Introduction to Genetics Developed by Mr. Neil Syrek

This Stand-Alone Instructional Resource is intended to introduce students to basic concepts in the field of Mendelian genetics, such as terminology, the segregation of parental alleles, and the use of punnett squares to predict genotypic and phenotypic ratios of offspring. This project was designed to be used with general education students in high school Biology. Typically these students would be in grades 9 or 10. The following Michigan High School Content Expectations will be met by this project: B4.1c Differentiate between dominant and recessive B4.1d Explain the genetic basis for Mendel’s laws of segregation and independent assortment. B4.1e Determine the genotype and phenotype of monohybrid crosses using a Punnett Square. In addition, the learners will understand the meanings of the terms gene, allele, homozygous and heterozygous. Furthermore they will be able to predict phenotypic and genotypic ratios of offspring and be able to predict the appearance of certain traits in individuals.

Consider this… A mother and father both have brown eyes But… Their child has blue eyes Is this even possible? How could this happen?

To begin to solve our problem… we will need to learn some genetics terminology Gene—the molecular information that codes for a certain trait (like eye color) Allele—a form or variation of a given gene (like blue or brown eyes) Side note: In most cases, individuals have a pair alleles for each gene—sometimes the alleles are the same, sometimes they are different Homozygous—what its called when the two alleles in the pair are the same (like two brown eye alleles) Heterozygous—what its called when the two alleles in the pair are different (like one brown eye allele, and one blue eye allele) Genotype—the genetic makeup or plan of an individual (can refer to a single trait or a collection of traits) Phenotype—the observable appearance of a trait or collection of traits that is the result of the genotype

Try to figure out these analogies 1. A blueprint is to a building, as a genotype is to ____________. A gene An allele A phenotype

Fantastic!! Genotype Phenotype An individual’s phenotype is the result of his genotype—just like a building is the result of a blueprint or plan

Sorry Let’s review: Gene—the molecular information that codes for a certain trait (like eye color) Allele—a form or variation of a given gene (like blue or brown eyes) Side note: In most cases, individuals have a pair alleles for each gene—sometimes the alleles are the same, sometimes they are different Genotype—the genetic makeup or plan of an individual (can refer to a single trait or a collection of traits) Phenotype—the observable appearance of a trait or collection of traits that is the result of the genotype

A convertible is to an automobile, as an allele is to a ___________. heterozygous gene phenotype

Way to go!! An allele is a variation of a gene like a convertible is a variation of an automobile

Sorry Hint: a convertible is a variation or type of automobile Gene—the molecular information that codes for a certain trait (like eye color) Allele—a form or variation of a gene (like blue or brown eyes) Heterozygous—what its called when the two alleles in the pair are different (like one brown eye allele, and one blue eye allele) Phenotype—the observable appearance of a trait or collection of traits that is the result of the genotype

More on Genotype For every gene, an individual has two alleles. In the case of eye color, the possible combinations are: Brown/Brown Brown/Blue Blue/Blue *Note: Eye color is a complex trait that in actuality is controlled by multiple genes. For this example, we must act as if there were only one gene for eye color.

What color eyes do you think someone with the Brown/Brown allele combination would have?

Right …so a Blue/Blue combination will cause what color eyes? or

Nope. It was brown …so a Blue/Blue combination will cause what color eyes? or

Good I think you’ve got it. Well now, what color eyes will a Brown/Blue allele combination cause?

Yes!! In the case of the Brown/Blue allele combination, brown eye color shows up at the expense of blue. Thus we can say that the brown allele is dominant over the blue one. The term used to describe the allele that is being masked, the blue one in this case, is recessive. In most cases, one allele is dominant over the other.

Sorry It was brown—but don’t worry you had no way of knowing In the case of the Brown/Blue allele combination, brown eye color shows up at the expense of blue. Thus we can say that the brown allele is dominant over the blue one. The term used to describe the allele that is being masked is recessive. In most cases, one allele is dominant over the other.

Scenario Luis has a genetic disorder called Huntington’s Disease (HD), which causes a degeneration of the central nervous system. Luis is married to Sandra who does not have the disorder. Before they decide to have a baby, they want to know the probability that the baby will inherit Luis’ disease. For more information on HD, visit the National Institutes of Health homepage

Some more info There are two alleles for the gene that causes HD: affected and normal The affected allele is dominant The normal allele is recessive For simplicity, we can abbreviate these alleles: ‘A’ for affected ‘a’ for normal We use the capital and lowercase forms of the same letter because they are variations of it, just as alleles are variations of a single gene. Now we must figure out Luis and Sandra’s genotype so we can work on probability.

