Inheritance and genetics Flashcards
what is a karyotype
ordered, visual representation of chromosomes in a cell
how is a karyotype laid out
from largest to smallest autosome, followed by sex chromosomes
what is nondisjunction
can occur at the first division during anaphase I, where the homologous chromosomes are not split apart, or at anaphase II, where the sister chromatids do not get split apart
effects of nondisjunction
Results in aneuploidy - abnormal number of chromosomes in a cell
what is aneuploidy
abnormal number of chromosomes in a cell
- loss or gain of one or a few chromosomes relative to the diploid
examples of aneuploidy
- Down syndrome- individuals have 3 sets of chromosome 21
- Klinefelter syndrome- An individual has two X chromosomes as well as a Y (coded as XXY)
- Turner syndrome- an individual with one X chromosome and no other chromosome (sex chromosomes coded as XO)
common methods of prenatal diagnosis of aneuploidy
- Amniocentesis - at 16-20 weeks a sample of the amniotic fluid taken and the cells centrifuged then analysed which will show how many copies of chromosome 21; 0.1% risk of miscarriage
- Chorionic Villus Sampling - 10-13 weeks a blood test measures protein levels using a screen; detects 90% of downs; 1% risk of miscarriage
signs of Downs syndrome
- growth failure
- mental retardation
- broad flat face
- short and broad hands
- congenital heart disease
signs of Klinefelter syndrome
- tall stature
- slightly feminised physique
- poor beard growth
- breast development
- testicular atrophy (wastage)
- mildly impaired IQ
signs of turner syndrome
- short stature
- widely spaced nipple
- poor breast development
- no menstruation
- rudimentary ovaries
what is a Barr body
condensed, inactive X chromosome which females don’t need
implications of Barr body (inactivated X chromosome)
- imaginary cellular mosaic in women, where cells could either have mother’s or father’s X chromosome active
- can result in certain cells of body being affected by disorders e.g. patches with no sweat glands
examples of chromosome rearrangement in humans
- Lejeune syndrome
- Williams-Beuren syndrome
- Philadelphia translocation
- Duchenne muscular dystrophy
- Familial down syndrome
what is polyploidy
possession of multiple entire sets of chromosomes
cause of familial Down syndrome and how does it behave at meiosis
arises from Robertson translocation from gene 14 to gene 21
causes of chromosomal aberrations (loss, gain or rearrangements of parts of chromosomes)
- deletion
- duplication
- inversion - segment is reversed
- translocation - segment moved from one chromosome to another
- can be reciprocal, in which non-homologous chromosomes exchanged segments
Duchenne muscular dystrophy
occurs when a piece of normally inactive X chromosome containing the allele for muscular dystrophy is translocated to the gene 21
Lejeune syndrome
children do not learn to speak caused by deletion of tip of chromosome 5
Williams-Beuren syndrome
caused by deletion chromosome 7 and results in lowered genetic product, reduced spatial/cognitive awareness, autism, ease with strangers
Philadelphia translocation
affects 95% of patients with chronic myeloid leukaemia due to TK (a gene product) overexpression- treatable with Gleevec in 90% of cases
Dihybrid cross
two genes involved, 9:3:3:1 ratio
test cross
used to determine genotype of unknown dominant phenotype
Mendel’s laws
- Law of segregation: Genes segregate at meiosis so that each gamete contains only one of the two possessed by the parent
- Law of independent assortment: alleles of different genes assort independently during gamete formation
monohybrid cross
one gene involved, 2:1:1 ratio
Lethal Alleles
Lethal alleles are those, that when present in a homozygous individual, cause death
Polymorphic
more than one form; there can be thousands of different alleles in a population but individuals can only have two
Incomplete dominance
Heterozygotes appear as though the parent genetics have blended; the heterozygote offspring can give rise to homozygous “non-blended” offspring
Co-dominance
- when both phenotypes exists side by side within the organism
- e.g. ABO blood antigen system
Epistasis
one gene effects the action of another - gene 2 cannot function unless gene 1 is expressed
Polygenic traits
phenotype controlled by many genes that have an additive effect; character appears continuous or quantitative
- e.g, skin colour, weight, milk yield, IQ, height
- has a normal distribution, more individuals are around the average value
