chapter 20 Genetic Inheritance Practice Flashcards

Genotype and Phenotype Fundamentals

• Genotype Defined: The genotype refers to the specific genes of an individual.

  • Genes: Segments of DNA that contain the code for a specific trait.
  • Locus (pl. Loci): The specific physical position or location of a gene on a chromosome.
  • Allele: An alternate form of a gene. For example, if the trait is eye color, one allele might code for blue eyes while another codes for brown eyes.

• Classification of Alleles:

  • Dominant Alleles: These alleles mask the expression of recessive alleles. Only one copy of a dominant allele needs to be present for the associated trait to be expressed in the phenotype.
  • Recessive Alleles: These alleles are only expressed when an individual has two copies of the allele (one on each chromosome of a homologous pair).

• Genetic Notation and Inheritance:

  • Notation: Dominant alleles are assigned an uppercase letter (e.g., $A$); recessive alleles are assigned a lowercase letter (e.g., $a$).
  • Example (Albinism): The allele for normal pigmentation is represented by $A$, and the allele for no pigmentation (albinism) is represented by $a$.
  • Inheritance: For every pair of chromosomes, one is received from each parent. Consequently, an individual inherits one allele from each parent for every trait.

• Types of Genotypes:

  • Homozygous Dominant: An individual possesses two dominant alleles, represented as $AA$. This occurs when both the sperm and egg carry the dominant trait.
  • Homozygous Recessive: An individual possesses two recessive alleles, represented as $aa$.
  • Heterozygous: An individual possesses one dominant allele and one recessive allele, represented as $Aa$.

• Phenotype Defined: The phenotype refers to the actual physical appearance of a trait. The genotype serves as the set of instructions that directs the phenotype.

Questions & Discussion: Section 21.1

• Define the following terms: gene, allele, locus, chromosome, dominant, and recessive.
  • Gene: A DNA segment coding for a trait.
  • Allele: Alternative version of a gene.
  • Locus: The specific spot on a chromosome where a gene resides.
  • Chromosome: Structure carrying genetic information.
  • Dominant: Allele that masks others.
  • Recessive: Allele expressed only in the absence of a dominant one.
• Describe the difference between genotype and phenotype.
  • Genotype is the genetic makeup (the alleles), while phenotype is the physical manifestation or observable trait.
• Summarize the three possible genotypes and their corresponding phenotypes.
  • Homozygous dominant ($AA$) shows the dominant phenotype.
  • Heterozygous ($Aa$) shows the dominant phenotype.
  • Homozygous recessive ($aa$) shows the recessive phenotype.

One-Trait and Two-Trait Inheritance Patterns

• Cross Types:

  • One-Trait (Monohybrid) Cross: Examines the inheritance patterns of a single set of alleles for a single characteristic.
  • Two-Trait Cross: Explores the inheritance patterns for two distinct characteristics.
  • Methodology: For both crosses, the first step is to determine the gametes of the parents.

• Gamete Formation:

  • Adults have $46$ chromosomes (arranged in $23$ homologous pairs, with one homologue from each parent).
  • Meiosis: During this process, homologous chromosomes are separated.
  • Gamete Content: Gametes (sperm and egg) contain only $23$ chromosomes. When gametes fuse during fertilization, the resulting individual returns to the full count of $46$ chromosomes.

• Monohybrid Cross Example: Freckles:

  • Alleles: $F$ is the dominant allele for freckles; $f$ is the recessive allele for no freckles.
  • Scenario 1 (No Freckles): Parents without freckles must have the genotype $ff$. They can only produce gametes containing $f$. Therefore, all children will be $ff$ and will not have freckles.
  • Scenario 2 (Heterozygous Cross): In a cross between two heterozygotes ($Ff \times Ff$):
    • Punnett Square: A tool used to predict outcomes where all possible sperm alleles are lined up on one side and egg alleles on the other.
    • Genotypic Ratio: $1\,FF:2\,Ff:1\,ff$.
    • Phenotypic Ratio: $3:1$ ($3$ with freckles, $1$ without).
  • Scenario 3 (Homozygous Dominant x Recessive): A man with genotype $FF$ and a woman with $ff$ will produce children who are all heterozygous ($Ff$) and have freckles.

• Determining Genotypes (Test Logic):

  • A person with a dominant phenotype could be $FF$ or $Ff$.
  • If a man with freckles reproduces with a woman who does not ($ff$):
    • If the man is $FF$, all children will have freckles.
    • If the man is $Ff$, each child has a $50\%$ chance of having freckles ($Ff$) and a $50\%$ chance of none ($ff$).
    • The birth of even one child without freckles proves the parent with the dominant phenotype is heterozygous.

• Probability Rules in Genetics:

  • The Product Rule: The chance of two different independent events occurring simultaneously is equal to the product of their separate probabilities. (Applied to "and" scenarios, e.g., $P(B \text{ and } C) = P(B) \times P(C)$).
  • The Sum Rule: Individual probabilities are added to determine the total probability for an event. (Applied to "or" scenarios, e.g., $P(B \text{ or } C) = P(B) + P(C)$).

