Genetics: Incomplete Dominance and Overdominance
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Questions and Answers

What is the phenotype exhibited by a heterozygote in incomplete dominance?

An intermediate phenotype between the two homozygotes.

Provide an example of incomplete dominance and explain the resulting phenotype.

The four o’clock plant exhibits pink flowers (CRCW) due to the mixing of the red (CR) and white (CW) traits.

In the F2 generation of incomplete dominance, what is the expected phenotypic ratio?

The expected phenotypic ratio is 1:2:1.

How does the trait of pea shape illustrate incomplete dominance?

<p>Microscopic examination shows that the round pea trait (R) exhibits incomplete dominance with respect to starch biosynthesis.</p> Signup and view all the answers

What phenotype does a heterozygous individual with one curly and one straight hair allele display?

<p>The individual displays wavy hair.</p> Signup and view all the answers

What is the genotypic ratio of offspring produced from two parents with homozygous genotypes in the case of hair types?

<p>The genotypic ratio is 1:2:1.</p> Signup and view all the answers

What defines a phenotype in the context of incomplete dominance?

<p>A phenotype is defined by the observable traits resulting from the mixing of alleles in the heterozygote.</p> Signup and view all the answers

How can the determination of dominance vary based on trait examination?

<p>Dominance may change when traits are closely examined, revealing complexities such as incomplete dominance.</p> Signup and view all the answers

What is overdominance and how does it relate to heterozygote advantage?

<p>Overdominance occurs when a heterozygote has greater reproductive success than either homozygote, illustrating the concept of heterozygote advantage.</p> Signup and view all the answers

Explain the implications of sickle-cell anemia as an example of overdominance.

<p>Individuals with the HbS allele suffer from sickle-cell anemia, which shortens their lifespan, but those with the HbAS genotype may have protection against malaria, highlighting the advantages of heterozygosity.</p> Signup and view all the answers

What are the two major consequences of sickle-shaped red blood cells in affected individuals?

<p>The two major consequences are a shortened lifespan of red blood cells, resulting in anemia, and clumping of sickle cells, which blocks capillary circulation.</p> Signup and view all the answers

How does co-dominance affect the traits expressed in offspring from a black and white chicken breeding?

<p>Co-dominance results in the offspring displaying both white and black feathers independently.</p> Signup and view all the answers

What distinguishes sex-influenced traits from sex-limited traits?

<p>Sex-influenced traits are dominant in one sex and recessive in the other, while sex-limited traits affect only one sex regardless of genotype.</p> Signup and view all the answers

Define co-dominance and how it differs from typical dominance relationships.

<p>Co-dominance is when both alleles in a heterozygote are expressed equally, without one being dominant over the other, unlike typical dominance where one allele masks the effect of the other.</p> Signup and view all the answers

Provide an example of a sex-influenced trait in cattle and explain its inheritance pattern.

<p>Scurs in cattle are a sex-influenced trait where the allele is dominant in males but recessive in females.</p> Signup and view all the answers

How does the presence of both HbA and HbS alleles illustrate the concept of overdominance?

<p>The presence of both HbA and HbS alleles in some individuals results in a survival advantage against malaria, demonstrating overdominance as these heterozygotes show greater fitness than either homozygote.</p> Signup and view all the answers

What is the significance of the blood type O in relation to dominant and recessive alleles?

<p>Blood type O does not express when A and B genes are present, indicating that A and B are dominant over O.</p> Signup and view all the answers

Explain how the concept of co-dominance can be applied to livestock with black and white coloring.

<p>When black and white animals breed, co-dominance results in offspring with both black and white spots.</p> Signup and view all the answers

What is meant by the term 'complementation' in genetics?

<p>Complementation is a phenomenon where two different parents with similar recessive phenotypes produce offspring with a wild-type phenotype due to contributions from different genes.</p> Signup and view all the answers

How does gene modification alter phenotypic outcomes?

<p>Gene modification occurs when an allele of one gene influences the phenotypic expression of alleles from another gene.</p> Signup and view all the answers

Describe gene redundancy and its effect on phenotype.

<p>Gene redundancy refers to the situation where the loss of function in one gene does not affect the phenotype, but the loss of function in two genes does result in an observable effect.</p> Signup and view all the answers

In the context of the sweet pea flower example, what does it mean when two genes complement each other?

<p>When two genes complement each other, it means that the mutations are in different genes, allowing for the contribution of a wild-type allele from each parent.</p> Signup and view all the answers

Explain the concept of epistasis in genetics.

