Studied by 0 peoplethe mechanism of inheritance
the way genes/traits are in inherited
blending inheritance
the thought that inherited traits resulted from a blend of parental characters. our modern understanding of inheritance is that traits are transmitted without blending or otherwise being altered as they pass from parents to offspring.
particulate inheritance
traits are determined by discrete units that are inherited intact through generations
Gregor Mendel (1822-1884)
Austrian Monk with training in plant breeding and mathematics. he studied the common garden pea (Pisum sativum) which breeds true for a number of traits. Mendel’s work revealed that heritable traits are specified discrete units. the units which are distributed into gametes in predictable patterns, are what are now called genes. unfortunately, his finding weren’t appreciated until after his death
transmission genetics
cytological data was used to relate chromosomal behavior during meiosis to Mendel’s principles of inheritance. This established this field: the study of how genes are transmitted from parents to offspring
Mendel’s experimental approach
Mendel’s success is attributed to his quantitative approach to experimental design and analytics. his pea plant was a model organism that was bisexual, easily self or cross pollinates, had a short generation time, inexpensive and easy to obtain, and had numerous traits that breed true.
Mendel’s hypothesis
particulate inheritance. individuals inherit two “units” of information about each trait (one unit from each parent). to test this Mendel crossed pea plants that bred true for different forms of the same trait [ seeds round v. wrinkled, seeds yellow v. green, purple or white petals, inflated or pinched fruit, unripe fruit green or yellow, axial or terminal flowers, long or short stems]
genes
heritable units of information about traits. Parents transmit these to offspring, each of these has a specific location (locus) on a chromosome
diploid cells (2n)
have pairs of genes on homologous chromosomes
mutation
a permanent change in a gene; may cause a trait to change
alleles
different forms of a gene
hybrid
offspring of a cross between two individuals that breed true for different forms of a trait
homozygous
an individual having two identical alleles for a particular gene (AA, aa)
heterozygous
an individual having two different alleles for a particular gene (Aa)
dominant/recessive
an allele is dominant if its effect masks the effect of a recessive allele paired with it. Capital letters signify dominant alleles; lowercase letter signify recessive alleles. Homozygous dominant (AA), homozygous recessive (aa), heterozygous (Aa)
gene expression
process by which information in a gene is converted to a structural or functional part of a cell or body. expressed genes determine traits
genotype
the particular alleles an individual carries; for example “Aa”
phenotype
an individual’s observable traits; for example “purple flowers” i.e. the outward appearance
P
parents
F
filial (offspring)
F1
first generation offspring of parents
F2
second generation offspring of parents
probability
a measure of the chance that a particular outcome will occur; can be expressed as a percentage (e.g. 50%) or on a scale of 0 to 1 (e.g. 0.5)
Mendel’s monohybrid experiments
crossing experiments that investigate the inheritance of a single trait (flower color); involves mating true-breeding individuals from two parent strains. Modern interpretation: monohybrid experiments investigate the inheritance of one gene with two alleles. Example: Mendel crossed plants that bred true for white flowers with plants that bred true for purple flowers; all F1 plants had purple flowers; when he crossed two F1 plants, ¾ of the F2 plants had purple flowers, ¼ had white flowers. This 3:1 pattern was found for all seven traits that he studied
Punnett Square
a grid used to calculate the probability of genotypes and phenotypes in offspring; vertical columns represent the female parent, and horizontal columns represent the male parent; the grid provides all possible random fertilization events
reciprocal cross
a breeding test that involves making crosses between a pair of parents (strain A and strain B) by using them in turn as the female and male parent. Mendel observed that F1 and F2 patterns are similar regardless of pollen and ovum source: the results of crosses are not sex dependent. Mendel proposed that the results were due to “particulate unit factors” for each of the seven traits; the factors (now called genes) serve as basic units of heredity and pass unchanged from generation to generation
test cross
a method of determining if an individual is heterozygous or homozygous dominant. Determines if an individual displaying dominant phenotype is homozygous or heterozygous for that trait. this cross always uses a homozygous recessive individual crossed with an unknown genotype individual expressing the dominant phenotype.
Mendel’s first postulate
Unit factors in pairs: genetic characters are controlled by unit factors existing in pairs in individual organisms.
Mendel’s second postulate
Dominance/recessiveness: pair of two unlike unit factors for single characteristic in individual, one unit factor dominant, the other recessive
Mendel’s third postulate
Segregation: Paired unit factors separate (segregate) randomly during gamete formation. Modern interpretation: diploid cells have pairs of genes on homologous chromosomes; the two copies separate during meiosis and end up in different gametes.
