genetics tutorial introduction punnett square – 1 trait punnett square – 2 traits product rule

Genetics TutorialIntroductionPunnett Square – 1 TraitPunnett Square – 2 TraitsProduct Rule

In this tutorial, you will learn:

Important terms used in genetics. How to use a Punnett square to

determine the outcome of a cross with 1 trait or 2 traits

How to use the product rule to determine the outcome of a cross with any number of traits.

Credits:Figures and images by N. Wheat unless otherwise noted.Funded by Title V-STEM grant P031S090007.

Introduction Information that will guide the

development of an organism is contained in that organism’s DNA. Every species has a characteristic number of DNA molecules called chromosomes.

Introduction An individual receives one complete set

of chromosomes from each parent, resulting in two complete sets. This is the diploid condition (2n).

Chromosomes Chromosomes

occur in pairs called homologous chromosomes. One from each

parent.

Genes are the functional unit of heredity

Chromosomes are made up of genes that code for traits.

A gene is found at a specific location or locus on a chromosome.

Genes & Alleles Different versions of genes are called

alleles. Purple flowers vs. white in pea plants

Gene = flower color, allele = white or purple

Genes & Alleles There can be any number of alleles for

a given gene, although an individual can have only two alleles(one on each homologous chromosome). A, B, O blood type in humans

Genes & Alleles Some traits are controlled by just one

gene, others are influenced by many genes (polygenic). Height in humans

A, B and O are _______ of the human blood type __________.

Genes, genes Genes, alleles Alleles, genes Alleles, alleles

Question 1

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Question 1

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Question 1

Homozygous & Heterozygous Since an individual has two sets of

chromosomes, it will have two copies of each gene (one originally coming from each parent). These two copies may be the same allele, or they may be different. Homozygous – both alleles are the

same. Heterozygous – two different alleles.

Which of the following represents the homozygous

condition? AA Aa aa Both AA and aa

Question 2

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your answer is complete. Try again!

Question 2


Question 2

Dominant & Recessive A trait is dominant if it is expressed in an

individual with one or two copies of the allele: Purple flower color in peas: P= purple p=

white. The dominant allele is represented by a capitol

letter, recessive by the lower case letter. PP – homozygous dominant – two copies

of the dominant (purple) allele. Pp – heterozygous – one purple allele, one

white allele (flowers appear purple).

Dominant & Recessive The trait is said to be recessive if it is

necessary for an individual to have two copies of the allele in order to express the trait. pp – two white flower alleles

(homozygous).

Purple flower color in peas is dominant over white. Which

of the following pairs of alleles would give purple

flowers? PP Pp pp Both PP and Pp

Question 3


Question 3

Now, which of the following pairs of alleles

would give white flowers? PP Pp pp Both PP and Pp

Question 4


Question 4

Genotype Genotype refers to the alleles that are

actually present. PP, Pp, pp in our flower color example. The purple phenotype may have PP or Pp

genotype.

Back to question 5

Phenotype Phenotype refers to the visible or

expressed characteristics of the trait. What does it look like? Purple or white for our flower color

example.

Back to question 8

Heredity – Passing on Traits

An individual can pass on genetic information to its offspring. In order to avoid doubling the number of chromosomes in each generation, cells must be created that carry only one set of chromosomes (haploid or 1n). An individual can pass along either of the

two alleles it carries for a trait, but not both. A Pp individual can pass on either P or p.

These haploid cells (eggs or sperm) are formed during meiosis.

Meiosis Meiosis is a type of

cell division that occurs in two parts.

Before division starts, each chromosome makes a copy of itself. These identical copies (called sister chromatids) remain connected at the centromere.

Meiosis In the first part of meiosis, homologous

chromosomes are separated.The cells are now haploid.

During the second part of meiosis, the sister chromatids are pulled apart.This part is like regular cell division

(mitosis). The division of one diploid cell will result

in 4 haploid cells.Each cell is different.

Heredity We can look at how traits are passed

from one generation to another individually or two at a time using a Punnett square.

Heredity For our example, we will use the ball

python. There are many mutations that breeders want to incorporate into their animals. Albino – a simple recessive trait Pinstripe – a dominant pattern mutation

Punnett Square – 1 Trait First let’s focus on

the albino trait. It is recessive so: AA & Aa

individuals will have normal coloration.

aa individuals will be albino.

Punnett Square – 1 Trait In a monohybrid cross we will cross two

animals that are heterozygous for albino. Aa x Aa We want to know, statistically, what kind of

offspring to expect. Each parent can donate only one allele for

the albino gene. A heterozygote (Aa) can donate either an A

or an a – not both. An albino must receive an a from both parents.

