Download Genetics

Genetics
Gregor Mendel
Genetic Concepts
• Phenotype
– Physical characteristics of organism
– Result of gene expression
– Biochemical properties of proteins determine
physical characteristics of organism
• Genotype
– Set of alleles present in genome of organism
– Alleles are versions of genes
Segregating Traits in Sweet Pea
Genetic Crosses
• Monohybrid
– cross between two variants of a single trait
– ABO type A x ABO type B
• Dihybrid
– cross between two variants of two traits
– ABO type A, Rh+ x ABO type B, Rh-
Mendel’s Monohybrid Crosses
Pea Color Trait
Generation
P
G
x
F1
Y
All Yellow
x
F2
¼ of progeny
green
What Does It Mean, Mendel???
• The disappearance of the green trait in F1 means it
is recessive
• The appearance of only the yellow trait in F1 means
it is dominant
• A recessive trait is seen if only recessive alleles of
that gene are present
• The dominant trait is seen if 1 or 2 alleles of that
gene are present
• THERE MUST BE TWO ALLELES OF EACH
GENE
• THE ALLELES SEPARATE DURING
REPRODUCTION
Mendel’s Dyhybrid Crosses
Pea Color Trait & Height Trait
Generation
P
G
x
Y
All Yellow &
Tall
F1
x
Mendel’s Dihybrid Cross
F2
9/16 yellow & tall
3/16 green & tall
3/16 yellow & short
1/16 green & short
What Does It Mean, Mendel???
• Yellow and Tall are dominant traits; green &
short are recessive
• EITHER ALLELE CONTROLLING COLOR
CAN ASSORT WITH EITHER ALLELE
CONTROLLING HEIGHT
Mendel’s Principles
• Law of Segregation
– There are two alleles (distinct copies) of each gene
in the genome of an organism
– The two alleles are separated during meiosis and put
into separate gametes
• Law of Independent Assortment
– Any allele of one gene may assort with any allele of
any other gene
– All possible allelic combinations are possible if
enough gametes are produced
Chromosomes & Genes
Homologous Chromosomes
Sister
Chromatids
Maternal
Homolog
Paternal
Homolog
Loci
Alleles
Non-Sister
Chromatids
Meiosis I
Crossing Over (Not with John Edward)
y
Y
z
Z
Meiosis II
Inheritance Patterns
• Complete Dominance
• Co-dominance
• Intermediate Dominance
Complete Dominance/Recessiveness
• Phenotype: ABO type A
• Genotype: IAIA, IAO
• Phenotype: ABO type B
• Genotype: IBIB, IBO
• Phenotype: ABO type O
• Genotype: OO
Co-Dominance
• Phenotype: ABO type A
• Genotype: IAIB
•
•
•
•
Both A & B alleles encode functional enzymes
Both enzymes work at same time
Both modifications are made simultaneously
Phenotype is combined effect of both A & B
enzymes
Intermediate
Dominance
• A dosage effect
• B better than B’
• BB > BB’ > B’B’
• Red, pink, white flowers
• RR > RW > WW
Linkage
• Genes on same chromosome are separated
by crossing over during meiosis I
• Genes located near each other, on the same
chromosome, are separated infrequently
• Often they are inherited together
• This is termed “being linked”
Chromosomal Sex Determination
• In most organisms, males & females have a
unique pair of chromosomes which
determine sex
• XY mechanism
– Males – XY; females - XX
• ZW mechanism
– Males – ZZ; females - ZW
• X0 mechanism
– Males – X_ ; females - XX
Sex Linkage
Normal
• Genes on the X or Y
chromosomes are
sex-linked
• A phenotype caused
by a gene on the X
chromosome will
appear in males more
frequently
Normal
DNA
Complementary Base
Pairing