Genetics: Mendel's Principles and Extensions to Mendelian Inheritance

A comprehensive set of 150 practice flashcards covering Mendelian genetics, laws of segregation and independent assortment, single-gene extensions, multi-gene interactions, epistasis, and continuous variation.

What is genetics defined as in the lecture notes?

The science of heredity.

What is a gene?

A region of DNA that encodes a protein or RNA, serving as the basic unit of heredity.

How is heredity defined in relation to genes?

Heredity is the way that genes transmit traits from parents to offspring.

Give an example of a human trait caused by a single change in a single gene.

Sickle-cell anemia.

Give an example of a human trait caused by complex interactions between many genes.

Facial features.

According to Mendel's general principles, why is observable variation essential?

Observable variation is essential for following genes from one generation to another.

Which plant species did Mendel use in his inheritance experiments?

Pisum sativum (pea plant).

What term describes the pairs of contrasting traits that Mendel studied in pea plants?

Antagonistic pairs.

What are the seven antagonistic traits studied by Mendel in pea plants?

Seed color, seed shape, flower color, pod color (unripe), pod shape (ripe), stem length, and flower position.

In Mendel's pea plant experiments, what is the dominant phenotype for unripe pod color?

Green.

What is the dominant phenotype for pea seed shape?

Round.

What is the dominant phenotype for pea flower color?

Purple.

What is the dominant phenotype for stem length in Mendel's pea plants?

Long.

What is a monohybrid cross?

A cross observing a single trait.

How did Mendel's monohybrid crosses disprove the blending hypothesis?

Traits did not become mixed or changed in offspring; the recessive trait reappeared unchanged in the $F_2$ generation.

What is a reciprocal cross?

A cross in which the traits of the male and female parents are swapped.

How is a dominant trait defined in the context of pure-breeding parents?

The dominant trait is the form of the trait that appears in the $F_1$ progeny.

How is a recessive trait defined in the context of pure-breeding parents?

The recessive trait is the form of the trait that is hidden in the $F_1$ progeny.

What are alternative forms of a single gene called?

Alleles.

What is a monohybrid individual?

An individual with two different alleles for a single trait.

What does Mendel's Law of Segregation state regarding gamete formation?

The two alleles for each trait separate during gamete formation so that each gamete carries only one allele.

What phenotypic ratio is observed in the $F_2$ progeny of a Mendelian monohybrid cross?

$3:1$ (dominant to recessive).

What genotypic proportions are present in the $F_2$ generation of a monohybrid cross?

$\frac{1}{4}$ true-breeding dominant, $\frac{1}{2}$ hybrids (heterozygous), and $\frac{1}{4}$ true-breeding recessive.

What is the product rule of probability?

The probability of two independent events occurring together is the product of their individual probabilities: $P(1 \text{ and } 2) = P(1) \times P(2)$.

What is the sum rule of probability?

The probability of either of two mutually exclusive events occurring is the sum of their individual probabilities: $P(1 \text{ or } 2) = P(1) + P(2)$.

Using the product rule, what is the probability of obtaining a $YY$ homozygous dominant offspring from a $Yy \times Yy$ cross?

$\frac{1}{2} \times \frac{1}{2} = \frac{1}{4}$.

Using probability rules, what is the total probability of obtaining a $Yy$ heterozygous offspring from a $Yy \times Yy$ cross?

$\frac{1}{4} + \frac{1}{4} = \frac{1}{2}$.

What is the definition of a phenotype?

An observable characteristic of an organism.

What is the definition of a genotype?

The pair of alleles present in an individual for a specific gene.

What is the difference between a homozygote and a heterozygote?

A homozygote has two identical alleles for a gene, whereas a heterozygote has two different alleles.

How is a dominant allele defined through a heterozygous individual?

The allele that defines the phenotype of the heterozygote is the dominant allele.

What does the notation $Y-$ represent in genetics?

An individual displaying the dominant phenotype with an unknown second allele (either $YY$ or $Yy$).

What is a testcross and why is it used?

A cross between an individual with a dominant phenotype and a homozygous recessive individual, used to determine the unknown genotype of the dominant individual.

If an individual with genotype $Y-$ is testcrossed with $yy$ and all progeny are yellow, what was the unknown genotype?

Homozygous dominant ($YY$).

If an individual with genotype $Y-$ is testcrossed with $yy$ and offspring show a $1:1$ ratio of yellow to green, what was the unknown genotype?

Heterozygous ($Yy$).

What is Mendel's Law of Independent Assortment?

During gamete formation, different pairs of alleles segregate independently of each other.

What is the expected phenotypic ratio in the $F_2$ generation of a Mendelian dihybrid cross ($Yy\,Rr \times Yy\,Rr$)?

