Unit A Lesson 13: Inheriting Traits
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Unit A Lesson 13: Inheriting Traits |
Learning Targets |
Inquiry Question: How do you get blue eyes if both your parents have brown eyes?
Traits are passed from parents to offspring, but it is rarely a simple process to determine the traits of a single child.
Traits are passed from parents to offspring, but it is rarely a simple process to determine the traits of a single child.
Page 50 to 52 in your textbook will help you answer these questions.
- What is an allele?
- What are dominant traits?
- What are recessive traits?
- What is a Punnett square?
- What is a purebred?
- What s a hybrid?
- What is discrete variation?
- What is continuous variation?
- How does the environment affect inherited characteristics?
Page 50 to 52 in your textbook will help you answer these questions about inheriting traits.
Introduction
Traits are inherited from the previous generation, which inherited traits from its previous generation, which inherited traits from its previous generation, which…. You can see where this is going!
How does this help us understand why siblings look different but similar? Can understanding traits and inheritance help us get closer to developing a purple cow? Two gametes are involved in fertilization, one from the female and the other from the male. The moment they unite, fertilization has occurred.
Each gamete carries half the number of chromosomes. When the gametes unite, the chromosomes pair up because of similarities in the genes. A single form of a gene-pair is called an allele. A gene-pair is formed when two alleles pair up. Alleles can be classified as dominant or recessive. Remember that alleles are always paired.
At this level of science, the very basics of genetics are covered. As you progress through your science studies you will encounter more complex examples of inheritance, for example, some traits are influenced by more than just one pair of genes.
Complete Dominance: Dominant vs. Recessive
A dominant trait is visible when two dominant alleles are paired or a dominant allele is paired with a recessive allele. Dominant alleles are represented by capital letters. Recessive traits become visible only when two recessive alleles are present. Recessive alleles are represented by a lowercase letters. Genes come in pairs, so there are two letters together.
Earlobes
Dominant Trait
- Dominant Trait – having free earlobes (unattached at the bottom).
- A capital “E” is used to represent the dominant allele for “unattached” earlobes. The "E" is used because it is the first letter of the word "earlobe"; capital "U" could also be used representing "unattached".
- E = unattached earlobes
Recessive Trait
- Recessive Trait – having attached earlobes.
- A lowercase “e” is used to represent the recessive allele. Small “e” is used because it indicates that it is the opposite of the dominant allele. We would not use the letter ”a” for attached earlobe.
- e = attached earlobes
During reproduction the mother (egg) and father (sperm) each contribute one allele for a trait.
The mother might contribute an E or an e, and the father might contribute an E or an e, depending on what their DNA contains.
The offspring of the parents will be one of the following:
The mother might contribute an E or an e, and the father might contribute an E or an e, depending on what their DNA contains.
The offspring of the parents will be one of the following:
If both parents contribute an E:
Dominant Allele (E) + Dominant Allele (E) = (EE)
Dominant Trait
Dominant Allele (E) + Dominant Allele (E) = (EE)
Dominant Trait
- The person will have unattached earlobes.
If one parent contributes an E, and one an e:
Dominant Allele (E) + Recessive Allele (e) = (Ee)
Dominant Trait
Dominant Allele (E) + Recessive Allele (e) = (Ee)
Dominant Trait
- The person will have unattached earlobes. As long as they possess one dominant allele, the dominant trait is expressed. The attached earlobe allele remains hidden or masked in the person’s DNA.
If both parents contribute an e:
Recessive Allele (e) + Recessive Allele (e) = (ee)
Recessive Trait
Recessive Allele (e) + Recessive Allele (e) = (ee)
Recessive Trait
- The person will have attached earlobes.
NOTE: If there is a capital letter in the allele pair, then the dominant trait is visible.
Interactive
Genetics and heredity study how traits are passed from one generation to the next. Genetics studies the traits, or characteristics, of an organism based on the information found in its chromosomes and DNA.. Heredity studies how this genetic information
gets passed along from one generation to the next.
Watch these two BrainPOP videos on Genetics and Heredity.
