Genetics, populations, evolution and ecosystems Flashcards
What is meant by the term genotype?
Genetic constitution of an organism
What is meant by the term phenotype?
The expression of this genetic constitution (genotype)
and its interaction with the environment
What are alleles and how do they arise?
Variations of a particular gene (same locus)→ arise by mutation (changes in DNA base sequence)
How many alleles of a gene can be found in diploid organisms?
● 2 as diploid organisms have 2 sets of chromosomes (chromosomes are found in homologous pairs)
○ But there may be many (more than 2) alleles of a single gene in a population
Dominant allele
Always expressed (shown in the phenotype)
Recessive allele
Only expressed when 2 copies present (homozygous recessive)
/ NOT expressed when dominant allele present (heterozygous)
Codominant alleles
Both alleles expressed / contribute to phenotype (if inherited together)
Homozygous
Alleles at a specific locus (on each homologous chromosome) are the same
Heterozygous
Alleles at a specific locus (on each homologous chromosome) are different
Monohybrid cross
inheritance of one phenotypic characteristic coded for by a single gene
Dihybrid cross
inheritance of two phenotypic characteristics coded for by two different genes
Explain the evidence from a pedigree diagram which would show that the
allele for [named phenotype] is dominant
● [Named phenotype] parents [n & n] have child [n] WITHOUT [named phenotype]
● So both parents [n & n] must be heterozygous / carriers of recessive allele
○ If it were recessive, all offspring would have [named phenotype]
Explain the evidence from a pedigree diagram which would show that the
allele for [named phenotype] is recessive
● Parents [n & n] WITHOUT [named phenotype] have child [n] WITH [named phenotype]
● So both parents [n & n] must be heterozygous / carriers of recessive allele
What is a sex-linked gene?
A gene with a locus on a sex-chromosome (normally X)
Explain why males are more likely to express a recessive X-linked allele
This assumes males are XY and females are XX, as in humans. In some organisms, it is swapped. In these cases,
females (XY) would be more likely to express a recessive X-linked allele.
● Females (XX) have 2 alleles → only express recessive allele if homozygous recessive / can be carriers
● Males (XY) have 1 allele (inherited from mother) → recessive allele always expressed
Explain the evidence from a pedigree diagram which would show that the
allele for [named phenotype] on the X-chromosome is recessive
● Mother [n] WITHOUT [named phenotype] has child [n] WITH [named phenotype]
● So mother [n] must be heterozygous / carrier of recessive allele
Explain the evidence from a pedigree diagram which would suggest that
[named recessive phenotype] is caused by a gene on the X chromosome
Only males tend to have [named recessive phenotype].
Explain the evidence from a pedigree diagram which would show that the
gene for [named phenotype] is not on the X chromosome
● [Named phenotype] father [n] has daughter [n] WITHOUT [named phenotype]
● Father [n] would pass on allele for [named phenotype] on X chromosome so
daughter [n] would have [named phenotype]
OR
● [Named phenotype] mother [n] has son [n] WITHOUT [named phenotype]
● Mother [n] would pass on allele for [named phenotype] on X chromosome so
son [n] would have [named phenotype]
Explain how autosomal linkage affects inheritance of alleles
● Two genes located on same autosome (non-sex chromosome)
● So alleles on same chromosome inherited together
○ Stay together during independent segregation of homologous chromosomes during meiosis
● But crossing over between homologous chromosomes can create new combinations of alleles
○ If the genes are closer together on an autosome, they are less likely to be split by crossing over
In fruit flies, the genes for body colour and for wing development are not
on the sex chromosomes. The allele for grey body colour, G, is dominant
to the allele for black body colour, g. The allele for long wings, L, is
dominant to the allele for short wings, l.
A cross was carried out between flies with grey bodies & long wings
(heterozygous for both genes) and flies with black bodies & short wings.
The result of this cross was 225 offspring with a grey body & long wings
and 220 with a black body & short wings. Explain these results.
● The two genes are linked
/ autosomal linkage
● No crossing over occurs /
genes are close together
● So only GL and gl
gametes produced / no
Gl and gL gametes
produced / no Ggll and
ggLl offspring produced
What is epistasis?
Interaction of (products of) non-linked genes where one masks / suppresses the expression of the other.
Describe when a chi-squared (X^2) test can be used
● When determining if observed results are significantly different from expected results (frequencies)
○ Eg. comparing the goodness of fit of observed phenotypic ratios with expected ratios
● Data is categorical (can be divided into groups eg. phenotypes)
Suggest why in genetic crosses, the observed phenotypic ratios obtained in
the offspring are often not the same as the expected ratios
● Fusion / fertilisation of gametes is random
● Autosomal linkage / epistasis / sex-linkage
● Small sample size → not representative of whole population
● Some genotypes may be lethal (cause death)
Describe how a chi-squared value can be calculated
O = frequencies observed
E = frequencies expected (multiply total n with each expected ratio as a fraction)
χ2 = (O-E)^2
sum of ——–
E