Mendel’s dihybrid experiments
Crossing experiments that simultaneously investigate the inheritance of two traits (e.g., “flower color” and “pod color”). Individuals that breed true for two different traits are crossed (AABB x aabb). F2 phenotype ratio is 9:3:3:1 (four phenotypes). Individually, each trait has an F2 ratio of 3:1. Inheritance of one trait does not affect inheritance of the other
Mendel’s fourth postulate: independent assortment
Mendel’s dihybrid experiments showed that “units” specifying one trait segregated into gametes separately from “units” for other traits. : In general, each gene is sorted into gametes independently of other genes. This is consistent with random assortment of chromosomes. Exception: Genes that have loci very close to one another on a chromosome tend to stay together during meiosis.
The product rule
The probability of two or more independent events occurring simultaneously is equal to the product of their individual probabilities. Helpful for predicting the outcome of crosses. For example, the probability of tossing two coins and getting heads on both: (Probability of heads on Coin 1 and Coin 2) = (PH1 : PH2) = (1/2)(1/2) = 1/4 = 25%
forked-line method (branch diagram)
Method used to solve crosses involving any number of gene pairs (assuming all gene pairs assort independently of each other). Easier to use than Punnett square for analysis of inheritance of larger number of traits. Uses simple application of laws of probability established for dihybrid cross. Each gene pair assumed to behave independently during gamete formation
trihybrid cross (three-factor cross)
The processes of allele segregation and independent assortment can be applied to three (or more) traits at the same time.
the sum rule
For mutually exclusive events, the probability (P) of either event occurring is the sum of their individual probabilities: P(A or B) = P(A) + P(B). For example, the probability of pulling an ace or a king from a deck of 52 cards: 4/52 + 4/52 = 1/13 + 1/13 = 2/13. In monohybrid F2 crosses (Aa x Aa), the probability of producing an offspring of dominant phenotype is: P(A-) = P(Aa or AA) = 2/4 + 1/4 = 3/4 or 75% chance.
Mendel rediscovered
Mendel published his work in 1866. His work was often cited, yet the significance was not recognized until 35 years later
Walter Fleming
described chromosomes in salamander nuclei in 1879. this set the stage for additional discoveries
Walter Sutton and Theodor Boveri
in 1902 these cytologsists independently published findings on chromosome behavior during meiosis, formalized as the Chromosome theory of inheritance
Chromosome Theory of Inheritance
Inherited traits (1) are controlled by genes residing on chromosomes, (2) are faithfully transmitted through gametes, and (3) maintain genetic continuity from generation to generation
Pedigree
A chart similar to a family tree. Shows genetic relationships among family members over two or more generations. Pedigree analyses reveal patterns of inheritance for traits under investigation (for example: dominant, recessive, sex-linked, etc.)
pedigree symbols
proband
individual for whom a pedigree was constructed
transmission genetics
has two main postulates: (1) Genes are present on homologous chromosomes, and (2) chromosomes separate from each other and assort independently during gamete formation. Classic ratios are 3:1 (monohybrid crosses) and 9:3:3:1 (dihybrid crosses). These ratios are modified when gene expression does not adhere to simple dominant/recessive mode, or when more than one pair of genes influences a single character.
alleles
alternative forms of the same gene
wild type (normal)
allele that occurs more frequently in population; often but not always dominant.
mutations
the ultimate source of new alleles; these type of alleles contain modified genetic information and often specify an altered gene product
loss-of-function mutation
mutation in gene causes change in conformation enzyme, thus, changing affinity. If loss is complete, the mutation results in a null allele
gain-of-function mutation
Some mutations may enhance function of wild-type allelic function; increases quantity of gene product by affecting regulation of transcription of gene.
neutral mutation
the gene product present no change to phenotype
standard conventions for allele symbols
(1) Initial based on recessive trait; (2) lowercase for recessive, uppercase for dominant; (3) alleles are italicized. Example: Dwarf: d, Tall: D. In the absence of dominance, other methods are used. Uppercase italic letters and superscripts are used to denote alternative alleles. Examples: R 1 and R 2 , L M and L N , I A and I
genetic nomenclature
this diverse system is used to o identify genes in various organisms. The symbols reflect the function of a gene or the disorder caused by mutant gene. Examples: cdk for cyclin dependent kinase gene (involved in cell-cycle regulation in Yeast); leu is a mutation that interrupts the synthesis of leucine (wild-type gene, designated leu+ ); dnaA: bacterial gene involved in DNA synthesis; Capital letters are used to name genes in humans: BRCA1: Gene associated with breast cancer.
incomplete/ partial dominance
One allele is not fully dominant; each genotype has its own phenotype. The heterozygote’s phenotype is somewhere between the two homozygotes, resulting in a 1:2:1 phenotype ratio in F2 offspring. Example: Snapdragon color: RR is red, rr is white, and Rr is pink.