You will need a username and password to access these videos.
Watch these two BrainPOP videos on Genetics and Heredity.
You will need a username and password to access these videos.
- Username: 0099
- Password: student
Consider a real-life example involving hair colour and offspring.
Look at Figure 1. This female has blonde hair and this male has brown hair. Which is the dominant colour? Brown? Blonde? What colour of hair would their offspring have?
Look at Figure 1. This female has blonde hair and this male has brown hair. Which is the dominant colour? Brown? Blonde? What colour of hair would their offspring have?
Remember that a dominant allele will always be expressed (show up or be visible) in the offspring when paired with a recessive allele. But, it takes two recessive alleles to pair for the recessive trait to appear in the offspring.
The colour brown is dominant and is represented by a capital ‘B’. You will remember from the readings and videos that we indicate recessive alleles by a lower case letter of the dominant allele. The recessive allele in this case is represented as a lower case ‘b’. The small “b” represents that a recessive allele is present. All alleles come in pairs (known as a gene-pair); therefore, the choices of gene-pairs in this situation would be as follows:
The colour brown is dominant and is represented by a capital ‘B’. You will remember from the readings and videos that we indicate recessive alleles by a lower case letter of the dominant allele. The recessive allele in this case is represented as a lower case ‘b’. The small “b” represents that a recessive allele is present. All alleles come in pairs (known as a gene-pair); therefore, the choices of gene-pairs in this situation would be as follows:
- BB – brown hair (two dominant alleles for brown hair)
- Bb – brown hair (one dominant allele for brown hair and a recessive allele for blonde hair) [The recessive allele for blonde hair remains hidden or masked in the person’s DNA.]
- bb – blonde hair (two recessive alleles for blonde hair)
Figure 1– There is no way to know if brown hair or blond hair is dominant without examining the offspring.
NOTE: The pattern of inheritance for hair colour has been simplified for this course.
Punnett Squares
Situation #1 with Mr. Blue and Mrs. Blue. The Punnet square in figure 2 shows the genetic possibilities for the hair colour of their children.
Situation #1 with Mr. Blue and Mrs. Blue. The Punnet square in figure 2 shows the genetic possibilities for the hair colour of their children.
Mr. Blue has the gene-pair Bb (a dominant allele and a recessive allele he received from his parents). Mr. Blue has a capital "B" allele in his hair colour gene, which means he has brown hair.
Mrs. Blue has the gene-pair bb (two recessive alleles she received from her parents). Recall that two recessive alleles are needed to make a recessive trait visible. Blonde hair is a recessive trait.
Figure 2 – Punnett Square of Bb x bb
Looking at this Punnett square, we would say the children of Mr. Blue and Mrs. Blue have a 2 out of 4 (or 50/50, or 50%) chance of having blonde hair.
Watch
Watch "What are Punnett
Squares?" Pay close attention at 1:42...this is where specific instructions are given on how to fill in a Punnett square.
Watch the video "Learn Biology: How to Draw a Punnett Square"
Watch the video "Dominant vs. Recessive"
Punnet Squares (continued)
In the previous Punnett square example we crossed a brown hair male Bb with a blonde hair female bb. Can you think of another situation that two parents with the same hair colours may have in terms of their genes? The brown-haired parent could be BB. The other parent could be bb, which would give that person blonde hair. What are the chances that their offspring would have blonde hair?
Situation #2 with Mr. Green and Mrs. Green. The Punnet square in figure 3 shows the genetic possibilities for the hair colour of their children.
In the previous Punnett square example we crossed a brown hair male Bb with a blonde hair female bb. Can you think of another situation that two parents with the same hair colours may have in terms of their genes? The brown-haired parent could be BB. The other parent could be bb, which would give that person blonde hair. What are the chances that their offspring would have blonde hair?
Situation #2 with Mr. Green and Mrs. Green. The Punnet square in figure 3 shows the genetic possibilities for the hair colour of their children.
Mr. Green has brown hair that he inherited from his parents. He is not carrying a recessive allele for blonde hair, so the letters “BB” represent his gene-pair. Mr. Green has two capital ‘B’s, which means he has brown hair.
