Genetics Exam 2 (Ch. 4,5,7)

What are features of Turner Syndrome?

(45, X0) Phenotypically female; female external genitalia and internal ducts; ovaries are rudimentary; underdeveloped breasts; short stature and cognitive impairment

What are the features of Klinefelter Syndrome?

(47, XXY) tall, long arms and legs, large hands and feet, internal ducts are male, rudimentary testes fail to produce sperm, feminine development not suppressed; enlarged breast common and rounded hips

Coupling heterozygote vs. Repulsion heterozygote

Coupling heterozygote-dominant alleles on both chromosomes Repulsion heterozygote-mixed dominant/recessive alleles on chromosomes

What is the impact of X inactivation in humans?

1) Females are mosaics for all heterozygous X-linked alleles. 2) Red-green color blindness is an X-linked recessive disorder. 3) Hemizygous males are fully color-blind in all retinal cells 4) Heterozygous females display mosaic retinas, with patches of defective color perception and patches of normal perception

What two postulates are the basic principles of gene transmission?

1) Genes are present on homologous chromosomes 2) Chromosomes segregate and assort independently

What are the 4 types of mutations?

1) Loss of function mutation-gene encodes for an enzyme but mutation causes reduction or elimination of that enzyme 2) Null allele-complete loss of enzyme function (Tay-Sachs disease) 3) Gain of Function mutation- Mutation enhances function of wild type, quantity of gene product increases (for example, conversion of proto-oncogenes which regulate cell cycle to oncogenes where regulation is overridden by excess gene product which leads to a cancerous cell) 4) Neutral mutations- No change to the phenotype, no change to the organisms evolutionary fitness

What are the three criteria of three-point mapping?

1) Parent must be heterozygous for all three genes under consideration. 2) Phenotypic class must reflect genotype of gametes of parents. 3) Sufficient number of offspring must be produced for representative sample. The three-point testcross is a powerful technique that can be used to map the location of a gene with respect to two other genes. Performed enough times, this can be used to map an entire chromosome.at

What are fixes to an imbalance (unequal dosage of x -linked genes in male/female) in gene number?

1) X-linked genes work half as hard in females than in males 2) X-linked genes work twice as hard in males than females 3) one copy of the x chromosome is inactivated (humans)

What are the 3 forms of reproduction?

1) asexual-bacteria, archaea, unicellular eukaryotes 2) sexual-most diploid eukaryotes; involves meiosis, formation of gametes, and fertilization 3) sexual differentiation-male and female; evident via phenotypic dimorphism

What are some mutations that are not visible?

1) sterile mutations-mutations that prevent reproduction 2) lethal mutations-mutations which interfere with vital functions. Essential genes are required for survival; if the gene is heterozygous is can be tolerated but homozygous recessive will not survive. Both impact the phenotypic ratio among offspring

What does it mean to have more than 2 alleles? i.e., 3 alleles?

3 different heterozygous genotypes and 3 different homozygous genotypes

What is required for proof that genes are located on chromosomes?

A certain gene needs to be unambiguously linked to a certain chromosome, the gene must be defined by a mutant allele and the chromosome can be distinguished morphologically, and the pattern of gene transmission has to reflect chromosome behavior during reproduction

What is interference?

A crossover in one region inhibited a crossover nearby

What causes Tay-Sachs disease?

A null allele mutation located on chromosome 15 and encodes for the alpha subunit of Hex-A enzyme. TSD results from the loss of activity of a single Hex-A enzyme. Without functional Hex-A, the lysosomes don't break down GM2 in nerve cell membranes so GM2 aggregates within the neurons in the brain. Heterozygous carriers (Aa) only produce 50% of the normal amount of Hex-A but this is not enough to show symptoms of the disorder.

What is an example of penetrance?

A recessive disease is 70% penetrant. This means an individual with genotype aa is 70% likely to develop the disease. Therefore 70% of people will show symptomes will 30% will not show symptoms because of reduced or incomplete penetrance.

What are characteristics of Huntington disease?

