Quiz 5 Flashcards
differences between law of segregation and law of independent assortment
law of segregation: for monohybrids; dependent assortment
law of independent assortment: for dihybrids; independent assortment
law of segregation
2 alleles for each gene segregate during gamete formation (like a coin flip)
law of independent assortment
each pair of alleles segregates independently of other pairs of alleles during gamete formation (when on different, nonhomologous chromosomes)
complications with Mendelian genetics
- inheritance patterns are more complex
- many characters are not just one gene with 2 allele types
- BUT principles of segregation and independent assortment still apply
situations when inheritance may deviate from Mendelian patterns
- alleles not completely dominant/recessive (incomplete dominance, co-dominance)
- gene has >2 alleles
- gene produces multiple phenotypes (pleitropy)
- multiple genes affect a single phenotype (polygenic inheritance)???
3 degrees of dominance
complete dominance: phenotypes of a heterozygote and dominant homozygote are identical
incomplete dominance: phenotype of F1 hybrids is somewhere between phenotypes of the 2 parental varieties
co-dominance: 2 dominant alleles affect the phenotype in separate, distinguishable ways
subjectiveness of phenotypes
- alleles are simply variations in a gene’s nucleotide sequences that change the way it’s expressed
- for any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype
what is Tay-Sachs disease
- accumulation of lipids in the brain and spinal cord due to deficiency of an enzyme that is required to break down fatty substances
- fatal; destroys brain and spinal cord
different types of dominance for Tay-Sachs disease
- organismal level: recessive allele
- biochemical level: phenotype (i.e. enzyme activity) is incompletely dominant (may be in-between level)
- molecular level: alleles are codominant
example of recessive alleles being more common
polydactyly (extra fingers/toes) is dominant, but only seen in 1/400 babies born in the US - recessive allele far more prevalent!
example of gene with more than 2 allelic forms
- 4 human blood phenotypes are determined by 3 alleles for enzyme I that attaches either A or B carbs to red blood cells
Type A: Ia Ia, Ia i
Type B: Ib Ib, Ib i
Type AB: Ia Ib
Type O: i i
pleiotropy
genes have multiple phenotypic effects
- e.g. multiple symptoms of hereditary conditions (CF, sickle cell anemia)
epistasis
gene at one locus alters phenotypic expression of gene at 2nd locus
- e.g. lab (and other mammals) coat color depends on 2 genes; one determining pigment color (black B or brown b) and one determining if pigment will be deposited in hair (color C or no color c)
quantitive characters
characters that vary in a population along a continuum (e.g. skin color, height)
***usually indicates polygenic inheritance
polygenic inheritance
additive effect of 2 or more genes on a single phenotype
- phenotype is usually a quantitive character
- e.g. human skin color
example of environmental impact on phenotype
hydrangea flowers of the same genotype vary from blue to pink depending on soil acidity–varies from Mendelian genetic patterns
norm of reaction
range of phenotypes for a genotype (e.g. hydrangea color) influenced by the environment (also called multifactorial)
***norm of reaction is broadest for polygenic characters
multifactorial inheritance
phenotypes are influenced by both genetic and environmental factors
what makes up an organism’s phenotype, and what does it reflect?
- phenotype includes physical appearance, internal anatomy, physiology, and behavior
- phenotype reflects genotype and unique environmental history
why are humans bad subjects for genetic research?
- several human traits follow Mendelian patterns but
* generation time is long
* parents produce too few offspring
* breeding experiments unacceptable (eugenics)
pedigree and their uses
family trees (called pedigree charts) that describes interrelationships or people across generations
- can be used to trace inheritance patterns of particular traits
- we can use these to predict the probability of specific phenotypes
general info and example of recessively inherited conditions/diseases
- condition only shows up in homozygous recessive
- heterozygotes called carriers (recessive allele but phenotypically normal)
- if recessive allele is rare, the chance of 2 carries mating is generally low
example: albanism–lack of pigmentation in skin and hair
impact of consanguineous matings on recessive diseases
- consanguineous matings increase chance of mating between 2 carries of the recessive allele
- therefore many societies/cultures have laws or taboos against marriage between close relatives
cystic fibrosis prevalence, impact, and symptoms
- most common lethal genetic disease in the US; strikes 1/2500 of European descent
- CF allele results in defective/absent chloride transport channels in plasma membrane, leading to chloride buildup outside of cell
- symptoms include mucus buildup in internal organs and abnormal absorption of nutrients in the small intestine
sickle-cell disease prevalence, impact, symptoms
- found in 1/400 African Americans, caused by substitution of a single amino acid that causes deformed hemoglobin proteins in RBCs
- symptoms include physical weakness, pain, organ damage, paralysis
- 1/10 African Americans are carriers, prevalence is so high for a detrimental allele because heterozygotes are less susceptible to malaria parasite (but may have some symptoms)
dominantly-inherited disorders and examples
dominant alleles that cause these disorders are generally rare, they arise by mutation
- Achondroplasia: form of dwarfism caused by rare dominant allele
- Huntington’s disease: degenerative disease of the nervous system
Huntington’s disease
- causes deterioration of the nervous system–fatal
- autosomal dominant genetic disease; person needs only 1 defective allele
- parent with a dominant allele had 50% chance of passing it to child, it’s often passed on because there’s no phenotypic effect until age 35-40
multifactorial disorders
many diseases have genetic and environmental components (although little understood about genetics of most)
i.e. heart disease, diabetes, cancer, alcoholism, mental illness
role of genetic testing and counseling
- genetic counselors can use Mendelian genetics, probability, and even tests to help people concerned with family history of a specific disease
