Chapter 10 Questions
Describe the relationships among chromosomes, DNA, genes and alleles.
Chromosomes consist of DNA and associated proteins. The DNA in a chromosome is divided into genes, which are sequences of nucleotides that encode proteins. Alleles are different versions of a gene.
How do meiosis, fertilization, diploid cells, and haploid cells interact in a sexual life cycle?
Specialized diploid cells undergo meiosis, a type of cell division that produces haploid cells. Haploid cells, in turn, combine during fetilization to form the diploid zygote, which is the first cell of the next generation.
Why did Gregor Mendel choose pea plants as his experimental organism?
Mendsel chose pea plants because they are easy to grow, develop quickly, produce many offspring, and have many traits that appear in two forms that are easy to distinguish. It also is easy to hand-pollinate pea plants, so an investigator can control which plants mate with one another.
Distinguish between Dominant and Recessive
Dominant alleles appear in a phenotype whenever they are present; Recessive alleles contribute to the phenotype only if no dominant alleles are present.
Distinguish beween Heterozygous and Homozygous
An individual is homozygous for a gene if both alleles are identical; in a heterozygous individual, the two alleles for a gene are different.
Distinguish between Phenotype and Genotype
An organism's phenotype is its appearance, the genotype is the alleles an individual possesses.
Distinguish between Wild-Type and Mutant Alleles
The Wild-Type allele is the most common form of a gene in a population, a mutant allele arises when a gene undergoes a mutation.
What is a monohybrid cross and what are the genotypic and phenotypic ratios expected in the offspring of the cross?
A monohybrid cross is a mating between two individuals that are each heterozygous for one gene. The genotypic ratio expected in a monohybrid cross is 1:2:1 (Homozygous Dominant: Heterozygous :Homozygous Recessive) The Phenotypic Ratio is 3:1 (Dominant: Recessive)
How are Punnett Squares helpful in following inheritance of single genes?
Punnett squares show the genotypes of each parent as well as the geotypes of potential offspring. Phenotypic and genotypic ratios of offspring can be predicted from the data in Punnett squares.
What is a Test Cross and why is it useful?
A test cross is a mating between a homozygous recessive individual and an individual of unknown genotype. The genotype of the unknown parent can be deduced from the ratio of phenotypes among the offspring.
How does the Law of Segregation reflect the events of meiosis?
The Law of Segregation says that the two alleles of each gene are packaged into separate gametes. This law is a consequence of the events of anaphase 1 in meiosis, during which homologous chromosomes split up and move into separate cells.
What is a dihybrid cross and what is the phenotypic ratio expected in the offpring of the cross?
In a dihybrid cross, two individuals that are each heterozygous for two genes are mated. The phenotypic ratio that is expected is 9:3:3:1. (Dominant for both genes: Dominant for one gene and recessive for the other: Recessive for one gene and dominant for the other" recessive for both genes)
How does the law of independent assortment reflect the events of meiosis?
The law of independent assortment states that the segregation of alleles for one gene does not affect the segregation of alleles for another gene on a separate chromosome. This law is a consequence of the events of metaphase 1 in meiosis, during which each homologous pair of chromosomes is oriented independently of other chromosome pairs.
How can the product rule be used to predict the results of crosses in which multiple genes are studied simultaneously?
The product rule allows you to estimate the odds that an offspring will have a cerain combination of alleles for multiple genes by multiplying the probability of inheriting each allele.
How do patterns of inheritance differ for unlinked versus linked pairs of genes?
The inheritance patterns of unlinked genes is predictable (and follows Mendelian genotype and phenotype ratios) since allele combinations are not affected by crossing over. However, when pairs of genes are linked, they are carried on the same chromosome and are inherited together. Crossing over during meiosis may or may not separate linked genes, so it is not possible to predict genotype and phenotype ratios unless crossover frequencies are known.
What is the difference between recombinant and parental chromatids, and how do they arise?
When crossing over occurs during gamete formation (meiosis), one chromatid per chromosome exchanges genetic material with its homolog. The result is two recombinant chromatids (which have a mixture of maternal and paternal alleles) and two parental chromatids (which retain the original allele combinations of each chromosome).
How do biologists use crossover frequencies to map genes on chromosomes?
The father apart two linked genes are on a chromosome, the more frequently they will cross over. Analyzying crossover frequentcies for multiple pairs of traits reveals which genes are close together and which are far apart. This information is used to deduce linkage maps, which show the relative positions of genes on chromosomes.
How do incomplete dominance and codominance increa the number of phenotypes observed in a population?
Incomplete dominance and codominance produce phenotypes that are intermediate between or combinations of those produced by homozygous dominant and homozygous recessive individuals.
Differentiate between pleiotropy and epistasis
Pleiotropy occurs when one gene contributes to multiple phenotypes. The protein encoded by the gene may enter several different biochemical pathways or affect more than one body part or process. Epistasis occurs when one gene's product masks the effects of another gene.