Allele H is a dominant lethal allele (rare) in humans; one copy of this allele results in death by progressive degeneration of nervous system, dementia, and early death. Onset of disease is about age 40. To persist in a population, affected individuals must produce before the lethal allele is expressed. If all affected individuals die before reaching reproductive age, the mutant gene will not be passed onto the next generation and disappears from the population

What proves that alleles can change phenotypes in different ways?

Alleles are transmitted from parent to offspring following Mendelian rules, but they often do not display a dominant/recessive relationship that follows Mendelian rules

What are heteromorphic chromosomes?

Are dissimilar (sex chromosomes X and Y in mammals)

What are caveats of gene mapping?

As the distance between two loci increases, inaccuracy of the estimate increases. This is because double crossovers are not always detected.

How is sex determined in reptile species?

By the incubation temperature of the eggs during embryonic development

What are sex chromosomes?

Characterize one sex or the other in a wide range of species

What are features of a human karyotype?

Chromosomes are labelled by size, number of chromosomes is constant for a species; humans have 23 pairs: 22 autosomes and 2 sex chromosomes

What are characteristics of the threshold effect?

Common in enzyme disorders, normal phenotypic expression results, certain level (50% or less) of gene product is attained, it is the most common and has greater than 50% threshold in Tay Sachs disease

What are alleles?

Different versions of a gene that impact how a gene is expressed. If the gene is expressed (translated) into a protein, an alternative allele might be expressed into a different gene product (a different protein). Different proteins (or lack of proteins) impact the phenotype differently.

What are the symbols for dominant, recessive, mutant, and wild type alleles?

Dominant- one italic uppercase letter or uppercase letter followed by a lowercase letter (D) or (Wr) Recessive- one italic lowercase letter or two italic lowercase letters (d) or (wr) Mutant- italic letter (e) or e/e Wild type- italic letter and plus (e+) or +/e

Sex-limited inheritance vs. sex-influenced inheritance

Even if genes are not located on a sex chromosome, patterns of gene expression can be affected by the sex of an individual. Sex-limited inheritance: expression of a phenotype is limited to one sex Sex-influenced inheritance: expression of a phenotype is influenced by sex, but not limited to one sex or the other.

What are the characteristics of pleiotropy?

Expression of a single gene has multiple phenotypic effects. One example is Marfan syndrome which is a genetic disorder of connective tissue; a single autosomal dominant mutation in gene that encodes the protein fibrillin and results in multiple phenotypic effects: affected individuals tend to be tall, thin, with long arms, legs, fingers, and toes, flexible joints, and scoliosis.

What is an example of variable expressivity?

Eye color-The production of the protein Melanin varies; brown eye color is fully penetrant because we all have some amount of brown in our eyes but the expressivity of brown eyes is variable. Eyes with very little melanin in the iris are blue (there is no blue pigment). Eyes with a bit more melanin are green hazel or light brown and those with high concentration of melanin are medium or dark brown. B brown and b are dominant and recessive and follow Mendelian rules but with variable expressivity

What is an example of sex-limited inheritance?

Feather plumage in chickens: Hen feathering is controlled by a dominant allele expressed in BOTH sexes, while cock feathering is controlled by recessive alleles expressed ONLY in males. Actual expression can be modified by the individual's sex hormones.

What characterizes sex chromosomes in humans?

Females are XX Males are XY During meiosis in males, the X and Y chromosomes pair like homologous chromosomes. Sex chromosomes are heteromorphic. Autosomes are the same for the two sexes.

What are characteristics of X-linked inheritance in pedigrees?

Focus on hemizygosity; an X-linked recessive allele is always expressed in males, but never passed from father to son. Homozygous females pass the trait to all songs but not to their daughters unless the father is also affected.

What are observable outcomes of crossing over?

Formation of chiasmata in late prophase (can be seen in cytological analysis). Recombination between genes on opposite sides of the crossover point (cannot be seen directly, but seen indirectly in the next generation)

In humans what do capital italicized letter like BRAC1 mean?

Gene names

What are characteristics of gene action?

Genes act in the context of the genotype/in the context of other genes. Genes also act in the context of the environment. Genes function in a biological and physical environment. Examples are Pattern baldness (sex), temperature sensitive alleles, conditional alleles, and phenylketonuria (diet).

