Chapter 4 Modification of Mendelian Ratios

What are the two postulates of Mendelian genetics?

1. Genes are present on homologous chromosomes. 2. Chromosomes segregate from each other and assort independently during gamete formation.

What happens to Mendelian ratios when gene expression does not follow simple dominant/recessive patterns?

Classic 3:1 and 9:3:3:1 ratios are modified.

What is sex linkage in genetics?

It refers to genes located on the X and Y chromosomes.

What are the three types of mutations that can alter phenotypes?

1. Loss-of-function mutation 2. Gain-of-function mutation 3. Neutral mutation.

What is a wild-type allele?

It is the normal allele that occurs more frequently in the population.

What is a mutant allele?

An allele that contains modified genetic information and often specifies an altered gene product.

What is a loss-of-function mutation?

A mutation that results in a null allele if the loss of function is complete.

What characterizes a gain-of-function mutation?

It enhances the function of the wild-type allele, often generating a dominant allele.

What is a neutral mutation?

A mutation where the gene product presents no change to the phenotype.

How are recessive alleles symbolized in genetic notation?

The initial letter of the recessive trait is lowercase and italicized.

How are dominant alleles symbolized in genetic notation?

Uppercase letters are used to denote dominant alleles.

What is the symbol for the dwarf trait in genetics?

d (recessive allele)

What does the symbol e represent in Drosophila genetics?

The ebony mutant phenotype.

What does e+ represent in Drosophila genetics?

The normal gray (wild-type) phenotype.

What is the phenotypic ratio of F2 generation when red snapdragons are crossed with white snapdragons?

1/4 red, 1/2 pink, 1/4 white.

What is the outcome of incomplete dominance in genetics?

An intermediate phenotype results from the cross between parents with contrasting traits.

What is codominance in genetics?

A condition where both alleles in a heterozygote are expressed clearly.

What is indicated by the notation Wr+/Wr in Drosophila genetics?

It represents a heterozygote for the wrinkled wing trait.

What does the notation e/e indicate in Drosophila genetics?

It indicates an ebony homozygote (mutant).

What is the significance of using uppercase italic letters and superscripts in genetic notation?

They denote alternative alleles when no dominance exists between them.

What is the relationship between genotype and phenotype in incomplete dominance?

Each genotype has its own distinct phenotype.

Give an example of codominance in humans.

The MN blood group, characterized by the presence of antigens on the surface of red blood cells, is an example of codominance.

What are the two forms of glycoprotein alleles in the MN blood group?

The two forms are designated as M and N.

What are multiple alleles?

Multiple alleles refer to three or more alleles of the same gene that exist in a population, leading to unique modes of inheritance.

Which blood group system in humans is an example of multiple alleles?

The ABO blood group system.

What are the three alleles responsible for ABO blood phenotypes?

The three alleles are IA, IB, and i.

What is the relationship between IA and IB alleles?

IA and IB are codominant to each other and both are dominant over the i allele.

What defines essential genes?

Essential genes are absolutely required for the survival of an organism.

What is a lethal allele?

A lethal allele is a mutation in an essential gene that has the potential to cause the death of an organism.

How does a recessive lethal allele affect survival?

A recessive lethal allele can be tolerated in heterozygotes, where one wild-type allele is sufficient for survival, but homozygous recessive individuals will not survive.

What is a dominant lethal allele?

A dominant lethal allele is one where the presence of just one copy of the allele results in death.

What is an example of a dominant lethal allele in humans?

Huntington disease is caused by a dominant autosomal allele (H) that leads to progressive degeneration of the nervous system.

How do combinations of two gene pairs affect inheritance ratios?

Different modes of inheritance combined can result in modified ratios that deviate from the classic 9:3:3:1 ratio.

What is gene interaction?

Gene interaction occurs when several genes influence a particular phenotypic characteristic.

What is the phenotypic outcome of mating two heterozygous MN individuals?

The offspring may exhibit all three blood types: M, N, and MN.

What happens when two yellow mice (homozygous recessive lethal) are crossed?

The offspring ratio will be 2/3 yellow and 1/3 agouti.

What is the result of crossing an agouti mouse with a yellow mouse?

The offspring will be 50% yellow and 50% agouti.

