Inheritance Flashcards
Genotype
Genetic constitution of an organism
e.g. BB, Bb, bb (eye colour)
Phenotype
The expression of the genotype and its interaction with the environment
e.g. brown eyes
Explains why genetically identical twins may have diverging phenotypes, especially as they get older and are exposed to more environmental stimuli
Allele
Different versions of a gene as the sequence of bases are different
In a diploid organism, the alleles at a specific locus may be either homozygous or heterozygous
Alleles may be dominant, recessive or codominant
Locus
The fixed position of a gene on a chromosome
Homozygous
Both alleles at a specific locus on each homologous chromosome are the same
e.g BB or bb
Heterozygous
Both alleles at a specific locus on each homologous chromosome are different e.g. Bb
Dominant
In heterozygous state, its the allele that is expressed in the phenotype
Recessive
In heterozygous state, its the allele that isn’t expressed in the phenotype
Only expressed if homozygous recessive
Codominant
Both alleles for same gene in heterozygous organism contribute to phenotype
Homozygote
An organism that carries two copies of the same allele
e.g. BB, bb
Heterozygote
An organism that carries two different alleles
e.g. Bb
Genetic diagrams
Used to illustrate possible genotypes (and thus, phenotypes) of offspring from two parents, and the probability of occurrence of each
Contains parent phenotypes/genotypes, parent gamete genotypes, and Punnet square (offspring genotypes, labelled with phenotypes)
Monogenic inheritance
Inheritance of a characteristic controlled by a single gene
e.g. inheritance of wing length in fruit flies
Dihybrid crosses
Inheritance of two different characteristics controlled by two different genes
An allele from one pair of chromosomes can enter a gamete with either allele from the other pair because of independant segregation
e.g. possible genotypes of gametes from AaBb adult: Ab, AB, ab, aB
Crosses involving codominance
Both alleles in a heterozygous individual contribute to phenotype
e.g. coat colour in cattle: a red and white allele will end up in a speckled coat as both alleles are expressed
Crosses involving multiple alleles
Individuals only have 2 alleles of a gene (2 homologous chromosomes) but there may be more than 2 alleles in the population
There is usually a dominance hierarchy
Crosses involving sex linkage
A gene is ‘sex-linked’ if its locus is on a sex-chromosome
Specific characteristics are more likely to be inherited in either male or female offspring
Genes are more likely to be X linked (found on X chromosome)
Females (XX) have two copies of the gene/allele and only express the recessive allele if homozygous recessive
Males (XY) have only have one copy of the gene/allele and it cant be heterozygous so the recessive X-linked allele can only be expressed if only one copy is present
Crosses involving autosomal linkage
Two genes carried on same autosome (non-sex chromosome) is autosomal linkage
Genes stay together during independant segregation of chromosomes in meiosis I
Assuming there is no crossing over, all linked genes remain together during meiosis and therefore linked genes pass onto the gametes (and offspring) together
Because two autosomally linked genes are inherited together, a higher proportion of offspring will have parents genotype and phenotype
Closer together on autosome means they are more closely linked and are therefore less likely to split up during crossing over
Crosses involving epistasis
Interaction of non-linked genes where one masks the expression of the other
Chi-squared test what is it
Statistical test to find out whether the difference between observed and expected data is due to chance
Due to the random nature of gamete fusion, these are rarely 100% accurate predictions
Chi-squared test when to use it
When the data is in categories (i.e. discrete variation)
When the data indicates absolute numbers (frequencies not %)
Chi-squared test how to apply it
Define the null hypothesis
No significant difference between observed/expected data/frequencies i.e. difference is due to cahnce
Calculation of chi-squared value
Determine the number of degrees of freedom (number of categories - 1)
Determine critical value at 5%
Critical value greater than 5% → difference is significant so reject null hypothesis
Critical value smaller than 5% → difference is not significant so accept null hypothesis
