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Questions and Answers
What will happen to a dominant trait if it is absent in both parents?
What will happen to a dominant trait if it is absent in both parents?
Carriers of dominant traits can exist without expressing them.
Carriers of dominant traits can exist without expressing them.
False
What is a dihybrid cross?
What is a dihybrid cross?
A mating between heterozygotes for two traits.
According to Mendel's Law of ______, alleles assort independently during the formation of gametes.
According to Mendel's Law of ______, alleles assort independently during the formation of gametes.
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Which of the following represents the possible gametes for a heterozygous organism with a genotype AaBb?
Which of the following represents the possible gametes for a heterozygous organism with a genotype AaBb?
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In a two-trait cross, the segregation of different allele pairs is dependent.
In a two-trait cross, the segregation of different allele pairs is dependent.
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What does it mean if a trait is recessive?
What does it mean if a trait is recessive?
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Match the following terms with their definitions:
Match the following terms with their definitions:
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What is the primary purpose of a test cross?
What is the primary purpose of a test cross?
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A test cross is performed by crossing an individual with a dominant phenotype with another individual that is also dominant.
A test cross is performed by crossing an individual with a dominant phenotype with another individual that is also dominant.
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List two examples of dominant traits in humans.
List two examples of dominant traits in humans.
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In a test cross, the unknown genotype is crossed with an individual that has the ______ phenotype.
In a test cross, the unknown genotype is crossed with an individual that has the ______ phenotype.
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What would the offspring phenotype ratio be if an individual with genotype Bb is test crossed with an individual with genotype bb?
What would the offspring phenotype ratio be if an individual with genotype Bb is test crossed with an individual with genotype bb?
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All human dominant traits follow a simple inheritance pattern.
All human dominant traits follow a simple inheritance pattern.
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Match the following genetic disorders with their classification:
Match the following genetic disorders with their classification:
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In many dominant traits, organisms with two copies of the dominant allele may exhibit a ______ phenotype compared to those with just one copy.
In many dominant traits, organisms with two copies of the dominant allele may exhibit a ______ phenotype compared to those with just one copy.
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What is the phenotype ratio for the offspring in the given scenario?
What is the phenotype ratio for the offspring in the given scenario?
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The genotype ratio of the offspring is 100% Aa.
The genotype ratio of the offspring is 100% Aa.
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In a test cross, what is the significance of crossing an individual with a homozygous recessive?
In a test cross, what is the significance of crossing an individual with a homozygous recessive?
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A probability of _____ means an event is certain to happen.
A probability of _____ means an event is certain to happen.
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What does a probability of 0 represent?
What does a probability of 0 represent?
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Match the following terms with their correct definitions:
Match the following terms with their correct definitions:
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How is a Punnett square structured?
How is a Punnett square structured?
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Actual offspring frequencies always match expected frequencies exactly.
Actual offspring frequencies always match expected frequencies exactly.
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Study Notes
Genes
- Units of information about specific traits
- Passed from parents to offspring on chromosomes (DNA)
- Each gene has a specific location (locus) on a chromosome
Alleles
- Different molecular forms of a gene
- New alleles arise by mutation
- Different alleles of the same gene produce basically the same protein or RNA molecule during gene expression, but one or more amino acids may be changed, resulting in a different cellular function.
Example: Phenylketonuria (PKU)
- A gene locus contains the information to produce an enzyme that degrades the amino acid phenylalanine.
- The most common allele produces a normal enzyme.
- The PKU allele results in a non-functional enzyme, causing a mutation in the DNA.
- Individuals with only PKU alleles cannot degrade phenylalanine, resulting in the disorder.
Key Terms
- Trait: A physical characteristic (e.g., flower color, blood type)
- Wild-type allele: The most common allele in a population (often called "normal allele")
- Mutant allele: A rare allele in a population (often called "non-normal allele")
- Dominant allele: Masks the effect of other alleles; usually shown with a capital letter
- Recessive allele: Masked by other alleles; usually shown with a lower-case letter
- Zygote: The first diploid cell produced by fertilization
- Cross: Mating between individuals to produce offspring
- Self-pollinate: In plants, male and female gametes originate from the same plant
- Cross-pollinate: In plants, the male gametes of one plant are delivered to the female gametes of a different plant
- Punnett Square: A chart used to determine the possible combinations of alleles in offspring
Genotype and Phenotype
- Genotype: The specific combination of alleles carried by an individual
- Phenotype: The version of a trait displayed by an individual (e.g., yellow vs. green)
Allele Combinations (Genotypes)
- Homozygous: When both alleles at the same gene locus are identical (“true-breeding”)
- Heterozygous: When two different alleles are present at the same gene locus (a “hybrid”)
Heredity and Genetics
- Heredity: Transmission of traits from one generation to the next
- Genetics: The scientific study of heredity
Mendel and Peas
- Gregor Johann Mendel is considered the father of genetics
- Chose peas as his organism to study inheritance
- Peas have many simple dichotomous traits (e.g., flower color, seed shape).