Tay-Sachs disease
an example of incomplete dominance in humans. a biochemical disroder. Homozygous recessives are affected by a fatal lipid-storage disorder. Hexosaminidase A activity is absent. The enzyme is involved in lipid metabolism. Normal heterozygotes: one copy of mutant gene, and 50% enzyme activity compared to homozygous normal noncarriers.e (autosomal recessive): Lipid metabolism disease involving abnormal enzyme hexosaminidase A. Newborn is normal for few months, then developmental disability, paralysis, blindness, and death by age 3.
multiple allele systems
Genes with three or more alleles in a population. A diploid individual can only have two alleles per gene, but in a population, there may be more than two alleles. Examples: Eye color in fruit flies; ABO blood types in humans. Many alleles are known to occur for practically every locus in fruit flies (Drosophila). A recessive mutation that causes white eyes was discovered in 1912. Over 100 other alleles have since been found; eye colors range from white (no pigment) to deep ruby to orange to sepia
codominance
two nonidentical alleles are both fully expressed in heterozygotes; neither is dominant nor recessive. Examples: ABO blood types, MN blood groups
blood types in humans
Blood type is due to genetically determined differences in molecules on the surface of red blood cells. These surface labels are called antigens. Our immune systems make antibodies (Y-shaped proteins) that target non-self antigens. There are three different alleles in human populations: I A , I B , and i; I A , I B are codominant: both alleles are expressed in heterozygotes (genotype I A I B , blood type AB). A person with type AB can receive blood from any donor (universal recipient), since they lack anti-A and anti-B antibodies. A person with type O is a universal donor, since their blood does not present any surface antigens. However, since a person with type O blood has neither A nor B antigens, their immune system treats both type A and type B cells as foreign; therefore, they can only receive type O. MN blood group in humans: characterized by antigen glycoprotein found on surface of red blood cells. Two forms exist (M and N), resulting in three blood types: M, N, and MN. Autosomal locus on chromosome 4, with two alleles: L M and L N .
lethal alleles
mutations resulting in nonfunctional gene product. these types of alleles reveal essential genes (required for survival). Mutations resulting in nonfunctional gene product can be tolerated in heterozygous individuals. One wild-type allele is sufficient for survival. Homozygous recessive individuals do not survive. Some lethal alleles result in a distinctive mutant phenotypes. For example, a mutation in mice causes yellow coat color; varies from normal agouti (wild-type) coat phenotype: homozygous yellow coats are lethal (mice die before birth); heterozygous individuals have yellow coats (mutation behaves dominant for coat color)
dominant lethal allele
one copy results in death. Example: Huntington disease. Allele H. Onset of disease in heterozygotes (Hh) delayed; late onset around age 40; gradual nervous and motor degeneration.
gene interactions
Phenotypes controlled by multiple genes. Gene interactions: phenotypic characters are sometimes influenced by many different genes. Not necessarily due to direct interaction of genes with each other. Multiple genes may contribute products that function together or in combination.
epistasis
when two or more gene products influence a trait. Typically, one gene product suppresses the effect of another. Example: Coat color in dogs. Interaction between genes B and E. Alleles B and b designate coat color (black or brown/chocolate); recessive allele e suppresses color (resulting in a yellow coat) in homozygotes (ee)
pleiotropy
one gene product influences two or more traits. Example: Marfan syndrome (connective tissue protein: fibrillin), sickle cell anemia, Phenylketonuria (PKU; toxic buildup of amino acid phenylalanine), Variegate porphyria (an autosomal dominant disorder: inadequate metabolism of porphyrin results in toxic buildup; numerous phenotypic effects: abdominal pain, muscular weakness, fever, racing pulse, insomnia, vision issues
phenotype=Genotype + environment
the phenotypic expression of a trait is influenced by genotype and the environment.
penetrance
is the percentage of individuals with the mutant genotype who actually exhibit the mutant phenotype. For example, if 25 percent of mutant individuals exhibit the wild-type phenotype, then the mutant gene is said to have a penetrance of 75 percent. In contrast, expressivity reflects the range of expression of the mutant phenotype. For example, flies homozygous for the mutant eyeless gene yield phenotypes that range from presence of normal eyes to absence of one or both eyes. Both penetrance and expressivity appear to be the result of a combination of background genetics and environmental effects.
position effects
the physical location of gene in relation to other genetic material may influence its expression. For example, translocation or inversion events modify expression. If a gene is relocated to condensed or genetically inert chromosome (heterochromatin), it may be silenced.
temperature effects
For example, Siamese cats and Himalayan rabbits have darker fur on cooler areas of the body (tail, feet, and ears). This is because certain enzymes lose catalytic function at higher temperature. Tyrosinase is a copper-containing enzyme present in plant and animal tissues that catalyzes the production of melanin. The enzyme works at low temperatures. Black fur absorbs light and solar heat
onset of genetic expression
Some genetic traits become apparent at different times during an organism’s life span. Prenatal, infant, preadult, and adult phases in humans require different genetic information. Many severe inherited disorders are not manifested until after birth.
