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6.1.2 Patterns of Inheritence (Patterns of inheritance: (phenotypic ratios…
6.1.2 Patterns of Inheritence
Variation
environmental contribution:
diet
etiolation: process in flowering plants grown in partial or complete absence of light.
chlorosis: loss of the normal green coloration of leaves of plants, caused by iron deficiency in lime-rich soils, disease, or lack of light.
accent
genetic contribution
mutagens can increase rate of mutations:
physical agents e.g. xrays, gamma, UV
chemical agents e.g. aromatic amides, nitrous acid
biological agents e.g. some viruses, food contaminants
chromosome mutations:
deletions: part of a chromosome is lost
inversions: section of chromosome break off, turn 180 degrees then attach back on; all genes may not be expressed
translocation: part of one chromosome breaks off and attaches to another chromosome
duplication: piece of chromosome duplicated; overexpression of gene
non-disjunction: one pair of chromosomes fails to separate so one gamete has an extra chromosome
aneuploidy: chromosome number is not an exact multiple of the haploid number for that chromosome; may be due to non-disjunction during meiosis
polyploidy: if a diploid gamete is fertilised by a haploid gamete, the resulting gamete will be triploid
sexual reproduction (meiosis)
random fusion of gametes at fertilisation
allele shuffling during prophase 1
crossing over
independent assortment during metaphase/anaphase 1/2
gametes produced by meiosis are genetically different ; are haploid, so only one allele of each gene is present
discontinuous
genetic variation producing discrete phenotypes; two or more non-overlapping categories e.g. shoe size, blood type
usually monogenic
sometimes alleles of two genes interact to govern a single characteristic; different alleles at single gene locus have a large effect on the phenotype, or different gene loci have different effects on the characteristic
continuous
variation that produces phenotypic variation where the quantitative traits vary by very small amounts between one group and the next e.g. height, weight
polygenic; many genes involved
alleles of genes have small effect on phenotype so have an additivie effect
Patterns of inheritance:
monogenic inheritance: determined by a single gene
dihybrid inheritance: involving two gene loci
multiple alleles: characteristic for which there are three or more alleles in the populations gene pool
sex linkage: gene present on (one of) the sex chromosomes
codominance: where both alleles present in the genotype of a heterozygous individual contribute to the individuals phenotype
phenotypic ratios
autosomal linkage: gene loci present on the same autosome (non sex chromosome) that are often inherited together
sex linkage
epistasis: interaction of non-linked gene loci where one masks the expression of the other
allele: a version of a gene
heterozygous: not true breeding; having different alleles at a particular gene locus on a pair of homologous chromosomes
homozygous: true breeding; having the same alleles at a particular gene locus on a pair of homologous chromosomes
chi squared test
degrees of freedom = number of categories -1
null hypothesis rejected if calculated value > critical value
null hypothesis accepted if calculated value < critical value
Sammer Sheikh
upto dihybrid crosses