L2,3,4(Mendelian Genetics + Pedigree) Flashcards
(31 cards)
Monohybrid crosses
Mono-single trait
Hydrid-traits different in each parent
Example: dwarf vs. tall plants in Mendel’s studies
Results of monohybrid crosses
F1 progeny have only one parental trait
F2 progeny have both parental traits in a 3:1 ratio
Commonly used terms
Phenotype: observable characteristic
Genotype: set of alleles for a given phenotype
Homozygote: two identical alleles(YY)
Heterozygote: two different alleles(Yy) and defines the dominant allele
Genotype vs. Phenotype in heterozygotes
Cross of Yy x Yy peas:
- genotype ratio: 1:2:1
-phenotype ratio: 3:1
Testcross can reveal unknown genotype
Is genotype with dominant phenotype(Y-) heterozygous or homozygous?:
- testcross with yy
Mendel’s principles
Principle of Dominance: one set of allele can mask the presence of another. dominant alleles masks traits of another, while recessive are the alleles that are masked by another.
Principle of Segregation: Alleles seperate during a cross, so gametes get one allele or the other(not both)
Principle of Independent Assortment: Alleles segregate independently/randomly of each other
Dihybrid crosses
two different traits, each with two alleles
example: yellow, round peas x green, wrinkled peas
phenotypic ratio shows a 9:3:3:1 ratio.
genotypic ratio shows 1:1:1:1 ratio
Methods for calculating predictions
Punnett square: used to track one or two gene segregation patterns. clear and keeps track of everything. cumbersome with more complicated patterns
Forked line method: Creates comprehensive list of trait ratios. more convenient than punnett square for multiple gene crosses.
Calculating probabilities:
- product rule: probability of two independent events occuring together is product of individual probabilities. eg. P(1 and 2) is P1 x P2
- sum rule: probability og ywo mututally exclusive events occurring together is sum of individual probabilities. eg. P(1 or 2) is P1 + P2
Chi-square test
measures how close experimental results are to predicted set of results of a particular hypothesis(null hypothesis)
sum of (observed - expected)^2 / expected
degree of freedom: number of result categories - 1
if x^2 > critical value then reject null hypothesis
if x^2 < critical value then accept null hypothesis
Pedigree
Chart that shows details of relationships between members of families and can reveal inheritance patterns for a trait
Hereditary patterns: autosomal dominant/recessive. x-linked dominant/recessive, Y linked, maternal/paternal imprinting, mitochondrial
Autosomal recessive
Equally likely to affect males and females, skipped generations are common, consanguinous parents(same ancestor) more likely to have affected offspring
examples of diseases: huntingtons disease, marfan syndrome
Autosomal dominant
Equally likely to affect males and females, unaffected parents never have affected offsprings, affected have at least one affected parent, no skipped generation
examples of diseases: cystic fibrosis, sickle cell anemia, tay-sachs disease
X-linked recessive
Mutation never passes from father to son, daughters of affected males are carriers, half of sons of carriers inherit the trait, generations can be skipped, males more likely to be affected
examples of diseases: color blindness, Duchene muscular dystrophy, hemophilia A and B
X-linked dominant
Unaffected never have affected offspring, no skipped generation, affected males always have affected daughters but no affected sons, affected mothers have half of sons and daughters are affected, equally likely to affect males and females
examples of diseases: Hypophosphatemia, bilateral ptosis
Y-linked
all male offspring of affected males are affected, only males are affected
examples of diseases: reduced fertility
Genomic imprinting
Caused by epigenetic differences in alleles inherited from male and female parents
Expression of an allele depends on parent that transmits it
- paternally imprinted: paternal allele not transcribed
- maternally imprinted: maternal allele not transcribed
Mitochondrial inheritance
Mutations passed down from mother to children
examples of diseases: Leber/s hereditary optic neuropathy, myoclonic epilepsy and ragged fiber disease
Challenges to mendelian genetics
traits exhibit patterns that don’t follow mendel’s rules
- no definitively dominant/recessive allele
- more than two alleles exist
- multiple genes involved
- gene-environment interactions
Extensions to mendelian genetics
Dominance is not complete
- incomplete dominance
- codominance
genes can have more than 2 alleles
pleiotropy: one gene may contribute to several characteristics
Incomplete dominance
phenotypic ratios reflect genotype ratios(1:2:1)
traits controlled by 2 alleles of one gene, one allele is partially dominant over other, heterozygote appears to be even mixture of two homozygous traits, alleles are semi-dominant
example: crossing pure breeding red and white plants lead to all pink F1 progeny, F2 progeny has 1:2:1 phenotype ratio
sickle cell anemia: HBBA/HBBA is unaffected, HBBA/HBBS shows some sickle cell trait, HBBS/HBBS is sickle cell disease
Co-dominance
Traits controlled by 2 or more alleles of one gene, each alleles phenotype is fully expressed in heterozygote
example with blood type: locus has 3 alleles that affect antigens on RBCs.
- IA creates A antigen, anti-B antibody
- IB creates B antigen, anti-A antibody
- i doesnt create antigen(type O), both anti A/B antibody
IA and IB alleles are co-dominant(IA/IB individuals have both A and B sugars)
Allelic series
Rabbit example:
- c+ is completely dominant
- ch is completely dominant over c
- ch is semi-dominant with cch
- ccc is semi-dominant with c
hypomorphic alleles(ch and cch) are partly functional
amorphic alleles are non-functional
Recessive lethal
heterozygotes live, homozygotes die
switches mendelian ratio to 2:1
Allele relationships
genes encode products(RNA and proteins)
diploid organisms have two alleles of each gene
how product function determines phenotype
