Mendelian Genetics

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Mendelian genetics is the study of how traits are inherited from parents to offspring, based on the work of Gregor Mendel (1822-1884). Mendel's experiments with pea plants revealed the basic laws of heredity that still form the foundation of genetics today.

Gregor Mendel, an Austrian monk, conducted careful experiments with pea plants for over 8 years. By tracking specific traits across generations, he discovered patterns that explained how characteristics pass from parents to offspring.

Key Vocabulary

Term	Definition	Example
Gene	A unit of hereditary information located on a chromosome	The gene for pea plant height
Allele	Different versions of a gene	Tall (T) and short (t) are alleles for height
Dominant	An allele that masks the effect of another allele	T (tall) is dominant over t (short)
Recessive	An allele that is masked by a dominant allele	t (short) only shows when TT is absent
Genotype	The genetic makeup (allele combination)	TT, Tt, or tt
Phenotype	The observable physical trait	Tall or short
Homozygous	Having two identical alleles	TT or tt
Heterozygous	Having two different alleles	Tt

Mendel's Laws

Law of Segregation

Each organism has two alleles for each trait. During gamete (sex cell) formation, these alleles separate so each gamete receives only one allele. When fertilization occurs, offspring receive one allele from each parent.

Law of Independent Assortment

Genes for different traits are inherited independently of each other (assuming they are on different chromosomes). The inheritance of one trait does not affect the inheritance of another trait.

Punnett Squares

A Punnett square is a diagram used to predict the genotypes and phenotypes of offspring from a genetic cross. Parent alleles are placed on the top and side, and possible offspring combinations fill the boxes.

SAT/ACT Connection

Genetics problems appear frequently on science sections. You may need to interpret Punnett squares, calculate probability ratios, or predict offspring traits. Practice reading genetics diagrams and understanding dominant/recessive relationships.

Examples

Work through these genetics problems step by step.

Example 1: Monohybrid Cross (Tt x Tt)

Problem: Two heterozygous tall pea plants (Tt) are crossed. What are the possible genotypes and phenotypes of offspring?

Step 1: Set up the Punnett square

	T	t
T	TT	Tt
t	Tt	tt

Step 2: Count genotype ratios

1 TT : 2 Tt : 1 tt (or 25% : 50% : 25%)

Step 3: Determine phenotypes

TT = Tall, Tt = Tall, tt = Short

Phenotype ratio: 3 Tall : 1 Short (75% tall, 25% short)

Example 2: Test Cross

Problem: A tall plant with unknown genotype is crossed with a short plant (tt). All offspring are tall. What is the genotype of the unknown parent?

Analysis:

Short plant must be tt (homozygous recessive)
All offspring are tall, meaning all received at least one T
If parent were Tt, some offspring would be tt (short)
Since all offspring are tall, all must have received T from the unknown parent

Answer: The unknown parent must be TT (homozygous dominant).

Example 3: Determining Genotype from Phenotype

Problem: In guinea pigs, black fur (B) is dominant to brown fur (b). A black guinea pig has a brown parent. What is the black guinea pig's genotype?

Analysis:

The brown parent must be bb (only way to show recessive trait)
The brown parent contributed b to the offspring
The black offspring must have received b from the brown parent
For the offspring to be black, it must also have B

Answer: The black guinea pig is Bb (heterozygous).

Example 4: Dihybrid Cross Ratio

Problem: In pea plants, yellow seeds (Y) are dominant to green (y), and round shape (R) is dominant to wrinkled (r). If YyRr x YyRr, what ratio of offspring will be yellow and round?

Analysis:

For a dihybrid cross (YyRr x YyRr), we can use the rule of independent assortment:

Probability of Yellow (YY or Yy): 3/4
Probability of Round (RR or Rr): 3/4
Probability of Yellow AND Round: 3/4 x 3/4 = 9/16

Answer: 9/16 (or about 56%) of offspring will be yellow and round.

Example 5: Probability Calculation

Problem: Two carriers of a recessive disorder (Aa x Aa) have children. What is the probability their first child will be affected?

