4.3 INHERITANCE

Gene

Section of DNA codes for polypeptide / functional protein

Allele

Different forms of the same gene

Locus

The specific position of a gene on a chromosome

Dominant allele

Allele always expressed in phenotype when only one present

Recessive allele

Only expressed when homozygous

Codominant allele

Both alleles are fully expressed in the phenotype in heterozygous

Genotype

Genetic composition of an organisms (its alleles)

Phenotype

Observable characteristics produce by genotype + environment

Homozygous vs heterozygous

Homo - 2 identical alleles AA or aa

Heter - 2 different allel Aa

F1 generation

First filial generation - offspring of parental P cross

Usual genetically identical (if homozygous parents)

F2 generation

Offspring of F1 cross

Shows variation

Autosomes

Chromosomes that are not sex chromosomes (same in male and female)

Sex chromosomes

Determine sex

XX female

XY male

Mendels principle

Controlled breeding experiments in pea plants

• Traits are controlled by discrete unites (genes)

• Genes come in pairs (alleles)

• One allele is inherited from each parent

Mendels laws

1. Segregation

Alleles pairs separate during gamete formation (meiosis) so each gamete has one allele

2. Law of independent assortment

Alleles of different genes are inherited independently if genes are on different chromosome

Standard ration heterozygote

Aa x Aa

3 dominant: 1 recessive

1 AA : 2 Aa : 1 aa

Test crosses

Determine genotype of individual showing dominant

Unknown dominant x homozygous recessive (Ax x aa)

• All dominant → AA

• 1:1 ration → heterozygous Aa

Linkage

Genes located on the same chromosome are said to be linked and ae inherited together

• dont follow mendels assortment

Why does linkage break mendelian ratios

• Genes physically connected on same chromosome

• Tend to be inherited together

How does variation still occur in linkage

Crossing over

• Prophase 1 meiosis

• Homologous chromosomes exchange

• Recombinant chromosome

Recombinant chromosome

Offspring new combinations of alleles not found in parents

How does variation occur in meiosis

Independent assortment

• Chromosomes line up randomly in metaphase 1

Crossing over

• Exchange alleles between homologous in prophase 1

Dihybrid inheritance standard cross and ratio

AaBb x AaBb

9: 3 : 3 :1

Difference dihybrid with linkage

• No independent assortment

• Parental combinations are more common

• Fewer recombinants

Conditions for chi squared test

• Discontinuous data

• Observed vs expected values

• Sample large enough

• Data independent events

Null hypothesis of chi squared

There is no significant difference between observed and expected results any difference is due to chance

Level of probability 5%

5% chance result is due to chance

Conclusion chi squared test

χ² ≤ critical value

• Accept null

• Due to chance

χ² > critical value:

• reject null

• significant difference not due to chance

Sex linkage

Gene located on a sex chromosome (usually X chromosome ) inheritance associated with sex

• Most on X

• Y chromosome smaller lacks corresponding allele

Why do sex linked recessive conditions affect males more

Males XY

• Only one X chromosome

• No second allele on Y to mask recessive →recessive x always expressed

Females XX

• 2 alleles present

• recessive masked by dominant

Examples of sex linked conditions

Haemophilia

• Blood clotting disorder

• Caused by recessive allele on X

Duchenne muscular dystrophy

• Muscle wasting disease

• Recessive x allele

Alleles of haemophyllia

• Xᴴ = normal allele

• Xʰ = haemophilia allele

Male:

• XʰY = affected

Female:

• XʰXʰ = affected

Why males are affected more

• Only one X chromsome

• recessive allele expressed immediately

Why are females less affected

Need to recessive alleles to show condition

Why does Y chromsome matter

• Lacks most genes present on X

• cant mask X -linked recessive

Mutation definition

Random, permanent change in genetic material DNA can affect genotype and sometimes phenotype

Gene point mutation

Change in the sequence of DNA bases with a single gene

• Substitution (one base replaced)

• Insertion (extra base added)

• Deletion (base removed)

Effect of gene point mutation

• Change amino acid sequence

• Produce non functional protein

• Cause disease / no effect

Sickle cell anaemia - gene point mutation

Mutation beta globin gene change haemoglobin structure

• Red blood cells sickle - shaped

• Reduce oxygen transport

Chromosonal mutation

Changes in chromosome number or structure affecting many genes

Down syndrome

Cause non disjunction during meiosis

Chromosome 21 does not separate

• trisomy 21 ( 3 copies of chromosome 21)

Non disjunction occurs in

anaphase I or II

Mutations occur when and result

• Crossing over (prophase I)

• Non disjunction (anaphase II)

→ Genetic variation

→new alleles or abnormal chromosome numbers

Mutations and evolution

Mutations → New alleles → genetic variation→ Natural selection → evolution

Mutagens

Agents that increase mutation rate

• Gamma radiation

• UV radiation

• X- rays

• Polycyclic hydrocarbons

Carcinogens

• mutagens that cause cancer

Cancer

Proto- oncogenes

• Normal genes controlling cell division

Oncogenes

• Mutated proto-oncogens cause uncontrolled cell division

• tumour formation and uncontrolled mitosis

Epigenetics

The study of changes in gene expression caused by mechanisms other than changes in DNA base sequence

(DNA sequence SAME gene expression changes)

Genes can be swtiched

On → expressed

Off → Not expressed

DNA Methylation

Methyl groups are added to DNA bases

→ Prevents transcription factors / enzymes binding to DNA

• transcription decreases

• Gene expression reduced

Histone

Proteins DNA wraps around in chromosomes

Histone modification

Tight coiling DNA is less accessible

• Less transcription

• Decreases gene expression

Loose coiling DNA is more accessible

• Increased transcription

• Increased gene expression

Why do epigenetics matter

Different cells contain same DNA but express different genes

• (Muscle, nerve, liver)all same DNA → different proteins produce

Epigenetics

• does NOT change base sequence

• DOES change transcription

• affects protein synthesis

• can alter phenotype

Sex linkage definition

Genes on X and Y

2 Individuals with genes DE on same chromosome the offspring showed four different types of phenotype

Some phenotypes very common and some rare

• Incomplete linkage

• Genes far apart on same chromosome

• 4 Types of gametes produced (not in equal numbers)

• Small num of recombinants - large number parental

• Recombinants equal

Similar bone structure how does this support darwin’s theory of evolution

• Bone structures follow similar pattern through millions of years

• Arisen from common ancestor

• Natural selection

Mutation definition and causes

Random change in DNA base sequence / chromsome structure

• Create genetic variation

• Radiation ( UV, X-ray gamma)

• Carcinogens cigarette smoke asbestos

Gene mutations

Change in base sequence

• Substitution, insertion, deletion (cause frameshift)

• Effect → change amino acid sequence → change protein

• Some silent, degenerate code no affect on phenotype

• Sickle cell anaemia, cystic fibrosis

Chromosome mutation

Change whole chromosome - many genes

• Deletion, inversion, translocation, nondisjunction

• Occur during crossing over meiosis

• Severe fatal, disorders

• Down syndrome trisomy 21