Genetics
Defined
• The study of genes and their
transmission from one generation
to the next.
Genes
• portions of the chromosome
– many genes per chromosome
• approx 25,000 human genes
• only 2% of chromosome is genes!
– 98% “junk” DNA
• code for traits
– genetically determined characteristics
(things that make you unique)
Genes
• portions of the chromosome
– many genes per chromosome
• approx 25,000 human genes
• only 2% of chromosome is genes
– 98% “junk” DNA
• code for traits
– genetically determined characteristics
(things that make you unique)
traits
Chromosomes
• 23 pairs of chromosomes
– 22 pairs of autosomes
– 1 pair of sex chromosomes (XX or
XY)
• autosomes are homologous
• look alike
• same genes in same locations (loci)
• two of each gene
– one from mother, one from father
Homozygous or Heterozygous?
• homozygous
(homo = same)
– same genes for a trait
• heterozygous
(hetero = different)
– different genes for a trait
• allele
– variant of a gene
Phenotype vs Genotype
• phenotype
– the physical expression of genes
• appearance, e.g. attached earlobe
• genotype
– the genes present in an individual
• the set of alleles a person possesses
– determines phenotype
Simple dominant/recessive
relationship
• dominant gene
– always expressed
– signified by upper case letter
• recessive gene
– only expressed if homozygous
– signified by lower case letter
Father of Genetics
1823-1884
St Augustine monastery,
Brno
pea traits
flower parts
Results from Mendel's Experiments
Parental Cross F1 Phenotype F2 Phenotypic Ratio F2 Ratio
Round x Wrinkled 5474 Round:1850
Seed Round Wrinkled 2.96:1
6022 Yellow:2001
Yellow x Green Seeds Yellow Green 3.01:1
Red x White Flowers Red 705 Red:224 White 3.15:1
Tall x Dwarf Plants Tall l787 Tall:227 Dwarf 2.84:1
Punnett square
• used to predict frequencies of offspring
• one trait = 2 x 2 square
– two traits (dihybrid) = 4 x 4 square
• female gametes along one side
– possible eggs
• male gametes along other side
– possible sperm
Incomplete dominance
• new phenotype appears
– 1:2:1 ratio, not 3:1, in F2
• examples
– flower color (red x white)
– curly & straight hair
– sickle cell anemia
sickled RBC’s
Codominance
• both alleles apparent in
phenotype
• examples
– blood types (A, B, o)
– cattle color (RW fur)
blood types
• type A AA or Ao
• type B BB or Bo
• type AB AB
• type O oo
Polygenic inheritance
• More than one set of genes determines
phenotype
– continuous variation of phenotypes
• e.g., eye color, height
– bell shaped curve
• distribution of phenotypes
Sex linked traits
• genes for traits on X chromosome
• Y chromosome
– not considered a homologue
– recessive X genes expressed
• color blindness
– cones for color lacking
– typically red-green
X and Y chromosomes
problem 1
problem 2
problem 3
problem 4
problem 5
problem 6
problem 7
problem 8
problem 9
The End