genetics exam 1 review

paralogs

- A species may have two or more copies of similar genes - These copies are not identical due to the accumulation of random changes during evolution, called paralogs

Principle of Segregation

- Two alleles for a gene segregate during gamete formation and are rejoined at random, one from each parent, during fertilization - Physical basis for allele segregation is the behavior of chromosomes during meiosis

Pleiotropy can be caused because

-The gene product can affect cell function in more than one way -The gene may be expressed in different cell types -The gene may be expressed at different stages of development

A dominant pattern of inheritance predicts that

1. An affected individual will have inherited the gene from at least one affected parent 2. Alternatively, the disease may have been the result of a new mutation that occurred during gamete formation

There are three possible explanations for overdominance at the molecular/cellular level

1. Disease resistance • A microorganism will infect a cell if certain cellular proteins function optimally • However, heterozygotes have one altered copy of the gene - Have altered protein function - Not enough to cause serious side effects, but is enough to prevent infections - Ex: Tay Sachs disease and TB; PKU and fungal toxins 2. Homodimer formation - Composed of two subunits, encoded by the same type of gene, but the alleles of that gene can be different - A1A2 heterozygotes make A1A1 and A2A2 homodimers, but A1A2 homodimer may have better functional activity for some proteins 3. Variation in functional activity - A gene, E, encodes a metabolic enzyme - Allele E1 encodes an enzyme that functions better at lower temperatures - Allele E2 encodes an enzyme that functions better at higher temperatures - E1E2 heterozygotes produce both enzymes - An advantage is that the combination of both enzymes function over a wider temperature range than either E1E1 or E2E2 alone

Why was the garden pea advantageous?

1. It existed in several varieties with distinct characteristics 2. Its structure allowed for easy crosses where the choice of parental plants could be controlled

The mechanism of x-chromosome inactivation

1. Random X inactivation occurs early in development. During X chromosome inactivation, the DNA becomes highly compacted - Most genes on the inactivated X cannot be expressed When this inactivated X is replicated during cell division: - Both copies remain highly compacted and inactive - X inactivation is passed along to all future somatic cells

Independent assortment

1. Two genes on different chromosomes segregate their alleles independently. 2. The inheritance of an allele of one gene does not influence which allele is inherited at a second gene.

A recessive pattern of inheritance makes two important predictions

1. Two normal heterozygous individuals will have, on average, 25% of their offspring affected 2. Two affected individuals will produce 100% affected offspring

phenotype of incomplete dominance in f2

1:2:1

___ of the normal protein is enough to accomplish the protein's cellular function

50% - The heterozygote may actually produce more than 50% of the functional protein

In 1949, Murray Barr and Ewart Bertram identified what?

A highly condensed structure in the interphase nuclei of somatic cells in female cats but not in male cats - later known as the barr body

Gene redundancy

A pattern in which the loss of function in a single gene has no phenotypic effect, but the loss of function of two genes has an effect. Functionality of only one of the two genes is necessary for a normal phenotype; the genes are functionally redundant. - the phenomenon in which an inactive gene is compensated for by another gene with a similar function

True-breeder

A variety that produces the same trait over several generations

Essential genes

Absolutely required for survival. The absence of their protein product leads to a lethal phenotype

X Linked traits patterns

An affected male and unaffected female have no affected offspring, but females are carriers An affected female and unaffected male have all male offspring affected and females all carriers

In 1960 Susumu Ohno proposed what?

Barr body is a highly condensed X chromosome

What is the pattern of methylation?

Both parents inherit one methylated and one unmethylated gene, which is maintained in somatic cells. Methylation is removed in gamete forming cells

What is the genotype of mothers that produce dextral offspring?

Dd or DD

In 1961, Mary Lyon proposed what?

Dosage Compensation in mammals occurs by inactivation of a single X chromosome in females

Expressivity and Incomplete Penetrance is due to?

Environment Other 'modifier' genes

Explanation of gene redundancy

Gene duplication • A species may have two or more copies of similar genes • These copies are not identical due to the accumulation of random changes during evolution, called paralogs • If one gene is missing, a paralog may be able to carry out the missing function Proteins that are involved in a common cellular function • When one of the proteins is missing, the function of another protein may be increased to compensate for the missing protein and thereby overcome the defect

phenotype of pea plants (incomplete dominance) with microscope

Heterozygotes look round, but they only have half the amount of starch found in homozygous dominants

Incomplete penetrance

In the case of dominant traits, this pattern occurs when a dominant phenotype is not expressed even though an individual carries a dominant allele. example: individual who carries the polydactyly allele but has a normal number of fingers and toes.

