1. Watch this video
2. Study this summary
- Genes and Alleles:
- Genes are the basic units of heredity, made of DNA sequences that code for proteins, which determine traits.
- Alleles are alternative versions of a gene, which may vary in their DNA sequence and result in different traits.
- Mendel’s Principle of Segregation:
- Every individual has two alleles for each gene, located on homologous chromosome pairs.
- These alleles separate during gamete formation (meiosis), so each gamete contains only one allele.
- Dominant vs. Recessive Alleles:
- Dominant alleles are always expressed in the phenotype. In the Mendelian notation system, they’re represented by capital letters (e.g., A).
- Recessive alleles only show their effect when an individual is homozygous recessive. They’re represented by lowercase letters (e.g., lowercase letter a).
-
Homozygotes and Heterozygotes
- Homozygote: An individual who has two identical alleles for a gene.
- Examples: AA (homozygous dominant) or aa (homozygous recessive).
- Homozygous dominant individuals express the dominant phenotype, while homozygous recessive individuals express the recessive phenotype.
- Heterozygote: An individual who has two different alleles for a gene (e.g., Aa).
- In heterozygotes, the dominant allele is expressed in the phenotype, masking the effect of the recessive allele.
- In the case of diseases such as sickle cell anemia or cystic fibrosis, heterozygotes are also called carriers. That’s because they carry the defective allele in their genes and can pass it on to their offspring.
- Homozygote: An individual who has two identical alleles for a gene.
- Genotype and Phenotype:
- Genotype refers to the genetic makeup (e.g., BB, Bb, bb).
- Phenotype refers to observable traits (e.g., eye color).
Example: A person with brown eyes (dominant phenotype) but a heterozygous genotype (Bb) can have blue-eyed children if their partner is homozygous recessive (bb). Note: eye color inheritance is somewhat more complicated.
- Monohybrid Crosses:
- A monohybrid cross involves two heterozygous individuals (e.g., Pp x Pp).
- Using a Punnett square, the resulting phenotypic ratio of offspring is 3:1 (dominant:recessive).
- Genotypic ratios are: 1 homozygous dominant, 2 heterozygous, and 1 homozygous recessive.
3. Master these flashcards
[qdeck]
[h]Mendelian Genetics, Basic Concepts
[q]What are genes?
[a]Genes are the basic units of heredity, consisting of DNA sequences that code for proteins and determine traits.
[q]What are alleles?
[a]Alleles are alternative versions of a gene, differing in DNA sequence and potentially resulting in different traits.
[h]Mendel’s Principle of Segregation
[q]What is Mendel’s principle of segregation?
[a]It states that every individual has two alleles for each gene, and these alleles separate during gamete formation (meiosis), ensuring each gamete contains only one allele.
[h]Homozygous vs. Heterozygous
[q]What does homozygous mean?
[a]Homozygous means having two identical alleles for a gene (e.g., AA or aa).
[q]What does heterozygous mean?
[a]Heterozygous means having two different alleles for a gene (e.g., Aa).
[q]How do homozygous and heterozygous individuals differ?
[a]Homozygous individuals have identical alleles for a gene, while heterozygous individuals have two different alleles.
[h]Dominant vs. Recessive Alleles
[q]What is a dominant allele?
[a]A dominant allele is expressed in the phenotype if present, even when paired with a recessive allele (e.g., A).
[q]What is a recessive allele?
[a]A recessive allele is only expressed in the phenotype when the individual is homozygous recessive (e.g., aa).
[q]How do dominant and recessive alleles differ?
[a]Dominant alleles mask the effect of recessive alleles when both are present.
[h]Genotype and Phenotype
[q]What is a genotype?
[a]A genotype is an organism’s genetic makeup, describing its allele combinations (e.g., AA, Aa, or aa).
[q]What is a phenotype?
[a]A phenotype is an organism’s observable traits, such as eye color.
[q]How do genotype and phenotype differ?