Luis’ genotype Through a study of family history, we find out that Luis is heterozygous for Huntington’s Disease. In other words, he has one affected allele and one normal allele What do you think his genotype would be? A A A a a a

Yes!! Nice job. Since Luis is heterozygous for the trait, he has two different alleles. A a

Sorry Think about it. If Luis is heterozygous, that means he has two different alleles (or two different forms of the same letter). Remember: ‘A’ for affected ‘a’ for normal

Sandra’s genotype Sandra does not have the disease, so she is homozygous for the recessive trait. What would Sandra’s genotype be? A A A a a a

Yes!! Since Sandra is homozygous, her alleles are the same. Also, because she has the recessive trait, we use the lowercase letters. a a

Sorry Remember, Sandra is homozygous, so her alleles are the same. In addition, she has the recessive trait, which is represented by lowercase letters.

called a Punnett square. Now that we know their genotypes, we can figure out the probability that their offspring will have HD. To do this, we can use a tool called a Punnett square.

Punnett squares To begin, insert the father’s alleles at the top of the table. Remember, Luis’ genotype was A a A a

Now insert the mothers alleles along the side. Sandra’s genotype is a a.

Filling in the table A a A a a a The four remaining squares represent the possible genotypes of the offspring To fill them in, simply drop the father’s allele down from the top, and drag the mothers allele over from the side—sort of like a multiplication table. What letters do you think would go in this space? a a A A A a A a A a a a

Right on!! Since Luis would donate the ‘A’ allele, and Sandra would send the ‘a’ allele, that square would be filled with A a.

Good try…but no The allele that Luis would donate to that square is all the way at the top of the table (A), while the allele that Sandra would donate is all the way to the left (a). Try again

Filling in the table A a A a a a Almost finished… What letters do you think would fill in the last space? a a A A A a A a A a a a

Way to go!! Both Luis and Sandra donated the ‘a’ allele to that space

Try again Hint: look at the top of the table above that square for one allele, and all the way to the left for the other.

The results Okay, we’ve got our Punnett square filled in Now what does it all mean? Some simple math will allow us to interpret the results. A a A a a a

Two of the four squares representing the offspring show the genotype ‘A a,’ which results in the affected phenotype. Since two out of four is the same as 50%, we can say there is a 50% chance that Luis and Sandra will have a child with Huntington’s Disease. A a A a a a

Try this If instead of the square we just completed, we had one that looked like this, what would be the probability of an offspring being affected with Huntington’s Disease? A) 25% B) 50% C) 75 % A a A A A a a a

Nice job!! Since HD is a dominant trait, either the ‘A A’ or the ‘A a’ genotype will result in an affected individual. 3 out of the 4 squares, or 75%, coded for the affected phenotype.

Try again Remember, since HD is a dominant trait, either the ‘A A’ or the ‘A a’ genotype will result in an affected individual. Also, 3 out of the 4 squares coded for the affected phenotype.

Now its time for the big quiz I’m ready. Lets go! I want to review the vocabulary first. I want to review Punnett squares again.

Let’s revisit our opening scenario Recall that two parents, each with brown eyes, had a blue eyed baby. The brown eyes allele is dominant. The blue eyes allele is recessive. We’ll use ‘B’ for the brown allele, and ‘b’ for the blue allele Which of these genotypes would result in the blue eyes phenotype? B B B b b b

Yes!! Blue eyes is the recessive trait, so it is represented by two lowercase letters.

Sorry Remember, blue eyes is the recessive trait. Even one brown eyes allele will result in the brown eyes phenotype.

Next question Recall that two parents, each with brown eyes, had a blue eyed baby. The brown eyes allele is dominant. The blue eyes allele is recessive. We’ll use ‘B’ for the brown allele, and ‘b’ for the blue allele Which of these Punnett squares could represent this scenario? (click here) (click here) (click here) B b B b b b B b B B B b b b B b B B B b

Great!! That was the only choice showing both parents with brown eyes, and a possibility of blue eyed offspring. Next question In this scenario, both parents had a ___________ genotype. A) homozygous B) heterozygous C) phenotype B b B B B b b b

Think again Remember, both parents have brown eyes

Think again You’re partially right. Both parents have brown eyes, but… Remember, the baby has blue eyes

Well done!! Since the parents each had different alleles, they would be heterozygous

Try again The genotype of both parents is ‘B b’ The alleles are different

Last question What was the probability that these parents would have an offspring with blue eyes? A) 100% B) 50% C) 25% B b B B B b b b

Spot on!! Only 1 of the 4 squares showed the blue eyed phenotype. That’s 25%. You are a genetics genious! Back to beginning

Try again The only genotype that will result in blue eyes is ‘b b’

Allele—a form or variation of a given gene (like blue or brown eyes) Gene—the molecular information that codes for a certain trait (like eye color) Allele—a form or variation of a given gene (like blue or brown eyes) Side note: In most cases, individuals have a pair alleles for each gene—sometimes the alleles are the same, sometimes they are different Homozygous—what its called when the two alleles in the pair are the same (like two brown eye alleles) Heterozygous—what its called when the two alleles in the pair are different (like one brown eye allele, and one blue eye allele) Genotype—the genetic makeup or plan of an individual (can refer to a single trait or a collection of traits) Phenotype—the observable appearance of a trait or collection of traits that is the result of the genotype Back to Quiz