with bb.For this example lets consider a genotype of BB crossed with bb.

B B

b

b

• 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 Next, fill in the boxes by copying the column and row head-letters column and row head-letters downdown and and acrossacross into into

the empty spaces. the empty spaces.

B B

b B

B

B

Bb

b

b

b

b

Punnett SquaresPunnett Squares

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

Lets say:

W- dominant white

w- recessive violet

W wW

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)

Usually write the capital letter first

w

W W W w

W w w w

Red hair (R) is dominant over blond hair (r). Make Red hair (R) is dominant over blond hair (r). Make a cross between a heterozygous red head and a a cross between a heterozygous red head and a blond.blond.

RrRr rr rr

RrRr rr rr

R r

r

r

What percentage of the offspring will have red hair? 50%

Let’s try some more…Let’s try some more…

In pea plants, tall pea plants (T) are In pea plants, tall pea plants (T) are dominant dominant

over short pea plants (t). Construct a Punnett over short pea plants (t). Construct a Punnett

Square for a heterozygous tall pea plant and Square for a heterozygous tall pea plant and a short pea plant.a short pea plant.

Tt tt

Tt tt

T t

t

t

What are the percentage of phenotypes?

50% tall

50% short

Black eyes (R) is dominant over red eyes Black eyes (R) is dominant over red eyes (r) (r)

in rats. Make a cross between a in rats. Make a cross between a homozygous rat with black eyes and a homozygous rat with black eyes and a rat with red eyes. rat with red eyes.

Rr Rr

Rr Rr

R R

r

r

What is the possibility of a red eye off springs?

0%

ReferencesReferences http://www.athro.com/evo/gen/punnett.htmlhttp://www.athro.com/evo/gen/punnett.html http://www.kidshealth.org/kid/talk/qa/what_is_gene.htmlhttp://www.kidshealth.org/kid/talk/qa/what_is_gene.html http://brookings.k12.sd.us/biology/ch%2011%20genetics/http://brookings.k12.sd.us/biology/ch%2011%20genetics/

punnettpractice.ppt#1punnettpractice.ppt#1 http://www.usoe.k12.ut.us/CURR/Science/sciber00/7th/genehttp://www.usoe.k12.ut.us/CURR/Science/sciber00/7th/gene

tics/sciber/punnett.htmtics/sciber/punnett.htm http://www.biotechnologyonline.gov.au/images/http://www.biotechnologyonline.gov.au/images/

contentpages/karyotype.jpgcontentpages/karyotype.jpg