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Chapter 14 & 15 - Coggle Diagram
Chapter 14 & 15
14.4 - Many human traits follow Mendelian patterns of inheritance
Recessively Inherited Disorders
Heterozygotes can pass their recessive allele to offspring and are therefore called carriers
Sickle-Cell Disease
Affects one out of 400 African Americans
Substitution of a single amino acid in the hemoglobin protein of red blood cells; in low oxygen envrionments, proteins aggregate into long fibers that conform the cell into a sickle-cell shape
Cells clump and clog blood vessels, causing weakness, pain, organ damage, etc.
Incompletely dominant; heterozygous individuals (1/10 African Americans) may suffer some symptoms
Cystic Fibrosis
4% of people with European descent are carriers
Allele coding for membrane protein involved in the transport of chloride ions is defective
Causes buildup of thick, sticky mucus in lungs, digestive tract, pancreas, etc.
With antibiotics and various therapies, victims can now on average live into 40s
Dominantly Inherited Disorders
Achondroplasia
Type of dwarfism
%99.99 of the population is homozygous recessive in this instance
Huntington's Disease
No observable effect until ages 35-40
Degenerative disease of the nervous system; irreversible and fatal
Affects 1/10,000 people
Multifactorial Disorders
Disorders with both a genetic and environmental influence
Ex. Heart disease, diabetes, cancer
Genetic Testing and Counseling
Inheritance of genetic complications may be determined before pregnancy/during early stages with mendelian genetics, probability rules, carrier tests, fetal tests, and newborn screening
14.1 - Mendel used the scientific approach to identify two laws of inheritance
Mendel's Experimental, Quantitative Approach
Character
Heritable feature that varies among individuals
Ex. flower color
Trait
Variants of a character
Ex. purple
True breeding
Describes production of the same variant over several generations
Hybridization
The crossing of two true-breeding varieties
F1 generation
Offspring of P1; filial generation
F2 generation
Offspring of self-pollinated or cross-pollinated F1 hybrids - second filial generation
3:1 ratio
P generation
The true-breeding parents; parental generation
The Law of Segregation
Alternative versions of genes account for variations in inherited characters
Ex. purple and white flowers
Alleles
Sequence of nucleotides at specific locus on chromosome
An organism inherits two alleles of a gene - one from each parent
Dominant allele
Determines organism's phenotype
Recessive allele
No noticeable effect on phenotype unless duplicated
A somatic cell in a diploids organism inherits two sets of chromosomes (one from each parent) - therefore a locust is represented twice
The law of segregation: the two alleles for a heritable character segregate during gamete formation and end up in different gametes
Egg/sperm only receive one allele per character present in diploid organism
Distribution of copies from homologous chromosomes during meiosis
Heterozygous
Two different alleles
Ex. Pp
A cross between heterozygotes is a monohybrid cross in reference to a particular character
Homozygous
Pair of identical alleles
Ex. homozygous dominant: PP
Genotype
Genetic makeup
Phenotype
Physical appearance
Punnett square: diagrammatic device for predicting allele composition
The Law of Independent Assortment
Monohybrid
Organism heterozygous for a specified character
Ex. Pp
Dihybrid
Ogranism heterozygous for two specified characters
Ex. YyRr
Alleles segregate independently of other alleles during gamete formation
Applies only to genes located on different chromosomes, or genes that are very distant from one another on the same chromosome
14.2 - Probability laws govern Mendelian inheritance
Multiplication rule
Probability of two separate events can be multiplied to derive the probability of both events occurring
Ex. 0.5 x 0.5 = 0.25 (25% chance of both events occuring)
Addition rule
Probability of one mutually exclusive event will occur can be derived by adding individual prooperties
Ex. 0.25 + 0.25 = 0.5 (50% chance of either event occuring)
Solving Complex Genetics Problems with the Rules of Probability
Uses probability laws and rules to determine likelihood of genetic inheritance; eliminates need for elaborate Punnett squares
Ex. probability of YYRR = 1/4(probability of YY) x 1/4(probability of RR) = 1/16
15.1 - Mendelian inheritance has its physical basis in the behavior of chromosomes
Chromosome theory of inheritance
Mendelian gene shave specific loci; the chromosomes undergo segregation/independent assortment
Hereditary factors were theoretical until chromosomal processes of mitosis and meiosis were discovered
Morgan's Choice of Experimental Organism
Thomas Hunt Morgan was the first to provide solid evidence
Used a species of fruit fly, Drosophila melanogaster, for his study; bred multiple generations
Species produced several offspring, can produce generations every two weeks, and have only four pairs of chromosomes
Wild type
The most common observed phenotype in natural populations of species