- Many pea traits are true-breeding.
- Male and female reproductive structures are in one flower, making it easy to manipulate for specific crosses.
- Each pea is a new individual, allowing for evaluation of thousands of mating pairs.
Mendel's Experimental Method
- Obtain true-breeding strains for each trait
- Cross-fertilize the true-breeding strains to produce the first filial generation (F1)
- Allow the F1 offspring to self-fertilize to produce the second filial generation (F2) and subsequent generations.
- Count the number of offspring with each trait in each generation.
One-Trait Crosses
- Crosses to study variations of a single trait.
- Starts with true-breeding pea strains for 7 different traits.
- Mendel performed reciprocal crosses.
F1 Generation
- Offspring produced by crossing two true-breeding strains.
- All F1 plants resembled one parent (in all traits studied).
- The trait displayed in the F1 generation was referred to as dominant, and the alternative trait was recessive.
- No intermediate traits were present.
F2 Generation
- Offspring resulting from self-fertilization of F1 plants
- The recessive trait reappeared in some F2 individuals.
- A 3:1 ratio for dominant to recessive phenotypes (proportions of traits) was consistently observed in the F2 generation when performing these crosses.
- The ratio suggested parents had two copies of every trait-determining factor (alleles).
Looking Closer at the F2 Generation
- The 3:1 phenotype ratio is actually caused by a 1:2:1 genotype.
- Includes true-breeding dominant, not-true-breeding dominant (hybrids), and true-breeding recessive plant.
Defining Monohybrid Crosses
- P generation: True-breeding purple crossed with true-breeding white
- F1 Generation: All purple
- F2 Generation: Mixture of purple and white plants with a 3:1 ratio approximately.
Mendel's Five-Element Model
- Parents transmit discrete factors (genes) that determine traits.
- Each characteristic has two copies, one from each parent.
- Alleles are alternative versions of genes with variations.
- Alleles remain discrete.
- The presence of an allele does not always guarantee expression (phenotype); dominant vs recessive.
Mendel's First Law: Law of Segregation
- During meiosis, gametes (sperm or egg) carry only one allele for each trait due to allele pairs separating (segregating)
- This is the physical basis for segregation, resulting from chromosome behavior during meiosis.
- Two alleles from two different gametes are joined at random during fertilization.
Setting Up a Punnett Square
- Decide on symbols for each allele and create a key
- Write out the phenotypes and genotypes of the parents
- Determine the alleles present in each possible gamete for each parent
- Create a Punnett square by placing gamete genotypes across the top and down the left for the other parent
- Use the Punnett square to determine the possible offspring genotypes and probabilities.
- Calculate phenotype ratio.
Probability Basics
- Chance events are described by probability.
- Probability is given as a fraction or decimal.
- A probability of 0 means an event will not occur, while 1 means it is certain.
- Anything in between is a possibility.
Probability vs. Random Offspring
- Chance outcomes depend on the number of ways an outcome can be achieved.
- Outcomes may not always match expectations based on probability but are closer to probabilities when performed many times.
Test Crosses
- Determine the genotype of an individual with a dominant phenotype
- Cross the unknown with an individual carrying a recessive allele
- This shows how many offspring have what phenotypes; dominant or recessive.
How the Test Cross Works
- Determine whether the organism is homozygous or heterozygous for a given trait
- Mendel verified these results for numerous traits
- Each parent carries two alleles
- Alleles segregate equally and randomly
- Gametes recombine during fertilization
Human Inheritance Patterns
- Most human traits are controlled by interactions between multiple gene loci; however some traits are controlled by only one gene.
- Pedigree Analysis: Shows inheritance pattern for traits over many generations.
- Shows traits are dominant or recessive
- Used to deduce the genotypes of family members.
- Useful in genetic counseling
Dominant Traits in Pedigrees
- Dominant traits may be more difficult to notice because their presentation is not always obvious.
- Dominant traits appear in every generation and might be missing in parents but reappear in offspring if the alleles for the trait are present.
