Topic 7.1 - 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 -> arise by mutation
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
Describe the dominant allele
Always expressed (shown in the phenotype)
Describe the recessive allele
Only expressed when 2 copies present (homozygous recessive) / NOT expressed when dominant allele present (heterozygous)
Describe the codominant alleles
Both alleles expressed / contribute to phenotype
Describe the term homozygous
Alleles at a specific locus are the same
Describe the term heterozygous
Alleles at a specific locus are different
What does a monohybrid cross show?
Inheritance of 1 phenotypic characteristic coded for by a single gene
What does a dihybrid cross show?
Inheritance of 2 phenotypic characteristics coded for by 2 different genes
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 (2)
1) Females (XX) have 2 alleles -> only express recessive allele if homozygous recessive / can be carriers
2) Males (XY) have 1 allele (inherited from mother) -> recessive allele always expressed
Explain how autosomal linkage affects inheritance of alleles (3)
1) 2 genes located on same autosome (non-sex chromosome)
2) So alleles on same chromosome inherited together
- Stay together during independent segregation of homologous chromosomes during meiosis
3) 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 / suppresses the expression of the other
Describe when a chi-squared (X^2) test can be used (2)
1) When determining if observed results are significantly different from expected results
- E.g. comparing the goodness of fit of observed phenotypic ratios with expected ratios
2) Data is categorical (can be divided into groups e.g. phenotypes)
Suggest why in genetic crosses, the observed phenotypic ratios obtained in the offspring are often not the same as the expected ratios (4)
1) Fusion / fertilisation of gametes is random
2) Autosomal linkage / epistasis / sex-linkage
3) Small sample size -> not representative of the whole population
4) Some genotypes may be lethal (cause death)
Describe how a chi-squared value can be analysed (3)
1) Number of degrees of freedom = number of categories -1 (e.g. 4 phenotypes = 3 degrees of freedom)
2) Determine critical value at p = 0.05 (5% probability) from a table
3) If X^2 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
