Explain what is meant by the terms Genotype and Phenotype
Genotype- genetic constitution of an organism
Phenotype-the expression of this genetic constitution and its interaction with the environment
Define the term allele
Different versions of a gene
Homozygous and Heterozygous
Homo- both alleles at a specific locus on each homologous chromosome are the same
Hetero-both alleles at a specific locus on each homologous chromosome are different
Alleles may be….
Dominant- in heterozygous this is expressed
Recessive-only expressed if homozygous recessive
Codominant-both alleles for the same gene in heterozygous organism contribute to phenotype
Monohybrid crosses
Used to illustrate possible genotypes (and thus, phenotypes) of
offspring from two parents, and the probability of occurrence of each
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 independent segregation.
Codominance
Both allele in a heterozygous individual contribute to phenotype
Crosses involving multiple alleles
Individuals only have 2 alleles of a gene but there may be more than 2 alleles in a population
Crosses involving sex-linkage
A gene is ‘sex-linked’ if its locus is on a sex -chromosome
- Specific characteristic more likely to be inherited in either
male or female offspring
- Females (XX) have two copies of the gene / allele
- Only express recessive allele if homozygous recessive
- Males (XY) have 1 copy of the gene / allele
- Can’t be heterozygous; express recessive X-linked allele if only one copy present,(inherited from mother)
Autosomal linkage
-Two genes carried on the same autosome
-Genes stay together during independent segregation
-if no crossing over all linked genes remain together during meiosis
-therefore passed onto gametes
-higher proportion of offspring will have parents genotype and phenotype
-closer together=more likely linked
Crosses involving epistasis
Interaction of non-linked genes where one masks the
expression of the other
Chi-sqaured
- Chi-squared test = statistical test to find out whether the
difference between observed vs expected data is due to chance - i.e. ‘goodness of fit’ of a hypothetical model
- When to use the chi-squared test:
- The data are in categories (i.e. discrete variation)
- The data indicate absolute numbers (frequencies not %)
Limitation of Chi-squared
- Due to the random nature of gamete fusion, these are rarely 100% accurate predictions
How do you do a chi-squared test?
- Define null hypothesis
- No significant difference between observed/expected data/frequencies
- I.e. difference is due to chance - Calculation of chi-squared value
Where O = frequencies observed
E = frequencies expected
= sum of - Determine the number of degrees of freedom
(number of categories - 1) - Determine critical value at p = 0.05 (5% probability) from a table given
How do you interpret the results of a Chi squared
test?
a. If calculated value of Chi-squared is larger than the critical value at p = 0.05
b. Hence there is less than 5% probability that the differences between the observed and expected data are due to chance i.e. difference is significant
c. Reject the null hypothesis
OR
a. If calculated value of Chi-squared is smaller than the critical value at p = 0.05
b. Hence there is more than 5% probability that the differences between the observed and expected data are due to chance i.e. not significant
c. Accept the null hypothesis
