3.7.1 - Inheritance Flashcards
Topic 7
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
When Alleles at a specific locus (on each homologous chromosome) are the same
Heterozygous alleles
When Alleles at a specific locus (on each homologous chromosome) are different
What do monohybrid crosses show?
inheritance of one phenotypic characteristic coded for by a single gene
What do dihybrid crosses show?
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, both parents would be homozygous in order to express phenotype and 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
> and child is therefore homozygous (recessive alleles must be homozygous pairs to express phenotype)
- if the two genes are on two different chromosomes (the genes are not linked), an allele from one pair of
chromosomes can enter a gamete with either allele from the other pair.
> what is this due to?
independent segregation
of homologous chromosomes during meiosis.
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 / 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]
> this assumes males are XY and females are XX, as in humans. There has been a question previously
about birds where males are XX and females are XY. In this case, swap father for mother and son for daughter.
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 (products of) non-linked genes where one masks / suppresses the expression of the other
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 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)
Describe how a chi-squared value can be calculated
X^2 = sum of (O-E)^2/E
O = frequencies observed
E = frequencies expected (multiply total n with each expected ratio as a fraction)
