Mendelian Genetics Backcross and Testcross
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Questions and Answers

In a backcross involving a pea plant, if the F1 generation (Dd) is crossed with a homozygous dominant parent (DD), what would be the expected genotypic ratio of the offspring?

  • 1 DD : 1 Dd (correct)
  • 1 DD : 2 Dd : 1 dd
  • All Dd
  • 3 DD : 1 dd

A researcher performs a testcross on a plant with an unknown genotype, using a homozygous recessive plant. If the offspring show a 1:1 phenotypic ratio of dominant to recessive traits, what can be inferred about the genotype of the unknown plant?

  • The plant's genotype cannot be determined from this cross.
  • The plant is homozygous dominant for the trait.
  • The plant is homozygous recessive for the trait.
  • The plant is heterozygous for the trait. (correct)

In the F2 generation of Mendel's experiment with pea plants, what proportion of tall plants are expected to be heterozygous (Dd)?

  • 1/3
  • 2/3 (correct)
  • 1/2
  • 1/4

What is the primary purpose of performing a backcross, especially one using a fully recessive parent, in genetic experiments?

<p>To identify the genotype of an individual expressing a dominant trait. (C)</p> Signup and view all the answers

Why is the phenotype of a heterozygote crucial in determining the relationship between two alleles in Mendelian genetics?

<p>It shows whether one allele is dominant, recessive, or exhibits incomplete dominance/co-dominance. (C)</p> Signup and view all the answers

What critical innovation allowed Mendel to accurately interpret the results of his genetic experiments?

<p>Developing pure lines. (A)</p> Signup and view all the answers

In Mendel's experiments, what consistently occurred in the F1 generation when crossing two pure lines with contrasting traits?

<p>Expression of only the dominant phenotype. (B)</p> Signup and view all the answers

What does the term 'phenotype' refer to in the context of genetics?

<p>The physical appearance of a trait. (D)</p> Signup and view all the answers

In the context of Mendelian genetics, what is a recessive allele?

<p>An allele that is masked by a dominant allele in a heterozygote. (B)</p> Signup and view all the answers

Mendel concluded that hereditary determinants are of a 'particulate nature.' What term do we use today to describe these determinants?

<p>Genes (D)</p> Signup and view all the answers

What is the significance of the F2 generation in Mendel's experiments?

<p>It reveals reappearance of recessive traits. (C)</p> Signup and view all the answers

If a plant with round seeds (RR) is crossed with a plant with wrinkled seeds (rr), and the offspring (Rr) all have round seeds, what can be inferred?

<p>Round seeds are dominant. (C)</p> Signup and view all the answers

A scientist repeats one of Mendel's experiments, crossing pure lines of tall and dwarf pea plants. However, their F2 generation displays a continuous variation in height rather than distinct 'tall' and 'dwarf' phenotypes. What is the most likely explanation for this observation?

<p>The height is determined by multiple genes, not just one. (A)</p> Signup and view all the answers

In Mendelian genetics, what distinguishes an allele from an allelic pair?

<p>An allele is one alternative form of a gene, while an allelic pair is the combination of two alleles for a specific gene. (A)</p> Signup and view all the answers

How does a heterozygote differ from a homozygote in Mendelian genetics?

<p>A heterozygote contains one of each member of the gene pair, while a homozygote contains only one allele at the allelic pair. (C)</p> Signup and view all the answers

In the context of Mendelian genetics, what is the significance of the Punnett square?

<p>It is a graphical representation of the possible genotypes and phenotypes of offspring based on the parents' genotypes. (C)</p> Signup and view all the answers

According to Mendel's First Law, the law of segregation, what process occurs during gamete formation?

<p>Each member of the allelic pair separates from the other member to form the genetic constitution of the gamete. (D)</p> Signup and view all the answers

What does the genotypic ratio of 1 DD : 2 Dd : 1 dd in the F2 generation indicate about the offspring?

<p>It describes the ratio of allele combinations present in the offspring's genetic makeup. (A)</p> Signup and view all the answers

If Mendel selfed the F2 plants and his law of segregation was correct, what would he predict about the results?

<p>The results would align with the predicted genotypic ratios based on the segregation of alleles. (C)</p> Signup and view all the answers

How does the concept of 'union of gametes at random' influence genetic outcomes in Mendelian inheritance?

<p>It means any sperm can fertilize any egg, leading to diverse combinations of genes in the offspring. (C)</p> Signup and view all the answers

What is the relationship between genotype and phenotype in the F2 generation of Mendel's pea plants?

<p>Genotype influences phenotype, but different genotypes can sometimes result in the same phenotype (e.g., DD and Dd both being tall). (D)</p> Signup and view all the answers

In a dihybrid cross, if the parental generation consists of plants with genotypes AABB and aabb, what does the term 'dihybrid' refer to?