Lesch-Nyhan disease
(X-linked recessive): Abnormal nucleic acid metabolism leads to increased uric acid in blood and tissues. Intellectual disability, palsy, and self-mutilation of lips and fingers; due to mutation of gene-encoding enzyme HGPRT. Newborns normal for 6–8 months prior to onset of first symptoms.
Duchene muscular dystrophy
y (DMD): X-linked recessive; associated with progressive muscular wasting; diagnosis 3–5 years of age; fatal in early 20s.
Huntington disease
e (autosomal dominant): Affects frontal lobes of cerebral cortex; progressive cell death occurs over period of more than a decade; brain deterioration accompanied by spastic uncontrolled movements, intellectual and emotional deterioration, and ultimately death; onset around age 45.
genetic anticipation
Genetic disease has earlier onset and increased severity with each succeeding generation. Example: Myotonic dystrophy (DM)
Myotonic dystrophy (DM)
Most common type of adult muscular dystrophy. Autosomal dominant. Mildly affected individuals develop cataracts as adults, but little or no muscular weakness; severely affected individuals demonstrate more extensive myopathy and may be intellectually disabled. Increased severity and earlier onset with successive generations of inheritance. Explanation for DM: A particular region of the DM gene (a short trinucleotide DNA sequence) is repeated a variable number of times, and unstable. Normal individuals have 5 copies; mildly affected individuals have 50 copies; and severely affected copies have over 1000 copies. The size of the repeated segment increases in successive generations.
Extranuclear Inheritance
Inheritance patterns that involve genetic material outside of the nucleus, resulting in variation from traditional biparental inheritance. Examples: organelle heredity (due to genes in mitochondria and chloroplasts) and maternal effects (due to extranuclear genetic information transmitted in the cytoplasm of the maternal gamete). Following fertilization, developing zygote’s phenotype can be influenced by gene products produced by the mother and transmitted from maternal parent via ooplasm.
heteroplasmy
y describes the situation in which two or more organelle variants exist within the same cell, and the organelle populations are variable among cells. A given cell may or may not have mutant genes in organelles; thus, the phenotype may not be revealed in every cell or tissue.
chloroplast heteroplasmy
White, green, and variegated (white and green) leaves occur in the Four-O’Clock Plant (Mirabilis jalapa). The maternal plant determines leaf coloration; related to presence of normal/mutant chloroplasts. White areas contain no chlorophyll.
mitochondrial heteroplasmy
Poky in Neurospora and Petite in Saccharomyces. Neurospora crassa (bread mold): slow-growing mutant strain pokey had impaired mitochondrial function. Saccharomyces cerevisiae (yeast): deficiency in cellular respiration, indicating mutations in mitochondrial DNA, results in smaller colonies.
human mitochondrial genome
contains 16,569 base pairs. Gene products include 13 proteins required for aerobic cellular respiration, 22 transfer RNAs (tRNAs) required for translation, and 2 ribosomal RNAs (rRNAs) required for translation. Mitochondrial disorders in humans: MERRF: myoclonic epilepsy and ragged-red fiber disease: Disorder due to mutation in mitochondrial genes: the gene encoding tRNA^Lys . Interferes with translation, leading to disorders. Affected cells exhibit heteroplasmy: contain mixture of normal and abnormal mitochondria
maternal effect (maternal influence)
Offspring’s phenotype of particular trait under control of mother’s gene products present in egg ooplasm. Nuclear genes of female gamete are transcribed, and the genetic products accumulate in egg ooplasm: the genetic products get distributed among new cells, and potentially influence patterns or traits in early development. Example: Embryonic development in Drosophila. Protein products of maternal-effect genes may in turn activate other genes, ultimately leading to normal embryo. Upon fertilization, the maternal gene products specify molecular gradients: determine spatial organization and development of the embryo.
Bicoid (bcd) gene
responsible for anterior development of fly. Embryos from mothers that are homozygous recessive fail to develop anterior areas; heterozygous mothers develop normally, even if genotype of embryo is homozygous recessive. Thus the genotype of female parent, not genotype of embryo, determines the phenotype of offspring. maternal effect.
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