• Two-Trait Crosses (Dihybrid):

  • Maternal and paternal homologues separate independently during Meiosis I.
  • Example Genotype: A person with $FfSs$ (heterozygous for freckles and finger length) produces four types of gametes in equal numbers: $FS$, $fs$, $Fs$, and $fS$.
  • Dihybrid Cross ($FfSs \times FfSs$):
    • The expected phenotypic ratio for simple dominance is $9:3:3:1$.
    • $9$ individuals with both dominant traits (Freckles, Short fingers).
    • $3$ individuals with one dominant and one recessive (Freckles, Long fingers).
    • $3$ individuals with the other dominant and recessive (No freckles, Short fingers).
    • $1$ individual with both recessive traits (No freckles, Long fingers).
  • Calculations via Product Rule:
    • Probability of freckles = $\frac{3}{4}$; Probability of no freckles = $\frac{1}{4}$.
    • Probability of short fingers = $\frac{3}{4}$; Probability of long fingers = $\frac{1}{4}$.
    • Freckles and Short: $\frac{3}{4} \times \frac{3}{4} = \frac{9}{16}$.
    • Freckles and Long: $\frac{3}{4} \times \frac{1}{4} = \frac{3}{16}$.
    • No Freckles and Short: $\frac{1}{4} \times \frac{3}{4} = \frac{3}{16}$.
    • No Freckles and Long: $\frac{1}{4} \times \frac{1}{4} = \frac{1}{16}$.
  • Special Cross: $FfSs \times ffss$ (heterozygous for both traits crossed with homozygous recessive for both) results in a $1:1:1:1$ phenotypic ratio.

Questions & Discussion: Section 21.2

• Explain how the results of a dihybrid cross are related to the events of meiosis.
  • During meiosis, homologous pairs align and separate independently (independent assortment), allowing alleles for different traits to distribute into gametes in all possible combinations.
• Predict the genotype of children if one parent is $ffSS$ and the other is $FFss$.
  • Parent 1 produces $fS$ gametes; Parent 2 produces $Fs$ gametes. All children will be $FfSs$.
• Explain how gametes are formed by meiosis using a dihybrid cross as an example.
  • In a dihybrid ($AaBb$), meiosis ensures that each gamete receives one allele from each gene pair, resulting in $AB$, $Ab$, $aB$, and $ab$ gametes.

Inheritance of Genetic Disorders and Pedigrees

• Origins of Disorders: Many diseases result from DNA mutations passed via sperm or egg. Mutations may be inherited from parents (affected or carriers) or may be new mutations.

• Autosomal Disorder Patterns:

  • Autosomal Dominant: Individuals with genotypes $AA$ or $Aa$ have the disorder.
  • Autosomal Recessive: Only individuals with the genotype $aa$ have the disorder.

• Pedigrees: Tools used to determine the inheritance pattern of a condition.

  • Symbols: Squares = Males; Circles = Females; Shaded = Affected.
  • Lines: Horizontal between symbols = Mating; Vertical downward = Offspring.

• Autosomal Recessive Characteristics:

  • Carrier: A heterozygote ($Aa$) who carries the recessive allele but possesses a dominant, unaffected phenotype.
  • Most affected children have unaffected parents (who are carriers).
  • Two affected parents will always have affected children.
  • Close relatives (consanguinity) who reproduce are more likely to have affected children.

• Autosomal Dominant Characteristics:

  • Affected children usually have at least one affected parent.
  • Heterozygotes ($Aa$) are affected.
  • Two affected parents can produce an unaffected child (if both are $Aa$).
  • Two unaffected parents will not have affected children.

• Specific Autosomal Recessive Disorders:

  • Tay-Sachs Disease: Common among Ashkenazic Jewish populations. Caused by a lack of the lysosome enzyme hex A, leading to fatty acid protein buildup in the brain. Symptoms: blindness, seizures, paralysis. No cure; death usually by age $5$.
  • Cystic Fibrosis (CF): Most prevalent in white populations. Caused by a defective chloride ($Cl^-$) channel encoded by the CFTR allele on chromosome 7. $1$ in $29$ whites in the US are carriers. Since $Cl^-$ cannot pass, water does not follow, causing thick mucus in bronchial tubes and pancreatic ducts.
  • Sickle-cell Disease: Prevalent among African Americans. Caused by abnormal hemoglobin differing by a single amino acid. Red blood cells (RBCs) become sickle-shaped, living only $2$ weeks (vs normal $4$ months). They clog capillaries, causing anemia, tissue damage, and joint pain. Heterozygotes are generally normal except during dehydration or oxygen deprivation.

• Specific Autosomal Dominant Disorders:

  • Marfan Syndrome: Defect in fibrillin, an elastic connective tissue protein. Symptoms include long limbs/fingers, caved-in chest, and weakened aorta walls.
  • Osteogenesis Imperfecta: Weak, brittle bones caused by mutations in two genes for type I collagen, which provides bone rigidity.
  • Huntington's Disease: Progressive brain cell degeneration caused by mutated huntingtin protein clumping in neurons. Symptoms (uncontrolled movements, dementia) don't appear until late $30s$ or $40s$. Survival is usually $15$ to $20$ years post-onset.

