Introduction to Genetics using Punnett...

Agenda

Karyotype video ~2 minutes

karyotype activity ~10 minutes

Quiz on Mitosis and Meiosis ~17minutes

Punnett Square notes

Punnett Square Practice Problems

Genetics Using Punnett Squares

Targets

Determine the probability of a particular trait in an offspring based on the genotype and phenotype with 85% accuracy

Early Genetics

• The study of genetics

began with observations made by Gregor Mendel.

• After noticing that the flowers his pea plants were either violet or white, Mendel began to study the segregation of 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 gene…• A gene carries

information that determines your traits.

Traits are characteristics you inherit from your parents.

• Genes are located in chromosomes.

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

Continued…

• In humans, a cell’s nucleus contains 46 individual

chromosomes or 23 pairs of chromosomes.

• Half of the chromosomes

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

Here is the detailed structure of a chromosome

Karyotype-male or female?

Definitions• Allele- alternate version of the same gene

• Genotype- the genes of an organism for one specific trait

• Phenotype- the physical appearance of a trait in an organism

Definitions• 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.

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

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

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).

• Example 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).

Female Male

Important to know before getting started:

Punnett’s Square

Scientists use a Punnett’s square to determine the possible genetic outcomes for the

offspring that result from the combination of the parent’s genes.

Black colored fur is the dominant (B) trait.

We will use the Punnett’s Square to determine the offspring of guinea pigs. The

offspring will either be black or white.

Generation 1

B B

b

b

To complete the Punnett’s square we combine the gene from mom with the gene from dad. We always write

the dominant gene first.

Bb

Generation 1

B B

b

b

In this case we have a dad with black fur and a mother with white fur. Because black is the dominant gene,

we write it with a capital ‘B’.

Generation 1

B B

b

b

White fur is a recessive trait. It is written with a lowercase ‘b’. It does not matter what letter we choose

to represent a gene, but capital letter is always dominant and lowercase is always recessive.

Generation 1

B B

b

b

We write the dominant gene first because it “masks” the recessive gene. Therefore, the color of the guinea

pig with the genes Bb would be black.

Bb

Generation 1

B B

b

b

Copy this Punnett’s square into your notebook. Try and fill out the remaining offspring on your own. When you

are done, go to the next slide.

Bb

Generation 1

B B

b

b

Bb Bb

Bb Bb

We say an individual is heterozygous when it has two different genes.

What percentage of these offspring are heterozygous?

Generation 1

B B

b

b

Bb Bb

Bb Bb

100% are heterozygous black.

We will now take one female and one male from this generation to cross for our second generation.

Generation 2

B b

B

b

What will the gene combinations be for these offspring?

Copy this into your notebook and try to fill out the Punnett’s square. Continue when you are done.

Generation 2

B b

B

b

What you have completed is the genotype for all the offspring. Genotype refers to the gene combination

that an individual has.

Can you figure out what color these offspring will be?

BB

Bb

Bb

bb

Generation 2

B b

B

b

What you have just determined is called the phenotype. The phenotype is what we see as a result

of an individual's genes.

BB

Bb

Bb

bb

black black

black white

Generation 2

B b

B

b

BB

Bb

Bb

bb

We say an individual is homozygous when it has two of the same genes.

Can you find a homozygous black guinea pig?

Generation 2

B b

B

b

BB

Bb

Bb

bb

The entire Punnett’s square represents all possible outcomes. That means each small box represents 25%

of the offspring.

What percentage of the offspring are homozygous black?

This individual is homozygous black.

Generation 2

B b

B

b

BB

Bb

Bb

bb

25% of the offspring are homozygous black. Try the next two on your own:

____% are homozygous white

____% are heterozygous black

Generation 2

B b

B

b

BB

Bb

Bb

bb

25% are homozygous black.

25% are homozygous white

50% are heterozygous black

Notice: This will always add up to 100%.

Punnett Squares

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 review 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

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 column and row head-letters down and across into

the empty spaces.

B B

b B

B

B

Bb

b

b

b

b

Punnett Squares

Now that we have learned the basics of genetics lets 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)

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 a cross between a heterozygous red head and a blond.

Rr rr

Rr rr

R r

r

r

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

t

t

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 Rr

Rr Rr

R R

r

r

What is the possibility of a red eye off springs?

0%

Practice Problem #1

Predict the fur color of the offspring of a brown heterozygous hamsters and a white homozygous hamster.

Use any letter you like.

Practice Problem #1

Bb bb

Bb bb

B b

bb

Practice Problem #2

Predict the offspring of 2 white homozygous cows. Black is dominant over white.

Use any letter you like.

Practice Problem #2

bb bb

bb bb

b b

bb

Practice Problem #3

Predict the offspring of 2 Grey Heterozygous rats. Gray is dominant over white, which is recessive.

Use any letter you like.

Practice Problem #3

GG Gg

Gg gg

G g

Gg

Practice Problem #4

Predict the pea shape of the offspring of one Heterozygous Round Pea plant and one homozygous wrinkled pea plant. Round is dominant over wrinkled.

Use any letter you like.

Practice Problem #4

Rr rr

Rr rr

R r

rr

Practice Problem #5you don’t have to write this one down…

What are the chances of having a boy or a girl?

How would you solve this one?

Practice Problem #5

XX XY

XX XY

X Y

XX

ReviewHeredity by Brainpop

1What do we call different forms a gene may have for a trait?

2What height of pea plant (tall or short) did Mendel find to be dominant?

3Which kind of trait can’t be passed from parent to child?a)Eye color b) height c) broken arm

Bellwork

Long hair (S) is dominate over short-haired (s). Cross a heterozygous cat carries with long-haired traits and mates with a carries short-haired cat. Use a Punnett square to determine the probability of one of their offspring having long hair.

References

http://www.athro.com/evo/gen/punnett.html

http://www.kidshealth.org/kid/talk/qa/what_is_gene.html

http://brookings.k12.sd.us/biology/ch%2011%20genetics/punnettpractice.ppt#1

http://www.usoe.k12.ut.us/CURR/Science/sciber00/7th/genetics/sciber/punnett.htm

http://www.biotechnologyonline.gov.au/images/contentpages/karyotype.jpg