[a]Genotype refers to genetic makeup, while phenotype refers to observable traits resulting from that genetic makeup.
[h]Monohybrid Crosses
[q]What is a monohybrid cross?
[a]A monohybrid cross is a genetic cross between two heterozygous individuals (e.g., Aa x Aa), examining inheritance of a single trait.
[q]What does a Punnett square show for a monohybrid cross?
[a]It shows a **phenotypic ratio** of 3:1 (dominant:recessive) and a **genotypic ratio** of 1:2:1 (homozygous dominant:heterozygous:homozygous recessive).
[/qdeck]
4. Tackle these quizzes
4.1 Genetics Vocabulary Check
[qwiz]
[h]Genetics Vocabulary Check
[q]The founder of genetics was an Austrian monk named Gregor [hangman]. In his honor, the basic principles of genetics are referred to as [hangman] genetics.
[c]TWVuZGVs[Qq]
[c]TWVuZGVsaWFu[Qq]
[q] An allele that always shows up in the offspring is a(n) [hangman] allele.
[c]ZG9taW5hbnQ=[Qq]
[q] An alternative form of a gene is called a(n) [hangman].
[c]YWxsZWxl[Qq]
[q]A(n) [hangman] allele will only show up in the offspring if the offspring inherits two copies of that allele.
[c]cmVjZXNzaXZl[Qq]
[q] An organism that has two identical alleles for a particular gene is known as a(n) [hangman].
[c]aG9tb3p5Z290ZQ==[Qq]
[q] An organism that has two different alleles for a particular gene is known as a(n) [hangman].
[c]aGV0ZXJvenlnb3Rl[Qq]
[q]Another term for a heterozygote refers to the fact that they possess the allele, don’t display the trait, and can pass it on to the next generation. That term is [hangman].
[c]Y2Fycmllcg==[Qq]
[q] A unit of heredity that is passed from parent to offspring is known as a(n) [hangman].
[c]Z2VuZQ==[Qq]
[q] The chance that a heterozygote parent will pass on a recessive or a dominant allele is one in [hangman] (write out the number).
[c]dHdv[Qq]
[q] The chance that the offspring of two heterozygotes will inherit two recessive alleles is one in [hangman](write out the number).
[c]Zm91cg==[Qq]
[q] Your underlying genes make up your [hangman]. Your appearance is your [hangman].
[c]Z2Vub3R5cGU=[Qq]
[c]cGhlbm90eXBl[Qq]
[q]A person with a genotype that would be symbolized as Aa or Cc is
[c]aG9tb3p5Z291cyBkb21pbmFudA==[Qq]
[f]Tm8uIEhvbW96eWdvdXMgbWVhbnMgdGhlIHR3byBhbGxlbGVzIGFyZSB0aGUgc2FtZS4gSW4gdGhpcyBjYXNlLCB0aGUgdHdvIGFsbGVsZXMgYXJlIGRpZmZlcmVudC4gQSBob21venlnb3VzIGRvbWluYW50IGdlbm90eXBlIGxvb2tzIGxpa2UgdGhpczogU1MsIEFBLCBHRywgZXRjLiBBbHdheXMgdHdvIHVwcGVyY2FzZSBsZXR0ZXJzLg==[Qq]
