Autosomal Inheritance Flashcards
1
Q
What are Mendel’s Laws of Inheritance?
A
- Each gamete receives one of the two alleles in a 1:1 ratio
- an individual carries 2 copies of each autosomal gene
- 2 copies = not identical (can have dominant/recessive genes)
-
Alleles of genes on different chromosomes segregate independently of each other
- Independent assortment
2
Q
What is Autosomal Recessive Inheritance?
A
-
Autosomal recessive phenotypes present only in:
- homozygotes
- compound heterozygotes
- 2 defective alleles
-
Punnett Square
- 1 DD : 2 Dd : 1 dd
- affected = 1/4 (dd)
- ‘carrier’ = 1/2 (2Dd)
- normal = 1/4 (DD)
- unaffected = 3/4 (DD + 2Dd)
- unaffected ‘carrier’ = 2/3
- 1 DD : 2 Dd : 1 dd
-
Pedigree
- Skips generations
- 1st gen = affected, 2nd gen = carriers, 3rd gen = affected
- Skips generations
3
Q
How are detrimental or Lethal Alleles maintained in a Population?
A
-
Maintained via recessive traits in heterozygotes
- often present with no abnormal phenotype
- Example: Huntington Disease
- Presents late in life, after mating occurs
4
Q
What is Autosomal Dominant Inheritance?
A
-
Heterozygotes exhibit phenotype
- Homozygotes may not be observed
- Males/Females affected equally
-
Punnett Square
- 2Aa:2aa = 50/50 chance of inheritance
-
Pedigree
- Every generation
- Every affected person has an affected parent
- No carriers; You have it or you don’t
5
Q
If both parents are heterozygous for 2 genes, what is the probability of having a homozygous recessive child?
A
-
Independent Assortment
- Mendel’s 2nd Law
- Probabilty of simultaneous independent events = PRODUCT of the probability of each event in isolation
- __RrYy x RrYr —> rryy (1/4 x 1/4 = 1/16 chance)
6
Q
Basis of Dominance and Recessivity
A
-
Recessive traits = “loss of function”
- hypo-
- amorphs
-
Dominant traits = “gain of function”
- hyper-
- neomorphs
(Generally speaking, but can deviate at times…)
7
Q
What is linkage?*
A
-
Alleles of 2 loci do NOT sort independently
- 2 loci are in close proximity on the same chromosome
-
“Tightly linked” pairs of loci co-inherit w/ high frequency
- Not all have same frequency of inheritance
Concept:
- Genes = widely separated = sort independently
- high probabililty of recombination
- Genes = close proximity = exhibit linkage
- local phenomenon
- Crossover –> gene combo different than parents
- Double crossover –> gene combo same as parents
Linkage Maps:
- Hypothesizing gene orders & map distances
- Closer 2 genes are to each other = LESS likely to crossover
- Farther 2 genes are to each other = MORE likely to crossover
Conclusion:
- Observed recombination frequencies = infer distances
- Higher frequency of recombination = farther apart
- Lower frequency of recombindation = closer together
- 50% = indistinguishable from indepenedent assortment
Note: suppression of recombination at centromeres & telomers
-
Site of recombination = random
- Happens during prophase of meiosis
- Crossover btw homologous chromosomes
8
Q
Why make pedigrees?*
A
-
Pedigrees
- standardized symbolic representation of family structure & phenotypes
- track most likely pattern of inheritance
- defective alleles = more likely to be inherited than introduced by marriage or mutation in each generation
9
Q
What are exceptions to Mendel’s Law of distinct dominant/recessive patterns of inheritance?
A
- Observed phenotypic ratios may vary because of interactions between:
- Penetrance & expressivity
- qual variation of geno/pheno correlation
- Epistasis
- one gene effects expression of another
- Allelic combos
- lethal combinations/incomplete & co-dominance
- Genetic heterogeneity
- phenotype common to many mutated genes
- Pleiotropy & Phenocopy
- Mis-attributed paternity
- New mutations
- Germ-line mosaicism
- mutation present in gonads & gametes (but not somatic tissue)
- Multifactorial inheritance
- more than one gene involved
- Genetic imprinting
- gene expression depends on which parent a gene is inherited from
10
Q
Penetrance & expressivity
A
-
Variable penetrance
- when a mutant allele is present but not expressed in
all members of a population - “all‐or‐none” phenomenon
- Example: polydactyly = only 80% of individuals with the mutant allele actually show the abnormal phenotype
- when a mutant allele is present but not expressed in
-
Variable expressivity
-
severity of effects of a mutant allele varies among individuals of a population.
- Example: familiar hypercholesterolemia = individuals in the same family show different blood cholesterol levels and other adverse consequences
-
severity of effects of a mutant allele varies among individuals of a population.
Note: Root causes may be diverse due to effects of alleles of other genes aka “genetic background”
11
Q
When one gene affects the expression of a second gene…
A
Epistasis
-
Example = blood group antigen presentation
- H gene = epistatic to the ABO gene
- attaches “linker” molecule to cell surface where A or B antigens are attached
- hh gentype = no H protein = O blood type
12
Q
What are lethal alleles?
A
- A phenotypic class does not survive to reproduce
- Example = spontaneous abortion
13
Q
What are Multiple Alleles?
A
- Many varients or degrees of a phenotype occur
- Examples: PKU & CF (many alleles = resulting in phenotype)
14
Q
What is Incomplete Dominance?
A
- Heterozygote’s phenotype is intermediate between those of 2 homozygotes
- Example: familial hypercholesterolemia or achondroplasia (malformation of skeleton)
15
Q
What are Codominant Alleles?
A
-
Both alleles are expressed in the heterozygotes in phenotypically identifiable ways
- Example: AB blood type
16
Q
What is Pleiotropy?
A
- One gene mutation leads to phenotype with many symptoms, or gene controls several functions;
- Example: porphyria variegata