<p>Epistasis is a genetic interaction where the phenotypic effects of one gene are masked by another gene, leading to alterations in observed traits.</p> Signup and view all the answers

What is the significance of the alleles 'cc' and 'pp' in the purple pigment production of sweet pea flowers?

<p>'cc' produces no functional enzyme C, and 'pp' produces no functional enzyme P, which both stop the production of purple pigment.</p> Signup and view all the answers

How do epistatic interactions often arise in cellular functions?

<p>Epistatic interactions typically arise when two different proteins are involved in a common cellular function, such as in an enzymatic pathway.</p> Signup and view all the answers

What distinguishes dominance from epistasis in genetic terms?

<p>Dominance refers to the interaction between alleles of the same gene, whereas epistasis involves the interactions between alleles of two different genes.</p> Signup and view all the answers

What is the inheritance pattern of lethal alleles, and how do they typically affect offspring ratios?

<p>Lethal alleles are usually inherited in a recessive manner and may lead to deviations from expected Mendelian ratios due to the absence of certain genotypes in the offspring.</p> Signup and view all the answers

Define pleiotropy and provide an example of a gene that exhibits this phenomenon.

<p>Pleiotropy is when a single gene influences multiple phenotypic traits. An example is the cystic fibrosis transmembrane conductance regulator (CFTR) gene, which affects lung function, digestion, and sweat characteristics.</p> Signup and view all the answers

What distinguishes a conditional lethal allele from a semilethal allele?

<p>Conditional lethal alleles cause death only under specific environmental conditions, while semilethal alleles kill only a portion of the population.</p> Signup and view all the answers

Give an example of a sex-limited trait and explain its significance.

<p>An example of a sex-limited trait is the ornate plumage of roosters, which is significant as it demonstrates sexual dimorphism in birds.</p> Signup and view all the answers

What is the role of essential genes in the context of lethal alleles?

<p>Essential genes are required for survival, and mutations in these genes can lead to the formation of lethal alleles, resulting in death.</p> Signup and view all the answers

Describe how gene interactions can influence phenotypes.

<p>Gene interactions occur when multiple genes affect a single trait, leading to variations in characteristics such as height and pigmentation influenced by both genetic and environmental factors.</p> Signup and view all the answers

In what way can embryonic development impact coat color patterns in mammals?

<p>Embryonic development influences coat color patterns through the expression of genes like MITF, which affects the migration and survival of melanoblasts.</p> Signup and view all the answers

Explain the term 'epistasis' and its effect on inheritance.

<p>Epistasis is when the alleles of one gene mask or alter the phenotypic effects of alleles from another gene, influencing phenotypic ratios in genetic crosses.</p> Signup and view all the answers

What are the potential effects of a lethal allele on cell division?

<p>Lethal alleles can prevent cell division, potentially causing death at an early developmental stage or later in life.</p> Signup and view all the answers

How do mutations in nonessential genes differ from those in essential genes?

<p>Mutations in nonessential genes may not lead to death but can still benefit the organism, while mutations in essential genes usually result in lethal alleles that affect survival.</p> Signup and view all the answers

What is responsible for sexual dimorphism in humans, and how is it observed?

<p>Sexual dimorphism in humans is primarily due to the presence of reproductive organs such as ovaries in females and testes in males.</p> Signup and view all the answers

How does a dominant mutation such as that found in Manx cats affect the survival of homozygous individuals?

<p>The dominant mutation in Manx cats shortens the tail but is lethal when homozygous, thus those individuals do not survive.</p> Signup and view all the answers

How do temperature-sensitive lethal alleles function?

<p>Temperature-sensitive lethal alleles result in the death of an organism under specific temperature conditions, often due to misfolded proteins at higher temperatures.</p> Signup and view all the answers

What is overdominance and how may it benefit heterozygotes?

<p>Overdominance occurs when heterozygotes have greater reproductive success than either homozygote. Benefits may include increased resistance to infections, enhanced protein function, or broader protein functionality.</p> Signup and view all the answers

Define codominance and provide an example.

<p>Codominance is when heterozygotes express both alleles simultaneously without forming an intermediate phenotype. An example is the AB blood type, where both A and B alleles are expressed.</p> Signup and view all the answers

Explain sex-influenced inheritance.

<p>Sex-influenced inheritance refers to alleles that may be recessive in one sex and dominant in the opposite sex. This pattern is influenced by sex hormones affecting gene expression.</p> Signup and view all the answers

What characterizes sex-limited inheritance?