$9:3:3:1$.

What proportion of $F_2$ progeny in a Mendelian dihybrid cross exhibit recombinant phenotypes?

$\frac{6}{16}$ (or $\frac{3}{8}$), made up of $\frac{3}{16}$ yellow wrinkled and $\frac{3}{16}$ green round.

In a dihybrid cross of $Yy\,Rr \times Yy\,Rr$, what proportion of offspring will be yellow and wrinkled ($Y-rr$)?

$\frac{3}{4} \times \frac{1}{4} = \frac{3}{16}$.

What formula determines the number of different gametes produced by an individual heterozygous for $n$ traits?

$2^n$.

How many different gamete types can an individual with the genotype $Aa\,Bb\,Cc\,Dd$ produce?

$2^4 = 16$.

What is the expected proportion of $AA\,bb\,Cc\,Dd$ progeny from a cross of $Aa\,Bb\,Cc\,Dd \times Aa\,Bb\,Cc\,Dd$?

$\frac{1}{4} \times \frac{1}{4} \times \frac{1}{2} \times \frac{1}{2} = \frac{1}{64}$.

What enzyme is encoded by Mendel's pea shape gene ($R$ allele)?

Starch branching enzyme 1 (Sbe1), which converts amylose to amylopectin.

What gene product does the dominant $Y$ allele encode in pea plants to produce yellow seeds?

Stay-green (Sgr) protein, an enzyme that breaks down chlorophyll.

What is the incidence of Cystic fibrosis among Caucasians, and which chromosome is affected?

$1/2000$ Caucasians; chromosome 7.

What is the incidence and cause of Tay-Sachs disease?

$1/3000$ Eastern European Jews; missing enzyme causing buildup of fatty deposits in the brain (chromosome 15).

What pattern of inheritance does Huntington disease show in human pedigrees?

Vertical inheritance pattern (it is a single-gene dominant trait on chromosome 4).

In human pedigrees, what does a double line between a male and female symbol indicate?

Consanguineous mating.

What pattern of inheritance do rare recessive traits typically display in family pedigrees?

A horizontal pattern of inheritance (affected individuals appear in single generations among siblings of unaffected carrier parents).

Under what condition can two affected parents produce unaffected children?

When the trait is dominant and both parents are heterozygous.

What is complete dominance?

A dominance relationship where the hybrid ($F_1$) phenotype is identical to one of the two homozygous parents.

What is incomplete dominance?

A dominance relationship where the hybrid ($F_1$) phenotype is intermediate between the phenotypes of the two homozygous parents.

What is codominance?

A dominance relationship where the hybrid ($F_1$) expresses traits from both homozygous parents simultaneously.

When pure-breeding red snapdragons ($A^1A^1$) are crossed with pure-breeding white snapdragons ($A^2A^2$), what is the phenotype of the $F_1$ generation?

All pink ($A^1A^2$).

What phenotypic ratio results from an $F_1 \times F_1$ cross of snapdragons exhibiting incomplete dominance ($A^1A^2 \times A^1A^2$)?

$1$ red : $2$ pink : $1$ white ($1:2:1$).

How does the biochemical mechanism explain incomplete dominance in snapdragon flower color?

One normal allele ($A^1$) produces half the amount of pigment-producing enzyme compared to two normal alleles, resulting in a lighter (pink) phenotype.

In lentils, what is the phenotype of $C^S C^D$ hybrids resulting from a cross between pure-breeding spotted ($C^S C^S$) and dotted ($C^D C^D$) parents?

Spotted and dotted.

What is the $F_2$ phenotypic ratio when codominant $C^S C^D$ individuals are selfed?

$1$ spotted : $2$ spotted and dotted : $1$ dotted ($1:2:1$).

Do variations in dominance relations alter the physical transmission of alleles during meiosis?

No, dominance relations reflect protein function, not allele transmission; Mendel's Law of Segregation still applies.

How many alleles can a single diploid individual carry for a specific gene?

A maximum of two alleles.

What alleles determine the ABO blood group system in humans?

Three alleles: $I^A$, $I^B$, and $i$.

What sugars are expressed on the surface of red blood cells in an individual with genotype $I^A I^B$?

Both A-type sugar and B-type sugar (codominance).

Why is the $i$ allele recessive to both $I^A$ and $I^B$ in the ABO blood group system?

The $i$ allele produces no functional sugar-attaching enzyme (no sugar attached).

How many genotypes and phenotypes exist for the human ABO blood group system?

Six genotypes ($I^A I^A$, $I^A i$, $I^B I^B$, $I^B i$, $I^A I^B$, $ii$) and four phenotypes (A, B, AB, O).