Mrs. Green has the gene pair bb (two recessive alleles she received from her parents). Recall that two recessive alleles are needed to make a recessive trait visible. Blonde hair is a recessive trait.
Figure 3 – Punnett Squareof BB x bb
In all four squares, a dominant allele and a recessive allele are present (Bb). The children of Mr. Green and Mrs. Green have a 4 of 4 or 100% chance of having brown hair. (Remember that there can be various shades of brown, but the hair
would be classified as brown. Therefore, children of Mr. Green and Mrs. Green have no chance of having blonde hair.
Interactive
Two famous species used in studies of genetics and heredity are pea plants and fruit flies. These organisms are excellent for studying because they breed easily, they reproduce quickly, and they have easily identifiable traits.
Click here to access the activity. Please note this activity requires Flash, if you cannot get it to work on your computer, don't worry, this activity is optional, just skip it and move on.
Follow the directions indicated on “Inheritance: It Runs in the Family” to work through an example of breeding fruit flies with dominant and recessive characteristics.
Click here to access the activity. Please note this activity requires Flash, if you cannot get it to work on your computer, don't worry, this activity is optional, just skip it and move on.
Follow the directions indicated on “Inheritance: It Runs in the Family” to work through an example of breeding fruit flies with dominant and recessive characteristics.
After working through dominant and recessive genes that lead to traits becoming visible or remaining hidden or masked, we often make some bold assumptions. Often, we presume that, if a gene is dominant, it must have a high frequency rate.
In other words, we think it should appear often and be very visible.
How often do you see a person with six fingers or toes? Polydactyl means having many digits (which is six), and it is considered a dominant allele. However, it does not appear often, so it has a low frequency rate. Dwarfism is another dominant allele that is not as common as regular height allele. Note that because an allele is dominant does not mean it has a high frequency rate.
How often do you see a person with six fingers or toes? Polydactyl means having many digits (which is six), and it is considered a dominant allele. However, it does not appear often, so it has a low frequency rate. Dwarfism is another dominant allele that is not as common as regular height allele. Note that because an allele is dominant does not mean it has a high frequency rate.
Watch
Watch the National Geographic video "
Dwarfism” to see a dominant, but not common, trait.
Purebreds and Hybrids
The previous section was about dominant and recessive traits. Dominant traits are expressed with the gene-pairs in the form of two capital letters (such as GG) or a capital letter paired with a lower case letter (such as Gg). In both these allele combinations, the dominant G trait is expressed in the offspring. Scientists categorize gene-pairs into purebreds (homozygous dominant (GG) or homozygous recessive (gg)) and hybrids (heterozygous (Gg).
A purebred plant or animal has specific identifying features that can be traced to its origins and continues to be coveted by breeders.
Humans are most familiar with purebred dogs, such as German shepherds, Doberman pinschers, poodles, Bernese mountain dogs, and cocker spaniels. A purebred has two dominant or two recessive alleles, such as GG or gg. On the other hand, a hybrid has one dominant and one recessive allele (Gg). Are there any drawbacks with breeding purebreds? Does breeding for those desired traits have negative consequences?
A hybrid is a cross between two different purebreds with each of their own distinctive characteristics. Many dog breeders breed purebred dogs of various breeds to get a hybrid dog with desirable characteristics. A common cross-breed is the standard purebred poodle with the purebred Labrador retriever, which produces a hybrid labradoodle!
The previous section was about dominant and recessive traits. Dominant traits are expressed with the gene-pairs in the form of two capital letters (such as GG) or a capital letter paired with a lower case letter (such as Gg). In both these allele combinations, the dominant G trait is expressed in the offspring. Scientists categorize gene-pairs into purebreds (homozygous dominant (GG) or homozygous recessive (gg)) and hybrids (heterozygous (Gg).
A purebred plant or animal has specific identifying features that can be traced to its origins and continues to be coveted by breeders.
