BIOL112 - Tammy’s Lectures

Inheritance

Transmission of traits from one generation to the next

Genetics

The study of heredity and genetic variation

Gene

Unit of heredity

Allele

Variant of a gene - you inherit alleles, not genes

Loci

Location of a gene on a chromosome

Homologous pairs

Genes on 1 chromosome from 1 parent that are in the same place as genes on other chromosome from other parent

Asexual reproduction

Single individual parent passes copies of all of its genes alleles to offspring. = genetically identical parent + offspring

Sexual reproduction

2 parents pass copies of half their alleles to offspring = genetically varying parents + offspring

Non-sister chromatids

Different chromatid from each parent

Sister chromatid s

Copied chromatid of 1 parents chromosome

Meiosis I

Homologous chromosomes separate

Meiosis II

Sister chromatids separate

Interphase

Centromeres, chromosomes replicate

Prophase I

Tetrads form, synapsis occurs, centrosomes more to poles, microtubules form and attach

Metaphase I

tetrads arrange on metaphase I plate

Anaphase I

Homologous chromosomes move to poles

Telophase I / cytokinesis

2 daughter haploid cells form, nuclear envelope reforms, sister chromatids still attached

Prophase II

Spindle forms, attach to centromeres, centrosomes move to poles

metaphase ||

Chromosomes arrange on metaphase II plate

Anaphase II

Centromeres separate, sister chromatids separate and move to poles

Telophase II / cytokinesis

4 haploid daughter cells leftover

3 mechanisms of variation transmission between generations.

Independent assortment, crossing over, random fertilization

Independent Assortment

Alleles for different genes segregate independently during gamete formation. The allele the gamete receives for 1 gene does hot influence the allele received for another gene

Crossing over

The exchange of genetic information between 2 non sister chromatids in a homologous pair

Random fertilisation

It is completely random which sperm fertilises which egg

The inheritance of more DNA from one parent than another...

Can be attributed to the behaviour of chromosomes during meiosis and random fertilisation

Particulate mode of inheritance

Parents pass on discrete heritable units (genes) that retain their separate identities in offspring

Why did Mendel use peas?

Able to strictly control crosses with fertilisation and they were available in many varieties

Character

Heritable feature that varies among individuals (e.g. flower colour)

Trait

Variant of a character (e.g. Purple/white flowers)

How was particulate model proven correct?

White flowers reappeared in 2nd generation. Purple flower dominant, white flower recessive

Mendels model of variation rule 1

Alternative versions of genes account for variation in heritable characters

Mendels model of variation rule 2

Organisms inherit two alleles for each character - one from each parent

Mendels model of variation rule 3

If 2 alleles differ, dominant allele determines organisms appearance - recessive allele has no noticeable affect

Law of segregation

2 alleles for a character separate (segregate) during gamete formation and end up in different gametes

Law of segregation in detail

1. Each cell has/carries 2 alleles for each character. 2.the 2 alleles separate / segregate during gamete formation. 3. Offspring inherit one allele from each parent

Ratio of genotypes and phenotypes

G: 1:2:1 P: 3:1

Law of independent assortment

Means seeds have other alleles that aren't connected to each other - 9:3:3:1 ratio

Complete dominance

One allele expressed as phenotype -e.g peas

Codominance

All alleles expressed as phenotype - e.g. Blood types

Incomplete dominance

Heterozygote for 2 different alleles - something in the middle expressed as phenotype -e.g. For colour, only half amount of enzymes present to make pigment

Pleiotropy

One gene, multiple phenotypic effects - amount of allele present can determine severity of expression of a gene, such as HD CAG repeats

Polygenic inheritance

Multiple genes: additive effect of 2 or more genes on single phenotypic character - for example, skin pigmentation

Epistasis

Multiple genes: gene at one locus alters phenotypic expression of gene at another locus

Phenotypic plasticity

Range of phenotypes produced by single genotype due to environmental influences

Why was natural selection not widely accepted as a mechanism of evolution?

Darwin could not explain now heritable variation appears & how heritable variation is transmitted between generations - Mendel helped show these things!