<p>The resulting offspring of the parental cross, having a genotype of AaBb. (A)</p> Signup and view all the answers

What was the primary purpose of Mendel performing a dihybrid cross, such as the one with yellow, round seeds and green, wrinkled seeds?

<p>To analyze whether a relationship exists between different allelic pairs. (A)</p> Signup and view all the answers

In the F2 generation of Mendel’s dihybrid cross, a 9:3:3:1 phenotypic ratio was observed. What does the '1' represent?

<p>The proportion of offspring displaying both recessive traits. (D)</p> Signup and view all the answers

Given that 'G' represents the allele for yellow seed color and 'W' represents the allele for round seed shape, what genotype would result in a green, wrinkled seed phenotype?

<p>ggww (B)</p> Signup and view all the answers

Consider a dihybrid cross where the F1 generation is GgWw (Yellow, Round). If two F1 individuals are crossed, what proportion of the F2 generation would you expect to have the genotype GgWw?

<p>$4/16$ (A)</p> Signup and view all the answers

In a co-dominant inheritance pattern, what is the phenotypic expression of a heterozygote?

<p>Displays a unique phenotype where both alleles contribute equally and distinctly. (A)</p> Signup and view all the answers

In a pedigree analysis, what is a key characteristic of a trait controlled by dominant gene action?

<p>Affected individuals usually have at least one affected parent. (C)</p> Signup and view all the answers

If a plant species exhibits incomplete dominance and red flowers (RR) are crossed with white flowers (rr), what would be the expected phenotype of the F1 generation?

<p>All pink flowers (C)</p> Signup and view all the answers

In pedigree analysis, which of the following observations would suggest that a trait is inherited through recessive gene action?

<p>Unaffected parents can have affected offspring. (D)</p> Signup and view all the answers

What is the fundamental difference between co-dominance and complete dominance?

<p>Co-dominance involves both alleles being fully expressed, while complete dominance involves one allele masking the effect of another. (D)</p> Signup and view all the answers

Why is pedigree analysis particularly useful when studying human genetics?

<p>Progeny data from several generations is often limited and human generation time is long. (B)</p> Signup and view all the answers

Mendel's Law of Independent Assortment is based on the observation of:

<p>The segregation of two or more genes during gamete formation. (B)</p> Signup and view all the answers

In the context of genetics, dominance refers to:

<p>The ability of one allele to mask the expression of another allele in a heterozygote. (A)</p> Signup and view all the answers

Flashcards

Phenotypes

Observable traits of an organism, arising from genotype.

Genotypes

Genetic makeup of an organism, indicating alleles present.

Backcross

Cross between an F1 hybrid and a homozygous parent to test genotype.

Testcross

Cross of an individual with a homozygous recessive to determine its genotype.

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Monohybrid cross

A breeding experiment between parents differing at one gene pair.

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Dihybrid Cross

A genetic cross between parents differing in two pairs of alleles.

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Dihybrid

An individual heterozygous for two pairs of alleles (AaBb).

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F1 Generation

The first generation offspring from a parental cross, all yellow and round.

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F2 Generation

The second generation from a dihybrid cross, resulting in a 9:3:3:1 phenotypic ratio.

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Allelic Pairs

Pairs of alleles that may show different traits in a dihybrid cross.

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Gamete

A reproductive cell carrying one allele from a gene pair.

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Allele

An alternative form of a gene; e.g., tall or dwarf in pea plants.

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Homozygote

An individual with two identical alleles in an allelic pair.

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Heterozygote

An individual with two different alleles in an allelic pair.

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Punnett Square

A diagram used to predict genetic ratios in offspring from a cross.

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Genotypic Ratio

The ratio of different genotypes produced in offspring.

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Mendel's First Law

The law of segregation; alleles separate during gamete formation.

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Dominance

The ability of one allele to express its phenotype over an alternate allele.

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Co-dominance

A relationship where both alleles contribute to the phenotype of a heterozygote.

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F2 Phenotypic Ratio

The expected ratio of phenotypes in the second generation is 1/4 Red, 1/2 Pink, 1/4 White.

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Pedigree Analysis

A method to study the inheritance of genes using family trees and symbols.

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Dominant Gene Action Rules

Traits with dominant action show affected individuals having an affected parent and appear in every generation.

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Recessive Gene Action Rules

Traits with recessive action can have unaffected parents producing affected offspring.

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Mendel's Law of Independent Assortment

States that genes segregate independently during gamete formation.

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Snapdragon Flower Colour

Example of co-dominance where red and white alleles produce pink flowers.

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Mendel's Second Law

Law of Independent Assortment; alleles for different traits segregate independently.

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Dominant allele

An allele that expresses its trait even when a recessive allele is present.

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Recessive allele

An allele whose trait only appears when two recessive alleles are present.

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Pure Line

A group of organisms that breed true for a specific trait.

