Mendel Flashcards
Inheritance patterns are often more complex than predicted by simple Mendelian genetics.
many heritable characters are not determined by only one gene with two alleles.
-relationship between genotype and phenotype is rarely as simple as in the pea plant characters Mendel studied
Law of independent assortment
each pair of alleles segregates independently of each others pair of alleles during gamete formation
(this law applies only to genes on different, non homologous chromosomes)
Inheritance of characters by a single gene may deviate from simple mendelian patterns in the following situations:
- when alleles are not completely dominant or recessive
- when a gene has more than two alleles
- when a gene produces multiple phenotypes
Degrees of dominance:
1: complete dominance
2: incomplete dominance
3: co dominance
1 complete dominance
occurs when phenotypes of the heterozygote and dominant homozygote are identical
2 incomplete dominance
the phenotype of F1 hybrids is somewhere b/w the phenotypes of the two parental varieties
3 codominance,
two dominant alleles affect the phenotype in separate, distinguishable ways
Relation b/w dominance and phenotype
- a dominant allele does not subdue a recessive allele; alleles don’t interact
- alleles are simply variations in a gene’s nucleotide sequence
- for any character, dominance/recessiveness relationships of alleles depend on the level at which we examine the phenotype
Most genes exist in populations in more that _____ allelic forms
two
ex. blood group use three alleles. iA iB i ( which are associated with carbohydrates)
some traits may be determined by two or more genes
ex. in mice and many other mammals, coat colour depends on two genes
epistasis
a gene at one locus alters the phenotypic expression of a gene at a second locus
quantitative characters
are those that vary in the populations along a continuum
Quantitative variation usually indicates …
polygenic inheritance (an additive effect of two or more genes on a single phenotype EX, skin colour on humans)
Another departure from mendelian genetics arises when the phenotype for a character depends on…
environment as well as genotype
Ex. hydrangea flowers of the same genotype range from blue-violet to pink, depending on soil acidity
Mendelian inheritance has its physical basis in the behaviour of chromosomes
- miltos and meiosis were first described in the late 1800’s
- chromosome theory