Recessive Traits in Pedigrees
- Recessive traits can disappear in one generation but may reappear in later generations showing they are present in the parent, potentially hidden due to heterozygosity.
Two-Trait Crosses
- Crosses looking at the inheritance of two traits simultaneously.
- Involves true breeding parents, and their offspring with two genes under investigation (flower color and stem height for example)
Mendel's Second Law: Law of Independent Assortment
- In 2-trait crosses, different allele pairs assort independently during gamete formation (alleles for one trait do not affect the alleles for another trait).
- This independent assortment is due to the independent alignment of different homologous chromosome pairs in metaphase I during meiosis
What You Need to Know About 2-Trait Crosses
- You don't need to perform 2-trait crosses or Punnett squares on a test.
- You need to be able to identify the gametes produced for traits.
- You should know how alleles at different gene loci assort independently.
Determining Possible Gametes in Two-Traits with Heterozygotes
- Heterozygotes (AaBb): Determine possible gametes by using independent segregation.
- Determine if A or a comes from the allele pair, and B or b comes from the allele pair.
Mendel's Parental & Dihybrid Crosses
- Parental cross begins with true-breeding (homozygous) parents.
- These true-breeding parents are used for F1 dihybrid cross.
- F2 generation contains 9:3:3:1 phenotypic ratio.
Mendel's Second Law in Action
- Each trait will show a 3:1 phenotypic ratio.
- Traits are inherited independently.
- Segregation and recombination of one trait do not affect segregation and recombination of another trait.
Tremendous Variation
- The number of different genotypes an individual can inherit from two parents is enormous due to independent assortment.
Extensions to Mendel
- Mendel's model assumes each trait is controlled by one gene, the gene has two alleles, and there is a clear dominant-recessive relationship.
- Most genes do not fit these requirements.
Polygenic Inheritance
- Multiple genes control a trait's phenotype.
- The phenotype is influenced by the cumulative contributions of those genes.
- Traits show continuous variation and can be represented as quantitative traits (e.g., height).
Pleiotropy
- One allele affecting multiple traits.
- Effects are difficult to predict due to unknown functions.
- Seen in human conditions like cystic fibrosis and sickle-cell anemia
Multiple Alleles
- More than two alleles are common in a population at one gene locus.
- Each allele has distinct effects on phenotype expression.
- Examples include ABO blood types.
Dominance Relations
- Complete dominance: The dominant allele completely masks the recessive allele
- Incomplete dominance: The heterozygote phenotype is an intermediate between the homozygotes
- Codominance: Both dominant alleles are expressed in heterozygotes
Incomplete Dominance
- The heterozygote phenotype is an intermediate between the homozygotes (e.g., pink flowers from a cross between red and white).
- Capital and lower-case letter choices cannot be used; phenotypes are not dominant.
Homeotic Mutants
- Affect development.
- Alleles affect gene loci, regulating the expression of other genes.
- Mutant alleles can affect numerous traits (e.g., insect limbs on the head instead of antennae).
Genetics of ABO Blood Types
- Codominance and multiple alleles influence ABO blood-type inheritance.
- The ABO blood type gene controls the structure of certain glycolipids on red blood cells.
- IA and IB are codominant alleles.
- The i allele is recessive, meaning only when both alleles are recessive, does a person have type O blood.
ABO and Transfusions
- Recipients' immune systems can attack blood cells with unfamiliar surface antigens.
- Antibodies cause blood cells to clot, which can kill the recipient.
- Type O blood is considered a universal donor.
Epistasis
- The product of one gene affects the expression of another gene.
- This often occurs in metabolic pathways where the outcome depends on multiple enzymes working in a series
Gene Linkage
- Linked genes are located close together on a chromosome and do not always assort independently, as they are often inherited together.
- Crossing over reduces linkage but if too close together, the genes are less likely to be separated, resulting in linkage.
- Sex linkage occurs when a gene locus is on a sex chromosome (X or Y), which affects inheritance differently in males and females.
Inheritance of Sex Linked Traits
- The sex of an offspring influences how they inherit traits, due to the differing makeup of sex chromosomes.
- Males receive an X chromosome from their mother.
- Females receive two X chromosomes, one from each parent.
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Description
Test your understanding of genetic principles, particularly dominant and recessive traits, as well as dihybrid crosses. This quiz covers key concepts from Mendelian genetics and the significance of test crosses. Perfect for students studying genetics in high school or college-level courses.