Recessive human diseases

Individuals who exhibit the disease are either homozygous for a recessive allele or hemizygous (for X-linked genes in human males). The disease symptoms result from a defect in the amount or function of the normal protein.

When do these stages of X inactivation occur?

Initiation: Occurs during embryonic development. The number of X-inactivation centers (Xics) is counted and one of the X chromosomes remains active and the other is targeted for inactivation. Spreading: Occurs during embryonic development. It begins at the Xic and progresses toward both ends until the entire chromosomes is inactivated and becomes a Barr body. Maintenance: Occurs from embryonic development through adult life. The inactivated X chromosomes is maintained as such during subsequent cell divisions

Three stages of X in activation

Initiation: one of the X chromosomes is targeted for inactivation Spreading: the chosen X chromosome is inactivated Maintenance: the inactivated X chromosome is maintained as such during future cell divisions

Dosage compensation in birds

It appears that the Z chromosome in males does not undergo condensation and dosage compensation like one of the X chromosomes in female mammals

Different studies have shown variation in gene expression of some Z-linked genes in male and female birds. Provide an example of this.

Males express twice as much of an enzyme, aconitase, as females

Multiple Alleles

Many genes have multiple alleles - commonly found in natural population - three or more forms of a gene that code for a single trait - for genes present in a single copy/haploid genome, a maximum of two alleles are found in any particular diploid individual - Ex: ABO blood type

dosage compensation

Mechanism in which X chromosome inactivation equalizes gene expression between males and females.

Law of Segregation

Mendelian law stating that two alleles for each trait separate during meiosis - the paired factors for a given character segregate randomly so that half of the gametes receive one factor and half of the gametes receive the other

pleiotropy

Multiple effects of a single gene on the phenotype of an organism ex: cystic fibrosis

Incomplete dominance

Occurs when the heterozygote has a phenotype that is intermediate between either corresponding homozygote. For example, a cross between homozygous red-flowered and homozygous white-flowered parents produces heterozygous offspring with pink flowers.

The Lyon Hypothesis was put to the test at a cellular level by 3 scientists. How was their experiment performed?

Original Lyon hypothesis: an adult female who is heterozygous for the fast and slow G-6-PD alleles should express only one of the two alleles in any particular somatic cell and its descendants, but not both. Experiment: they analyzed the expression of a human X-linked gene (G-6-PD, an enzyme used in sugar metabolism). This gene was detected through gel electrophoresis - One G-6-PD allele encodes an enzyme that migrates very quickly ("fast" enzyme) -Another allele encodes an enzyme that migrates more slowly ("slow" enzyme) The two types of enzymes have minor differences in their structures that do not significantly affect G-6-PD function - Heterozygous adult females produce (not express!) both types of enzymes - Hemizygous males produce either the fast or the slow type Findings: All nine clones expressed one of the two types of G-6-PD enzyme, not both These results are consistent with the hypothesis that - X inactivation has already occurred in any given epithelial cell - AND this pattern of inactivation is passed to all of the cell's progeny

Sex-influenced inheritance

Refers to the effect of sex on the phenotype of the individual. Some alleles are recessive in one sex and dominant in the opposite sex Ex: scurs

Binomial Expansion Equation

Represents all of the possibilities for a given set of unordered events

What is snail coiling due to?

The maternal effect gene that exists as dextral (D) and sinistral (d) alleles

hybridization

The mating or crossing between two individuals that have different characteristics Purple-flowered plant X white-flowered plan

How is penetrance measured?

The measure of penetrance is described at the population level: If 60% of heterozygotes carrying a dominant allele exhibit the trait, the trait is 60% penetrant

Norm of reaction

The range of phenotypes produced by a single genotype, due to environmental influences. - Eye facet # in fruit flies decrease as temperature increases

How many different alleles determine which antigen(s) are present on the surface of red blood cells?

There are three different alleles that determine which antigen(s) are present on the surface of red blood cells Allele I A, produces antigen A Allele I B, produces antigen B Allele i, does not produce either antigen

Codominance

This pattern occurs when the heterozygote expresses both alleles simultaneously without forming an intermediate phenotype. Ex: an individual carrying the A and B alleles will have an AB blood type.

