Genetics Flashcards
what is a gene
section of DNA on a chromosome coding for one or more polypeptides
what is an allele
one of a number of alternative forms of a gene
what are nucleotides
-complex chemicals made up of an organic base, a sugar and phosphate
-they are basic units of which the nucleic acids DNA and RNA are made
what is a genotype
-genetic constitution of an organism
-determines the limits within which the characteristics of an individual may vary
what is a phenotype
-observable/biochemical characteristics of an organism
-result of interaction between the expression of the genotype and the environment
what is a dominant allele
allele that is always expressed in the phenotype of an organism
what is a recessive allele
allele that isn’t expressed in the phenotype. Only expressed if homozygous recessive
what is a codominant allele
two alleles both contribute to the phenotype
what is a locus
position of a gene on a particular DNA molecule
what is meant by homozygote
alleles of a particular gene are the same
what is meant by heterozygote
alleles of a particular gene are different
what is meant by the term carrier
individual carrying an allele not expressed in phenotype
what is meant by the term multiple allele
-gene that has two or more alleles
-organism will have 2 alleles on each homologous pair of chromosomes
define monohybrid inheritance
where one phenotypic characteristic is controlled by a single gene
what is the expected phenotypic ratio from crossing two heterozygous parents (monohybrid)
3:1
example of codominance
inheritance of sickle-cell anaemia
what is sickle cell anaemia
→ a condition where a faulty haemoglobin allele
causes red blood cells to be sickle shaped
→ in the heterozygous genotype, some cells
are sickle shaped but some are normal
what is the expected phenotypic ratio for crossing two heterozygous parents (codominance)
1:2:1
what is an example of multiple alleles
human blood group has three alleles
which blood groups are codominant and which are recessive
-group a + group b = codominant
-group 0= recessive
define dihybrid inheritance
where two phenotypic characteristics are determined by two different genes present on two different chromosomes at the same time
what is the expected phenotypic ratio from crossing two parents heterozygous for both genes (dihybrid)
9:3:3:1
what can be an example of dihybrid crossing
inheritance of wing length and wing colour in an insect
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 ( in humans)
● 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
what is epistasis
a gene interactions-> one gene masks the expression of another gene
what is a n example of epistasis
flower colour
Describe when a chi-squared (X2) 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 analysed
- Number of degrees of freedom = number of categories - 1 (eg. 4 phenotypes = 3 degrees of freedom)
- Determine critical value at p = 0.05 (5% probability) from a table
- If X2 value is [greater / less] than critical value at p < 0.05
● Difference [is / is not] significant so [reject / accept] null hypothesis
● So there is [less / more] than 5% probability that difference is due to chance
