Chromosoml Basis of Inheritance pt1

Progeria

• A mutation ins lamin A gene = causes rapid premature aging

• Children appear normal at birth but develop symptoms early in life

• life expectancy is 14 yrs

Lamin A gene

• Protein: Stabilizes the nuclear membrane

• The nuclear envelope supports chromosome organization

• Mutations weaken the nuclear envelope and disrupts cell division

Lamin A mutation

• Cell is structurally unstable

• Chromosomes are poorly organized during mitosis

• CELL DIVIDE ABNORMALLY

• Lead to premature aging

Some genes dont follow law of independent assortments

• Law states

  • Genes for diff. traits assort independently

  • Alleles separate into gamete independently of one another

  • This happens when genes are located in different chromosomes

Linked Genes

• Genes located on the same chromosome

• Genes close toegtehr tend to be inherited toegther

• They DONT assort independently during meiosis

Chromosomes contain MANY genes

• Genes are linear order along chromosomes

• Genes physically close remain together

• Physical organization influences inheritance

Independent Assortment vs Linkage

• Genes on diff. chromosome assort independently

• Genes on the same chromosome MAY be inherited together

  • Crossing over can sometime separate linked genes

  • All based on DISTANCE between genes

Thomas Hunt Morgan

• Discovered genetic linage

• He studied inheritance in fruit flies and revealed exceptions to Mendel’s laws

• Connected genes to chromosomes

Fruit Flies

• Drosophila melanogaster

• Easy to distinguish sex

• SHORT lifespan

• They reproduce quickly

• Produce MANY offspring

• Their chromosomes are large and easy to study

• Many visible mutant traits

Morgans Experiment

• He studied 2 traits in fruit flies

  • Eye color

    • Red (wild types) and Purple (mutant)

  • Wing shape

    • Normal wings and Vestigial wings

• These traits are controlled by diff. genes

Test Cross (Morgan)

• (Normal wings + Red eyes) x (Vestigial wings + Purple eyes)

• The offspring didn’t match expected Mendelian rations if genes assorted independently

  • 1: 1: 1: 1

• Most offspring has the same traits as the parent flies

  • Very few has new combination of traits

  • This suggest that the 2 genes are linked

Parental offspring

• Have the same trait combinations as the parents

• In linked genes they appear most frequently

• They are the chromosomes that didn’t undergo recombination (crossing over)

Recombinant offspring

• Have new combinations of traits

• Result of crossing over

• Happens less frequently when genes are linked

Crossing Over

• Happens during prophase 1 of meiosis

• This creates new alleles combination

• Increases genetic diversity

Genetic Recombination

• Recombination = crossing over = homologous chromosomes exchange DNA

• Gametes carrying recombination chromosome create new trait combination

Recombination Frequency

• Depends on the distance between genes

  • Genes farther apart recombine OFTEN

• Calculated from percentage of recombinant offspring

  • The value determins the DISTANCE between genes

• RF = (# of Recombinants) / (# of Offspring) x (100)

Genetic Mapping

• The work done to show relative location of genes

• Use recombination frequency to map genes

• Genes are arranged according to that frequency

Linkage Maps

• The product of Genetic mapping that shows the order of genes on a chromosome

• Distance between genes is based on recombination frequency

  • The maps don’t show EXACT physical distance

Map Units and Centimorgans

• Distance between genes in a linkage map is measured in map units/centimorgans

• 1 map unit = 1 centimorgan = 1% recombination frequency

• Greater recombination frequency indicate greater gene distance.

Widely Separated genes (limitations of linkage maps)

• Double crossover can reduce accuracy

• Genes far apart may appear to assort independently

  • This is strange because independent assortment is for UNLINKED genes on DIFFERENT chromosomes

• As recombination frequency approached 50% genes appear unlinked even though they’re one the same chromosome

Max recombination frequency

• CANNOT exceed 50%

• Beyond this point genes appear to assort independently and be on different chromosomes

Modern Genetics

• Combines linkage maps + genome sequencing

• Genome sequencing reveals the exact physical position of genes

Importance of Genetic Mapping

• helps identify genes responsible for diseases

• Allows study of genome organization

• Provides insight into chromosome structure

Genetic Variation

• Created by Recombination

• New allele combinations arise in offspring

• ESSENTIAL FOR EVOLUTION

Recombination and Evolution

• Genetic variation increases adaptability

• Populations with more variation respond better to environmental changes