Questions & Discussion: Section 21.3

• List parent genotypes if all family members are affected: (a) recessive, (b) dominant.
  • (a) Both parents must be $aa$. (b) At least one parent must be $AA$, or both are $Aa$ (though in a small family, it's possible all children happen to be affected).
• Predict chances that homozygous normal parents for CF will have a child with CF.
  • The chance is $0\%$, as they can only pass on the normal dominant allele.
• Why are autosomal recessive disorders higher in one race/culture?
  • Often due to the "founder effect" or high frequency of specific alleles within a gene pool that historically reproduced mainly within that same population.

Beyond Simple Inheritance Patterns

• Polygenic Inheritance: Traits controlled by multiple sets of alleles (e.g., skin color, height). The effects are additive, resulting in a continuous variation of phenotypes visible as a bell-shaped curve.

  • Skin Color: Influenced by over $100$ genes and environmental factors like sunlight.

• Environmental Influence (Multifactorial Traits): Polygenic traits subject to environmental factors.

  • Examples: Cleft lip/palate, clubfoot, schizophrenia, diabetes, allergies, cancers.
  • Himalayan Rabbits: Melanin-producing enzymes are active only at low temperatures, resulting in dark fur only at extremities (ears, nose, paws).

• Pleiotropy: A single mutant gene affects multiple, seemingly unrelated traits. Marfan syndrome is a prime example, affecting the skeleton, heart, eyes, lungs, and skin.

• Incomplete Dominance: The heterozygote has a phenotype intermediate between the two homozygotes.

  • Familial Hypercholesterolemia: Genes control LDL cholesterol receptors. Two mutated alleles result in no receptors (death before age $30$); one mutated allele results in half the receptors.

• Codominance: Both alleles are equally expressed in the heterozygote. Example: Blood type $AB$.

• Multiple-Allele Inheritance: ABO Blood Types:

  • The gene exists in several allelic forms: $I^A$, $I^B$, and $i$.
  • $I^A$ and $I^B$ are dominant over $i$, but codominant with each other.
  • Type A: Genotypes $I^A I^A$ or $I^A i$.
  • Type B: Genotypes $I^B I^B$ or $I^B i$.
  • Type AB: Genotype $I^A I^B$.
  • Type O: Genotype $ii$.

Sex-Linked Inheritance

• Chromosomal Context: Humans have $23$ pairs: $22$ pairs of autosomes and $1$ pair of sex chromosomes ($XX$ for females, $XY$ for males). The Y chromosome contains the male-determining gene.

• Sex-Linked Traits: Controlled by genes on sex chromosomes.

  • X-linked: Genes on the X chromosome. Most sex-linked traits are X-linked.
  • Y-linked: Genes on the Y chromosome.

• Inheritance Rules:

  • Males always receive their X-linked allele from their mother. The Y from the father does not carry a corresponding allele.
  • X-linked disorders are usually recessive. Females need two recessive alleles to be affected, while males need only one.

• X-Linked Notation (Color Blindness):

  • $X^B X^B$: Female, normal vision.
  • $X^B X^b$: Carrier female, normal vision.
  • $X^b X^b$: Female, color blind.
  • $X^B Y$: Male, normal vision.
  • $X^b Y$: Male, color blind.

• Pedigree for X-Linked Recessive disorders:

  • More males than females are affected.
  • The condition often skips a generation, passing from grandfather to grandson via a carrier daughter.
  • If a woman is affected, ALL her sons will be affected.

• Specific X-Linked Recessive Disorders:

  • Color Blindness: Red-green form; affects $1$ in $12$ males and $1$ in $200$ females in the US.
  • Duchenne Muscular Dystrophy: Absence of dystrophin. Calcium leaks into cells, activating enzymes that dissolve muscle fibers. Death usually by age $20$ to $25$.
  • Fragile X Syndrome: Most common inherited intellectual disability and common cause of autism. Physical traits include prominent jaw and flexible joints.
  • Hemophilia: Blood does not clot. Type A (lack of factor VIII) and Type B (lack of factor IX).

Questions & Discussion: Section 21.5

• Solve: A man and woman with normal vision have a color-blind boy and girl. List parent genotypes.
  • This is actually impossible if the father has normal vision and it is X-linked recessive color blindness. A normal-vision father ($X^B Y$) always gives a normal $X^B$ to his daughters. The girl could only be color-blind if the father was $X^b Y$.
• Can a woman with an X-linked dominant disorder have an unaffected child?
  • Yes, if she is heterozygous ($X^A X^a$), she has a $50\%$ chance of passing the normal $X^a$ to her children.
• Why are X-linked disorders more common than Y-linked disorders?
  • The X chromosome is significantly larger and carries many more genes than the Y chromosome, which primarily carries genes related to male development.