Define gene
specific sequence of DNA nucleotides in DNA which codes for polypeptide/RNA >
each gene occupies a specific gene locus on chromosome & determines specific trait in organism
Define locus
position of gene on chromosome/within DNA molecule >
2 alleles of a gene occupy the same gene locus on a pair of homologous chromosomes
Define allele
alternative form of gene >
responsible for determining contrasting characteristics of gene; each allele possesses unique nucleotide sequence, only 1 can occur @ given locus. diff. in nucleotide sequence of each allele, when translated produces diff gene product resulting in diff phenotypes
Define dominant allele
allele that expresses phenotype even in presence of an alternative allele >
in heterozygotes, expression of gene product of dominant allele masks the effect of gene product expressed by recessive allele
Define recessive allele
allele that expresses phenotype only in presence of another identical recessive allele
Define codominance
2 diff dominant alleles of same gene expressed independently of one another & influence phenotype in heterozygote >
occurs when heterozygote has phenotype that is diff from phenotypes of either homozygotes; >2 phenotypes possible
Define incomplete dominance
form of intermediate inheritance, one allele for specific trait not completely expressed over alternative allele >
occurs when heterozygote has phenotype that is distinct from & often intermediate to phenotypes of 2 homozygotes
Define homozygous
diploid condition where alleles @ given locus on pair of homologous chromosomes are identical, either homozygous dominant/recessive >
homozygous individual breeds true i.e. same characteristic as offspring receive same allele from parents
Define heterozygous
diploid condition where alleles @ given locus on pair of homologous chromosomes are diff >
expression of dominant allele masks effect of recessive allele >
heterozygous individuals do not breed true
Define genotype
combination of alleles situated @ corresponding loci on homologous chromosomes >
determines phenotype observed of an organism
Define phenotype
observable trait of an organism due to expression of genotype >
traits arise from interaction b/w genotype & environment in which organism develops; environmental factors may influence phenotype of organisms
Define test cross
cross b/w individual of unknown genotype/heterozygote to a homozygous recessive individual; offspring ratio used to deduce genotype of unknown/ linkage r/s of heterozygous loci
Define linkage
2 or more genes on same chromosome that do not assort independently in meiosis, likely inherited together >
closer gene loci of 2 genes on chromosome, less likely crossing over occurs b/w 2 genes on homologous chromosomes
Explain how genes from 1 generation are inherited to the next via gametes
meiosis occurs in all organisms carrying out sexual reproduction producing haploid gametes for reproduction >
results in production of cells w/ half no. of chromosomes as parent cell so each gamete contains only 1 copy of each gene >
male, female gametes fuse to form diploid zygote during fertilisation, zygote contains 2 sets of chromosomes >
thus 2 copies of each gene @ same loci on 2 homologous chromosomes
How is genotype linked to phenotype?
genes control phenotypes of organisms, diff alleles account for variation >
expression of genes gives rise to pdts that affect reactions, physiological function of organisms >
DNA(genotype) transcribed to mRNA, translated to protein(phenotype)
State steps in drawing genetic diagram
1. Define alleles
2. list down parental phenotype then parental genotype
3. list down gametes, MUST BE CIRCLED
4. list down F1 genotype then F1 phenotype
5. list down gametes
6. prepare Punnett Square
7. list down F2 genotype, F2 phenotype then phenotypic ratio
State dihybrid ratio from crossing of 2 parents heterozygous @ both gene loci
9:3:3:1
State dihybrid ratio from crossing heterozygous @ both gene loci x homozygous @ both gene loci
1:1:1:1
Explain deviation of observed test cross results from expected dihybrid test cross ratio
state expected ratio VS observed ratio >
account for gametes carrying parental type >
account of recombinant gametes
e.g. for autosomal linkage,
2 genes for X & Y are linked, alleles of 2 genes are inherited together as 1 linkage group, results in higher proportion of gametes carrying parental types
>
crossing over b/w 2 linked genes on non-sister chromatids of homologous chromosomes occur; crossing over is chance event, resulting in lower proportion of recombinant gametes >
observed ratio majority parental combinations, minority recombinant combinations
Explain the term epistasis in this context.
Explain the interaction between the genes/alleles that resulted in the described
phenotypes >
Identify the genotypes that led to the phenotypes >
All phenotypes observed must be accounted for
State ratio of recessive epistasis
9:3:4
State ratio of dominant epistasis
12:3:1
State ratio of duplicative recessive epistasis
9:7
Outline how to read chi-squared test results
state null hypothesis & alternate hypothesis >
calculate expected values >
obtain calculated chi-squared value through formula >
determine degrees of freedom i.e no. of phenotypic classes - 1 >
determine level of significance(0.05) >
refer to distribution table; determine critical x2 value @ P=0.05, determine probability of calculated x2 value, state conclusion in terms of null hypothesis
define continuous variation
many phenotypes in a population show complete gradation from 1 extreme to the other w/o any break
define discontinuous variation
variation shows clear cut differences with no intermediates between phenotypes