Chapter 17 – Genetics: Patterns of Inheritance

A comprehensive set of question-and-answer flashcards covering Mendel’s laws, Punnett squares, chromosome theory, inheritance patterns (simple, incomplete, codominant, X-linked), dihybrid ratios, quantitative traits, environmental effects, and sex determination mechanisms.

Who is considered the father of modern genetics and what organism did he study?

Gregor Mendel; he performed historic inheritance experiments with garden pea plants (Pisum sativum).

List two reasons why pea plants were an advantageous model for Mendel’s studies.

(1) They possessed many clearly-variable traits and (2) they normally self-fertilize, allowing easy creation of true-breeding lines; additionally, their large flowers permit controlled cross-fertilization.

Name Mendel’s seven pea-plant traits.

Flower color, flower position, seed color, seed shape, pod color, pod shape, and plant height.

What is the male reproductive structure of a pea flower called and what does it produce?

The stamen; it produces pollen.

What is the female reproductive structure of a pea flower that receives pollen?

The stigma.

Describe the basic procedure Mendel used to make a controlled cross-fertilization.

He removed the stamens from an immature flower to prevent self-pollination, then brushed pollen from a second plant onto the exposed stigma of the first plant.

In Mendelian terminology, how many copies of each gene does a diploid individual possess?

Two copies—one on each homologous chromosome.

What term is used for alternative versions of the same gene?

Alleles.

Differentiate between a dominant and a recessive allele.

A dominant allele masks the effect of a recessive allele and is expressed in heterozygotes; a recessive allele is expressed only when both alleles are recessive (homozygous recessive).

Define genotype.

The genetic composition of an individual (e.g., TT, Tt, or tt).

Define phenotype.

The observable physical, biochemical, or behavioral traits produced by gene expression (e.g., tall or dwarf plants).

Provide the three possible genotypes for a single gene with dominant allele T and recessive allele t.

TT (homozygous dominant), Tt (heterozygous), tt (homozygous recessive).

What is a single-factor (monohybrid) cross?

An experimental cross that follows the inheritance of only one trait at a time.

State Mendel’s Law of Segregation.

During gamete formation, the two alleles for each gene separate so that each gamete carries only one allele.

Which meiotic event provides the physical basis for Mendel’s Law of Segregation?

Separation of homologous chromosomes during meiosis I.

What genotypic ratio is expected in the F₂ generation of a monohybrid cross between two heterozygotes (Tt × Tt)?

1 TT : 2 Tt : 1 tt.

What phenotypic ratio did Mendel observe in the F₂ generation for single-trait crosses?

Approximately 3 dominant phenotype : 1 recessive phenotype.

What is a Punnett square used for?

To predict the genotypic and phenotypic ratios of offspring from a genetic cross by systematically combining parental gametes.

Explain the purpose of a testcross.

To determine whether an individual with a dominant phenotype is homozygous dominant or heterozygous by crossing it with a homozygous recessive individual.

What outcome from a testcross indicates the unknown parent is heterozygous?

If any offspring display the recessive phenotype.

Define a dihybrid cross.

A genetic cross that follows the inheritance of two different traits simultaneously (individuals are heterozygous for both genes).

State Mendel’s Law of Independent Assortment.

Alleles of different genes assort independently of one another during gamete formation.

Which meiotic mechanism explains independent assortment?

Random alignment of homologous chromosome pairs on the metaphase plate during meiosis I.

What phenotypic ratio is expected in the F₂ generation of a dihybrid cross showing independent assortment (YyRr × YyRr)?

9:3:3:1 (nine with both dominant traits, three with first dominant/second recessive, three with first recessive/second dominant, one with both recessive traits).

What does the term locus refer to?

The specific physical location of a gene on a chromosome.

Summarize the Chromosome Theory of Inheritance in one sentence.

Genes are located on chromosomes, which segregate and assort independently during meiosis, thereby explaining Mendel’s laws.

Why does a recessive allele often fail to affect phenotype in a heterozygote?

One functional copy of the dominant allele usually produces enough functional protein to yield the dominant phenotype.

Describe incomplete dominance.

A pattern where the heterozygote exhibits an intermediate phenotype between the two homozygotes, e.g., red × white flowers producing pink offspring.

Give an example of incomplete dominance discussed in lecture.

Pink four-o’clock flowers resulting from a cross between red-flowered (CR) and white-flowered (CW) plants.

Define codominance and provide an example.

Both alleles in a heterozygote are fully expressed simultaneously; example: human ABO blood type AB expresses both A and B antigens.

Differentiate discrete traits from quantitative traits.

Discrete traits have clearly defined categories (e.g., purple vs. white flowers), whereas quantitative traits show continuous variation influenced by many genes and the environment (e.g., height, skin color).

What is polygenic inheritance?

A single phenotypic trait determined by the additive effects of multiple genes.

How can environment influence phenotype?

Environmental conditions such as temperature, nutrition, or light can alter the expression of genetic potential, producing different phenotypes from the same genotype (e.g., plants of identical genotype growing to different heights at different temperatures).

List four chromosomal sex-determination systems mentioned in the lecture.

X-Y (mammals), X-O (certain insects), Z-W (birds, some fish), haplodiploid (bees).

Why are X-linked recessive disorders more common in human males?

Males are hemizygous for X-linked genes, so a single recessive allele on the X chromosome will be expressed because no second X allele is present to mask it.

Define hemizygous.

Having only one copy of a particular gene in an otherwise diploid cell; typically applies to X-linked genes in males.

What genetic defect causes Hemophilia A and how is it inherited?

A recessive mutation in an X-linked gene encoding a blood-clotting protein; it follows an X-linked recessive inheritance pattern.

Explain epistasis in one sentence.

A genetic interaction in which alleles of one gene mask or modify the phenotypic expression of another gene.

What genotype ratio results from blending two heterozygous parents in a monohybrid cross?

1 homozygous dominant : 2 heterozygous : 1 homozygous recessive.

How many different gamete types can a YyRr individual produce and what are they?

Four: YR, Yr, yR, yr.

If a plant’s tall phenotype could be TT or Tt, which cross would reveal its genotype?

Cross it with a homozygous recessive dwarf plant (tt) in a testcross.

What percentage of functional protein P is produced in a Pp genotype, and what is the resulting flower color?

50 % of normal protein P is produced, yet the phenotype is still purple because that amount suffices.

Why did Mendel not detect gene linkage in his pea experiments?

The genes he studied happened to be on different chromosomes or far apart on the same chromosome, so they assorted independently.

What is the molecular explanation for incomplete dominance producing a pink phenotype in CR CW flowers?

Half the normal amount of pigment-producing enzyme is synthesized, yielding an intermediate (pink) color rather than full red.

What two components are necessary for phenotype formation according to the ‘plan and environment’ concept?

Genotype provides the blueprint, while the environment furnishes resources (nutrients, energy) to execute that blueprint.