What antibodies are present in the blood serum of an individual with Type O blood?

Antibodies against both A and B antigens.

What antibodies are present in the serum of an individual with Type AB blood?

No antibodies against A or B antigens.

Which blood type is considered a universal recipient for red blood cell transfusions?

Type AB.

Which blood type is considered a universal donor for red blood cell transfusions?

Type O.

What is a dominance series?

A hierarchical ranking of multiple alleles of a gene based on their dominance relative to one another.

What is the dominance series for seed coat pattern in lentils?

marbled-1 > marbled-2 > spotted = dotted > clear.

What genes code for human histocompatibility antigens (HLA)?

HLA-A, HLA-B, and HLA-C.

What cells lack human histocompatibility antigens (HLA)?

Red blood cells (RBC) and sperm cells.

How many alleles exist for each HLA gene in the human population, and how are they expressed?

Each gene has 400–1200 alleles; all alleles are codominant.

What is the typical mutation frequency per gene per gamete per generation?

$10^{-4}$ to $10^{-6}$.

What is an allele frequency?

The percentage of the total number of gene copies for a specific allele in a population.

What is the wild-type ($+$) allele?

The most common allele in a natural population.

What is a monomorphic gene?

A gene that has only one common wild-type allele in a population.

What is a polymorphic gene?

A gene that has more than one common allele in a population.

What are common variants?

High-frequency alleles of a polymorphic gene.

Is the agouti gene ($A$) in mice monomorphic in wild populations?

Yes, only the $A$ allele exists in the wild, though 14 different alleles exist in laboratory strains.

Define pleiotropy.

The phenomenon in which a single gene determines several distinct and seemingly unrelated phenotypic traits.

How does a mutation in a single gene cause both respiratory problems and sterility in some Maori men?

The mutated gene encodes a protein required for the structure and function of both cilia (respiratory) and flagella (sperm).

What effect does the $A^Y$ allele of the mouse agouti gene have on coat color and viability?

$A^Y$ is dominant to $A$ for yellow coat color, but recessive to $A$ for lethality (homozygous $A^Y A^Y$ mice die in utero).

What altered phenotypic ratio in offspring indicates the presence of a recessive lethal allele in a monohybrid cross?

A $2:1$ phenotypic ratio among surviving progeny.

Why are pure-breeding yellow ($A^Y A^Y$) mice impossible to produce?

Two copies of the $A^Y$ allele cause embryonic lethality.

How many mutant alleles of the $\beta$-globin gene have been identified?

Over 400 mutant alleles.

At the protein level (polypeptide production), what dominance relationship exists between the $\text{Hb}^A$ and $\text{Hb}^S$ alleles?

Codominance (both A and S proteins are produced in heterozygous carriers).

At high altitude, what dominance relationship is observed between $\text{Hb}^A$ and $\text{Hb}^S$ regarding red blood cell shape and concentration?

Incomplete dominance (carriers show mild sickling and lower RBC counts).

Regarding susceptibility to malaria, what dominance relationship exists between $\text{Hb}^S$ and $\text{Hb}^A$?

$\text{Hb}^S$ is dominant to $\text{Hb}^A$ (heterozygotes are resistant to malaria).

How does incomplete dominance alter the standard $3:1$ phenotypic ratio in an $F_1 \times F_1$ monohybrid cross?

It changes the ratio to $1:2:1$, where phenotypic ratios coincide with genotypic ratios.

How do recessive lethal alleles alter the standard $3:1$ phenotypic ratio in an $F_1 \times F_1$ cross?

They change the ratio to $2:1$.

What is delayed lethality?

A situation where a lethal allele causes death later in life rather than during embryonic development (e.g., Huntington disease).

What phenotypic ratio is expected when crossing two spotted/dotted $C^S C^D$ lentils?

$1$ spotted ($C^S C^S$) : $2$ spotted/dotted ($C^S C^D$) : $1$ dotted ($C^D C^D$).

What is the primary cause of cystic fibrosis at the molecular level?

Abnormal CFTR protein leading to defective chloride ion channel transport across cell membranes.

What is the phenotypic incidence of sickle-cell anemia among African-Americans?

$1/625$ African-Americans.

What is the chromosome location for the gene responsible for Phenylketonuria (PKU)?

Chromosome 12.

What is the incidence of Hypercholesterolemia in French Canadians?

$1/122$ French Canadians.

How does a dominant allele typically alter phenotype at the molecular level?

It usually encodes a normally functioning protein.

What does a recessive allele typically represent at the protein level?

A protein that is nonfunctional or produced in reduced amounts.

In a human pedigree, what symbol represents a deceased individual?

A square or circle with a diagonal slash through it.