What is the significance of the IA and IB alleles in the ABO blood group?

IA and IB alleles produce their respective antigens on red blood cells, while the i allele does not produce any antigens.

What is the phenotypic effect of a homozygous recessive lethal allele in heterozygotes?

It may result in a distinctive mutant phenotype, such as a yellow coat color in mice.

How can phenotypes be influenced in genetics?

Phenotypes can be influenced by multiple genes and their interactions.

What is the expected dihybrid ratio when two traits are inherited independently?

The expected dihybrid ratio is 9:3:3:1, unless modified by other inheritance patterns.

What is the role of antigens in blood types?

Antigens, such as A and B, are glycoproteins on the surface of red blood cells that determine blood type.

What is epistasis in genetics?

Epistasis is when the expression of one gene masks or modifies the effect of another gene pair.

What is the Bombay phenotype?

The Bombay phenotype is a homozygous recessive condition where the first locus masks the expression of the second locus, specifically the mutant F U T1 gene masking the A and B alleles.

What is pleiotropy?

Pleiotropy is when the expression of a single gene has multiple phenotypic effects.

Give an example of a condition that demonstrates pleiotropy.

Marfan syndrome, which results from an autosomal dominant mutation in the gene encoding the connective tissue protein fibrillin, leading to lens dislocation, increased risk of aorta aneurysm, and lengthened long bones.

What does X-linkage refer to?

X-linkage describes genes located on the X chromosome, which exhibit unique patterns of inheritance.

What is hemizygous in the context of X-linked genes?

Hemizygous refers to males who cannot be homozygous or heterozygous for X-linked genes, meaning alleles present on their X chromosome will be expressed.

Who demonstrated X-linked inheritance through studies of Drosophila?

Thomas Hunt Morgan demonstrated X-linked inheritance in studies of white-eyed mutations in Drosophila.

What is the crisscross pattern of inheritance?

The crisscross pattern of inheritance occurs due to X-linkage, where phenotypic traits controlled by recessive X-linked genes are passed from homozygous mothers to all sons.

What are X-linked traits?

X-linked traits are traits controlled by genes on the X chromosome, such as color blindness.

How does color blindness inheritance work in humans?

Color blindness is passed from a mother to all sons but not to daughters.

What are lethal X-linked recessive disorders?

Lethal X-linked recessive disorders are observed only in males, while females can only be heterozygous carriers who do not develop the disorders.

What is Duchenne muscular dystrophy?

Duchenne muscular dystrophy is a lethal X-linked recessive disorder that typically has an onset prior to age 6 and is lethal around age 20, occurring only in males.

What is sex-limited inheritance?

Sex-limited inheritance is when the expression of a specific phenotype is absolutely limited to one sex, caused by an autosomal gene.

Provide an example of sex-limited inheritance.

Genes for milk production are only expressed in females.

How does an individual's sex influence phenotype in sex-influenced inheritance?

In sex-influenced inheritance, patterns of gene expression are affected by the individual's sex, even when the genes are not on the X chromosome.

What is the role of hormones in sex-influenced inheritance?

The expression of mutations in autosomal genes is dependent on the hormone constitution of the individual.

What is the significance of the first locus in epistasis?

The first locus in epistasis masks the expression of the second locus, affecting the overall phenotype.

What is the relationship between X-linked genes and male expression?

Males express alleles present on their X chromosome because they are hemizygous for X-linked genes.

What type of inheritance pattern was observed in reciprocal crosses of Drosophila?

Reciprocal crosses between white-eyed and red-eyed flies did not yield identical results, indicating a relationship between inheritance pattern and the sex of the parent carrying the mutant allele.

What is the effect of a homozygous recessive condition on phenotype?

A homozygous recessive condition can mask the expression of other alleles, as seen in the Bombay phenotype.

How does the Y chromosome compare to the X chromosome in terms of gene presence?

The Y chromosome lacks most genes that are present on the X chromosome.

What is the inheritance pattern of hen-feathering in chickens?

Controlled by a dominant allele expressed in both sexes.

How is cock-feathering inherited in chickens?

Controlled by a recessive allele expressed only in males.

What influences the expression of phenotype in sex-influenced inheritance?

The sex of the individual.

What is an example of sex-influenced inheritance in humans?

