Chromosome Variation and Population Genetics

Question 1

Variations in Chromosomes

Answer 1

• both in number and in the structure of chromosomes themselves play an important role in evolution
• these variations are identified by constructing a karyotype

Question 2

Karyotype Preparation

Answer 2

• 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)

Question 3

Karyotype

Answer 3

• chromosome abnormalities that can be passed to progeny
• extra, missing or abnormal chromosomes
• Y chromosomes

Question 4

How do chromosomes vary?

Answer 4

• 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

Question 5

Chromosome Rearrangements

Answer 5

• 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

Question 6

Unbalanced Gene Dosage

Answer 6

• 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

Question 7

Deletion

Answer 7

• 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

Question 8

Inversions

Answer 8

• 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

Question 9

Translocations

Answer 9

• 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)

Question 10

Robertsonian Translocation

Answer 10

1. The short arm of one acrocentric chromosome
2. is exchanged with the long arm of another
3. creating a large metacentric chromosome
4. and a fragment that often fails to segregate and is lost

Question 11

Aneuploidy

Answer 11

• 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

Question 12

Polyploidy

Answer 12

• extra set of chromosomes
• autopolyploidy (from same species)
• allopolyploidy (from different species)

Question 13

Autopolyploidy

Answer 13

• extra sets of chromosomes from single species
• frequently creates unbalanced gametes
• usually sterile

Question 14

Allopolyploidy

Answer 14

• 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

Question 15

The Gene Pool

Answer 15

• total number of genes (alleles) of every individual in an interbreeding population
• each generation represents a sample of gene pool

Question 16

Genetic Variation

Answer 16

• genetic variation can be measured at a number of scales
• phenotype

Question 17

Why study population genetics?

Answer 17

• 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

Question 18

Genotype frequencies

Answer 18

• 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

Question 19

Allele Frequencies

Answer 19

• Number of copies of allele/number of copies of all alleles at locus
• p = f(A)
• q = f(a)
• Freq(R) = 1 - Freq(r)

Question 20

Hardy-Weinberg Equilibrium

Answer 20

• Hypothetical, non-evolving population - no change in allele frequencies
• model (null hypothesis)
• natural populations rarely in H-W equilibrium

Question 21

Evolution of Populations

Answer 21

• 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

Question 22

Hardy-Weinberg Law

Answer 22

• 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

Question 23

Testing for Hardy Weinberg

Answer 23

• Observed vs. Expected
• Chi-squared test
• The KEY to Hardy-weinberg problems lies with the homozygous recessive individuals

Question 24

No-random mating

Answer 24

• 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

Question 25

Gene Mutations

Answer 25

• Mutations can change the frequencies of alleles in populations
• forward and backward mutations

Question 26

Migration and Gene Flow

Answer 26

• 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

Question 27

Genetic Drift

Answer 27

• 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

Question 28

Founder effect and bottlenecks

Answer 28

• Original population > bottleneck event > survivors/selected individuals > new population
• genetic drift is associated with a bottleneck event

Question 29

Isolation

Answer 29

• isolation due to natural or anthropogenic barriers
• isolation on ‘islands’

Question 30

Effects of genetic drift

Answer 30

• increase or decrease number of alleles in a population
• may reduce genetic variation
• alleles become fixed
• increase genetic differences between populations