Inheritance 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 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
○ But there may be many 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)
Codominant alleles
Both alleles expressed in the phenotype
Homozygous
Alleles at a specific locus are the same
Heterozygous
Alleles at a specific locus are different
What do monohybrid crosses show?
inheritance of one phenotypic characteristic coded by a single gene
What do dihybrid crosses show?
inheritance of two phenotypic characteristics coded by two different gene
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
● Females (XX) have 2 alleles → only express recessive allele if homozygous recessive
● Males (XY) have 1 allele (inherited from mother) → recessive allele always expressed
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?
Interaction of non-linked genes where one masks and suppresses the expression of the other
Describe when a chi-squared (X2) test can be used
● When determining if observed results are significantly different from expected results (frequencies)
● 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
● 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
- Determine critical value at p = 0.05 (5% probability) from a table
- If X2 value is / less
than or equal to critical value at p < 0.05
● Difference is significant so we reject null hypothesis
● So there is less than 5% probability that difference is due to chance