Ex. red eyes in Drosophila meleanogaster
Mutant types
Traits alternative to the wild type
Ex. white eyes in Drosophila melanogaster
Correlating Behavior of a Gene's Alleles with Behavior of a Chromosome Pair: Scientific Inquiry
When Morgan bred the white eyed male with a red eyed female, half of the male offspring had white eyes, and none of the female offspring had white eyes
Suggested that the mutation occurred on exclusively on the x chromosome
Morgan's experiment supported Chromosomal theory of inheritance (specific genes carried on specific chromosomes)
15.2 - Sex-linked genes exhibit unique patterns of inheritance
The Chromosomal Basis of Sex
Two XX chromosomes code for female
XY chromosomes code for male
Only regions of the Y chromosome homologous with the X are short segments on either end
Homologous regions allow for pairing and homolog behavior during meiosis
Sex-linked genes
A gene located on either sex chromosome
X-linked gene
Genes located on X chromosome
About 1,100 genes
Y-linked gene
Genes located on Y chromosome
78 identified genes
Inheritance of X-Linked Genes
If a trait is caused by a recessive allele on the X-chromosome, a female will only express the phenotype if she is homozygous for that allele
If a male receives a recessive X-linked gene, he will automatically express that gene, as he has no other chromosome to potentially mask the allele
Not homozygous or heterozygous, but hemizygous
Much more common in males
Color blindness
Duchenne muscular dystrophy
Affects 1/5000 males born in U.S.
Progressive weakening of muscles and loss of coordination
Life expectancy of mid 20's
Lack of muscle protein called dystrophin
Hemophilia
Absence of protein involved in blood clotting
Causes prolonged bleeding in case of injury
X Inactivation in Female Mammals
Almost all of one X-chromosome present in a female becomes inactive in early development
Barr body
Condensed inactive chromosome
Chromosome which condenses is random in each cell
If a female is heterozygous for an X-linked trait, she will be a mosaic for the character
15.3 - Linked genes tend to be inherited together because they are located near each other on the same chromosome
How Linkage Affects Inheritance
Morgan crossed flies with grey bodies and normal-sized wings (wild type) with flies with black bodies and vestigial wings (mutant)
Resulting proportions of traits suggested body color and wing type did not assort independently
Suggested that the genes encoding these traits were linked; or near to each other on a chromosome
Genetic recombination
Production of offspring with combinations of traits that differ from what is found in either P generation parent
Genetic Recombination and Linkage
Recombination of linked genes: Crossing over
Occurs while replicated homologous chromosomes exchange segments
Occasionally breaks physical connection between two linked genes
New combinations of alleles: Variation for natural selection
Crossing over and independent assortment contribute greatly to genotypical and phenotypical diversity
Diversity allows natural selection to take place; genetic combinations better suited for survival are more likely to be passed on
Recombination of unlinked genes: Independent assortment of chromosomes
Parental types
Offspring which match phenotypes with a parent
Recombinant types
Offspring with new phenotypical combinations which do not match the parents
Mapping the Distance Between Genes Using Recombination Data: Scientific Inquiry
Alfred H. Sturtevant (Morgan's student)
Created genetic maps, which demonstrate an ordered list of loci along a chromosome
Determined that recombination frequency was dependent on the distance between genes on a chromosome
Distance measured in map units
Linkage map
A genetic map based on recombination frequency, which will increase in tandem with the distance between the two alleles
Can only provide an approximate picture of a chromosome
Linkage may not be reflected via the genetic crossing, as recombination frequency can have a maximum value of 50%
15.4 - Alterations of chromosome number or structure cause some genetic disorders
Abnormal Chromosome Number
Nondisjunction
Error occurring during meiosis I or II in which a gamete receives two of the same type of chromosome and receives no copy
Aneupoloidy
Condition in which a zygote has an abnormal number of a particular chromosome
Monosomic
Describes a zygote with a missing chromosome
Trisomic
Describes a zygote with an extra chromosom
Polyploidy
Term describing an organism with more than two complete chromosome sets in all somatic cells
Alterations of Chromosome Structure
Duplication
Broken fragment of chromosome reattaches to another chromatid
Inversion
Broken fragment of chromosome reattaches to original chromosome but in reverse orientation
Deletion
Chromosomal fragment is lost
Translocation
Broken fragment of chromosome joins a nonhomologous chromosome
Deletions and duplications most likely to occur during meiosis
Duplications and translocations tend to be harmful
Human Conditions Due to Chromosomal Alterations
Down Syndrome
Affects 1/830 children in the U.S.