what determines phenotype
genotype and environment
Pleiotropy
one gene has many effects e.g. sickle cell gene produces many symptoms; colouration pattern and cross eyes of Siamese cats and produced by the same gene
polymorphism
one gene for a particular trait has many different alleles. however an individual can only has two alleles, one from each parent
linked gene and features
- genes found on same chromosome so do not assort independently
- Off spring are not in normal test cross 1:1:1:1 ratio; instead two heavily favoured genotypes and 2 minority recombinant genotypes
- if less than 50% of offspring show recombinant phenotype (more than 50% show parental phenotype), we can assume genes are linked
features of
autosome
chromosome that does not determine sex
sex chromosome
chromosome that determines sex of individual
inheritance pattern of X linked recessive gene
- males more likely to show trait than females because they don’t have a backup X chromosome
- father with the trait will transmit the mutant allele to all daughters but to no sons
- carrier woman who mates with a normal male will pass the mutation to half her sons and half her daughters
- carrier woman mates with a male with the trait, there is a 50% chance that each child will have the trait
- e.g. haemophilia
crossing over
- Occurs during meiosis I
- Two chromatids of a tetrad (one from each pair) cross over at random points and swap genetic material
- causes recombinant phenotypes
- the proportion of recombination gametes is called combination frequency
recombinant chromosome
chromosome in an offspring that has a genotype not found in either parent, due to crossing overcrossing over in meiosis.
how is recombination frequency used for gene mapping
- The smaller the distance between two genes- the less likely chiasma will be formed between them
- Distant (unlinked) genes have a recombination frequency of 50% (half recombinant and half parental types)
- Close genes have recombination frequencies of between 0-50%
- There is a near linear relationship between distance and recombination frequency
population
ocalised group of individuals of the same species
gene pool
total aggregates of genes (and their alleles) in the population at one time
hardy-weinberg theory
allele frequencies remain constant over time unless acted upon by evolutionary forces
assumptions of hardy-weinberg theory
- No migration
- No mutation
- No natural selection
- random mating
- large population size
hardy-weinberg equation
- allele frequency: p + q = 1
- genotypic frequency: p2 + 2pq + q2 = 1
how fast does random genetic drift occur in small populations
rapidly
what causes changes in allele frequency
what does telocentric mean
when centromere is at extreme end of the chromosome
what does acrocentric mean
when centromere is at the near end of the chromosome
what is an autotriploid
three of the same chromosomes
what is an autotetraploid
four of the same chromosomes
what is an allotetraploid
four sets of chromosomes
what is an allohexaploid
six sets of chromosomes
natural selection
individual has a favourable trait which gives it an advantage in its environment so is more likely to reproduce and pass on its favourable allele
genetic drift
random change in allele frequency in a small population over generations due to sampling errors
bottleneck effect
- sudden environmental change which causes a drastic reduction in population size
- causes different allele frequency which is not representative of original population
- undergo genetic drift due to small population
founder effect
- small population from original group gets separated/isolated
- different allele frequency not representative of original population
- genetic drift acts
cline
- gradual change in phenotypes across geographic areas
- slightly different environment causes slightly different phenotypes
- slightly different allele frequency
- still same species as long as they can interbreed
migration
individual from one population moves to another population
- introduce new alleles, changes allele frequency
- changes population size
mutation
- only source of new alleles in gene pool
- normally harmful
- acted upon by natural selection
stabilising selection
reduces variation but does NOT change the mean
directional selection
changes the mean value towards one extreme
disruptive selection
favours the two extremes producing two peaks
selection selection
individuals mate with individuals of opposite sex with certain traits they find attractive, selecting for those certain traits
