Topic 10: Genetics and Inheritance Flashcards
List two causes of variation within a gene pool. [2]
- Mutation
- Sexual reproduction / random fertilization
Describe how variation contributes to evolution by natural selection. [3]
- Variation: different phenotypes between individuals in a population/species
- Competition for survival
- Some individuals have more advantageous characteristics
- Favourable characteristics passed on to next generation
Outline causes of speciation. [3]
- Divided gene pool
- Reproductive isolation
- Due to temporal/behavioural/geographical isolation
- Different natural selections or selective pressures
Discuss the role of genes and chromosomes in determining individual and shared character features of the members of a species. [7]
Genes
- mutation causes genetic differences
- genes have multiple alleles
- different alleles give different characteristics
- alleles are dominant or recessive
- both alleles influence the characteristic with co-dominance
- all members of a species are genetically similar
- genes inherited from parents
- not all genes are expressed (epigenetics/methylation/acetylation)
Chromosomes
- same locus
- same number of chromosomes within a species
- some individuals have an extra chromosome
- X and Y chromosomes determine the gender
- independent assortment gives new combinations of genes
Outline the process of speciation [4]
- splitting of a species
- reproductive isolation
- temporal/behavioural/geographical isolation
- polyploidy
- gene pools separated
- gradualism: changes accumulating over long periods
- punctuated: changes over a short time
Outline how reproductive isolation can occur in an animal population. [3]
- sympatric or allopatric
- temporal: different populations reproduce at different times, e.g. different flowering patterns
- behavioural: different in courtship/mating rituals
- geographical: populations separated by natural or man-made barriers, e.g. mountains
- polyploidy
Describe the process of crossing over. [2]
- occurs in prophase I
- homologous chromosomes line up and form bivalents
- breaking and rejoining of chromatids
- exchange of DNA between non-sister chromatids
Explain the reason for linked genes not following the pattern of inheritance discovered by Mendel. [2]
- linked genes are on the same chromosome
- Mendel’s genes were on different chromosomes
- inherited together, no independent assortment
- only separated by crossing over, fewer recombinants
Outline the processes that occur during the first division of meiosis. [6]
- PMAT
- chromosome number halved (diploid to haploid)
- homologous chromosomes pair up to form bivalents
- crossing over between non-sister chromatids
- nuclear envelope breaks down at end of prophase I
- bivalents align on the spindle equator in metaphase I
- spindle fibres attach to centromeres
- homologous chromosomes pulled to opposite poles in anaphase I
- nuclear envelopes reform in telophase I
Compare the information that could be deduced when the genotypes are presented as AaBb or AB/ab. [2]
- both indicate the same phenotype
- AB/ab indicates linked genes
- AB/ab indicates genes located on the same chromosome
- no independent assortment unless crossing over occurs
Deduce one possible recombinant offspring of individual AB/ab after a test cross. [1]
Aabb or aaBb
Deduce the reason for the person developing as a female. [1]
No Y chromosome
The probability of extinction of a species increases if the population is small with low genetic variation.
State two processes that cause population size to decrease. [2]
- mortality (diseases, natural disasters, predation)
- emigration
Explain how meiosis promotes variation. [3]
- in prophase I, crossing over between homologous chromosomes
- in metaphase I, random alignment of bivalents along spindle equator
- second division of meiosis separates alleles further
- combinations of alleles in gametes is unlimited (2^n)
Outline the inheritance of haemophilia in humans. [2]
- sex-linked on the X chromosome
- recessive alleles
- more common in males
- heterozygous females are carriers
Using an example, describe polygenic inheritance. [3]
- more than one gene contributes to some characteristics
- as the number of genes increases, so do phenotypes
- continuous variation
- e.g. human skin colour due to differing amounts of melanin
Describe the causes of Down syndrome. [5]
- non-disjunction
- during meiosis
- chromosomes fail to separate in meiosis I
- some gametes have an extra chromosome
- individual has 47 chromosomes
- trisomy 21
- increased probability with mothers of older age
Describe how human skin colour is determined genetically. [5]
- polygenetic inheritance
- many genes contribute
- due to the amount of melanin in the skin
- combination of alleles determines the phenotype
- continuous variation
- phenotypes do not follow Mendel’s ratios of dominant/recessive
- environment affects gene expression
Explain the causes of sickle-cell anaemia. [8]
- gene mutation
- due to base substitution
- changes code on DNA
- change in transcription
- DNA changes from CTC to CAC
- change in translation/polypeptide chain
- tRNA adds the wrong amino acid to the chain
- glutamic acid replaced by valine
- produces abnormal haemoglobin
- abnormal erythrocyte shape
- lower ability to transport oxygen
Explain, using a named example, how polygenic inheritance gives rise to continuous variation. [2]
- e.g. human skin colour (amount of melanin varies)
- controlled by alleles from more than three genes
- alleles are co-dominant
- many possible combinations
Describe the inheritance of colour blindness in humans. [3]
- sex-linked
- gene on the X chromosome
- more common in males
- allele is recessive
- heterozygous females are not colour blind, homozygous females are colour blind
Outline the relationship between Mendel’s Law of Independent Assortment and meiosis. [6]
- independent assortment of unlinked genes
- unlinked genes located on different chromosomes
- genes inherited independently
- presence of one allele does not affect presence of other allele
- seen in dihybrid crosses
- phenotypic ratio 9:3:3:1 in double heterozygous cross
- ratio shows equal probability of all genes
- random orientation of homologous chromosomes
- orientation of one bivalent does not affect orientation of other bivalents
- occurs in meiosis I, metaphase
Outline the formation of chiasmata during crossing over. [5]
- chiasmata formed during prophase I of meiosis
- pairing of homologous chromosomes
- chromatids break
- non-sister chromatids exchange alleles
- X-shaped structure formed
- chiasma formed at position of crossing over
- chiasmata becomes visible when homologous chromosomes unpair
- chiasma holds chromosomes together until anaphase
Explain the cause of sickle cell anemia and how this disease affects humans. [8]
- base substitution mutation from GAG to GTG
- mutation of a subunit of hemoglobin
- mRNA copies DNA mutation, substitutes wrong amino acid in hemoglobin
- glutamic acid substituted by valine
- causes distorted hemoglobin protein (Hb^S)
- causes distorted/sickling shape change of red blood cells
- block capillaries, block blood flow
- cannot carry enough oxygen for the body
- homozygous leads to severe anemia
- heterozygous has less anemia
- heterozygous provides protection against malaria
Outline the inheritance of blood types in the ABO blood system in humans. [4]
- i, I^A, and I^B are alleles
- I^A/I^B are dominant, i is recessive
- Group A: I^A i or I^A I^A, Group B: I^B i or I^B I^B, Group O: ii
- I^A and I^B co-dominant, so Group AB: I^A I^B
- one allele inherited from each parent
Explain how genetic variation between the individuals in a species can be generated. [7]
- mutation
- base substitution/change to base sequence of gene
- new alleles formed
- radiation/mutagens increase the chance of mutation
- meiosis
- new combinations of genes produced by crossing over
- independent assortment of homologous chromosomes/bivalents
- gametes from two parents combined in sexual reproduction
- random fertilization
- natural selection differs in reproductively isolated populations
- disruptive selection causes phenotypes to diverge
Outline one method that has been used successfully to clone an adult animal. [3]
- nucleus removed from egg cell
- body/somatic cells removed from donor
- enucleated egg and donor cell fused (egg cell nucleus replaced by body cell nucleus)
- resulting embryo implanted in uterus of surrogate