- use known family information to predict probabilities for certain conditions, although these probabilities always are changing as we learn more
types of genetic testing
fetal testing - more invasive
- aminocentesis: liquid that bathes the fetus tested
- chorionic villus samping (CVS): placenta sample
- also types of imaging, ultrasound or fetascopy
newborn screening
- simple tests routinely done in US hospitals for genetic info
discoveries after Mendel
- Mendel’s “hereditary factors” were chromosomes
- mitosis and meiosis first described in late 1800s
- chromosomal theory of inheritance followed; Morgan’s work proved it (flies)
- then Griffith’s DNA discovery (pathogenic/harmless bacteria)
- finally double helix discovered
chromosome theory of inheritance
- genes have specific loci on chromosomes
- chromosomes undergo segregation and independent assortment
Morgan’s first experiment
- bred mutant male flies with white eyes with wild type females with red eyes
- F1 gen had all red eyes, F2 had 3:1 ratio of red/white but only males had white
- concluded that white allele must be on x chromosome, supporting the chromosomal theory of inheritance
wild type vs mutant alleles
wild type are normal phenotypes in fly populations, mutant are alternatives
why were fruit flies ideal for Morgan’s experiments
- breed at high rate (new generation every 2 weeks)
- only 4 pairs of chromosomes
info on human sex chromosomes
- larger X and smaller Y (only ends of Y chromosome have regions homologous with X)
- each ovum contains X, sperm may contain X or Y
- SRY gene on Y codes for development of testes
*however, some animals have different methods for sex determination (XX or X, different chromosomes entirely)
sex-linked genes and recessive conditions more common in males
genes located on either sex chromosome
- sex chromosomes have many genes for characters unrelated to sex
- usually genes found on X chromosome
- recessive X-linked traits need only 1 copy to be expressed in males, causing certain recessive disorders to be more common in males (color blindness, muscular dystrophy, hemophelia)
X inactivation in female mammals
- in some mammalian females one of the X chromosomes in each cell is inactivated during cell development, condensing into a Barr body
- if females are heterozygous for an X gene, the phenotype will be a mosaic for that character (i.e. calico cat)
- very rare for males, usually sterile
Morgan’s 2nd experiment
- crossed flies differing in body color and wing size
- traits usually inherited together in specific combinations that parent phenotype had
- showed genes must be on the same chromosome if they don’t assort independently
- some nonparental phenotypes that showed up could be explained by genetic recombination (crossing over)
types of offspring observed in Morgan’s 2nd experiment
- parental types: phenotype matching parent phenotype
- recombinant types: new combinations of traits, nonparental from crossing over
frequency of recombination for genes on same/different chromosome
- 50% frequency of recombination for any 2 genes on different chromosomes
- genes far apart on same chromosome can have a frequency near 50% (genetically unlinked despite physical linkage)
chromosome alterations
- large-scale alterations often result in miscarriage or a variety of developmental challenges
- some caused by nondisjunction–pairs of homologous chromosomes don’t separate in meiosis (one gamete receives 2 of same chromosome and one gamete receives none)
- some are structural changes–caused by breakages
problems caused by nondisjunction
aneuploidy: fertilization of gametes with nondisjunction; offspring have variation in chromosome #
- monosomic zygote: only 1 copy of a chromosome
- trisomic zygote: 3 copies of a chromosome
polyplody: organism has > 2 full sets of chromosomes (common in plants but not animals, more “normal” in appearance than aneuploids
4 types of alterations of chromosome structure from breakage
deletion - removes segment
duplication - repeats segment
inversion - reverses segment
translocation - moves segment from one chromosome to another
human conditions due to aneuploidy
autosomal
- Down Syndrome: trisomy 21
sex chromosomes
- Hinefelter syndrome: extra chromosome in men, XXY
- Turner syndrome: monosome X, only viable monosomy in humans but females sterile
examples of human conditions due to chromosomal alterations
- cri du chat syndrome: deletion in chromosome 5; child has diminished mental development and catlike cry, death in infancy or early childhood
- some cancers (i.e. chronic mylogenous leukemia) caused by chromosomal translocations between chromosome 9 and 22 (called a Philadelphia chromosome!)
how does inheritance of organelle genes cause conditions
- cytoplasmic/extranuclear genes with circular DNA are found in organelles (mitochondria, chloroplasts, plastids)
- these genes are inherited maternally (cytoplasm comes from egg)
examples
- some defects in mitochondrial genes prevent cells from making ATP, resulting in diseases affecting muscular and nervous systems
- mitochondrial myopathy and Lever’s hereditary optic neuropathy
experiment that discovered DNA as the genetic material
- Frederick Griffith (1928) used 2 strains of bacteria, one pathogenic and one harmless
- mixed heat-killed remains of pathogenic bacteria with harmless living cells and some cells became pathogenic
- showed transformation: a change in genotype/phenotype due to assimilation of foreign DNA
- identification of molecules of inheritance had previously been a major challenge, this led to discovery of DNA as genetic material
discovery of DNA structure and its role
- Watson and Crick stole images from Maurice Wilkins and Rosalind Franklin (x-ray crystallography) to introduce the double-helical model of DNA present in all body cells
- hereditary info in DNA present in all cells of the body
- DNA directs biochemical, anatomical, physiological, and sometimes behavioral traits
more evidence for DNA as the genetic material (Chargaff)
- 1950 Erwin Chargaff hypothesized DNA composition varied among species
- evidence of diversity made DNA more credible as genetic material (different bases)
Chargaff’s rules
- base composition of DNA varies among species
- in any species, the number of A/T bases are equal and the number of G/C bases are equal
- the basis for these rules was not understood until the discovery of the double helix!!