<p>Sex-limited inheritance occurs when a trait is expressed in only one sex. An example is the development of mammalian breasts, which typically occurs in females.</p> Signup and view all the answers

What are lethal alleles and what usually causes them?

<p>Lethal alleles are those that can cause the death of an organism, often resulting from loss-of-function mutations in essential proteins. These mutations can lead to detrimental phenotypes.</p> Signup and view all the answers

How can sex hormones influence sex-influenced inheritance?

<p>Sex hormones can regulate the molecular expression of genes, affecting whether an allele is dominant or recessive in a given sex. This can lead to different phenotypic outcomes.</p> Signup and view all the answers

What role does protein function play in overdominance?

<p>In overdominance, heterozygotes may produce proteins that enhance function or provide wider functionality, contributing to their reproductive success. This can include resistance to diseases.</p> Signup and view all the answers

Why might codominance be advantageous for individuals?

<p>Codominance allows for the expression of both alleles, resulting in traits that may provide a fitness advantage or better adaptability in varying environments. It leads to greater phenotypic diversity.</p> Signup and view all the answers

Flashcards

Incomplete Dominance

Heterozygote's phenotype is between the two homozygotes' phenotypes.

Four o'clock flowers

Example of incomplete dominance, where red and white flowers create pink flowers.

Incomplete dominance ratios

F2 generation shows 1:2:1 phenotypic ratio instead of 3:1 in simple inheritance.

Pea shape and incomplete dominance

Although round peas seem dominant, microscopic examination reveals incomplete dominance in starch.

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Wavy hair

Example of incomplete dominance in humans, intermediate between curly and straight.

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Incomplete dominance in humans

The resulting phenotype is different from either parent's phenotype.

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Genotype ratio

Homozygous parents create offspring with genotypic ratio of 1:2:1.

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Phenotype ratio

Homozygous parents create offspring with phenotypic ratio of dominant: intermediate: recessive.

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Overdominance

When a heterozygote has greater reproductive success than either homozygote.

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Sickle-cell anemia

An autosomal recessive disorder causing abnormal hemoglobin, leading to misshapen red blood cells and anemia.

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HbA

Allele encoding normal hemoglobin.

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HbS

Allele encoding abnormal hemoglobin (sickle-cell).

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Co-dominance

Both alleles in a heterozygote express, neither being completely dominant or recessive.

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Co-dominance in Chickens

When a white-feathered chicken breeds with a black-feathered chicken, the offspring have both white and black feathers.

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Sex-influenced trait

A trait where an allele is dominant in one sex, but recessive in the other.

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Sex-limited trait

A trait that is expressed in only one sex.

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Autosomal trait

A trait determined by a gene located on one of the non-sex chromosomes.

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Complementation

Two parents with the same or similar recessive phenotypes produce offspring with a wild-type phenotype.

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Gene Modification

An allele of one gene changes the expression of alleles of another gene.

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Gene Redundancy

Loss of one gene has no effect on phenotype, but loss of two genes does.

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Epistasis

One gene masks the effect of another gene's alleles.

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How does epistasis differ from dominance?

Epistasis involves alleles of two different genes, while dominance involves alleles of the same gene.

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What is an example of epistasis?

In sweet peas, two genes are needed for purple flowers, with mutations in either gene leading to white flowers.

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Why is epistasis important?

It helps explain how complex traits are controlled by multiple genes interacting in a pathway.

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What happens when a gene is epistatic?

The epistatic gene masks the effect of the other gene, resulting in a different phenotype.

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Example: Bird Plumage

Roosters exhibit more elaborate plumage than hens, showcasing a sex-limited trait.

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Lethal Allele

A gene variant that can cause death in an organism, often due to mutations in essential genes.

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Essential Gene

A gene required for survival, with its absence resulting in a lethal phenotype.

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Nonessential Gene

A gene not critical for survival, but still beneficial to the organism.

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Conditional Lethal Allele

A lethal allele that only kills an organism under specific environmental conditions.

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Temperature-Sensitive Lethals

Lethal alleles that kill an organism at a specific temperature, but not at others. Misfolding of proteins at high temperatures is often the cause.

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Semilethal Allele

An allele that kills a portion of individuals in a population, not all.

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Manx Cat

A breed of cat with a shortened tail due to a dominant mutation. Homozygous individuals are lethal.

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Pleiotropy

The phenomenon where a single gene affects multiple traits in an organism.