Humans are most familiar with purebred dogs, such as German shepherds, Doberman pinschers, poodles, Bernese mountain dogs, and cocker spaniels. A purebred has two dominant or two recessive alleles, such as GG or gg. On the other hand, a hybrid has one dominant and one recessive allele (Gg). Are there any drawbacks with breeding purebreds? Does breeding for those desired traits have negative consequences?
A hybrid is a cross between two different purebreds with each of their own distinctive characteristics. Many dog breeders breed purebred dogs of various breeds to get a hybrid dog with desirable characteristics. A common cross-breed is the standard purebred poodle with the purebred Labrador retriever, which produces a hybrid labradoodle!
Purebred Poodle
Purebred Labrador Retriever
Crossbred Labradoodle
+
=
Many dog lovers like this hybrid because the labradoodle has the intelligence and non-shedding fur from the poodle, and the energy and loyalty common among purebred retrievers. It is the best of both worlds! There are many combinations of
hybrid dogs because humans are always crossing different breeds together to see what offspring are produced.
What do YOU think?
What other animal combinations are out there? Have you ever heard of a zonkey, liger or dzo? (Yes, they do exist!)
What other animal combinations are out there? Have you ever heard of a zonkey, liger or dzo? (Yes, they do exist!)
Figure 4 – A zonkey is a zebra-donkey hybrid.
Figure 5 – A liger is a lion-tiger hybrid.
Figure 6 – A dzo is a yak-cow hybrid.
- How do you feel about these hybrids?
- Is it right to develop hybrids that do not exist naturally in our world?
- Could a hybrid combination produce a purple cow?
Review some Punnett square examples from the last section. Note the difference is that the words purebred and hybrid are discussed. Which of the following Punnett Squares (1 or 2) produces a purebred blue-eyed offspring?
Punnett Square 1
Punnett Square 2
100% of the offspring have brown eyes because
in all squares there is a dominant allele (B for brown).
in all squares there is a dominant allele (B for brown).
75% of the offspring have brown eyes because
three of the squares have a dominant allele present (B for brown).
25% of the offspring have blue eyes because
there is no dominant allele in one square (bb for blue).
three of the squares have a dominant allele present (B for brown).
25% of the offspring have blue eyes because
there is no dominant allele in one square (bb for blue).
Try It!
Practice Worksheet: Punnett Squares
- DOWNLOAD this practice worksheet (S9_UA_S3_L13_punnett_squares). If you prefer to use a Google Drive or PDF version of the worksheet, click here.
- Complete the worksheet.
- When you are satisfied with your responses you can check your work by clicking on the "SUGGESTED ANSWERS" button below.
Wait! Don't view the suggested answers first.
This practice work is not for marks, it is meant to help you check your
understanding. Check the answers AFTER doing the questions! Keep the
practice worksheet for study purposes. If you don't understand
something, contact your teacher!
1.
Note: you might have used the letter R to represent rolling rather than T. Rr = dominant (can roll) and rr = recessive (cannot roll).
2.
Note: you might have used the letter R to represent rolling rather than T. Rr = dominant (can roll) and rr = recessive (cannot roll).
Percent that can roll tongues in offspring?
50% - because two squares have the dominant gene present
Percent that cannot roll tongues in offspring?
50% - because the two squares do not have the dominant gene present
2.
Percent of white hippos in offspring?
0%
Percent of grey hippos in offspring?
100%
Discrete and Continuous Variation
Recall that in Lesson 8 you were introduced into discrete and continuous variation. In this lesson, these concepts will be linked to patterns of inheritance.
Pages 28 and 29 in your textbook will help you understand discrete and continuous variation.
Some traits are discrete; you either have the trait or not.
A cleft in the chin is a discrete variant. No cleft is dominant over having a cleft.
A cleft in the chin is a discrete variant. No cleft is dominant over having a cleft.
Some traits are continuous; traits that have a range of expression such as height.
There are many different heights in your groups of family and friends.
There are many different heights in your groups of family and friends.
Freckles as a trait are dominant and continuous, plus individuals will get more freckles when they are exposed to sunlight. This is an example of a trait being influenced both by genetics and by the environment. Can you think of other traits
that are influenced by the environment?