Pattern baldness, where allele B is dominant in males and recessive in females.

What is penetrance in genetics?

The percentage of individuals showing at least some degree of expression of a mutant genotype in a population.

Define expressivity in genetics.

The range of expression of a mutant phenotype, influenced by genetic background and environmental effects.

What is an example of expressivity in Drosophila?

The eyeless mutation, where the phenotype ranges from normal eyes to absence of one or both eyes.

How can the physical location of a gene influence its expression?

Translocation or inversion events can modify expression by relocating the gene to condensed or genetically inert chromosome regions.

What temperature effects are observed in evening primrose flowers?

Red flowers at 23°C and white flowers at 18°C.

How does temperature affect the fur color of Siamese cats and Himalayan rabbits?

They have darker fur on cooler areas of the body due to enzymes functioning only at lower temperatures.

When do genetic traits become apparent during an organism's life span?

At different times, such as prenatal, infant, pre-adult, and adult phases.

What is an example of a severe inherited disorder that manifests after birth?

Tay-Sachs disease, which leads to mental retardation, paralysis, blindness, and death by age 3.

What is extranuclear inheritance?

Inheritance involving genes not found in the nucleus but in cytoplasmic organelles.

What is organelle heredity?

The phenotype of an organism can be affected by genes contained in the DNA of mitochondria and chloroplasts.

What is maternal inheritance?

The phenotype is determined by genetic information expressed in the maternal gamete.

In a cross between a blue-flowered plant (BB) and a white-flowered plant (bb), what is the expected offspring ratio?

All offspring will be light blue (Bb).

If a mother has type A blood and her son has type O blood, what are the possible blood types of the father?

The father must have type O or type A blood.

What is the expected offspring ratio in a cross of a blue-flowered plant (B) and a white-flowered plant (b) if the alleles are incompletely dominant?

All offspring will be light blue (Bb).

What factors can influence phenotypic expression?

Both genotype and environmental factors.

What is the significance of genetic background in expressivity?

It can lead to variations in the expression of a mutant phenotype.

How does temperature affect the expression of traits in certain organisms?

Temperature can influence the color of flowers and fur, as seen in evening primrose and Siamese cats.

What is the genotype of the cross that yields all light blue offspring?

The cross is B B × b b, resulting in all B b (light blue) offspring.

If a mother has type A blood and her son has type O blood, what are the possible blood types of her son's father?

Type A, B, or O.

Why must the father of a son with type O blood carry the O allele?

The son must inherit an O allele from each parent, so the father must carry the O allele.

What does a 2:1 ratio in offspring suggest about the presence of a lethal allele?

It indicates a 'missing' phenotypic class, suggesting a homozygous recessive lethal allele.

What is the best explanation for a cross between two short-tailed mice resulting in a 2:1 ratio of short-tailed to long-tailed offspring?

There is a recessive lethal allele.

What does the allele 'T' represent in the context of short-tailed mice?

The allele 'T' is dominant for short tail but recessive for lethality.

How is Duchenne muscular dystrophy inherited?

It is caused by a recessive X-linked allele.

From whom must a man with Duchenne muscular dystrophy have inherited the disorder?

He must have inherited it from his mother.

Why can a man with Duchenne muscular dystrophy only pass the disorder to his daughters?

He carries only one copy of the gene on his X chromosome, inherited from his mother.

In the Bombay phenotype, what does the A B O blood type depend on?

It is dependent on another gene.

What is the role of the F U T1 gene mutation in the Bombay phenotype?

The F U T1 gene mutation is epistatic to the A B O alleles, masking their expression.

What inheritance pattern is demonstrated by the presence of horns in heterozygous male sheep but not in heterozygous females?

This is an example of sex-influenced inheritance.

How does sex-influenced inheritance differ from sex-limited traits?

Sex-influenced traits can occur in either sex but act dominant in one sex and recessive in the other, while sex-limited traits are expressed in only one sex regardless of genotype.

What is the significance of the proband in a genetic study?

The proband is the starting point for a genetic study.

What does the term 'epistatic' refer to in genetics?

Epistatic refers to one gene masking the expression of another gene.

What are the possible blood types of a father if the mother has type A blood and the son has type O blood?

The father could be type A, B, or O.