[c]aGV0ZXJvenlnb3VzIC hhbmQgZG9taW5hbnQp[Qq]
[f]TmljZS4gQ2Mgb3IgQWEgZ2Vub3R5cGVzIGFyZSBoZXRlcm96eWdvdXMgKGFuZCBoZXRlcm96eWdvdGVzIGFsd2F5cyBkaXNwbGF5IHRoZSBkb21pbmFudCB0cmFpdCwgd2hpY2ggaXMgd2h5ICYjODIyMDtkb21pbmFudCYjODIyMTsgaXMgaW4gcGFyZW50aGVzaXMu[Qq]
[c]aGV0ZXJvenlnb3VzIChhbmQgcmVjZXNzaXZlKQ==[Qq]
[f]Tm8uIElmIHlvdSYjODIxNztyZSBoZXRlcm96eWdvdXMsIHlvdSYjODIxNztsbCBkaXNwbGF5IHRoZSBkb21pbmFudCB0cmFpdC4gWW91IGNhbiYjODIxNzt0IGJlICYjODIyMDtoZXRlcm96eWdvdXMgcmVjZXNzaXZlLiYjODIyMTs=[Qq]
[c]aG9tb3p5Z291cyByZWNlc3NpdmU=[Qq]
[f]Tm8uIEhvbW96eWdvdXMgcmVjZXNzaXZlIHBoZW5vdHlwZXMgbG9vayBsaWtlIHRoaXM6IGNjLCBzcywgYWEsIGV0Yy4gQWx3YXlzIHR3byBsb3dlcmNhc2UgbGV0dGVycy4=[Qq]
[q]A person with a genotype that would be symbolized as aa or cc is
[c]aG9tb3p5Z291cyBkb21pbmFudA==[Qq]
[f]Tm8uIEEgZG9taW5hbnQgZ2Vub3R5cGUgYWx3YXlzIGhhcyBhbiB1cHBlcmNhc2UgbGV0dGVyLiBBIGhvbW96eWdvdXMgZG9taW5hbnQgZ2Vub3R5cGUgbG9va3MgbGlrZSB0aGlzOiBTUywgQUEsIEdHLCBldGMu[Qq]
[c]aGV0ZXJvenlnb3VzIChhbmQgZG9taW5hbnQp[Qq]
[f]Tm8uwqAgSGV0ZXJvenlnb3VzIG1lYW5zIHRoYXQgdGhlIHR3byBhbGxlbGVzIGFyZSBkaWZmZXJlbnQuIEZvciBleGFtcGxlLCAmIzgyMjA7QWEmIzgyMjE7IGlzIGEgaGV0ZXJvenlnb3VzIGdlbm90eXBlLg==[Qq]
[c]aGV0ZXJvenlnb3VzIChhbmQgcmVjZXNzaXZlKQ==[Qq]
[f]Tm8uIEhldGVyb3p5Z291cyBtZWFucyB0aGF0IHRoZSB0d28gYWxsZWxlcyBhcmUgZGlmZmVyZW50LiBGb3IgZXhhbXBsZSwgJiM4MjIwO0FhJiM4MjIxOyBpcyBhIGhldGVyb3p5Z291cyBnZW5vdHlwZS4gQWxzbywgeW91IGNhbiYjODIxNzt0IGJlIGhldGVyb3p5Z291cyBhbmQgcmVjZXNzaXZlLiBIZXRlcm96eWdvdGVzIGFsd2F5cyBkaXNwbGF5IHRoZSBkb21pbmFudCB0cmFpdC4=[Qq]
[c]aG9tb3p5Z291cy ByZWNlc3NpdmU=[Qq]
[f]V2F5IHRvIGdvLiBIb21venlnb3VzIHJlY2Vzc2l2ZSBwaGVub3R5cGVzIGFsd2F5cyBoYXZlIHR3byBsb3dlcmNhc2UgbGV0dGVycy4=[Qq]
[x][restart]
[/qwiz]
4.2. Understanding Cystic Fibrosis
Cystic fibrosis is the most common genetic disease among North Americans of European descent. Complete the interactive reading/quiz below to learn about this disease and to consolidate your understanding of basic genetics concepts.
[qwiz]
[h]Cystic Fibrosis, Interactive Reading
[i]
[q labels=”top”]
Cystic fibrosis is caused by a defect in a protein found in cell membranes. The _______ for this protein is found on chromosome 7. That makes the gene an ____________ gene (because it’s not on the X or Y chromosome).
