BIO 260 Genetics Exam 1 Lecture 3-4

Extension of Mendelian Genetics Abridged STUD

Extensions of Mendelian Analysis

build on the same fundamentals but have some sort of variations; extensions of the basic themes

Extensions of Mendelian

- In complete Dominance -multiple allele; always two allele of every gene (dominant and recessive)- Codominance- X-lingage- Lethal alleles - gene interaction; more than one gene governing a trait

Mendelian genetics

there is something for everything; always a rule that can be bent or broken for every postulate

Unit factors...

occur in pairs

unit factors occurring in pairs

traits determines by genes, which occur in pairs in diploid individuals- one pair per trait- different versions of genes are alleles

one gene can

affect more than one trait

Dominance/ recessive relationship between different alleles

not always true

Segregation (of alleles) takes place...

during gamete formation (Meiosis 1)

Independent assortment (for multiple alleles)

-when talking about more than one gene- linkage

Extensions of Mendelian Analysis

Variations on the basic scheme in which one or another of the basic rules is modified or not in operation- not new mechanisms but different variations

Basic types of extensions

- Relationship between allele of a single gene- Relationship between 1 gene for a single trait - combination of the above

The relationship between alleles can

effect things if there are more than two options

other types of extensions

- sex-linkage (X-linkage)Lethal alleles

Incomplete, or partial or semi, dominance

- neither allele completely dominant over the other - heterozygotes display either parental (homozygous) phenotype but one that is intermediateEx: Red x White --> Pnk flowers - 1:2:1 genotype --> 1:2:1 phenotype - difference is in heterozygotes

Molecular Nature of Incomplete Dominance

often due to loss of function (lof) allele

dosage-dependent

threshold number depends on dose provided

threshold effect

a certain threshold has to be maintained

Co-dominace

- alleles produce distinct gene products; both of which are expressed- Heterozygotes display both parental (homozygous) phenotypes -molecular vs. Physiological levels - when you have both of the parental phenotypes being exhibited

Multiple alleles

For any gene that there is two versions of every gene- sometimes there are more than two; many types of different alleles-at a molecular level there would be some difference; difference in the actual DNA sequence - can result in phenotypic differences

Multiple alleles

more than two options

Multiple alleles

- when there are 3 or more alleles for a single locus - in diploids , can only have 2 alleles at a time - multiple alleles allow for greater variation of genotypes and potentially phenotypes

Multiple alleles are studied in the context of

population

Molecular Basis of ABO and Bombay Phenotype

- Epistatic mutation and cause the phenotypes to differ from generation- Genotype may or may not be equal to the phenotype

Lethal alleles

-allele that causes death- for some some gene you need to have tow lethal copies in order to show- genes that are expressed in every cell but is critical to basic cell life - can be dominant or recessive - can be early or late onset

Time of onset for lethal alleles

- time of onset says when the lethal phenotype manifests itself (anytime before it can produce and pass it on it is early - late onset is anything that is past reproduce maturity; but can be way past reproductive maturity

Lethal alleles that occur before early onset will...

never be passed on

Lethal alleles that occur during late onset...

may be passed on to other genertations

Early dominant

will never be passed on

Late dominat

can be passed on

X-linkage

type of sex linkage

In X-linkage

-the sex chromosomes are not homologous- XX are; XY are not - not true homologs yet they still pair up as if they were homologs; they align and synapse in a line - hemi-zygous- also result in different ratios than in autosomal

Hemi-zygous

meaning they are between homo and heterozygous

X-linkage

- unlike (non- or incompletely homologous) chromosomes involved in sex determination- X and Y chromosomes- X an Y have only small are of homology (for synapsis - often X has many genes are hemizygous

Reciprocal crosses yield different ratios

- Clear segregation based on sex- Reciprocal crosses test for x-linkage - get two different result with two different crosses- has a different kind of notation- you write the chromosome and the alleles goes as superscripts

Multiple gene scenarios

- independent- interactions

Independent multiple allele scenarios

- multiple traits - one or more genes have alternate modes of inheritance and thus result in modified ratios

interactions multiple allele scenarios

-shared, common trait- genes interactalleles may interact in alternative ways

Independent Multiple Genes

-only get this ratio if classic mendel applies - sorted independently- classic dihybrid cross --> 9:3:3:1- alternative modes of inheritance in 1 or more genes can yield modified ratios

Genes are...

a type of loci

Understanding Gene interaction problems

- the relationship with the gene and the loci- the relationship between allele- phenotype of a single trait often influenced by more than one gene

Common Gene interactions

1. Epistasis2. Complementation3. Additive4. Polygenic or Quantitive5. Suppression6. Redundancy Novel/ Synthetic7. others

Gene interactions

- understand the nature of the interaction- the relationship between genes/ loci- the relationship between allelesby their ratios you will know them- shirt cuts - based on what?-assumptions

Epistasis

- defined as when one locus can block the expression of another locus- Can be recessive or dominant - when one locus can block or override the expression at a second locus

Epistasis Ratio

9:3:3:1

Complementation

- one dominat alleles of each gene is required for phenotype- "both ... AND"

Complementation Ratio

9:7

Dominant Epistasis Ratio

12:3:1

Redundancy

- either gene with a dominant genotype can result in a single phenotype - non additive- "either... OR"- More than one way to get there

Recessive Epistasis Ratio

9:4:3

Additive

- each gene with a dominant genotype can have independent but equal contribution to a phenotype- genes can have an additive effect

additive ratio

9:3:3:1

polygenic or Quantitative

- like additive but each allele of each gene can have an equal but independent contribution to the phenotype- individual allele is contributing unit vs. gene

Polygenic or Quantitative Ratio

9:7 or 9:3:4

Novel/ Synthetic

- each gene with a dominant genotype has an independent and distinct phenotype or contribution and the presence of both contributions results in a novel synthesis- similar to additve but with sistinct contributions

Novel/ Synthetic Ratio

Threshold traits

- threshold traits are polygenic though they don't appear to be so - have a small number of discrete phenotypes - have environmental factors such as diet etc.

Discontinuos

Ex: Tall vs. Short, Green vs Yellow, Round vs. Wrinkled

Continuous

Infinite variation vs. discrete categories - note, meristic vs. Quantitative- how tall? how Short?

Polygenes

the number of genes involved

N=

the number of polygenes involved

Calculating the \# of polygenes

- Extreme ratio 1/4^n solve for n - end frequencies should be the same

Heritability

- tries to quantify the genetics vs. environment - How our genotype gives us in whichever range - looked at in population of individuals; and how much of that variability is derived from its components

high heritability

means it mostly comes from genetics

Low heritability

means it comes mostly from environment

Heritability

not fixed, can change with environment

phenotypic variance (Vp)

Vp= VG+VE +VGxE

Genotype variance

VG

environmental variance

VE

Genotype-by-environemt interaction variance

VGxE

two kinds of heritability

1. Broad Sense2. Narrow Sense

Broad Sense Heritability

- (H^2)- gives proportion of total phenotypic variance (Vp) attributed variance (Vg) within a curtain population in particular environment- H^2 = Vg/VpRanges from 0-1; 0= Vp mainly due to environment and 1= Vp mainly due to genotypic factors

Narrow Sense Heritability

(h^2)- distinguishes contributions from different types of genetic factors-additive variance (VA)- Dominance variance (VD)- interactive variance (V) (Often assumed to be negligible)- Vg= Va+VD+VI- focuses only on genetic contributions for addictive variance (VA)H^2= VA/VP+VA/(VE+VG)=VA/(VE+VA+VG)