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Gene Pair

Two alleles that determine a specific trait in an organism.

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Study Notes

Introductory Genetics I/ Genetics I

  • This course covers introductory genetics, specifically the field of Genetics I.

Mendelian Laws of Inheritance

  • Mendel's First Law (Law of Segregation): Describes monohybrid inheritance.
  • Mendel's Second Law (Law of Independent Assortment): Describes dihybrid inheritance.
  • Genetic analysis precedes Mendel, but his laws form the theoretical underpinning for our understanding of inheritance.

Mendel's Innovations

  • Developed pure lines: Populations consistently breeding true for a specific trait.
  • Counted results and kept statistical records: Crucial for analyzing inheritance patterns. Non-pure lines can obscure results.

Results from Mendel's Experiments

  • Round x Wrinkled Seeds: F₂ phenotypic ratio 2.96:1 and ratio 3:1
  • Yellow x Green Seeds: F₂ phenotypic ratio 3.01:1 and ratio 3:1
  • Red x White Flowers: F₂ phenotypic ratio 3.15:1 and ratio 3:1
  • Tall x Dwarf Plants: F₂ phenotypic ratio 2.84:1 and ratio 3:1

Terms and Results

  • Phenotype: The observable physical characteristics.
  • Mendel's pea plants exhibited phenotypic variations like: round/wrinkled seeds, yellow/green seeds, red/white flowers, tall/dwarf plants.

Mendel's Conclusions

  • Hereditary factors are particulate (genes): Discrete units of inheritance.
  • Each parent contributes a gene pair for each trait.
  • The F₁ generation has one dominant and one recessive allele
  • Each gamete carries only one member of each gene pair.
  • Gametes unite randomly.

Definitions of Mendelian Genetics

  • Allele: Alternative form of a gene.
  • Allelic pair: Combination of two alleles.
  • Homozygote: Individual with two identical alleles for a gene.
  • Heterozygote: Individual with two different alleles for a gene.
  • Genotype: The specific allelic composition for a given gene or set of genes.

Using Symbols

  • Symbols depict crosses of tall and short pea plants illustrating one parent genotype (DD) and another (dd) and their parental gametes, and F1 genotype.

The F2 Generation

  • Created by allowing F₁ plants to self-fertilize.
  • Depicted graphically using Punnett squares.
  • This explains Mendel’s First Law.

Genetic Ratios

  • Genotypic ratio of F₂: 1 DD : 2 Dd : 1 dd (1:2:1 ratio)
  • Phenotypic ratio of F₂: 3 tall : 1 dwarf or 3D_ : 1 dd

Confirmation of Mendel's First Law

  • Mendel’s self-fertilized F2 plants, confirming segregation.
  • F₂ phenotype data, genotypes and analysis, confirm the genotype of the F₂ individuals.

Backcross

  • Mendel crossed the F₁ heterozygote to a pure-line homozygous dwarf plant.
  • Backcross: Dd x dd

Testcross

  • Crossing an individual with a homozygous recessive parent.
  • Used to determine gene composition of the individual.

Monohybrid Cross

  • Cross between parents differing at a single gene pair.
  • Useful for describing the relationship between different alleles.

Co-dominance

  • Both alleles contribute to the phenotype in the heterozygote.
  • Four o'clock plant flower color example.
  • Red x White flowers producing pink flowers in the F₁ generation, and 1:2:1 ratio in F₂

Pedigree Analysis

  • A useful approach for studying human genetics where progeny data is limited.
  • Especially useful for studying species with long generation times.
  • Symbols used: Male, Female, Affected individual, Mating, Offspring, Twin types

Dominant Pedigree

  • Following phenotypic data and careful pedigree analysis will reveal whether the trait is dominant or recessive.

Recessive Pedigree

  • Traits following recessive patterns: unaffected parents can have affected offspring, and affected offspring can be female or male.

Mendel's Law of Independent Assortment

  • Followed the segregation of two genes across dihybrid crosses.
  • The law of independent assortment describes that members of different gene pairs segregate into gametes independent of each other.

Dihybrid Cross

  • Cross between parents differing at two pairs of alleles
  • (AABB x aabb)

The Chi-Square Test

  • A statistical test used to determine if experimental data fit theoretical ratios.
  • Determines if data supports or rejects a hypothesis based on experimental data.

Chi-Square Formula

  • Formula for calculating the Chi-Square value:

Degrees of Freedom (df)

  • Calculation of the degrees of freedom:
  • Formula: df = n-1, where 'n' represents the number of classes

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BIO 211 Introductory Genetics I/Genetics I Lecture Notes PDF

Description

Explore Mendelian genetics with questions on backcrosses, testcrosses, and genotypic and phenotypic ratios. Understand allele relationships and Mendel's experimental innovations. This quiz reviews the impact of backcrossing, test crossing and the importance of heterozygotes.

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