MENDEL’S GENETICS

MENDEL’S GENETICSPunnett Squares

& Probability

Before we begin,let’s get a few terms straight…

Phenotype: The PHysical APPEARANCE of an inherited trait. What you SEE.

Hint: look for descriptions (adjectives) or pictures.Examples: blue eyes, black hair, tall, short, hairy, smooth…

Genotype: Describes the 2 alleles you have for a gene. Your genetic make-up.

Hint: look for 2 letters together (eg. AA, aa, or Aa) or the words “homozygous” or “heterozygous”

EXAMPLE…

Gg = green pea pods

Genotype Phenotype(heterozygous)

PLEASE FIND A WHITEBOARD AND A MARKER

at your table!

PREPARE TO PRACTICE…

Practice: Genotype or Phenotype?(write G.T. or P.T. on whiteboard)

RR


RR Genotype!(specifically, a homozygous dominant genotype!)


Red hair


Red hair

Phenotype!


Phenotype! (An albino lion!)


Heterozygous


Heterozygous

Genotype!


Phenotype!

DOMINANT: A version of the gene that is ALWAYS shown if even one allele is present. Bossy, pushy, insists on being written 1st in a genotype… Hint: look for a CAPITAL LETTER if given.Remember: Dominant traits don’t surprise you- one of the parents MUST have shown the dominant phenotype if they had a dominant allele to pass on.

recessive: A version of the gene that’s totally HIDDEN if any dominant alleles are present. Hints: Look for a lower case letter if given. The ONLY way to LOOK recessive is to have NO dominant allele.Remember: If traits that seem to “pop up” or “skip a generation” because they weren’t seen in the parents, but appear in offspring, the “surprise” trait is recessive AND the parents were both heterozygous!

Practice: Dominant/Recessive(write D or R on whiteboard)

f


f

recessive(represented by a lower case letter)


Tt will look…


Tt will look…

Dominant! It has a T

Practice: Dominant/Recessive(write D or R on whiteboard)If a dove who is heterozygous for beak

color has an orange beak, are orange beaks DOMINANT or RECESSIVE?


If a dove who is heterozygous for beak color has an orange beak, are orange beaks DOMINANT or RECESSIVE?

DOMINANT (Heterozygous will always show

the dominant phenotype)

PLEASE PUT AWAY YOUR WHITEBOARD

AND MARKER !

(done with whiteboards, that is-the rest will be done on paper).

Genotypes: 3 options…- HOMOzygous Dominant (like AA):

“homo”= same (both alleles are dominant)*Phenotype will show DOMINANT

- HOMOzygous Recessive (like aa): “homo”= same (both alleles are recessive)

*only way to see a RECESSIVE phenotype.- HETEROzygous (like Aa): “hetero”= mixed

A mixture of 1 dominant & 1 recessive.*Phenotype will show DOMINANT

Other common terms seen in genetics problems….- True-breeding: Means homozygous.

(Describes a parent which has consistently similar offspring).

- Purebred: Means homozygous.- Carrier: Means heterozygous.

(They appear dominant, but also “carry” that sneaky, undetectable recessive allele which may be passed on to the next generation).

Test CrossIf something SHOWS a dominant phenotype,

you can’t be sure if they are AA or Aa.You can do a “test cross” by breeding it with

one that is homozygous recessive (aa).If the dominant parent was Aa, some of the

offspring will look recessive.If AA, none of the offspring will be recessive.

Test Cross

P = purplep = white

Practice decoding genetics problems…

“A red flower that is heterozygous is crossed with a white flower plant.”

a. Pick any letter of the alphabet & tell me the genotype (alleles) of the red plant: ____



a. Pick any letter of the alphabet & tell me the genotype (alleles) of the red plant: _Rr_

(whatever letter you chose is fine, but it is HETEROZYGOUS, so you need a capital & a lower case of SAME LETTER)



a. Pick any letter of the alphabet & tell me the genotype (alleles) of the red plant: _Rr_ *(any letter is fine)

b. What is the genotype of the white plant?: ______How can you know?



a. Pick any letter of the alphabet & tell me the genotype (alleles) of the red plant: _Rr_ *(any letter is fine)

b. What is the genotype of the white plant?: __rr__ (homozygous rec.)How can you know?