Causes short stature, distinguishable facial features, developmental delays, heart defects, and increased risk of other diseases
Trisomy 21
Average lifespan shorter than typical
Likelihood increases as age of female parent increases
Aneuploidy of Sex Chromosomes
Complicate genetic balance less than aneuploidy of autosomes
Klinefelter syndrome
XXY chromosomes occurring 1/650 live male births
Little sperm production, taller than average height, breast tissue, less muscle mass
Turner syndrome
X (Monosomy X) chromosomes occurring 1/2,500 female births
Only known viable monosomy in humans
Immature sex organs
Disorders Caused by Sexually Altered Chromosomes
Cri du chat
Deletion in chromosome five
Causes small head, unusual facial features, intellectually disability, and a cry reminiscent of a cat
Life expectancy is short; most die during infancy or early childhood
Chronic myelogenous leukemia (CML)
Exchange of chromosomal fragments between chromosome 22 and chromosome 9 during mitosis
Chromosome 22 is produced as a shortened version; new fused gene leads to uncontrolled cell cycle progression and creates cancer
Inheritance patterns are often more complex than predicted by simple Mendelian genetics
Extending Mendelian Genetics for a Single Gene
Degrees of Dominance
Complete dominance
Dominant allele overpowers recessive allele completely; heterozygous and homozygous dominant organisms are phenotypically indistinguishable
Ex. F1 generation of pea plants looking identical to one parent
Incomplete dominance
The alleles blend instead of demonstrating dominance over one another
Ex. red and white snapdragons producing pink offspring
Codominance
The alleles affect the phenotype in separate manners
Blood type is determined codominant alleles present on the surface of red blood cells (M and N molecules)
Relationship between dominance and phenotype
Relationship between dominant and recessive alleles determine examined phenotype
Ex. Tay-Sachs disease is only observable if two recessive alleles for the condition are inherited
Frequency of dominant alleles
Dominant alleles are not necessarily more common than recessive ones
Multiple alleles
Most characters involve more than two possible alleles
Pleiotropy
Property in which a gene affects multiple phenotypes
Ex. pleiotropic alleles responsible for both cystic fibrosis and sickle-cell disease
Extending Mendelian Genetics for Two or More Genes
Epistasis
Phenotypic expression of one gene at a locus affects the demonstration of a gene at a different locus
Ex. the gene for pigment deposition is epistatic to the genes for black/brown pigment in labs
Polygenic inheritance
Quantitative characters
Characters that vary along large continuum
An additive effect of two or more genes on a single phenotypic character
Ex. 700 phenotypical variations observed from 180 genes
15.5 - Some inheritance patterns are exceptions to standard Mendelian inheritance
Genomic Imprinting
Variation in phenotype which is dependent on whether the allele was inherited from the male parent or the female parent
A particular allele of a certain gene may be silenced depending which parent the gene was inherited from
Ex. the mouse gene for insulin-like growth factor is an imprinted gene; only the paternal allele is expressed
-CH3 methyl groups are added to cytosine nucleotides of one allele
Although spare, vital to embryonic development
Inheritance of Organelle Genes
Some genes are located outside of nucleus within organelles mitochondria and chloroplast
Do not display Mendelian inheritance
Carl Horrens (German scientist) observed that the coloration of yellow/white patches on a green plant was determined by the maternal parent only
Further research pigmentation related mutations were sourced to the DNA circles in organelles called plastids
A plant zygote will receive all plastids from the egg
Defects in mitochondrial DNA can impact ATP production, primarily affecting the nervous and muscular system
Mitochondrial myopathy
Causes weakness, intolerance of exercise, and muscle deterioration
Leber's hereditary optic neuropathy
Can cause sudden blindness in 20's or 30's
A method for eradicating an organelle disorder involves transferring the specific chromosomes of an egg of healthy female to that of an impacted egg of another female and replacing the chromosomal portion before fertilization, creating an embryo with three parents