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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
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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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
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Question 3
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Question 3
Now, which of the following pairs of alleles
would give white flowers? PP Pp pp Both PP and Pp
Question 4
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Question 4
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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
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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
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Question 6
Which of these Punnett squares is correct?
Question 7
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Question 7
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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
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Question 8
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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
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Question 9
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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
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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
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Question 11
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Question 11