It doesn’t show the dominant trait (red)

More practice decoding genetics problems…

“Two hybrid aliens with purple eyes mate. 3 of their 13 offspring have orange eyes.”

a.) What is the genotype of the parents? ____ Keyword that helped: _______



I chose P=purple, p=orange.a.) What is the genotype of the parents? _Pp_ Keyword that helped: _hybrid_



I chose P=purple, p=orange.a.) What is the genotype of the parents? _Pp_ Keyword that helped: _hybrid_b.) What is the genotype of the orange-

eyed offspring? _______How can you know?



I chose P=purple, p=orange.a.) What is the genotype of the parents? _Pp_ Keyword that helped: _hybrid_b.) What is the genotype of the orange-

eyed offspring? __pp__How can you know? It’s a “surprise”

phenotype, not seen in parents.

STEPS for solving genetics problems…#1: Read the question CAREFULLY!#2: Decide which phenotype is dominant, & which

is recessive (see hints in definitions above!).

#3: Make a key (like H= hairy, h= no hair),be sure to use only 1 letter of the alphabet per trait.

#4: Decide & write out what the genotypes of all the individuals involved are, if possible.(see hints in definitions above!)

#5: Do a PUNNETT SQUARE if needed.#6: READ question AGAIN- Does the answer make sense?

PUNNETT SQUARES: why?

- If you know the exact genotype of two parents, you can predict the possible genotypes and phenotypes of their offspring!

- Punnett squares can show the probability (likelihood) of an offspring’s genotype.

PUNNETT SQUARES: how to- Draw a Punnett Square frame.- Write the 2 alleles

of 1 parent on the side of the square.

- Write the 2 alleles of the other parent on the top of the square.

A

a

a

A

Example: Mom= Aa & Dad= Aa

PUNNETT SQUARES: how toFill in the boxes by:- Copying top alleles into each box in the

column belowA

A

Aa

a

a

a

A

PUNNETT SQUARES: how toFill in the boxes by:- Copying top alleles into each box in the

column below- Copy side alleles into each box in the row next to them* There should be 2 letters in each box when you are done!

A A A

A

Aa

a

a aa

a

A

PUNNETT SQUARES: Are used to determine Probability- The 2 letters in each box represent a possible

genotype of an offspring.

- To figure out the chances (probability) of a certain genotype being produced by a cross,1. Do the Punnett Square2. Count how many boxes show each genotype or

phenotype3. Divide that number by 4 (for monohybrid crosses)

- Probability can also be shown as a ratio (like “3 to 1” or 3:1)

PUNNETT SQUARES: Are used to determine Probability

which can be shown as a fraction, %, or ratio.

There are 4 boxes in a basic Punnett Square.

each box is ¼, or 25%

¼ =_______% 2/4 = _____%

¾ = ______ % 4/4= ______%

Fill in the Punnett Square to answer the questions.

If 1 parent’s genotype is Jj, and the other parent’s genotype is jj… (“Jj x jj”)

1. What percent of offspring is expected to be heterozygous?

________%

2. What percent of offspring is expected to show the dominant phenotype?

________%




________%


________%

J

j

j

j




________%


________%

J

j j

j

J

J j

j

j j

j

j




________%


________%

J

j j

j

J

J j

j

j j

j

j 50(Jj)




________%


________%

J

j j

j

J

J j

j

j j

j

j 50

50

(Jj)

PRACTICE

PRACTICE

PRACTICE!!!

The secret to masteringPUNNETT SQUARES:

Image sources & links:Give credit where credit is due! Albino lion:

http://4.bp.blogspot.com/_vnyw0Q0l7uU/RzBOCa5kTvI/AAAAAAAABEc/vG3L6_wfjLY/s400/AlbinoLeo.jpg

Six fingers: http://img5.travelblog.org/Photos/32088/214128/t/1647731-The-Six-fingered-man.com

Red hair: http://home.overflow.net.au/~nedwood/pauline.jpg Whiteboard done:

http://cache.lifehacker.com/assets/resources/2007/08/white-board.png Tim Russert: http://www.tv.com/news-documentary/tim-russerts-

whiteboard Punnett Square Practice:

http://www.usoe.k12.ut.us/CURR/Science/sciber00/7th/genetics/sciber/punnett.htm Clay Genetics: http://www.youtube.com/watch?v=IPPljCw5AW8

MENDEL’S GENETICS

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