Linkage vs. Recombination vs. Crossing over

Genes that are located on the same chromosome are inherited together; they are linked like color gene and length gene (all genes on a chromosome are a linkage group); alleles of chromosomally linked genes can be recombined by crossing over which results in recombination. The frequency of crossing over on a single chromosome is proportional to the distance between them.

What is the allelic dominance hierarchy?

Heterozygous combinations show that alleles are not equally dominant (or equally recessive). c+ is dominant over c^ch, c^h, and c. c^ch is partially dominant over c^h and c. c^h is dominant over c. c is recessive to everyone. Summary: c+>c^ch>c^h>c

Homogametic sex vs. Heterogametic sex

Homogametic-producing like chromosomes, zygotes with two X chromosomes, results in female offspring Heterogametic-producing unlike chromosomes, zygotes with one X and one Y chromosome, results in male offspring

What are the 3 different alleles for blood type?

IA (dom) IB (dom) and i (rec)

What are patterns of inheritance of sex-linked genes in humans?

If genes are located on the X or Y chromosomes, inheritance deviates from Mendelian principles. A male needs to inherit only one recessive allele to be affected by an X-linked trait (males are hemizygous). In contrast, a female needs to inherit two recessive alleles to be affected. Females can be carriers; if they inherit one recessive allele they are not affected, but they can pass on the allele to their sons and daughters

How do you find the distance between two genes?

If the recombination frequency is .18 then that mean that there are 18 map units (mu) which is equal to 18 centiMorgans (cm). 1 centiMorgan=1% recombination. Genes that are closer to each other on the same chromosome have a lower recombination frequency (because crossover is less likely to happen).

What is an example of epistasis?

In Labs, coat color depends on two genes: one determines pigment color and the other determines whether the pigment will be deposited in the hair

What is the principle of independent assortment?

In the case of 2 characters, each pair of alleles segregates independently of the other pair of alleles during gamete formation. The law of independent assortment depends on alternative orientations of chromosomes in metaphase 1. Creates 4 possible gametes with equal ratios 1:1:1:1

What are characteristics of carriers?

Individuals that are carriers are always female; the heterozygous genotype for a sex-lined trait is a carrier (because the recessive allele is usually the one that causes problems--this means that the heterozygotes are normal but they can pass on the recessive allele). A female must recieve the allele from both parents to be affected. The probability of affected females is higher when inbreeding occurs.

What is polygenic inheritance?

Inheritance pattern where a single character can be determined by multiple genes; there can be additive effects. For example, height and skin color in humans works this way.

What are the characteristics of Tay-Sachs disease?

It is a rare autosomal disorder where two unaffected heterozygous (Aa) parents have a 1/4 probability of having a child with TSD (aa). Infants are unaffected at birth until 6 mo old when progressive loss of mental and physical state leads to death by age 2-5.

What is chromosome mapping?

Linked genes can be mapped on a chromosome by studying how often their alleles recombine. Sturtevant estimated the distance between points on a chromosome by counting the number of crossovers between them. Points far apart should have more crossover between them than close together points. •Statistically, we measure the average numbers of crossovers in a particular chromosome region

What is an example of sex-influenced inheritance?

Male pattern baldness in humans: The sex of an individual determines how an allele works; Allele B behaves dominant in males and recessive in females: in BB genotype in females the phenotype is less pronounced

Why is sex in the real world not 50% male and 50% female?

One sex might have higher mortality due to infanticide, x-linked diseases, etc.

What are mutant alleles?

Modified genetic information that causes altered gene product. Arises through mutations and is responsible for the mutant phenotype.

What is a chromosome map and who first made them for drosophila?

Morgan and colleagues were able to determine which genes are located on which chromosomes. They drew maps implying that a chromosome is a linear array of genes.

What are visible mutations?

Mutations which change morphology; most of them are recessive.

What is codominance?

Neither allele is dominant; the heterozygote expresses the phenotypes of both homozygotes (RR + rr=Rr)

Nonrecombinant vs. Recombinant gametes

Nonrecombinant-parental, more common Recombinant-nonparental, through crossing over, less common

What are nutritional effects on phenotype expressivity?