If you inherit a working ________ (version) of this gene, you’ll be fine, even if you inherit one defective allele. That’s because the disease-causing allele is ___________. If you inherit the working, ____________ allele, then this allele will be translated into the working membrane protein. Your cells will function as they should, and there’ll be no build-up of the sticky mucus that underlies the disease.
[l]allele
[fx] No, that’s not correct. Please try again.
[f*] Good!
[l]autosomal
[fx] No, that’s not correct. Please try again.
[f*] Good!
[l]dominant
[fx] No, that’s not correct. Please try again.
[f*] Great!
[l]gene
[fx] No. Please try again.
[f*] Great!
[l]genotypes
[fx] No. Please try again.
[f*] Great!
[l]homozygous
[fx] No, that’s not correct. Please try again.
[f*] Great!
[l]heterozygous
[fx] No. Please try again.
[f*] Good!
[l]phenotype
[fx] No. Please try again.
[f*] Excellent!
[l]recessive
[fx] No. Please try again.
[f*] Excellent!
[q labels=”top”]
You can only have the disease if you inherit two defective alleles. Another way to say this is that to have cystic fibrosis, you have to be ______________ for the cystic fibrosis allele. Being homozygous means that you’ll have two copies of the allele. People who are _______________, with one dominant allele and one recessive allele, don’t get the disease. They have a normal ____________ (no sticky mucus).
[l]allele
[fx] No, that’s not correct. Please try again.
[f*] Good!
[l]autosomal
[fx] No, that’s not correct. Please try again.
[f*] Good!
[l]dominant
[fx] No, that’s not correct. Please try again.
[f*] Great!
[l]gene
[fx] No. Please try again.
[f*] Great!
[l]genotypes
[fx] No. Please try again.
[f*] Great!
[l]homozygous
[fx] No, that’s not correct. Please try again.
[f*] Great!
[l]heterozygous
[fx] No. Please try again.
[f*] Good!
[l]phenotype
[fx] No. Please try again.
[f*] Excellent!
[l]recessive
[fx] No. Please try again.
[f*] Excellent!
[q labels=”top”]
If we use C to represent the normal allele and c to represent the cystic fibrosis allele, then we have three ____________:
- _______________ dominant: CC
- ________________: Cc
- homozygous ___________: cc
[l]allele
[fx] No, that’s not correct. Please try again.
[f*] Good!
[l]autosomal
[fx] No, that’s not correct. Please try again.
[f*] Good!
[l]dominant
[fx] No, that’s not correct. Please try again.
[f*] Great!
[l]gene
[fx] No. Please try again.
[f*] Great!
[l]genotypes
[fx] No. Please try again.
[f*] Great!
[l]homozygous
[fx] No, that’s not correct. Please try again.
[f*] Great!
[l]heterozygous
[fx] No. Please try again.
[f*] Good!
[l]phenotype
[fx] No. Please try again.
[f*] Excellent!
[l]recessive
[fx] No. Please try again.
[f*] Excellent!
[q]Just to check your understanding. Cystic fibrosis is an autosomal [hangman] condition. That means you’ll only have the phenotype if you inherit [hangman] copies of the allele.
[c]cmVjZXNzaXZl[Qq]
[c]dHdv[Qq]
[/qwiz]
4.2 A Guided Punnett Square
Being able to solve genetics problems is an important AP Bio skill. The exercise below will show you how to complete a basic Punnett square
[qwiz]
[h]Guided Punnett Square: Autosomal inheritance
[q] In peas, the tall allele (T) is dominant to the short allele (t). Cross a plant that is homozygous tall with a plant that is heterozygous tall.
Step 1: Determine the genotypes of the parents
Parent 1) Homozygous tall: ___ ___
Parent 2) Heterozygous tall: ___ ___
Steps 2 through 4: Complete your Punnett square. Note that for this quiz, you have to place the alleles of the gametes of the 1st parent to the left of the Punnett square, and put the alleles of the second parent’s gametes on top. On paper, you could do it in either order.
| Parent 2 | |||
| __ | __ | ||
| Parent 1 | __ | ___ ___ | ___ ___ |
| __ | ___ ___ | ___ ___ | |
Step 5: Genotype ratio: _____ TT: ____Tt: ____tt
Step 6: Phenotype ratio: _____ Tall: ______Short
[l]T
[fx] No, that’s not correct. Please try again.