Step 1: Set up Punnett square

	A	a
A	AA	Aa
a	Aa	aa

Step 2: Identify affected genotype

Only aa shows the disorder (homozygous recessive)

1 out of 4 boxes shows aa

Answer: 1/4 or 25% probability the first child will be affected.

Practice

Apply your knowledge of Mendelian genetics.

1. An organism with genotype Bb is called:

A) Homozygous dominant B) Homozygous recessive C) Heterozygous D) Codominant

2. The physical expression of a genotype is the:

A) Allele B) Phenotype C) Chromosome D) Gene

3. If B = brown eyes (dominant) and b = blue eyes, what phenotype results from Bb?

A) Blue eyes B) Brown eyes C) Green eyes D) Half brown, half blue

4. In a cross Tt x tt, what percentage of offspring will be tall (T is dominant)?

A) 25% B) 50% C) 75% D) 100%

5. What is the genotype ratio from a Bb x Bb cross?

A) 1:1 B) 3:1 C) 1:2:1 D) 2:1:1

6. Two brown mice produce some white offspring. Brown (B) is dominant. The parents' genotypes are:

A) BB x BB B) Bb x bb C) Bb x Bb D) BB x Bb

7. According to Mendel's Law of Segregation:

A) Traits blend together B) Alleles separate during gamete formation C) All offspring are identical D) Dominant always beats recessive

8. A test cross involves crossing an individual with an unknown genotype with:

A) Another unknown individual B) A homozygous dominant individual C) A homozygous recessive individual D) A heterozygous individual

9. If both parents are Aa, what is the probability of having an aa offspring?

A) 0% B) 25% C) 50% D) 75%

10. Mendel's Law of Independent Assortment states that:

A) Genes are linked together B) Genes for different traits are inherited independently C) All alleles are dominant D) Offspring are clones of parents

Click to reveal answers

C) Heterozygous - Having two different alleles (B and b).
B) Phenotype - The observable trait resulting from genotype.
B) Brown eyes - Dominant allele (B) masks recessive (b).
B) 50% - Punnett: Tt, Tt, tt, tt = 50% tall (Tt).
C) 1:2:1 - 1 BB : 2 Bb : 1 bb.
C) Bb x Bb - Both must carry recessive b to produce bb offspring.
B) Alleles separate during gamete formation - Each gamete gets one allele.
C) A homozygous recessive individual - This reveals the unknown genotype.
B) 25% - 1/4 of offspring from Aa x Aa will be aa.
B) Genes for different traits are inherited independently - Separate genes assort independently.

Check Your Understanding

Reflect on these questions to deepen your understanding.

1. Why was it important that Mendel used pea plants with distinct, contrasting traits?

Reveal Answer

Distinct traits (like tall vs. short, not medium) made it easy to count and categorize offspring. If traits blended or varied continuously, Mendel would not have been able to identify the clear ratios (3:1, 1:2:1) that revealed inheritance patterns. The contrasting traits also helped him identify dominant and recessive relationships. His choice of organism was key to his success.

2. Explain why two parents with brown eyes can have a child with blue eyes.

Reveal Answer

If brown (B) is dominant and blue (b) is recessive, both parents could be heterozygous (Bb). They would have brown eyes because the dominant allele masks the recessive one. However, when both contribute their recessive allele (b) to an offspring, that child would be bb and have blue eyes. There's a 25% chance with each child if both parents are Bb.

3. How does a test cross help determine an unknown genotype?

Reveal Answer

A test cross mates the unknown individual with a homozygous recessive (aa) individual. If the unknown is homozygous dominant (AA), all offspring will be Aa (showing dominant phenotype). If the unknown is heterozygous (Aa), half the offspring will be Aa (dominant phenotype) and half will be aa (recessive phenotype). The offspring ratios reveal the unknown parent's genotype.

4. Why is it possible for siblings to look very different from each other genetically?

Reveal Answer

Independent assortment and the random combination of alleles mean each child receives a unique mix of parental genes. If parents are heterozygous for many traits, the number of possible combinations is enormous (2^n where n is the number of heterozygous genes). Each sibling represents one random outcome of the genetic lottery. Only identical twins share the same genetic combination.

🚀 Next Steps

Review any concepts that felt challenging
Move on to the next lesson when ready
Return to practice problems periodically for review