Inheritance Flashcards
What is the F1 generation - for true breeding parents?
When you cross x1 hom dom & x1 hom rec = all het (100%)
What is the F2 generation?
When you cross two heterozygotes from F1 generation = gives 3:1 ratio
What is the law of segregation?
Individuals possess x2 alleles for each characteristic - only one is passed to each offspring (in gametes)
What is the law of independent assortment?
Genes at different loci segregate independently - i.e., genes will sort into ‘groups’ independently which will form each gamete
What is the principle of dominance?
One allele can mask another allele - dominant masks recessive (in heterozygotes)
What can Mendelian inheritance be used to simply explain in human characteristics?
-Eye colour
-Hitchhiker’s thumb -> ability to bend thumb back nearly 90 degrees
-Tongue rolling into U-shape
-Earlobes - attached or free
-Earwax consistency - dry or wet
-Widow’s peak hairline (or no widow’s peak hairline)
How can Mendelian inheritance be useful in genetic diseases?
Useful for understanding role of inheritance
in genetic diseases
-> some which largely conform to simple Mendelian inheritance - of dominant & recessive alleles
-e.g., sickle-cell disease & Huntington’s disease
What are the different ways in which a genetic condition can be inherited?
-Autosomal dominant
-Autosomal recessive
-X-linked dominant
-X-linked recessive
-Y-linked
What is meant by autosomal dominant conditions?
-x1 mutated copy of gene in each cell = leads to autosomal dominant disorder
–> i.e., mutated gene is on an autosomal chromosome & this mutated gene holds dominance over the normal wild-type gene
-Can inherit this condition from affected parent
-Such conditions tend to not skip generations
-Gene loci = on autosome
What are the 2 types of autosomal dominance?
-Dominance (normal)
-Incomplete dominance
-Co-dominance
What is incomplete dominance - autosomal dominance?
Allele has partial effect over the other allele -> pink flower (red & white mixed)
What is co-dominance - autosomal dominance?
Both alleles are shown -> stripy flower (from 2 different coloured phenotype parents)
Examples of autosomal dominant conditions?
-Huntington’s disease
-Marfan syndrome
–> i.e., are caused by a dominant autosomal gene mutation
Example of autosomal sporadic condition?
Retinoblastoma (can be)
What is the chance of having Huntington’s if you have an affected (heterozygous) parent?
50%
–>assume heterozygous - as is v. unlikely would get x2 homozygous (in which case = 100%)
-As Huntington’s doesn’t manifest until later life - may have kids & pass on to them without knowing
What is meant by an autosomal recessive condition?
-Need both copies of a recessive gene to have the condition
-So need 2 carrier parents or 2 affected parents
-x2 carrier parents:
25% affected
50% carrier
25% unaffected
offspring
-Gender = irrelevant
Examples of autosomal recessive conditions?
-Cystic fibrosis
-Sickle cell disease
What is sickle cell disease?
-Blood disorder
-RBCs = distorted into sickle/crescent shape –> causes blood vs prematurely breaking down or getting stuck in v. small blood vs = causing anaemia
-Affects mostly people of African descent
-Heterozygous carriers express partial phenotype - so not all RBCs affected - some are normal
-Het = can get moderate symptoms
+ve of having sickle-cell disease or being carrier = gives malaria protection
What is meant by x-linked dominant?
-Gene loci = on X chromosome
-Allele masks other allele on X chromosome
-Disorders caused by mutated genes on X
chromosome
-Females - x2 muted X chromosome genes = needed to cause disease
-Males - x1 mutated X chromosome gene = needed to cause disease
Affected father
-Male kids - get their X chromosome from mother
-Female kids = always affected - as get their X chromosome from father
Affected mother
= 1:1:1:1 ratio of gender & aff or unaff
Example of X-linked dominant condition?
Fragile X syndrome
What is an X-linked recessive condition?
-Mutated gene = recessive & on X chromosome
-Females = can mask an affected gene on one X chromosome with dominant gene on other X chromosome = heterozygous (one normal & one mutant allele) = carrier - often no phenotype for this
-Males -> only need x1 affected X chromosome to be affected - as only have the one X chromosome
–> i.e., can’t get male carriers
What is a Y linked condition?
-Males only affected
-Females never affected
-No carriers
–> only males have Y chromosome & only have the one
–> so can only pass from father to son
Details on blood groups?
-3 major alleles = A, B, O
-4 phenotypes possible = O, A, B, AB
-O = silent
-A & B = make slightly different carbohydrate modifications
-If someone can’t make A or B - they possess IgM antibodies against that protein
–> = major concern in blood transfusion
What are mitochondrial linked conditions?
-Loci in mitochondrial DNA
-99% of time = passed from mother to child only
What are genetic pedigrees?
-Determine mode of inheritance for a condition
-Used for basic genetic risk calculations
Epigenetics
-Methylation
Acetylation
–> which decreases gene expression - gene silencing
DAIM
-Decreased acetylation
-Increased methylation
-All epigenetic changes = reversible
What is a karyotype?
= individual’s complete set of chromosomes
= lab produced image of all chromosomes - by staining - can identify large scale problems
Give the process of mitosis - PMAT.
-I -> chromosomes decondense
-P -> spindle condenses, centrioles appear & nuclear envelope breaks down
-M -> chromosomes align at equator
-A -> centromeres & sister chromatids part
-T -> spindle disassembles & nuclear envelope reforms & cytokinesis
= x2 identical diploid daughter cells (2n)
Process of meiosis.
-PI (early) -> synapsis (= lengthwise pairing of homologous chromosomes) & crossing over
-PI (late) -> chromosomes condense (become visible); spindle forms; nuclear envelope fragments; spindle fibres attach to chromosomes
-MI -> paired homologous chromosomes align across equator of cell
-AI -> homologous chromosomes separate to opposite poles of the cell
-TI -> nuclear envelope partially assembles around chromosomes; spindle disappears, cytokinesis divides cell = x2 intermediate daughter cells (2n)
-PII -> nuclear envelope fragments; spindle forms & fibres attach to both chromosomes
-MII -> chromosomes align across equator of cell
-AII -> sister chromatids* separate to opposite poles of the cell
-TII -> nuclear envelopes assemble around daughter nuclei; chromosomes decondense; spindle disappears
= x4 identical haploid daughter cells (1n)
How does genetic variation arise in meiosis?
-Independent assortment = each haploid cell contains 1 chromosome from each of the homologous pairs (either the maternal or paternal homolog) - maternal & paternal chromosomes are shuffled into novel combinations**
-Unique combinations of alleles, or dominant & recessive genes
-Divided sex cells undergo recombination to produce unique offspring
-Alleles for a trait separate when gametes are formed
-Chiasmata (crossing over): homologous maternal & paternal chromosomes exchange genetic information during prophase 1
–> exchange equivalent portions of genetic material between non-sister chromatids
