Chromosome Variation and Population Genetics Flashcards
1
Q
Variations in Chromosomes
A
- both in number and in the structure of chromosomes themselves play an important role in evolution
- these variations are identified by constructing a karyotype
2
Q
Karyotype Preparation
A
- G Banding (A-T base pairs)
- R Banding (C-G base pairs)
- Q Banding (C-G v A-T base pairs)
- C Banding (position of centromere)
3
Q
Karyotype
A
- chromosome abnormalities that can be passed to progeny
- extra, missing or abnormal chromosomes
- Y chromosomes
4
Q
How do chromosomes vary?
A
- mutations can be rearrangements or a change in the number of chromosomes
- they can also be changes in the number of sets of chromosomes
- Rearrangements: duplications, deletions, inversions, translocations
- Changes in the number of chromosomes (Aneuploidy): nullisomy, monosomy, trisomy, tetrasomy
- Changes in the number of chromosome sets (Polyploidy): autopolyploidy, allopolyploidy
5
Q
Chromosome Rearrangements
A
- duplications occur when part of the chromosome is doubled
- can cause problems in cell division
- homologous chromosomes no longer same length
- problems in chromosome pairing
- Tandem duplication: duplicated region next to original segment - EFEF
- Displaced duplication: some distance to original segment - AEFB
- Reverse duplication: duplication inverted - EFFE
- Major effect on phenotypes
- cause of disease
- additional copies of normal sequences
- additional gene dosage, more proteins, abnormal development
6
Q
Unbalanced Gene Dosage
A
- Developmental processes often require the interaction of many genes
- Development may be affected by the relative amounts of gene products
- Duplications and other chromosome mutations produce extra copies of some, but not all, genes
- Which alters the relative amounts (doses) of interacting products
- If the amount of one product increases but amounts of other products remain the same, developmental problems often result
7
Q
Deletion
A
- the loss of a chromosome segment
- a heterozygote has one normal chromosome and one chromosome with a deletion
- formation of deletion loop during pairing of homologs in prophase I
Phenotype effects
- heterozygote deletions dosage problem with duplications
- homozygote deletions lethal as lose essential gene(s)
- pseudodominance when recessive allele expressed
- haploinsufficient gene when two copies of gene needed to produce phenotype
8
Q
Inversions
A
- occur when DNA breaks and is reversed and then reinserted into the chromosome
- alters gene order and can destroy gene function
- Position effect: genes expressed at wrong time or in wrong order
- a heterozygote possesses one wild-type chromosome and one chromosome with a paracentric inversion
- in prophase I, an inversion loop forms
- a single cross-over within the inverted region
- results in an unusual structure
- one of the four chromatids now has two centromeres (dicentric) and one lacks a centromere (acentric)
- in anaphase I, the centromeres separate, stretching the dicentric chromatid, which breaks. the chromosome lacking a centromere is lost
- two gametes contain non-recombinant chromosomes: one wild type (normal) and one with inversion
- the other two contain recombinant chromosomes that are missing some genes; these gametes will not produce viable offspring
- conclusion: the resulting recombinant gametes are non-viable because they are missing some genes
9
Q
Translocations
A
- the movement of genetic material between non-homologous chromosomes or within the same chromosome
- non-reciprocal translocation: movement from one chromosome to another with no exchange
- reciprocal translocation: two way exchange of genetic material
- can disrupt gene function and gene expression (position effect)
10
Q
Robertsonian Translocation
A
- The short arm of one acrocentric chromosome
- is exchanged with the long arm of another
- creating a large metacentric chromosome
- and a fragment that often fails to segregate and is lost
11
Q
Aneuploidy
A
- change in the number of chromosomes
- nullisomy - loss of both members of homologous chromosomes (2n-2)
- monosomy - loss of single chromosome (2n-1)
- trisomy - gain single chromosome (2n+1)
- tetrasomy - gain two homologous chromosomes (2n+2)
- Most tolerated on the sex chromosomes; turners, triple x, klinefelter syndrome
- autosomal aneuploids (rarer): down’s syndrome, edward’s syndrome
12
Q
Polyploidy
A
- extra set of chromosomes
- autopolyploidy (from same species)
- allopolyploidy (from different species)
13
Q
Autopolyploidy
A
- extra sets of chromosomes from single species
- frequently creates unbalanced gametes
- usually sterile
14
Q
Allopolyploidy
A
- two species hybridize
- hybrid has same number of chromosomes, but these are non-homologous, so functionally haploid and sterile
- used in agriculture to breed desirable qualities
- hybrids may self sterilize
15
Q
The Gene Pool
A
- total number of genes (alleles) of every individual in an interbreeding population
- each generation represents a sample of gene pool
16
Q
Genetic Variation
A
- genetic variation can be measured at a number of scales
- phenotype
17
Q
Why study population genetics?
A
- What isn the genetic make up of the popuation?
- allele frequencies gene pool
- how variable is the population
- what is the population structure
- how different are populations
- what caused this
18
Q
Genotype frequencies
A
- we can describe the genetic composition of our population in terms of genotype frequencies or allele frequencies
- frequency or proportion of genotypes in a population
- number of individuals with genotype/number of individuals in the population
19
Q
Allele Frequencies
A
- Number of copies of allele/number of copies of all alleles at locus
- p = f(A)
- q = f(a)
- Freq(R) = 1 - Freq(r)
20
Q
Hardy-Weinberg Equilibrium
A
- Hypothetical, non-evolving population - no change in allele frequencies
- model (null hypothesis)
- natural populations rarely in H-W equilibrium
21
Q
Evolution of Populations
A
- Evolution = change of allele frequencies over time
Non-evolving population
1. very large population size (no genetic drift)
2. no migration (no gene flow)
3. no mutation (no genetic change)
4. random mating (no sexual selection)
5. no natural selection (everyone is equally fit)
- allele and genotype frequencies in a population will remain constant from generation to generation in the absence of other evolutionary influences
22
Q
Hardy-Weinberg Law
A
- in a largely randomly mating population, not affected by migration, mutations or natural selection
1. the frequencies of alleles in a population will not change
2. the genotype frequencies will stabilize in fixed proportions - p^2 + 2pq + q^2 = 1
23
Q
Testing for Hardy Weinberg
A
- Observed vs. Expected
- Chi-squared test
- The KEY to Hardy-weinberg problems lies with the homozygous recessive individuals
24
Q
No-random mating
A
- Inbreeding: mating between related individuals (consanguine matings)
- These affect the proportion of heterozygotes and homozygotes, not the frequency of alleles
- Assortive: more/less matings occur than predicted between individuals with the same genotype
25
Q
Gene Mutations
A
- Mutations can change the frequencies of alleles in populations
- forward and backward mutations
26
Q
Migration and Gene Flow
A
- Gene flow (migration)
1. Prevent populations from being too different
2. increases genetic variation - migration is affected by barriers, which can cause population isolation…genetic drift
27
Q
Genetic Drift
A
- no population is infinite in size
- each population has a limited proportion of the complete gene pool
- chance influences what alleles are present, and who will breed
- genetic drift is influenced by the effective population size (Ne)
- sampling error
28
Q
Founder effect and bottlenecks
A
- Original population > bottleneck event > survivors/selected individuals > new population
- genetic drift is associated with a bottleneck event
29
Q
Isolation
A
- isolation due to natural or anthropogenic barriers
- isolation on ‘islands’
30
Q
Effects of genetic drift
A
- increase or decrease number of alleles in a population
- may reduce genetic variation
- alleles become fixed
- increase genetic differences between populations