Overdominance

This pattern occurs when the heterozygote has a trait that confers a greater level of reproductive success than either homozygote - Also called heterozygote advantage Ex: sickle cell trait has malaria resistance

Time and lethal alleles

Time when lethal effect is exerted can vary • Many lethal alleles prevent cell division • These will kill an organism at an early age • Some lethal alleles exert their effect later in life

Product Rule

Used for ordered events 1. Calculate the individual probabilities 2. Multiply the individual probabilities

X Chromosome inactivation in mammals depends on two things:

X-Inactivation center (Xic) and Xist (X-inactive specific transcript)

What is the Xic

Xic is a short region on the X chromosome that plays a critical role for inactivation to occur

Can some genes escape inactivation? If yes, is dosage compensation necessary?

Yes. A few genes on the inactivated X chromosome are expressed in the somatic cells of adult female mammals - Xist - Pseudoautosomal genes Up to a quarter of X genes may escape inactivation. Dosage compensation is unnecessary because these genes are located on both the X and Y chromosomes.

Does genomic imprinting involve a marking process?

Yes. The imprinting of several genes is known to involve an imprinting control region (ICR) located near the imprinted gene

Sex chromosomes in birds

ZZ male, ZW female Z, a large chromosome containing many genes W, a micro chromosome containing few genes

Phenylketonuria

a genetic disorder in which the essential digestive enzyme phenylalanine hydroxylase is missing Inability to metabolize phenylalanine. The disease can be prevented by following a phenylalanine-free diet. If the diet is not followed early in life, the result can be severe mental impairment and physical degeneration.

Genomic imprinting

a phenomenon in which expression of an allele in offspring depends on whether the allele is inherited from the male or female parent

mechanism of maternal effect in snail coiling?

a) transfer of gene products from nurse cells to egg - The nurse cells express mRNA and/or protein from genes of the D allele (green) and the d allele (red) and transfer those products to the egg. - D gene products cause egg cleavage that promotes right-handed body plan (dextral) b) maternal effect in snail coiling - Recessive d gene products cause egg cleavage that promotes a left-handed body plan. - Even if the egg is fertilized by sperm carrying the dominant D allele, the sperm's genotype is irrelevant because the expression of the sperm's gene would be too late to change early embryonic development. (this is why mother only has effect on offspring phenotype!) c) orientation of the cleavage plane in the earliest stages of development carries through to the adult

mutant alleles

alleles that have been altered by mutation - often defective in their ability to express a functional protein - rare in natural populations

lethal allele

an allele whose expression results in the death of the individual organism expressing it - These alleles are typically the result of mutations in essential genes - They are usually inherited in a recessive manner

dihybrid cross

crossing individual plants that differ in two characters

monohybrid cross

crossing two variants of the same characteristic

What genotype produces sinistral offspring?

dd

T/F: X chromosomes don't need a Xic region for inactivation to occur

false

T/F: the genotypes of the father influence the phenotype of the offspring

false

Nonessential genes

genes that are not absolutely required for survival, although they are likely to be beneficial to the organism

Does the phenotype of snails depend solely on the mother's genotype or phenotype?

genotype

What are the 7 characteristics Mendel studied?

height (tall or dwarf) flower color (white or purple) seed color (yellow or green) seed shape (round or wrinkled) pod color (yellow or green) pod shape (smooth or constricted) flower position (axial or terminal)

Blending Theory of Inheritance

hypothetical inheritance pattern in which parental traits are blended together in the offspring to produce an intermediate physical appearance - Factors that control hereditary traits are malleable - They can blend together from generation to generation - this theory was refuted by mendel's garden pea experiments

Lyon hypothesis

in females with XX genotype, one X is inactivated after zygote formed during embryonic development

monoallelic expression

in the case of imprinting, refers to the phenomenon that only one of the two alleles of a given gene is transcriptionally expressed

Simple Mendelian

inheritance of alleles that obey Mendel's laws and follow a strict dominant/recessive relationship

X-linked

inheritance of genes that are located on the X chromosome

Epistasis

inheritance pattern in which the alleles of one gene mask the phenotypic effects of the alleles of a different gene.

