Topic 7 - Genetics, populations, evolution and ecosystems Flashcards
(32 cards)
What is meant by genotype
- genetic constitution of an organism
What is meant by phenotype
- expression of this genetic constitution 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 found in homologous pairs
-> but there may be many (more than 2) alleles of a single gene in a population
Describe the different types of alleles
- dominant allele -> always expressed and shown in phenotype
- recessive allele -> only expressed when 2 copies are present (homozygous recessive)
- codominant alleles -> both alleles expressed/contribute to phenotype
What is meant by homozygous and heterozygous
- 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
What do monohybrid and dihybrid crosses show
- 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
Worked examples for monohybrid crosses on quizlet
Explain the evidence from a pedigree diagram which would show that the allele for a named phenotype is dominant
- (named phenotype) parents have child without (named phenotype)
- so both parents 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 a named phenotype is recessive
- parents without (named phenotype) have child with (named phenotype)
- so both parents must be heterozygous/carriers of recessive allele
Worked examples for dihybrid crosses on quizlet
What is a sex linked gene
- gene with a locus on a sex chromosome (usually X)
Why are males 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
Worked example of monohybrid and dihybrid crosses with sex linkage and codominance on equizlet
Explain the evidence from a pedigree diagram which would show that the allele for a named phenotype on the X chromosome is recessive
- mother without named phenotype has child with named phenotype
- so mother must be heterozygous and carrier of recessive allele
Explain the evidence from a pedigree diagram which would suggest that the 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 a named phenotype is caused by a gene on the X chromosome
- named phenotype father has daughter without named phenotype -> father would pass on allele for named phenotype on X chromosome so daughter would also have named phenotype
- named phenotype mother has son without named phenotype -> mother would pass on allele for named phenotype on X chromosome so son would also 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 genes are closer together on an autosome they are less likely to be split by crossing over
Worked example for autosomal linkage on quizlet
What is epistasis
- interaction of products of non linked genes where one masks/suppresses the expression of the other
Worked example of dihybrid cross with epistasis
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 and cause death
How can a chisquared value be calculated
How can a chi squared value be analysed
1 - number of degrees of freedom = number of categories - 1
2 - determine critical value at p = 0.05 using table
3 - if X^2 value is greater/less than critical value at p<0.05 then
- difference is/not significant so reject/accept null hypothesis
- so there is less/more than 5% probability that difference is due to chance