[f*] Excellent!
[l]t
[fx] No. Please try again.
[f*] Great!
[l]0%
[fx] No, that’s not correct. Please try again.
[f*] Good!
[l]25%
[fx] No, that’s not correct. Please try again.
[f*] Correct!
[l]50%
[fx] No, that’s not correct. Please try again.
[f*] Good!
[l]75%
[fx] No, that’s not correct. Please try again.
[f*] Correct!
[l]100%
[fx] No. Please try again.
[f*] Great!
[q]In peas, the gene for purple flowers (P) is dominant to the gene for white flowers (p). What will happen if a plant homozygous for purple flowers is crossed with a plant homozygous for white flowers?
Step 1: Determine the genotypes of the parents
Parent 1) Homozygous purple flowers: ___ ___
Parent 2) Homozygous white flowers: ___ ___
Step 2: Draw the Punnett square (already done below)
Step 3: Place the alleles of the gametes that the 1st parent can make to the left of the Punnett square. Put the alleles that the 2nd parent can make on top. (note: these required positions (left and top) are solely so that this quiz program will work. On paper, you can put them in either position.
Step 4: Move the gametes to the right or downward into the square, simulating fertilization and zygote formation.
| Parent 2 | |||
| __ | __ | ||
| Parent 1 | __ | ___ ___ | ___ ___ |
| __ | ___ ___ | ___ ___ | |
Step 5: Genotype ratio: _____ PP: ____Pp: ____pp
Step 6: Phenotype ratio: _____ Purple-flowered, ____white-flowered
[l]P
[fx] No. Please try again.
[f*] Excellent!
[l]p
[fx] No, that’s not correct. Please try again.
[f*] Correct!
[l]0%
[fx] No. Please try again.
[f*] Correct!
[l]25%
[fx] No. Please try again.
[f*] Good!
[l]75%
[fx] No. Please try again.
[f*] Great!
[l]100%
[fx] No, that’s not correct. Please try again.
[f*] Correct!
[q]In peas, the gene for tall (T) is dominant to the gene for short (t). What genotypes and phenotypes will result in the offspring if two heterozygous plants are crossed with each other?
Step 1: Determine the genotypes of the parents
Parent 1) Heterozygous tall: ___ ___
Parent 2) Heterozygous tall: ___ ___
Step 2: Draw the Punnett square (I did that one for you)
Step 3: Place the alleles of the gametes that the 1st parent can make to the left of the Punnett square. Put the alleles that the 2nd parent can make on top. (note: these required positions (left and top) are solely so that this quiz program will work. On paper, you can put them in either position).
Step 4: Move the gametes to the right or downward into the square, simulating fertilization and zygote formation.
| Parent 2 | |||
| __ | __ | ||
| Parent 1 | __ | ___ ___ | ___ ___ |
| __ | ___ ___ | ___ ___ | |
Step 5: Genotype ratio: _____ TT: ____Tt: ____tt
Step 6: Phenotype ratio:
_____ Tall, _____medium, _____ short
[l]T
[fx] No. Please try again.
[f*] Great!
[l]t
[fx] No, that’s not correct. Please try again.
[f*] Great!
[l]0%
[fx] No, that’s not correct. Please try again.
[f*] Excellent!
[l]25%
[fx] No. Please try again.
[f*] Excellent!
[l]50%
[fx] No, that’s not correct. Please try again.
[f*] Excellent!
[l]75%
[fx] No. Please try again.
[f*] Correct!
[l]100%
[fx] No. Please try again.
[f*] Great!
[/qwiz]
What’s Next?
Proceed to the next tutorial: Dihybrid Crosses