Lethal alleles

lethal allele is one that has the potential of causing the death of an organism - most commonly loss-of-function alleles that encode proteins that are necessary for survival

Semilethal alleles

lethal alleles that kill some individuals but not all - Environmental factors and other genes may help prevent the detrimental effects of semilethal genes

Conditional lethal alleles

may kill an organism only when certain environmental conditions prevail - Temperature-sensitive (ts) lethals: a developing Drosophila larva may be killed at 30° Celsius - Typically caused by mutations that alter structure of the protein at the nonpermissive temperature

Imprinting control region (ICR)

methylated either in the oocyte or sperm The ICR contains binding sites for one or more transcription factors that regulate the imprinted gene For most genes, methylation causes inhibition of transcription

Sandhoff disease

missing protein: Hexosaminidase B Defect in lipid metabolism. Muscle weakness in infancy, early blindness, and progressive mental and motor deterioration

Albinism

missing the protein Tyrosinase which leads to the lack of pigmentation in the skin, eyes, and hair

Character

morphological characteristics of an organism ex: eye color

Dominant Mutants

much less common than recessive mutants. 3 reasons: 1. gain-of-function: Protein encoded by the mutant gene is changed so it gains a new or abnormal function 2. dominant-negative: protein encoded by the mutant gene acts antagonistically to the normal protein 3. haploinsufficiency: mutant is loss-of-function heterozygote does not make enough product to give the wild type phenotype *In some instances, a dominant allele does not influence the outcome of a trait in a heterozygote individual (incomplete penetrance)

Lesch-Nyhan syndrome

no Hypoxanthine-guanine phosphoribosyl transferase Inability to metabolize purines, which are bases found in DNA and RNA. Leads to self-mutilation behavior, poor motor skills, and usually mental impairment and kidney failure.

Cystic fibrosis

no chloride transporters Inability to regulate ion balance across epithelial cells. Leads to production of thick mucus and results in chronic lung infections, poor weight gain, and organ malfunctions

Gene interactions

occur when two or more different genes influence the outcome of a single trait - Morphological traits such as height weight and pigmentation are affected by many different genes in combination with environmental factors

When do epigenetic changes occur?

oogenesis, spermatogenesis, or early embryonic development

epigenetic inheritance

pattern in which a modification occurs in a nuclear gene or chromosome that alters gene expression - DNA SEQUENCE DOES NOT CHANGE, so the expression is not permanently changed over the course of many generations - epigenetic changes are caused by (structural and functional) DNA and chromosomal modifications

Gene modifier effect

phenomenon in which an allele of one gene modifies the phenotypic outcome of the alleles of a different gene.

Complementation

phenomenon in which two parents that express the same or similar recessive phenotypes produce offspring with a wild-type phenotype.

wild-type alleles

prevalent alleles in a natural population - typically encode proteins that function normally and are made in proper amounts - Genetic Polymorphism can produce more than one wild-type in large populations

Tay-Sachs disease

protein not produced: Hexosaminidase A Defect in lipid metabolism. Leads to paralysis, blindness, and early death.

Pseudoautosomal inheritance

refers to the very few genes found on both X and Y chromosomes Found in homologous regions needed for chromosome pairing

Trait

specific properties of a character ex: blue eyes

gene knockout

techniques to directly generate loss-of-function alleles genetic manipulation in which one or more of an organism's genes are prevented from being expressed loss-of-function alleles often have no effect due to gene redundancy

Environmental effects on phenotype

temperature, light, nutrition, chemicals that change the phenotype without changing the genotype - Some animals like the arctic fox change coat color - Grayish brown in summer, white in winter - This is an example of a temperature-sensitive allele

Expressivity

the degree to which a trait is expressed

particulate theory of inheritance

the genetic determinants that govern traits are inherited as discrete units that remain unchanged as they are passed from parent to offspring - hereditary elements are passed on in discrete units rather than "blended" together at each new generation

purpose of dosage compensation?

to offset differences in the number of active sex chromosomes

Sex-limited inheritance

trait occurs in only one of the two sexes. An example is breast development in mammals.

T/F Researchers have found that mammalian cells can count their X chromosomes and allow only one of them to remain active

true. additional X chromosomes are converted to barr bodies.

Linked assortment

two genes always stay associated with each other

Is dosage compensation different depending on the species?

yes, depending on the species, dosage compensation occurs via different mechanisms

X-linked genes

• Hemizygous in males • Only one copy • Males are more likely to be affected

Y-linked genes

• Relatively few genes in humans • Referred to as holandric genes • Transmitted only from father to son