How can the same phenotype stem from many different genotypes?
Each gene encodes one protein, but many different proteins may interact in a single metabolic pathway. A mutation in a gene encoding any of these proteins may produce a flawed metabolic pathway. In this way, different genotypes can produce the same phenotype (failure of the metabolic pathway to operate properly).
What determines a person's sex?
One pair of chromosomes, the sex chromosomes, determines a person's sex. A female has two X chromosomes; a male has an X and a Y chromosome.
What is the role of the SRY gene in sex determination?
The Y chromosome's SRY gene encodes a protein that acts as a master switch. The SRY protein turns on other genes, which direct the undeveloped testes to secrete the male sex hormone testosterone and to develop male structures. SRY also turns on a gene encoding protein that causes embryonic female structures to disasssemble. If a functional SRY gene is not present, an embryo will develop as a female.
Why do males and females express recessive X-linked alleles differently?
Each female has a pair of X chromosomes, wheras a male has only one X chromosome. Any trait a male has on his X chromosome will be expressed. Recessive alleles on an X chromosome of a female may be masked by dominant alleles on her other X chromosome.
Why does X inactivation occur in female mammals?
X inactivation prevents each cell of a female from receiving a "double dose" of proteins encoded by X chromosome genes.
How are pedigrees used to determine mode of inheritance?
By observing which individuals have a trait over multiple generations in the same family, it is often possible to determine whether a trait is autosomal or sex-linked and to determine whether it is dominant or recessive
How is polygenic inheritance different from codominance?
A polygenic trait is one that is controlled by multiple genes. Codominance reflects the relationship among multiple alleles of the same gene.
How can the environment affect a phenotype?
The environment can directly affect the expression of some genes. For example, temperature can influence the expression of temperature-sensitive alleles. The environment can also affect the phenotype in other ways, as when infectios agents intensify a genetic disorder. Many aspects of the phenotype, including temperment and physical health, reflect not only genes but also upbringing, nutrition, and many other environmental variables.
____ encode proteins: Mutations create new ____.
Genes / Alleles
A _____ is a continuous molecule of DNA with associated proteins.
Chromosome
In humans, a diploid cell contains __ homologous pairs of autosomes and __ pair of sex chromosomes.
22 / 1
Meiosis gives rise to _____
Haploid Cells.
_____ unites haploid gametes and restores the diploid number
Fetilization
Why did Gregor Mendel use peas to study inheritance?
Because they are easy to breed, develop quickly and produce abuntant offspring.
An __________ disorder can be inherited from one affected parent.
Autosomal Dominant
An ________ disorder must be inherited from both parents.
Autosomal Recessive
_ - Linked recessive disorders affect mostly males.
X
__________ trace phenotypes in families and reveal modes of inheritance.
Predigrees.
________ randomly shuts off all but one X chromosome in each cell.
X-Inactivation. This prevents the double dosing of proteins.
Females express an X-linked recessive disorder only if they inherit faulty alleles from ____ parent
Both
Males express ____ allele on their single X chromosome
Every
In _____ , the activity of one gene masks the effect of another.
Epistasis.
When multiple proteins participate in a biochemical pathway, mutations in genes encoding any of the proteins can produce the same ______
Phenotype
A _____ gene affects multiple phenotypes.
Pleiotropic
In the list of four terms below, which term is the SECOND most inclusive?
Genome
Allele
Chromosome
Gene
Chromosome
According to Mendel, if an individal is heterozygous for a gene, the phenotype will correspond to that of
The recessive trait alone
The dominant trait alone
A blend of the dominant and recessive traits
A Wild Type trait.
The dominant trait alone
If a plant that is heterozygous dominant for a gene is crossed with a plant that is homozygous recessive for the same gene, what is the probability that the offspring will have the same genotype as one of the parents?
100
50
25
0
0
Which of the following is a possible gamete for an individual with the genotype PP rr?
PP
P r
p r
rr
P r
Use the product rule to determine the chance of obtaining an offspring with the genotype Rr Yy form a dihybrid cross between parents with the genotype Rr Yy.
1/2
1/4
1/8
1/6
1/4
How can epistasis decrease the number of phenotypes ovserved in a population?
One gene may mask the expression of another gene.
One allele may mask the expression of another allele of the same gene
Genes may become silenced after exposure to certain environmental cues.
Genes may be inactivated if they are on an X Chromosome
One gene may mask the expression of another gene
Suppose a woman has a recessive X- Linked disease. Her husband does not have the disease. What is the chance that their second child has the disease?
100
50
25
0
50
How does X inactivation contribute to a persons phenotype?
It controls the number and kind of genes inherited on an X chromosome
It determines which X Chromosome is expressed in a male
It allows for the expression of either the maternal or the paternal X in different cells
It enhances the expression of Y-linked genes in males
It allows for the expression of either the maternal or the paternal X in different cells