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Pleiotropic Effects Example: Cystic Fibrosis

A single gene mutation affecting the CFTR protein leads to multiple symptoms, including thick mucus in lungs, blocked digestive enzymes, and salty sweat.

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Coat Color Patterns and Development

Coat patterns in mammals can be determined by the migration and survival of melanoblasts during embryonic development.

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Spotting Gene (MITF)

A gene that controls the distribution of melanocytes, affecting coat color patterns.

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Gene Interactions

The combined influence of multiple genes on a single trait.

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Sex-influenced inheritance

The sex of an individual influences the phenotypic expression of an allele. An allele may be dominant in one sex and recessive in the other.

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Sex-limited inheritance

A trait is expressed only in one sex, often due to sex-specific hormone regulation.

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How do heterozygotes gain benefits in overdominance?

Heterozygotes can have increased resistance to infections, produce more functional protein dimers, or have proteins that work in a wider range of conditions.

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What is the molecular basis of codominance?

Codominant alleles encode proteins with slightly different functions. The combined function of these proteins in the heterozygote results in a unique phenotype.

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How does sex influence gene expression?

Sex hormones regulate the molecular expression of genes. This can influence the phenotypic effects of alleles, leading to sex-influenced or sex-limited inheritance.

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Study Notes

Incomplete Dominance

  • Heterozygotes have an intermediate phenotype between the two homozygotes

  • Example: flower color in four-o'clock plants

    • Two alleles: CR (red) and CW (white)
    • Heterozygote (CRCW) is pink
    • F2 generation shows a 1:2:1 phenotypic ratio (not 3:1 as in simple Mendelian inheritance)

  • Pea shape

    • Mendel thought RR and Rr were round, rr wrinkled
    • Microscopic examination revealed incomplete dominance for starch biosynthesis

Overdominance (Heterozygote Advantage)

  • Heterozygotes have greater reproductive success than either homozygote.

  • Example: Sickle-cell anemia

    • Autosomal recessive disorder
    • Two alleles: HbA (normal hemoglobin) and Hbs (abnormal hemoglobin)
    • HbAHbS individuals have a mix of normal and abnormal hemoglobin
    • Heterozygotes (HbAHbS) exhibit resistance to malaria
    • Homozygotes (HbSHbS) have sickle-cell anemia and shorter lifespan
    • Homozygous (HbsHbs) individuals have sickle-shaped red blood cells, causing anemia and shorter lifespan

  • Phenotype falls outside the range of the parent phenotypes, e.g., if one homozygote is tall (TT) and the other is short (tt), the heterozygote may be extra tall (Tt).

Codominance

  • Both alleles in a heterozygote are fully expressed, resulting in a phenotype that combines aspects of both alleles.

  • Example: ABO blood types in humans

    • Three alleles (IA, IB, i)
    • IA and IB are codominant
    • ii has type O blood
    • IAIA or IAi has type A blood
    • IBIB or IBi has type B blood
    • IAIB has type AB blood
    • Alleles IA and IB are codominant; both are expressed in the heterozygote, resulting in type AB blood.

  • Spotting on livestock (e.g., cows, chickens) represents a phenomenon that is analogous to co-dominance in flowers, where both black and white traits are expressed independently.

Sex-Influenced Traits

  • An allele is dominant in one sex but recessive in the other.
  • Example: scurs in cattle
    • Dominant in males; recessive in females.

Sex-Limited Traits

  • Traits that are only expressed in one sex
  • Example: mammary glands (only expressed in females) or plumage characteristics (only expressed in males) in birds.
  • Sex hormones regulate molecular expressions

Lethal Alleles

  • Alleles that cause death of an organism.
  • Often result from mutations in essential genes, needed for survival
  • Typically inherited recessively (e.g., Manx cats)

Conditional Lethal and Semilethal Alleles

  • Lethal only under certain environmental conditions.
  • Example: temperature-sensitive (ts) lethals in Drosophila.

Gene Interactions (Pleiotropy)

  • When two or more genes influence a single trait
  • Example: height, weight, pigmentation
  • Multiple gene interactions often arise due to two or more proteins functioning in the same cellular pathway.
  • Example: cystic fibrosis

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Explore key concepts in genetics related to incomplete dominance and overdominance. This quiz discusses heterozygotes and provides examples such as flower color in four-o'clock plants and the advantage of heterozygotes in sickle-cell anemia. Test your understanding of these complex genetic interactions.

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