Nutritional mutations prevent synthesis of nutrient molecules in microbes; the phenotype is expressed or not depending upon diet. In humans, phenylketonuria causes loss of enzyme to metabolize amino acid phenylalanine which causes severe problems unless the person eats a low-Phe diet. Lactose intolerance is another example because the body cannot metabolize lactose.

What is special about types O and AB blood?

O is a universal donor, AB is a universal recipient

Nature vs nurture

Ongoing debate between how much influence genetics vs. environments have. Both contribute but at different ratios for bipolar disorder and eating disorder.

MSY vs. SRY Regions

Pairing regions critical to segregation of X and Y chromosomes during male gametogenesis. MSY is male specific region of the Y (nonrecombining region of the Y chromosome). SRY is sex determing region Y. It is located adjacent to PAR of the short arm of Y chromosome. Controls male development, encodes TDF protein, and encodes protein that triggers testes formation. At 6-8 weeks of develoment, SRY gene becomes active in XY embryos

What is the principle of segregation?

Pairs of alleles separate during gamete formation; fusion of gametes during fertilization creates allele pairs again. The law of segregation depends on separation of homologous chromosomes in anaphase I.

Penetrance vs. Expressivity

Penetrance-percentage of expression of the mutant genotype in a population Expressivity-Range of expression of mutant phenotype and result of genetic background differences and/or environmental effects

What are characteristics of gene-gene interactions?

Phenotypic characters can be influenced by many different genes and their products; it does not necessarily mean two or more genes directly interact with each other. Cellular function of numerous gene products contributes to development of common phenotype. For example, in humans over 50 genes are involved in ear development: mutations that interrupt the developmental steps can lead to the common phenotype of hereditary deafness in humans.

What are characteristics of phenotypic expression of traits?

Phenotypic expression is influenced by genotype and by the environment

What is the founder effect?

Polydactyly in Amish population; one immigrant carried a mutation for Ellis-van Creveld syndrome that results in polydactyly so today 5 of 1000 amish babies have this syndrome compared to 1 in 60,000 in the general population

What is an example of incomplete penetrance?

Polydactyly-This dominant mutation manifests only in some carriers; some individuals do not express the trait even though they have the appropriate genotype.

What are characteristics of inbreeding?

Probability of being homozygous for a recessive, harmful allele is increased and inbred lines can suffer from inbreeding depression (loss of vigor)

What are PARs?

Pseudoautosomal regions; present on both ends of a Y chromosome, share homology with regions on X chromosome, synapse and recombine with X during meiosis

How are recombination frequencies and genetic map distances used together?

Recombination frequencies can be used to infer genetic maps. We can estimate where crossover happened. With that we can estimate the distance between genes and map them. But: this only works as long as the genes are close together. When they are very far part, recombination frequency may not reflect the true map distance because of undetected multiple crossovers between two far apart genes. Distances of 20 cM or less are good estimators of the true genetic distance

What if a gene is located on a sex chromosome?

Sex and trait cannot be treated as two independent characters in a dihybrid corss because sex chromosomes do not behave like autosomes. Females can be homozygous or heterozygous but males cannot because they are hemizygous (XY)

How do you determine the gene sequence during mapping?

Single crossovers-used to determine distance between two linked genes Double crossovers-have double exchanges of genetic material, used to determine distance between three linked genes, and genes must be heterozygous for two alleles. These are two single independent events, so multiply the probabilities together.

What is the underlying basis for sex determination?

Specific genes, NOT entire chromosomes. Most of genes are located on the sex chromosomes but some are autosomal (located on autosomes)

What are examples of conditional mutations?

Temperature effects Evening primrose to cause different colors at different temperatures; siamese cats and himalayan rabbits have darker fur on cooler areas of the body. Their enzymes lose catalytic function at higher temperature.

What did Klinefelter and Turner syndromes lead scientists to conclude?

That the Y chromosome determines maleness in humans (Y is dominant over X)

What did Morgan prove?

That the gene for eye color was on the X chromosome of fruit flies; with that he was able to correlate the inheritance of that gene with the transmission of the X chromosome during reproduction

What is the Lyon hypothesis?