Single genes, multiple alleles

e.g. Coat colour

Linked genes

Found on same chromosome and tend to be inherited together

Recombination frequency equation

#recombinants divided by #of total offspring x 100 = %

Recombination frequency

A measure of how often 2 genes on the same chromosome are separated during meiosis due to a crossover event

What would a recombination frequency of 50% indicate

2 genes on different chromosomes- not linked

Sex chromosomes

Results of suppressed recombination between one autosomal pair around sex-determining locus on one automsome. Generally heterogametic, but sometimes cryptic. Can happen independently indifferent groups

Automsome

Any chromosome that is not a sex chromosome

Hemizygous

Express whatever is on here with allele-only has 1 allele

The closer together 2 genes are together on a chromosome

The less likely here is to be recombination

Why does a sex chromosome lose genes

Because they are not useful for sex determination - e.g. Y chromosome

Haplodiploid system

Males = unfertilised/haploid, females = fertilised/diploid

Difference between ZW and XY system

XY system = males are heterogametic ZW system = females are heterogametic

Intersex

A system where it appears there is a binary male/female but hen get individuals that don't follow that rule or have unexpected characteristics

Mendel didn't know...

- seed colour and seed shape loci found on different chromosomes

- seed and flower colour loci found on same chromosome but far apart so linkage not observed

Physically linked vs. Genetically linked

If 2 genes are found on the same chromosome but behave as though they aren't, they are physically linked but not genetically linked, so unlikely to be inherited together

Aneuploidy

Changes in chromosome number - one extra or one missing chromosome. Occurs in autosomes

Nondisjunction

Error in meiosis in which homologous chromosomes or sister chromatids fail to separate, resulting in aneuploidy

Down syndrome

Extra chromosome 21 - trisomy 21. Nondisjunction during meiosis 1

Polyploidy in animals

More than 2 complete chromosome sets due to meiotic error in diploid gametes - rare except fish amphibian species

Polyploidy in plants

Common - e.g. Wheat - 6 sets of chromosomes, 2 sets from each of 3 diff species. Meotic and mitotic error

Self fertilisation in plants

Common. If there is an error and we end up with a diploid egg and diploid sperm on a single plant and it selffertilizes = tetraploid offspring

Deletion

Removes a single chromosome segment - frameshift

Duplication

Repeats a segment of chromosome

Inversion

Reversal of a segment within a chromosome

Translocation

Moves a segment from one chromosome to a non homologous chromosome

Supergenes

No crossing over between inverted regions because they are not similar regions on homologous chromosomes anymore

Selection

May or may not act upon mutation

Chromosome structure changes lead to changes in....

Gene number (deletion +duplication) gene order (inversion) and gene location (translocation)

Origins of genetic variation

Sexual life cycle, mutation, chromosome changes

Genetics of conservation

Different populations of the same species can exhibit numerical or structural chromosome alterations. Hybrid offspring often sterile which can threaten captive breeding and/or reintroduction programmes.

Genetics of invasion

Contemporary and historical collections of ragweed used to track rapid adaptation of its invasive range in Europe. Large haploblocks underlie global spread across vast climatic gradients

Haploblocks

Indicative of chromosomal inversions. Regions of genome where recombination is relatively infrequent.

Chromosome changes do not:

Generate new alleles

3 types of chromosome changes

Change in number of chromosomes (aneuploidy)

Change in chromosome set (polyploidy)

Change in chromosome structure (deletion inversion etc.)

Impact of chromosome changes can be

Usually harmful, occasionally neutral, rarely beneficial - except polyploidy in plants as it is adaptive

The universal genetic code

Redundant, not ambiguous

Point mutations

Single nucleotide base is changed, inserted, or deleted

Silent mutation

Unnoticed due to redundancy of genetic code

Missense mutation

Wrong amino acid coded for

Nonsense mutation

Premature stop codon coded for - incomplete protein

Point mutations in introns

Generally silent because they do not code for proteins - they are removed 'spliced' during RNA processing

Exons

Genes in genome that code for proteins - 1.5% of genome. 98.5% of human genome does not code for proteins

Mutations in DNA

Original source of genetic variation. Generates new alleles

Impact of DNA mutations

Mostly neutral, sometimes harmful, rarely beneficial

Neutral changes in DNA

Occur in non-coding regions or if in coding regions, they are silent

Mutations in animals

Mostly occur in somatic cells - only those in gametic cells can be passed on to next generation

Why does most genetic variation arise

Due to behaviour of chromosomes during fertilisation and meiosis

Heterozygous advantage

Advantage for being heterozygous

Original source of variation

Mutation as it generates new alleles

Reference genomes

Trying to sequence all of the DNA for a particular species. When we want to make sure we are using correct genomic markers - never beautifully complete!

Microsattelites

Genetic markers - a tract of short highly repetitive DNA sequences found throughout the genome characterised by high mutation rates

Genomics

study of whole genomes, including genes and their functions

High quality reference genomes

Chromosomally assembled using genome scaffolding technologies and annotated using transcriptomes, prediction, or inference. Have many genomic markers.

Genomic Markers

Lots and lots, represent lots of genome