DNA & Heredity IIAlso known as...

Why you look like your mum

Last time....• What is “Genetics”?• Structure, function of DNA• Genetic code

Today....• Basics of inheritance• Mendelian genetics• Sex determination• Pedigrees

sex chromosomes

Human karyotype

Terminology• Locus (pl loci) = a location within the genome

• Allele (pl alleles) = one particular version of a gene, locus

• Diploid = cells contain two full homologous sets of chromosomes (2N, one from each parent)

• Haploid = cells contain one set of chromosomes only (1N, e.g. gametes)

• Gene = the smallest unit of inheritance, a region of DNA encoding an RNA or protein

Chromosomes & ploidy

3 nonhomologouschromosomes

centromere

3 pairs of homologouschromosomes

haploid (1N) diploid (2N)

Information content of DNA vs traits:Genotype: the information in DNA

Phenotype: physical expression of DNA

sickle phenotype normal phenotype

red blood cells

SS AS AA

heterozygote has a mix of red blood cell types=> A and S are codominant for this trait

Homozygote HomozygoteHeterozygote

More terminology...• Homozygous – having the same allele at a locus

• Heterozygous – having two different alleles at a locus

In heterozygotes:

• Dominant - allele that determines the phenotype

• Recessive – allele that has no noticeable effect on the phenotype

• Codominant – simultaneous expression of two different alleles

Why cells divideMitosis: simple cell division (2N 2N)

• growth of multicellular organism from single cell: zygote

• replacement of cells in organisms

• asexual reproduction of unicellular forms of life

Meiosis: reductional division (2N 1N)

• formation of sex cells (gametes: sperm/egg)

mitosis

meiosis

chromosomes consist of identical “sister chromatids”: replicated DNA

chromosome at onset of cell

division

Meiosis I

Meiosis II

...produces 4 haploid gametes

Meiosis1 diploid cell...

Mendelian genetics

Gregor Mendel

Mendelian genetics : the fundamentals

Principle of segregation:Alleles are placed in gametes at random

(50:50) and combine at random in progeny

Principle of independent assortment: Alleles at unlinked loci segregate independently

Principle 1 –SegregationAlleles segregate 50:50 and combine at random

Yy

y

Yy

Y y

Y yyYY

Yy

Yy

yyyy

Principle 2: Independent assortmentAlleles at unlinked loci assort independently

YySs

YS Ys yS ys

YS

Ys

yS

ys

YySs

YYSS

YYSs

YySSYYSs

YySsYySS

YYss YySs

YySs Yyss

Yyss

YySs

yySS

yySs

yySs

yyss

all gametecombinationspossible

all zygote combinationspossible(& predictable)

Sex Determination

What is sexual reproduction? The fusion of gametes

Many organisms have2 mating types or sexes(usually, but not always, male and female)

sea urchin egg & sperms

Sex DeterminationChromosomal sex determination

XX = female, XY = male

X Y

X XX XY

X XX XY

Dad

Mom

XB Y

Xb

Xb

XBXb

XBXb

XbY

XbY

X-linked & Y-linked loci do not assort independently from sex chromosomes

Single gene human disordersRecessive:AlbinismCystic fibrosisPhenylketonuriaSickle cell disease

Dominant:DwarfismAlzheimer’s (one type)Huntington’s disease

Single gene human disordersRecessive:Cystic fibrosis 1 in 1,800 European AmericansPhenylketonuria 1 in 10,000 in US & EuropeSickle cell disease 1 in 500 African americans

Dominant:Dwarfism 1 in 25,000Alzheimer’s (one type) unknownHuntington’s disease 1 in 25,000

Autosomal recessiveDeafness

X linked recessive

Hemophilia

Red-green color blind

Often Environment Matters!

Sometimes, genes and environment interact fairly simply

Temperature-dependent expression of coat color genes in both Himalayan rabbits and Siamese cats

Complications..... Incomplete dominance (parental phenotypes “blend”)

Pleiotropy (1 locus affects multiple traits)Complex traits (multiple loci encode a single trait)Epistasis (interactions between loci)Linkage (non-independent segregation of loci)

Phenotypic plasticity (expression of genotype depends on the environment)

& many more......