dna & heredity-ii
TRANSCRIPT
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......