The X chromosome that is inactivated is chosen randomly during early embryonic development

What are wild type alleles?

The allele which normally occurs in a population, most frequently, often (but not always) dominant. It is responsible for the wild type phenotype.

What are characteristics of epistasis?

The expression of one gene masks/modifies the phenotypic effect of another gene pair. Each step of development increases complexity of the organ; the expression is under the control and influence of many genes

What causes deviation from Mendelian genetics?

The law of independent assortment

What is recombination frequency?

The number of recombinant offspring/total number of offspring. For any two genes, recombination frequency cannot exceed 50%. Above 50% would mean the two genes are on two different chromosomes and assort independently. Below 50% would imply that the two genes are on the same chromosome (they are linked). Smaller frequency=closer the two genes are located to each other (crossing over is less common)

What are the characteristics of incomplete or partial dominance?

The phenotype of the heterozygote is midway between the phenotypes of the two homozygotes. One allele is partially, or incompletely, dominant over the other.

What determines sex in Drosophila?

The ratio of X chromosomes to autosomes (rather than Y chromosome)

What are caveats of using genetic map distances?

The true genetic distance depends on the average number of crossovers on a chromosome. Multiple crossovers can occur between two genes far apart. Some of these crossovers donot produce recombinant chromosomes.

How does the wrinkled gene show that the lack of enzyme/protein impacts the phenotype?

The wild type allele codes for the starch branching enzyme, while the mutant allele does not code for that enzyme. The lack of the starch branching enzyme causes a wrinkled appearance (more sucrose leads to water loss which leads to wrinkles)

Why are fruit flies one of the most popular genetic model organisms?

They are small, easy to maintain, and reproduce quickly. Mutant white eyes were first found in 1910 by Morgans wife and undergraduate Calvin Bridges

What did Bridges discover when he crossed exceptional white eyed females with red eyed males?

They produced exceptional white eyed females; he concluded that the exceptional flies were the results of abnormal X chromosome behavior during meiosis (later named nondisjunction, when X chromosomes occasionally fail to separate)

Who discovered the Chromosome Theory of Heredity? What does it state?

Thomas Hunt Morgan (identification of genes) and Edmund Beecher Wilson (movement of chromosomes). It states that all genes are located on chromosomes, Mendel's principles can be explained by the transmission of chromosomes during reproduction, and the meiotic behavior of chromosomes is the basis for Mendel's Principles of Segregation and Independent Assortment.

If there are two genes which do not follow Mendelian inheritance, then...

Two modes of inheritance can happen at the same time; Mendel's law of independent assortment still applies, but the phenotypic ratio will deviate from 9:3:3:1. This results in many variants of modified ratios

What are characteristics of variable expressivity? (expressivity is the degree, magnitude, or severity of a phenotype)

Variable expressivity-a trait is not manifested uniformly among individuals that show it. For example, all flies are heterozygous for the dominant mutation but the phenotypes vary from small eye to nearly wild type eye

What is the Chromosomal Theory of Inheritance?

Walter Sutton measured the 22 chromosomes of the lubber grasshopper and found 11 species sizes. He followed the chromosomes through cell division and observed that they paired up in the first meiotic division. Gametes only get 11 chromosomes and upon fertilization the chromosomes pair again. He concluded that chromosomes exist as identical pairs which constitutes the physical basis of the Mendelian Law of Heredity

What are two examples that show that the lack of or code for different proteins causes different phenotypes?

Wrinkled gene and Tay-Sachs Disease

What is a tetrad?

a pair of homologous chromosomes with a total of 4 homologous chromatids

What are recombination events dependent on?

chromosome length, position of centromere, species dependent, and many other factors

What genes are on human sex chromosomes?

more than 1000 genes on the female X chromosome; 397 possible genes on the male Y chromosome but fewer than 100 seem to be function (required for fertility), some genes are present on both X and Y mostly in the terminal region (pseudoautosomal genes)

How many alleles for eye color gene locus does drosophila have?

over 100 alleles (brick red is wild type/white is mutant allele as pigments do not get transported and deposited into the units of the compound eye)