Punnett Square – 1 Trait Place the alleles

that may be donated by each parent across the top and along the sides.

Fill in the boxes: 1AA - normal 2Aa – normal,

heterozygous for albino

1aa - albinoBack to question 8

Which is the genotype? Normal or albino AA, Aa, or aa Both are considered to be

the genotype. Neither of these is the

genotype.

Question 5

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genotype. Try again!

Question 5


Question 5

Let’s try another example! If we have a male that is heterozygous

for albino and an albino female, what kind of offspring do we get?

X

What is the genotype of this pairing?

Aa x Aa AA x aa Aa x aa aa x aa

Question 6


Question 6

Which of these Punnett squares is correct?

Question 7


Question 7

What are the phenotypes of the offspring? All normal appearing

offspring All albino offspring 3:1 normal to albino 2:2 normal to albino

Question 8

Sorry! That is incorrect. Review Phenotype Review Punnett squares Try again!

Question 8


Question 8

Punnett Square – 2 Traits We can also use the Punnett square to

track two traits at once. Remember each gamete (egg or sperm)

will contain one allele for each trait. So, the possible combinations of alleles

that we will place on our Punnett squares will always have one letter for each trait.

Punnett Square – 2 Traits In a dihybrid cross, both animals are

heterozygous for two traits – here, albino (recessive) and pinstripe (dominant). AaPp x AaPp The parents will have normal coloration

(Aa) and they will be Pinstripes (Pp).

Punnett Square – 2 Traits Again, we want to know, statistically,

what kind of offspring to expect. Each parent (AaPp) will donate either

an A or an a allele for the albino gene and either a P or a p allele for the pinstripe gene.

So every gamete will always contain ONE A(or a) and one P(or p).

FOIL We can use the FOIL method from math

to be sure that we have all of the possible combinations of alleles. First, Outer, Inner, Last

Punnett Square – 2 Traits Next, we fill in each square. By

convention, we put the alleles for one gene together followed by the second: Aapp not Apap

Also, any dominant alleles are placed before recessives. AaPp not aApP

Genotypes from Dihybrid Cross

The Punnett square gives us the genotypes that result from the cross.

Phenotypes from Dihybrid Cross

The phenotypes would be: 9 Pinstripe (A_P_) 3 Normal (A_pp) 3 Albino pinstripe (aaP_) 1 Albino (aapp)

Albino is a recessive trait, while pinstripe is a dominant trait.

The 9:3:3:1 phenotypic ratio is characteristic of a dihybrid cross.

Which of the following Punnett squares is correct for

this cross: AaPp x aapp

Question 9


Question 9

What is the ratio of phenotypes that would result from the cross? AaPp x aapp

9 normal: 3 albino: 3 pinstripe: 1albino pinstripe

4 pinstripe: 4 normal: 4 albino pinstripe: 4 albino

All albino pinstripe AaPp: 4 Aapp: 4 aaPp: 4 aapp

Question 10


Question 10

The Product Rule Punnett squares are very useful for

tracking one or two traits, but they can become unwieldy when looking at more than two traits.

The product rule is a simple way to determine the likelihood of getting a particular result from any cross, regardless of the number of traits involved.

The Product Rule To use the product rule, we determine

the likelihood of getting each trait individually, then multiply those probabilities together.

We’ll use our dihybrid cross example to start with. AaPp x AaPp

The Product Rule We need to look at the traits separately:

Aa x Aa There would be a 1 in 4 chance of hatching

an albino from this cross. Pp x Pp

Pinstripe is dominant so ¾ of the offspring will be pinstripes.

What is the chance of getting an albino pinstripe from this cross? ¼ x ¾ = 3/16

This is the same result that we got using the Punnett square.

The Product Rule We can look at as many traits as we

want using the product rule. Say we are interested in combining

these 4 traits: Pinstripe (dominant) – (PP, Pp, pp) Albino (recessive) – (AA, Aa, aa) Piebald (recessive) – (BB, Bb, bb) Hypo (recessive) – (HH, Hh, hh)

The Product Rule The parents have the following genotypes:

AaPpBbhh x aappBbHh Calculate probability of getting individual

traits: Albino – Aa x aa = ½ Pinstripe – Pp x pp = ½ Piebald – Bb x Bb = ¼ Hypo – hh x Hh = ½ ½ x ½ x ¼ x ½ = 1/32 = chance of getting

an animal that shows all 4 traits from this pairing.

What is the probability of getting a hypo albino piebald animal from these parents:

AaBbhh x Aabbhh Remember, albino, piebald, and hypo

are all recessive traits. ½ ¼ 1/8 1/16

Question 11


Question 11

genetics tutorial introduction punnett square – 1 trait punnett square – 2 traits product rule

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