L2: Extensions of Mendelism

similarities between behaviour of chromosomes during meiosis and genes during gamete formation

1. both genes and chromosomes exist in pairs

2. members of a gene pair and members of a chromosome pair seperate from each other during gamete formation

chromosomal theory of inheritance

inherited traits are controlled by genes residing on chromosomes transmitted through gametes, maintaining genetic community from gen to gen

why do some genes not follow mendelian rules

• some genes dont display clear dominant/recessive qualities

• more than one gene may influene the phenotype of a single characterisitic

• genes may be linked to sex chromosomes and not autosomes

• phenotypes may be influenced by the environment

• extranuclear inheritance

extranuclear inheritance

genes could lie on organelles that is not the nucleus

incomplete dominance

• when a cross between pure breeding parents with contrasting traits generates offspring with an intermediate phenotype

• red x white = pink

• dominance is incomplete

example of incomplete dominance

• four o’clocks or snapdragons

• when red is crossed with white flowers, pink is produced

• neither red or white is dominant

F2 gen of incomplete dominance

• 1:2:1 ratio of both genotype and phenotypes

• identical to complete dominance in a mendel’s monohybrid cross genotype ratio

molecular basis of incomplete dominance

insufficient expression of the gene or gene dosage

tay sach’s disease

• ex. of incomplete dominance

• heterozygotes, with only a single copy of the mutant gene are phenotypically normal

• only express 50% of the enzyme activitiy found in homozygous normal individuals

codominance

• phenotype of the heterozygote is not intermediate between the phenotypes of the homozygotes

• both phenotypes are expressed

• ex. MN blood type

codominance ratio

1:2:1

multiple alleles

• within a group of organisms, for some loci, more than 2 alleles are present

• ex. bunnies: brown > chinchilla > himalayan > albino

• ex. ABO blood types - multiple alleles show codominance

lethal alleles

an allele that causes the death of an organism during an early stage of development, so that certain genotypes can never be observed

recessive lethal mutation

• mutated allele can be tolerated in the heterozygous state

• one wild type allele may be sufficient to produce enough the essential product to allow survivial

• affect essential genes

• homozygous recessive individuals do not survive

dominant lethal mutation

• lethal in both homozygotes and heterozygotes

• very rare

• cause ectopic pregnancy or overexpression of a toxic product

phenotypic ratio of recessive lethal alleles

2:1

early onset lethal allele

result in the death of an organism at an early stage of life

late onset lethal alleles

• lethality happens in the late stages of life

• ex. huntingtons disease

conditional lethal alleles

• kills an organism under certain environmental conditions

• ex. a temp sensitive mutant protein

semi lethal allele

• kills only some individuals of the pop but not all

• ex. hemophilia - kills mostly men with (X^HY) and not women (X^HX)

gene interactions

• take place when genes at multiple loci determine a single phenotype

• the cellular functions of multiple gene products contribute to a common process or pathway

types of gene-gene interactions

• epistasis

• complementary gene action

• duplicate gene action

• supplementary/additive genes

general ratio of gene-gene interactions

9:3:3:1

epistasis

expression of one gene or gene pair masks or modifies the expression of another gene or gene pair

epistatic

gene that masks

hypostatic

gene that gets masked

phenotypic ratio of epistasis

9:3:4 (9:3:3:1)

recessive epistasis

presence of two recessive alleles inhibits the expression of an allele at a diff locus

dominant epistasis

only a single copy of an allele is req to inhibit the expression of an allele at a diff locus

phenotypic ratio of dominant epistasis

12:3:1

complementary gene interaction

• duplicate recessive epistasis

• 2 different genes with the same phenotype work together to produce a diff phenotype

• parents have to be homozygous

• F1 req atleast 1 dominant allele from both gene pairs to show trait

phenotypic ratio of complementary gene interaction

9:7

complementation test

• helps determine whether 2 mutations associated with a specific phenotype rep two diff forms of the same gene or are variations of two diff genes

• aka cis-trans test

allelic mutations

• mutations a and b occur at the same locus

• no complementation

• cis

non allelic mutations

• mutations a and b occur at different loci

• complementation

• trans

when are 2 genes said to be complementary

when mutations occur in two diff genes that together control a trait

duplicate gene action

• either A or B can show the phenotype

• only 1 gene req

phenotypic ratio of duplicate gene action

15:1

supplementary genes

• novel phenotypes result from the interactions of two genes

• if both alleles are present in double recessive condition (rrpp) or at least one dom condition (R_P_), new phenotypes are observed

F2 phenotypic ratio of supplementary genes

9:3:3:1

variable expressivity

when the degree of phenotypic expression in dominant or homozygus recessive form varies from one individual to another, the gene is said to have this

incomplete penetrance

• refers to a condition when a trait is not manifested in the detectable phenotype despite the presence of gene

• extreme case of variable expressivity

• ex. polydactyly

polydactyly

• presence of absence of extra digits - penetrance/incomplete penetrance

• presence of extra digits in all limbs or only hands or only feet - variable

factors responsible for incomplete penetrance and variable expressivity

• effect of external environment

• mosicism for X linked characters due to X inactivation

• gene-gene interactions

• effect of sex hormones

pleiotropy

• genes that affect multiple, unrelated phenotypes

• not to be confused with gene-gene interactions or polygenic traits

write the phenotypic ratios due to gene interactions

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